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The importance of offspring value: maternal defence in parasitoid contests
ANIMAL BEHAVIOUR, 2007, 74, 437e446 doi:10.1016/j.anbehav.2006.11.029
The importance of offspring value: maternal defence in parasitoid contests M AR L E` NE G O UB A UL T, DAN IEL S COTT & IA N C . W. HA R DY
School of Biosciences, University of Nottingham (Received 16 August 2006; initial acceptance 6 October 2006; final acceptance 3 November 2006; published online 20 August 2007; MS. number: 9080)
Parent investment theory predicts that parents should adjust their investment in offspring defence according to offspring value. For instance, parents should protect older, more valuable, offspring more intensively than younger offspring against a given risk of mortality. The benefits of protection may, however, vary with offspring age and parents should thus behave according to the harm that offspring would suffer in the absence of parental defence. Prior studies on this topic have focused mainly on birds and mammals with data on other taxa largely lacking. We used the parasitoid wasp Goniozus nephantidis to investigate the effect of offspring developmental stage on maternal defence against infanticidal conspecific females. On finding a host caterpillar, a G. nephantidis female paralyses it and lays eggs onto it approximately 1 day later, then remains with the offspring during development until the offspring pupate. If the host is encountered by a second female, classic ownereintruder contests ensue, with the loser being aggressively excluded from the vicinity of the host. We tested the effect of brood developmental stage on contest outcomes and also assessed the consequences of offspring age for their survival in the event that an intruder female wins the contest. We found that females defend younger offspring more than older offspring. Explanations for this behaviour are discussed in terms of ownereintruder asymmetries in resource value and the ‘harm to offspring hypothesis’. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: Goniozus nephantidis; maternal investment; offspring age/brood stage; pairwise contest; parasitoid wasp; parental care
In many animal species, offspring growth and survival depend on parental care, such as incubating offspring and then feeding and protecting them (Archer 1988; CluttonBrock & Godfray 1991). In some species predation is the major factor determining offspring mortality and predators may include conspecific adults that commit infanticide (Ricklefs 1969; Wolff & Peterson 1998; Koskela et al. 2000). Indeed, infanticide is an important source of juvenile mortality in more than 1300 animal taxa (Jenssen et al. 1989). Parental investment in offspring protection can be costly in terms of parents’ ability to reproduce, leading to a trade-off between investment in current and future offspring (Trivers 1972; Dawkins & Carlisle 1976). To maximize their lifetime reproductive success, parents should adjust current investment according to offspring reproductive value. Factors determining
Correspondence: I. C. W. Hardy, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, U.K. (email: [email protected]). 0003e 3472/07/$30.00/0
reproductive value should therefore influence decisions to take part in risky or energetically costly offspring defence behaviour (reviewed in Montgomerie & Weatherhead 1988; ¨ nen et al. 1995). For instance, parents Redondo 1989; Rytko should protect more intensively larger rather than smaller offspring groups (e.g. Carlisle 1985; Koskela et al. 2000). Similarly, since the probability of reaching maturity and the parental investment necessary to replace offspring at the equivalent developmental stage both increase with offspring age, parents should protect older rather than younger off¨ nen spring against a given risk of offspring death (e.g. Rytko et al. 1995; Jaroensutasinee & Jaroensutasinee 2003). Other factors such as the conspicuousness of the nest (‘vulnerability’ hypothesis; Onnebrink & Curio 1991) or the relative harm that offspring would suffer in the absence of parental care (‘harm to offspring’ hypothesis; Dale et al. 1996; Listøen et al. 2000) may also affect the cost/benefit balance of optimal parental defence. Theoretical and empirical studies on parental defence of offspring have mainly focused on vertebrates, especially
437 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 74, 3
birds, mammals and fishes (e.g. Archer 1988; Montgomerie & Weatherhead 1988; Redondo 1989; Clutton-Brock 1991; Maestripieri 1992). Far less is known about parental defence in invertebrates, although defence in crustaceans (Figler et al. 2001) and several species of insects (see Tallamy 1984; Tallamy & Wood 1986; Archer 1988) has been observed. Studies of parental protection in insects mainly concern evolution of parental care (Klemperer 1982; Scott 1990, 1994), especially when defending their offspring against predators and allospecific competitors (Tallamy & Denno 1981; Trumbo 1990; Cocroft 2002). Protection of offspring against conspecific competitors has only rarely been assessed (Hardy & Blackburn 1991; Field & Calbert 1999). In this study we link predictions from parental investment theory and from contest theory (e.g. Maynard Smith 1974; see below) by investigating factors influencing the outcome of contests between females in the context of offspring protection. Our study system is the wasp Goniozus nephantidis Muesebeck (Hymenoptera: Bethylidae), a gregarious ectoparasitoid of larvae of the coconut pest Opisina arenosella Walker (Lepidoptera: Oecophoridae) (Cock & Perera 1987). A female, on encountering a suitable host, stings and paralyses it, laying a clutch of 3e18 eggs (dependant on the size of the host larva; Hardy et al. 1992) about 24 h later. Females guard paralysed hosts during the preoviposition period (Petersen & Hardy 1996) and also after oviposition, while their offspring develop as larvae (Antony & Kurian 1960; Remadevi et al. 1981; Cock & Perera 1987). This guarding behaviour constitutes parental care via defence against superparasitism and multiparasitism (oviposition on the same host by conspecific and allospecific parasitoids, respectively), each of which can considerably reduce offspring survival (Hardy & Blackburn 1991): conspecific females gaining access to a previously parasitized host bearing a clutch of eggs, always commit ovicide by eating the eggs before depositing their own clutch (Hardy & Blackburn 1991). Thus the offspring of only one G. nephantidis female are present on a host at a given time, in contrast to the superparasitism without ovicide observed in many other parasitoid species. Hosts bearing broods at the larval stage of development are rarely superparasitized by conspecific females, and conspecific larvicide has been reported as absent but larvicide by allospecific female parasitoids may occur (Hardy & Blackburn 1991). When a female guarding a host (the ‘owner’) is encountered by a conspecific female (the ‘intruder’), the females usually engage in agonistic behaviour that escalates into brief, but violent, fights with the loser excluded from the vicinity of the host (Petersen & Hardy 1996; Stokkebo & Hardy 2000). Although injuries from such interactions appear to be rare, one instance of fatal fighting has been observed (Humphries et al. 2006). As predicted by game theoretic models (e.g. Maynard Smith 1974; Maynard Smith & Parker 1976; Hammerstein 1981; Enquist & Leimar 1987), the outcomes of G. nephantidis contests occurring during the preoviposition period are influenced by asymmetries between contestant females. These are the differences between females in (1) their ability to acquire and retain the resource (‘Resource-Holding Potential’, RHP) and (2) the value they place on the resource (‘Resource
Value’, RV). Specifically, larger G. nephantidis females are more likely to win contests (body size constituting a resource-uncorrelated component of RHP, Petersen & Hardy 1996). Prior owners are also advantaged, and tend to win against moderately larger intruders (Petersen & Hardy 1996). While owner-advantage could potentially be interpreted as a resource-correlated component of RHP, because of mechanistic advantages (e.g. Stutt & Willmer 1998) or evolved conventions (Maynard Smith 1982), these explanations are unlikely to apply to G. nephantidis: a more probable explanation is that ownership generates an ownereintruder resource value asymmetry because ownership stimulates egg maturation such that owners are more quickly able to exploit the host resource (Stokkebo & Hardy 2000). Goniozus nephantidis contest outcomes also appear to be influenced by RV asymmetries associated with contestant age and host size: in ownereintruder contests, older intruders are more successful, and in ownereowner contests (in which asymmetries in ownership status are absent) owners of the larger (higher quality) host are advantaged (Humphries et al. 2006). Our study primarily involved evaluating the effect of varying the developmental stage (reproductive value) of the brood of offspring that owner females defend during contest interactions, and extended the experiments of Hardy & Blackburn (1991) which did not assess effects of contestant size, egg load or host size (all of which have subsequently been shown to influence contest outcomes, see above). We also assessed two aspects of G. nephantidis behaviour that are likely to underpin an understanding of any changes in contest outcomes as offspring develop: the guarding time investment made by unchallenged mothers and the consequences of intruder take-over for offspring survival to maturity. Prior work has shown that undisturbed females remain with or near paralysed hosts before oviposition (Petersen & Hardy 1996) but there has been no experimental evaluation of the probability and timing of a female choosing to abandon offspring postoviposition.
METHODS
Rearing Procedure Goniozus nephantidis wasps were reared on larvae of the substitute host Corcyra cephalonica Stainton (Lepidoptera: Pyralidae). Cultures and experiments were carried out in a climate room (12:12 h light:dark cycle at 27 C) with relative humidity maintained by evaporation from a water bath. The host was the same as used in previous studies (Hardy & Blackburn 1991; Petersen & Hardy 1996; Stokkebo & Hardy 2000; Humphries et al. 2006) and the parasitoid strain was obtained from Sreekanth Reddy (University of Agricultural Sciences, Bangalore, India). All culturing procedures were the same as previously described (Stokkebo & Hardy 2000).
Time Invested by Unchallenged Mothers To evaluate how long the mothers stay with their brood when they are unchallenged and free to leave their brood,
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
2e3-day-old females (N ¼ 20) of known weight (to an accuracy of 0.01 mg) were individually provided with hosts weighing 30e40 mg until the hosts were paralysed. They were then placed, with their host, into a one-chamber plastic block covered with a transparent lid which was within a sealed transparent plastic 1-litre container. There was a slot through the plastic block which was left open, allowing females to leave the interior chamber, but not the larger container. This experimental set-up was very similar to that illustrated in Hardy et al. (1999), where it was used to study postemergence dispersal of G. nephantidis offspring. The location of each female was recorded every 12 h until the female died. Locations were classified as (1) in the chamber with the host, (2) in the slot or (3) outside the block (i.e. elsewhere within the 1-litre container). The containers were not moved to avoid disturbing the females. When visible with the naked eye, the developmental stage reached by any offspring was noted.
Maternal Success during Contests To determine the effects of female size, brood stage and host size on contest outcomes, we used 2e3-day-old mated females, kept in isolation in glass vials following emergence as adults to avoid any prior contest experience. After being briefly anaesthetized with carbon dioxide, they were marked on the dorsal surface of the thorax by a dot of red or yellow water-based acrylic paint. A C. cephalonica host larva was introduced into the tube of some of the females; having paralysed the host these females were termed ‘owners’. Hosts were of known weight (mean 33.59 mg) ranging between 20 and 46 mg, but the majority were between 30 and 40 mg. Three categories of owners, defined according to the developmental stage of their brood, were used in contest experiments: (1) owners of paralysed hosts which had not been oviposited on, (2) owners of hosts on which they had laid a clutch of eggs and (3) owners of hosts on which the brood had reached an advanced larval stage. The number of eggs or larvae present on the hosts was counted before the experiments. In contrast, those females that were not given prior access to a host were used as ‘intruders’ in contest experiments. Dyadic contests were set up between females of similar age, bearing different colour (red versus yellow) marks and coming from different broods. Contests were observed in the ‘contest block’ (illustrated and described in Petersen & Hardy 1996; see also Goubault et al. 2006): this apparatus has three chambers interconnected by a slot. Females can pass through the slot when it is not blocked by movable barriers. When contests were set up between two owners, both were placed with their hosts in the central chamber but were initially separated from each other by a barrier. In ownereintruder replicates, intruders were placed in a lateral chamber and owners with their hosts in the central chamber, and kept separate by a barrier. For both types of contests, after a settlement time of 30 min, the barriers were withdrawn allowing both females access to the entire interior of the contest block. Contests were recorded for 90 min by a video camera placed above the central
chamber. Immediately after the end of an observation period, the female in possession of (i.e. in close proximity to) the host(s) was considered as the winner (Petersen & Hardy 1996; Humphries et al. 2006) and the number of any eggs and larvae on the host(s) was checked. Both females were then weighed to an accuracy of 0.01 mg and dissected to determine their egg load (Stokkebo & Hardy 2000). A total of 60 ownereintruder contests were observed, with 20 replicates involving an owner belonging to each of the brood stage categories (see above). There were 60 ownereowner contest replicates: in each case the contestant females belonged to different brood stage categories, with 20 replicates for each combination.
Consequences of Contest Outcome for Offspring Survival We measured the survival of offspring in the presence of their mother or an intruder female. To do so, females of known weight were individually placed with a 30e40 mg host until it was either paralysed or bore eggs or advanced larvae. The host was then removed from the tube and then either replaced with the owner female (control: N ¼ 20 for each of the three host categories) or transferred to a tube with a nonowner female of known weight (N ¼ 20 for each of the three host types). To assess the developmental success until adulthood, the number of eggs and/or larvae on the hosts was checked every day until the emergence of new adults from their cocoons. When hosts bore a clutch of eggs at the time of transfer or replacement, these eggs were individually marked with eosin dye (Hardy & Blackburn 1991): this allowed distinction between eggs laid before and after experimental host handling.
Statistical Analysis Data were analysed using generalized linear modelling with Genstat statistical package (Version 8, VSN International, Hemel Hempstead, U.K.). Our general approach was to use (semi-)parametric analyses in which the assumed distribution of residuals was matched to the data rather than transforming data to fit standard Gaussian assumptions (Wilson & Hardy 2002). We followed backward stepwise procedures, with initial statistical models containing all explanatory variables of interest and the sequence of deletion from the model determined by parameter inspection, with only significant terms reentered to obtain a parsimonious ‘minimal adequate model’ (Crawley 1993; Quinn & Keough 2002; Wilson & Hardy 2002). When explanatory variables are highly mutually correlated, their simultaneous inclusion in a model can lead to interpretational problems because of ‘collinearity’ (Grafen & Hails 2002; Quinn & Keough 2002). We followed Humphries et al. (2006) in, therefore, excluding egg load (highly correlated with female weight) and number of offspring on host (correlated with host weight) from the main analyses (see Results). We used survival analysis (Crawley 1993) to examine the effects of female weight, host weight and number of offspring at the time at which females were first observed
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outside the observation block (see also Hardy et al. 1999). We used a Weibull distribution (suitable for timedependent hazard functions) after checking that this provided a significantly better fit to the data than an exponential distribution assuming a constant hazard (Crawley 1993). Factors influencing contest outcomes (a proportional response variable) were explored using logistic analyses (Hardy & Field 1998) except when initially checking that the colour of the paint mark had no influence on contest outcomes, where binomial tests were used (as in Petersen & Hardy 1996; Stokkebo & Hardy 2000). Log-linear analyses (suitable for ‘small count’ response variables, Crawley 1993) were used to investigate relationships between female egg load and weight, number of offspring and host weight and to compare the number of eggs laid by females on the different host types. The proportions of offspring surviving to adulthood when in the presence of their mother or another female were compared by logistic analyses.
RESULTS
Time Invested by Unchallenged Mothers Females tended to stay on or close to their host until their eggs hatched (Fig. 1). Once eggs hatched, females generally spent more time in the slot of the experimental block: they were usually near to the chamber rather than outside of the block and appeared to be guarding the entrance of the chamber. Females usually left the block about 24 h after their larvae had spun cocoons (Fig. 1). Times at which females were first observed out of the block were not distributed randomly but were timedependent (a Weibull model provided better fit than an Female in the observation block:
exponential model: G1 ¼ 22.35, P < 0.001) with the tendency to leave the block increasing from around day 6 (Fig. 1). The time at which females were first observed outside the block was not influenced by the female’s weight (G1 ¼ 1.78, P > 0.05), the weight of the host (G1 ¼ 1.99, P > 0.05) or the number of adult offspring that were produced from the host (G1 ¼ 1.51, P > 0.05).
Maternal Success During Contests Female characteristics: egg load and brood size Egg loads were highly variable but were positively correlated with female weight (F1,235 ¼ 13.07, P < 0.001, r2 ¼ 0.04) and affected by host type (F3,235 ¼ 26.58, P < 0.001, r2 ¼ 0.24) but the interaction between female weight and host type was not significant. Model simplification by aggregation of host type factor levels (Crawley 1993, page 190) showed that, for a given body size, females bearing a brood of advanced larvae had the largest egg loads (F1,237 ¼ 4.39, P ¼ 0.037) whereas those females bearing a clutch of eggs (i.e. having laid eggs in the last 24 h) had the least (F1,237 ¼ 57.02, P < 0.001). Intruders and females with paralysed hosts had intermediate and similar egg loads (F1,236 ¼ 0.01, P ¼ 0.94). Figure 2 illustrates the minimum adequate model obtained by this analysis. The number of eggs or larvae present on the hosts was influenced by host weight (F1,119 ¼ 22.00, P < 0.001) but not by the offspring developmental stage (i.e. eggs or larvae) (F1,119 ¼ 0.13, P ¼ 0.72).
Contests Ownereintruder contests. To determine the effects of female size, brood stage and host size on contest outcomes, we first considered contest outcome as a binary
With host In slot
Female out of the observation block:
Alive Dead
20
15 Number of females
440
10
5
0
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13
Brood stage
Time(days) Eggs
Larvae
Cocoons
Figure 1. Female location in relation to time and offspring development.
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
Regression Intruder + Paralysed Regression Eggs Regression Larvae
Intruder Paralysed Eggs Larvae 16
14
12
Egg load
10
8
6
4
2
0 0.4
0.6
0.8
1 1.2 Female weight
1.4
1.6
1.8
Figure 2. Relationship between female egg load and weight according to brood stage. The results for intruder females and females defending a paralysed host did not differ significantly and are represented by a single regression line. The observed data points for the four types of females are presented separately to show the variability between the treatments.
response: 1 ¼ intruder won or 0 ¼ intruder lost. We explored the effect of the (i) difference in weight between contestants (intruder weight minus owner weight), (ii) owner’s brood stage (paralysed host, host bearing eggs or host bearing larvae) and (iii) host weight, and their interactions. The probability of the intruder winning was enhanced by being larger than the owner (G1 ¼ 5.94, P ¼ 0.015). The probability of the intruder winning also depended on the stage of the owner’s brood (G2 ¼ 3.28, P ¼ 0.037): the intruders won more often against the owner of a brood of advanced larvae than the other types of owners (with eggs or paralysed host; Fig. 3). Contest outcomes were not affected by the weight of the owner’s host (G1 ¼ 0.80, P ¼ 0.37) or any interaction between the above explanatory variables. Furthermore, the number of offspring did not affect the probability of the intruder winning (simple logistic regression: G1 ¼ 0.04, P ¼ 0.83). Second, to assess the importance of ownership, we redefined the contest outcomes in terms of the colour of the winning female: 1 ¼ red wasp won, 0 ¼ red wasp lost (see Petersen & Hardy 1996; Stokkebo & Hardy 2000). The marking colour was unrelated to contest outcomes (red wasps won 35 of 60 contests, two-tailed binomial test: P ¼ 0.25). A simple logistic analysis using the colour of the winning female as the response variable showed that owners were more likely to win the contests (G1 ¼ 5.65, P ¼ 0.017). A multiple logistic analysis including
ownership, female weight difference and brood stage as explanatory variables confirmed that all these explanatory variables were involved in significant terms (interaction between ownership and female weight: G1 ¼ 4.17, P ¼ 0.041; interaction between ownership and owner’s brood stage: G2 ¼ 3.65, P ¼ 0.026). In three of the 20 replicates involving hosts bearing a brood of larvae, intruders were observed grasping the larvae in their mandibles and pulling them away from the host, which resulted in the death of part or all of the brood. Ownereowner contests. To assess the effect of hosts’, females’ and broods’ properties on contest outcomes, we defined the contest outcomes as a binary response: 1 ¼ female with brood at a later stage won, 0 ¼ female at a later stage lost. We then tested the following explanatory variables: (i) difference in host weight (host weight of the female at the later stage minus host weight of the female at the earlier stage), (ii) difference in female weight (weight of the female at the later stage minus weight of the female at the earlier stage) and (iii) treatment (paralysed host versus host with eggs, host with eggs versus host with larvae, and paralysed host versus host with larvae) and their interactions. Females at a later stage were more likely to win contests when their host was bigger than their opponent’s (G1 ¼ 5.05, P ¼ 0.025, Fig. 4). Brood stage treatment had a marginally nonsignificant effect on contest outcome (G2 ¼ 2.91, P ¼ 0.055): because of the
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Owner with paralysed host Owner with eggs Owner with larvae
Regression paralysed + eggs Regression larvae
1
0.75 Intruder take over
442
0.50
0.25
0 -0.8
-0.6
-0.4 -0.2 0 0.2 Intruder weight–Owner weight
0.4
0.6
Figure 3. Ownereintruder contests: probability of the intruder winning in relation to differences in female weight and brood stage. Results for the treatments in which an owner defends a paralysed host or a host bearing a clutch of eggs did not differ significantly and are represented by a single regression line. Data points have been vertically displaced from their binary positions to show the numbers of observations.
lack of precision in probability estimates obtained from logistic analyses (e.g. Crawley 1993, page 278) this result may be regarded as equivocal). There were similar likelihoods of a female with host bearing larvae winning against a female with a paralysed host and a female with larvae winning against a female with eggs. When the data from these two categories were pooled, contest outcomes were significantly influenced by brood stage (G1 ¼ 5.28, P ¼ 0.022): females with eggs won more frequently against females with paralysed hosts than females with larvae won against females with paralysed hosts or eggs (Fig. 4). Female size difference did not affect contest outcome (G1 ¼ 0.46, P ¼ 0.495) nor did any of the interactions between fitted main effects. The difference in the number of offspring produced by contestant females did not affect the probability of the female with offspring at the later developmental stage winning (simple logistic analysis: G1 ¼ 0.36, P ¼ 0.55).
Offspring Survival The probability of the mother’s eggs surviving to adulthood was not affected by host weight (F1,39 ¼ 3.09, P ¼ 0.09) but was considerably reduced when in the presence of a female other than the mother (Table 1). Intruders ate the mother’s eggs before laying their own clutch, causing a significant reduction of egg to larval survivorship (F1,39 ¼ 58.29, P < 0.001). The presence of intruders did not, however, affect survivorship from the larval to the cocoon stage (F1,23 ¼ 0.01, P ¼ 0.94) or the cocoon to adult survivorship
(F1,19 ¼ 0.00, P ¼ 0.99). Clutches of eggs laid by intruders were often smaller than the clutch originally laid by the mother (F1,39 ¼ 5.33, P ¼ 0.027) and intruder clutch size was unrelated to host weight (F1,39 ¼ 0.34, P ¼ 0.56). When offspring had reached the larval stage, the presence of an intruder did not affect their survival to adulthood (Table 1). Contrary to observations during ownereintruder contests, no intruders were seen pulling larvae away from their hosts. Host weight did not influence the probability of larvae surviving to the cocoon stage (F1,39 ¼ 0.01, P ¼ 0.91). The survival to adulthood of eggs laid on paralysed hosts by intruders was similar to that of the owners’ eggs (F1,39 ¼ 0.81, P ¼ 0.37) and was unaffected by host weight (F1,39 0.00, P ¼ 0.98; Tables 1 and 2). Intruder females laid larger clutches on paralysed hosts and hosts that bore eggs than on hosts that bore larvae (Table 2). We detected no influence of host type on the probability of intruders’ eggs surviving to adulthood (Table 2) but note that intruders laid eggs on hosts originally bearing larvae in only two replicates and none of their offspring survived. DISCUSSION As already shown by previous studies of femaleefemale contests in G. nephantidis (Hardy & Blackburn 1991; Petersen & Hardy 1996; Stokkebo & Hardy 2000; Humphries et al. 2006), we found advantages of being larger and of being the owner: owners usually retained their host against intruders, except when intruders were substantially larger. When the ownership asymmetry was absent
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
Eggs vs paralysed
Regression eggs vs paralysed
Larvae vs paralysed
Regression larvae vs paralysed + larvae vs eggs
Larvae vs eggs 1
Later stage female won
0.75
0.50
0.25
0 −12
−10
−8
−6
−4
−2
0
2
4
6
8
10
Later stage female's host weight–earlier stage female's host weight Figure 4. Ownereowner contests: probability of the female which was more advanced with offspring production winning ownereowner contests in relation to differences in host weight and brood stage. Results for treatments where females defend a host bearing larvae against either a female defending a paralysed host or a host bearing a clutch of eggs did not differ significantly and are represented by a single regression line. Data points have been vertically displaced from their binary positions to show numbers of observations.
(ownereowner contests), host size influenced the outcomes: females with larger host usually won (as observed by Humphries et al. 2006). Our results further show that the developmental stage of the brood affects contest outcome. According to the ‘offspring reproductive value’ hypothesis of parental investment theory, older broods should be
Table 1. Percentage of eggs or larvae surviving to adulthood according to which female (mother or intruder) was in the presence of the host Mother’s brood Paralysed host Eggs Larvae
more intensively defended by parents as reproductive value increases with brood age (Montgomerie & Weatherhead 1988; Rytko¨nen et al. 1995). In G. nephantidis, an increase in offspring value would contribute to the resource value (RV) of the host defended by owners and should consequently affect contest outcome. As expected, females
Table 2. Intruder’s clutch size and egg-to-adult survivorship in relation to the stage of the brood on the owner’s host
Female with host
Mean percentage of eggs/larvae surviving to adulthood (N )
Logistic analysis corrected for overdispersion
Owner’s brood
Mother
77.1 (20)
e
Paralysed host Eggs Larvae
Mother Intruder Mother Intruder
57.7 5.5 82.1 64.5
(20) (20) (20) (20)
F1,39¼35.0, P<0.001 F1,39¼2.93, P¼0.095
Average intruder’s clutch size (N )
Mean percentage of intruder’s eggs surviving to adulthood (N )
9.7 (20) 8.6 (20) 0.7 (20) F2,59¼72.70, P<0.001*
59.9 (20) 63.7 (20) 0 (2) F2,41¼2.28, P¼0.12y
*Log-linear analysis corrected for overdispersion. yLogistic analysis corrected for overdispersion.
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with hosts bearing a clutch of eggs were more likely to win against females protecting a paralysed host. However, females with hosts bearing larvae were not advantaged when competing with females with paralysed hosts or hosts with eggs. Indeed, females defending larvae were more likely to be defeated by intruders than were females defending younger broods. Thus, G. nephantidis females appear to protect their older (larval stage) broods less than their younger offspring, even though the reproductive value of older offspring is higher. Such behaviour, where parents take greater risks or make greater efforts to defend younger than older broods, is expected under the ‘harm to offspring’ hypothesis, which was developed for birds (the pied flycatcher Ficedula hypoleuca, Dale et al. 1996). This prediction is based on the observation that older offspring suffer less than younger offspring if unattended (i.e. no incubation, brooding or feeding) because they conserve energy more efficiently (Dale et al. 1996). In G. nephantidis, the analogous explanation is more directly related to offspring survival: successful intruders destroy clutches of eggs but only rarely damage broods of larvae (Hardy & Blackburn 1991; this study). Unattended larvae are consequently less ‘vulnerable’ than unattended eggs. Such vulnerability is, however, unlikely to be related to the probability of G. nephantidis offspring being encountered by intruders and our results cannot be explained by the ‘vulnerability hypothesis’ which was developed for birds on the basis that older chicks call louder, making the nest more conspicuous to predators (Onnebrink & Curio 1991). The avian ‘vulnerability hypothesis’ thus predicts the opposite to what was observed in G. nephantidis: parents should protect more intensively older rather than younger offspring. Results similar to ours have, to our knowledge, only been obtained in the bank vole Clethrionomys glareolus (Koskela et al. 2000). In this mammalian species, maternal defence intensity decreases as offspring age, probably because older pups are less vulnerable to infanticide committed by male adults. It is thought that being bigger and more mobile, older pups are harder to kill than newborn pups (Wolff 1985). In G. nephantidis, conspecifics also kill older larvae less frequently than eggs, but this could be because of greater difficultly of removing larvae from a host and/or to the difference in benefit they gain from infanticide. Indeed, eggs are likely to constitute a source of nutrients since ovicide is committed by ingestion (Hardy & Blackburn 1991; this study). In contrast, when larvae are killed by being pulled away from the host, infanticidal females did not appear to feed on them (M. Goubault, personal observation). Furthermore, infanticide is a way of restoring host quality by eliminating competitors for future offspring. Destroying eggs is likely to result in the acquisition of a host of similar quality to a paralysed host, but once a host bears larvae that have started to feed, less remains of the initial host resource. In the rare instances where the intruder females laid eggs on hosts that initially bore larvae, all their offspring died during the larval stage, probably because of a shortage of host resources. Thus, older (larval) offspring may be attacked less than eggs because they are more difficult to kill and provide no nutritional resource, and
because possession of a partially depleted host is of low benefit to intruders. Despite the reduction in risk from conspecific intruders as the brood develops, G. nephantidis females usually guard their broods until pupation (in common with some bethylid species but in contrast to others which abandon the host soon after oviposition; Mayhew 1997; Takasu & Overholt 1998). Their continued presence can protect larval offspring against predators or heterospecific competitors, such as the braconid wasp Bracon hebetor. Guarding larvae is known to reduce substantially, but not prevent, the detrimental (larvicidal) actions of B. hebetor females (Hardy & Blackburn 1991). Complementary experiments would be needed to test whether offspring defence intensity in G. nephantidis depends upon the species of competitor, as in some other bethylids (Pe´rez-Lachaud et al. 2002; Batchelor et al. 2005). In accord with previous work on G. nephantidis (Stokkebo & Hardy 2000; Humphries et al. 2006), we found a positive correlation between egg load and female weight. We did not detect egg maturation during the preoviposition period (owners with paralysed hosts did not have more eggs than intruders, contra Stokkebo & Hardy 2000). We found that female’s egg loads were higher when broods were at the larval stage than prior to oviposition (immediately after oviposition, egg loads were obviously depleted). These confirm recent egg-load observations (Humphries et al. 2006) and suggest that in nature brood guarding females anticipate future reproductive opportunities (in the laboratory, females are able to produce many broods, Hardy et al. 1992). In semelparous species (one reproductive event per lifetime), the reproductive value of offspring is likely to be higher than under iteroparity and mothers should always guard and fiercely defend their brood. Thus, the higher egg load of females that have already laid, the ability to reproduce repeatedly in the laboratory, the variable investment in brood protection according to the offspring developmental stage and the tendency to cease brood guarding as offspring mature, all countersuggest that G. nephantidis is (effectively) semelparous (Cock & Perera 1987; Hardy et al. 1992) and suggest anticipated future reproduction as an explanation for why G. nephantidis clutch size is smaller than the optimum calculated from laboratory-estimated fitness parameters and the assumption of semelparity (Hardy et al. 1992; see also Petersen & Hardy 1996; Mesterton-Gibbons & Hardy 2004). In conclusion, our results show that G. nephantidis females adjust maternal investment according to changes in both offspring reproductive value and the risk of their brood being destroyed by infanticidal intruders. Acknowledgments This work is an Anglo-French collaboration concerning optimal investment: we hope to have avoided the Concorde fallacy. We thank Tim Batchelor and David Heron for help and discussion, Julietta Marquez for technical assistance, and Sreekanth Reddy for providing G. nephantidis. This research was funded by grant BB/C504778/1 from the Biotechnology and Biological Sciences Research Council, U.K.
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References Antony, J. & Kurian, C. 1960. Studies on the habits and life history of Perisierola nephantidis Muesebeck. Indian Coconut Journal, 13, 145e153. Archer, J. 1988. The Behavioural Biology of Aggression. In: (Ed. by G. Barlow, P. P. G. Bateson & R. W. Oppenheim), Cambridge: Cambridge University Press. Batchelor, T. P., Hardy, I. C. W., Barrera, J. F. & Pe´rez-Lachaud, G. 2005. Insect gladiators. II: competitive interactions within and between bethylid parasitoid species of the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae). Biological Control, 33, 194e202. Carlisle, T. R. 1985. Parental response to brood size in a cichlid fish. Animal Behaviour, 33, 234e238. Clutton-Brock, T. H. 1991. The Evolution of Parental Care. In: (Ed. by J. R. Krebs & T. H. Clutton-Brock), Princeton, New Jersey: Princeton University Press. Clutton-Brock, T. & Godfray, C. 1991. Parental investment. In: Behavioural Ecology. 3rd edn (Ed. by J. R. Krebs & N. B. Davies), pp. 234e262. Oxford: Blackwell Scientific. Cock, M. J. W. & Perera, P. A. C. R. 1987. Biological control of Opisina arenosella Walker (Lepidoptera, Oecophoridae). Biocontrol News and Information, 8, 283e310. Cocroft, R. B. 2002. Antipredator defense as a limited resource: unequal predation risk in broods of an insect with maternal care. Behavioral Ecology, 13, 125e133. Crawley, M. J. 1993. GLIM for Ecologists. Oxford: Blackwell Scientific. Dale, S., Gustavsen, R. & Slagsvold, T. 1996. Risk taking during parental care: a test of three hypotheses applied to the pied flycatcher. Behavioral Ecology and Sociobiology, 39, 31e42. Dawkins, R. & Carlisle, T. R. 1976. Parental investment, mate desertion and a fallacy. Nature, 262, 131e133. Enquist, M. & Leimar, O. 1987. Evolution of fighting behaviour: the effect of variation in resource value. Journal of Theoretical Biology, 127, 187e206. Field, S. A. & Calbert, G. 1999. Don’t count your eggs before they’re parasitized: contest resolution and the tradeoffs during patch defense in a parasitoid wasp. Behavioral Ecology, 10, 122e127. Figler, M. H., Blank, G. S. & Peeke, H. V. S. 2001. Maternal territoriality as an offspring defense strategy in red swamp crayfish (Procambarus clarkii, Girard). Aggressive Behavior, 27, 391e403. Goubault, M., Batchelor, T. P., Linforth, R. S. T., Taylor, R. J. & Hardy, I. C. W. 2006. Volatile emission by contest losers revealed by real-time chemical analysis. Proceedings of the Royal Society of London, Series B, 273, 2853e2859. Grafen, A. & Hails, R. 2002. Modern Statistics for the Life Sciences. Oxford: Oxford University Press. Hammerstein, P. 1981. The role of asymmetries in animal contests. Animal Behaviour, 29, 193e205. Hardy, I. C. W. & Blackburn, T. M. 1991. Brood guarding in a bethylid wasp. Ecological Entomology, 16, 55e62. Hardy, I. C. W. & Field, S. A. 1998. Logistic analysis of animal contests. Animal Behaviour, 56, 787e792. Hardy, I. C. W., Griffiths, N. T. & Godfray, H. C. J. 1992. Clutch size in a parasitoid wasp: a manipulation experiment. Journal of Animal Ecology, 61, 121e129. Hardy, I. C. W., Pedersen, J. B., Sejr, M. K. & Linderoth, U. H. 1999. Local mating, dispersal and sex ratio in a gregarious parasitoid wasp. Ethology, 105, 57e72. Humphries, E. L., Hebblethwaite, A. J., Batchelor, T. P. & Hardy, I. C. W. 2006. The importance of valuing resources: host weight and contender age as determinants of parasitoid wasp contest outcomes. Animal Behaviour, 72, 891e898.
Jaroensutasinee, M. & Jaroensutasinee, K. 2003. Type of intruder and reproductive phase influence male territorial defence in wildcaught Siamese fighting fish. Behavioural Processes, 64, 23e29. Jenssen, T. A., Marcellini, D. L., Buhlmann, K. A. & Goforth, P. H. 1989. Differential infanticide by adult curly-tailed lizards, Leiocephalus schreibersi. Animal Behaviour, 38, 1054e1061. Klemperer, H. G. 1982. Parental behaviour in Copris lunaris (Coleoptera, Sacrabaeidae): care and defence of brood balls and nest. Ecological Entomology, 7, 155e167. Koskela, E., Juutistenaho, P., Mappes, T. & Oksanen, T. A. 2000. Offspring defence in relation to litter size and age: experiment in the bank vole Clethrionomys glareolus. Evolutionary Ecology, 14, 99e109. Listøen, C., Karlsen, R. F. & Slagsvold, T. 2000. Risk taking during parental care: a test of the harm-to-offspring hypothesis. Behavioral Ecology, 11, 40e43. Maestripieri, D. 1992. Functional aspects of maternal aggression in mammals. Canadian Journal of Zoology, 70, 1069e1077. Mayhew, P. J. 1997. Fitness consequences of ovicide in a parasitoid wasp. Entomologia Experimentalis et Applicata, 84, 115e126. Maynard Smith, J. 1974. The theory of games and the evolution of animal conflicts. Journal of Theoretical Biology, 47, 209e221. Maynard Smith, J. 1982. Evolution and the Theory of Games. Cambridge: Cambridge University Press. Maynard Smith, J. & Parker, G. A. 1976. The logic of asymmetric contests. Animal Behaviour, 24, 159e175. Mesterton-Gibbons, M. & Hardy, I. C. W. 2004. The influence of contests on optimal clutch size: a game-theoretic model. Proceedings of the Royal Society of London, Series B, 271, 971e978. Montgomerie, R. D. & Weatherhead, P. J. 1988. Risks and rewards of nest defence by parent birds. Quarterly Review of Biology, 63, 167e187. Onnebrink, H. & Curio, E. 1991. Brood defense and age of young: a test of the vulnerability hypothesis. Behavioral Ecology and Sociobiology, 29, 61e68. Pe´rez-Lachaud, G., Hardy, I. C. W. & Lachaud, J. P. 2002. Insect gladiators: competitive interactions between three species of bethylid wasps attacking the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae). Biological Control, 25, 231e238. Petersen, G. & Hardy, I. C. W. 1996. The importance of being larger: parasitoid intruder-owner contests and their implications for clutch size. Animal Behaviour, 51, 1363e1373. Quinn, G. P. & Keough, M. J. 2002. Experimental Design and Data Analysis for Biologists. Cambridge: Cambridge University Press. Redondo, T. 1989. Avian nest defence: theoretical models and evidence. Behaviour, 111, 161e195. Remadevi, O. K., Mohamed, U. V. K. & Abdurahiman, U. C. 1981. Some aspects of the biology of Parasierola nephantidis Muesebeck (Hymenoptera, Bethylidae), a larval parasitoid of Nephantis serinopa Meyrick (Lepidoptera, Xylorictidae). Polskie Pismo Entomologiczne, 51, 597e604. Ricklefs, R. E. 1969. An analysis of nesting mortality in birds. Smithsonian Contributions to Zoology, 9, 1e48. ¨ nen, S., Orell, M. & Koivula, K. 1995. Pseudo concorde falRytko lacy in the willow tit? Animal Behaviour, 49, 1017e1028. Scott, M. P. 1990. Brood guarding and the evolution of male parental care in burying beetles. Behavioral Ecology and Sociobiology, 26, 31e39. Scott, M. P. 1994. The benefit of parental assistance in intra- and interspecific competition for the burying beetle, Nicrophorus defodiens. Ethology, Ecology and Evolution, 6, 537e543. Stokkebo, S. & Hardy, I. C. W. 2000. The importance of being gravid: egg load and contest outcome in a parasitoid wasp. Animal Behaviour, 59, 1111e1118.
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Stutt, A. D. & Willmer, P. 1998. Territorial defence in speckled wood butterflies: do the hottest males always win? Animal Behaviour, 55, 1341e1347. Takasu, K. & Overholt, W. A. 1998. Brood guarding behavior and life history characteristics of Goniozus indicus Ashmead (Hymenoptera: Bethylidae), a larval ectoparasitoid of lepidopteran stemborers. Applied Entomology and Zoology, 33, 121e126. Tallamy, D. W. 1984. Insect parental care. BioScience, 34, 20e24. Tallamy, D. W. & Denno, R. F. 1981. Maternal care in Gargaphia solani (Hemiptera: Tingitidae). Animal Behaviour, 29, 771e778. Tallamy, D. W. & Wood, T. K. 1986. Convergence patterns in subsocial insects. Annual Review of Entomology, 31, 369e390.
Trivers, R. L. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man 1871e1971 (Ed. by B. Campbell), pp. 136e179. Chicago, Illinois: Aldine. Trumbo, S. T. 1990. Interference competition among burying beetles (Silphidae, Nicrophorus). Ecological Entomology, 15, 347e355. Wilson, K. & Hardy, I. C. W. 2002. Statistical analysis of sex ratios: an introduction. In: Sex Ratios: Concepts and Research Methods (Ed. by I. C. W. Hardy), pp. 48e92. Cambridge: Cambridge University Press. Wolff, J. O. 1985. Maternal aggression as a deterrent to infanticide in Peromyscus leucopus and Peromyscus maniculatus. Animal Behaviour, 33, 117e123. Wolff, J. O. & Peterson, J. A. 1998. An offspring-defense hypothesis for territoriality in female mammals. Ethology, Ecology and Evolution, 10, 227e239.
The importance of offspring value: maternal defence in parasitoid contests M AR L E` NE G O UB A UL T, DAN IEL S COTT & IA N C . W. HA R DY
School of Biosciences, University of Nottingham (Received 16 August 2006; initial acceptance 6 October 2006; final acceptance 3 November 2006; published online 20 August 2007; MS. number: 9080)
Parent investment theory predicts that parents should adjust their investment in offspring defence according to offspring value. For instance, parents should protect older, more valuable, offspring more intensively than younger offspring against a given risk of mortality. The benefits of protection may, however, vary with offspring age and parents should thus behave according to the harm that offspring would suffer in the absence of parental defence. Prior studies on this topic have focused mainly on birds and mammals with data on other taxa largely lacking. We used the parasitoid wasp Goniozus nephantidis to investigate the effect of offspring developmental stage on maternal defence against infanticidal conspecific females. On finding a host caterpillar, a G. nephantidis female paralyses it and lays eggs onto it approximately 1 day later, then remains with the offspring during development until the offspring pupate. If the host is encountered by a second female, classic ownereintruder contests ensue, with the loser being aggressively excluded from the vicinity of the host. We tested the effect of brood developmental stage on contest outcomes and also assessed the consequences of offspring age for their survival in the event that an intruder female wins the contest. We found that females defend younger offspring more than older offspring. Explanations for this behaviour are discussed in terms of ownereintruder asymmetries in resource value and the ‘harm to offspring hypothesis’. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: Goniozus nephantidis; maternal investment; offspring age/brood stage; pairwise contest; parasitoid wasp; parental care
In many animal species, offspring growth and survival depend on parental care, such as incubating offspring and then feeding and protecting them (Archer 1988; CluttonBrock & Godfray 1991). In some species predation is the major factor determining offspring mortality and predators may include conspecific adults that commit infanticide (Ricklefs 1969; Wolff & Peterson 1998; Koskela et al. 2000). Indeed, infanticide is an important source of juvenile mortality in more than 1300 animal taxa (Jenssen et al. 1989). Parental investment in offspring protection can be costly in terms of parents’ ability to reproduce, leading to a trade-off between investment in current and future offspring (Trivers 1972; Dawkins & Carlisle 1976). To maximize their lifetime reproductive success, parents should adjust current investment according to offspring reproductive value. Factors determining
Correspondence: I. C. W. Hardy, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, U.K. (email: [email protected]). 0003e 3472/07/$30.00/0
reproductive value should therefore influence decisions to take part in risky or energetically costly offspring defence behaviour (reviewed in Montgomerie & Weatherhead 1988; ¨ nen et al. 1995). For instance, parents Redondo 1989; Rytko should protect more intensively larger rather than smaller offspring groups (e.g. Carlisle 1985; Koskela et al. 2000). Similarly, since the probability of reaching maturity and the parental investment necessary to replace offspring at the equivalent developmental stage both increase with offspring age, parents should protect older rather than younger off¨ nen spring against a given risk of offspring death (e.g. Rytko et al. 1995; Jaroensutasinee & Jaroensutasinee 2003). Other factors such as the conspicuousness of the nest (‘vulnerability’ hypothesis; Onnebrink & Curio 1991) or the relative harm that offspring would suffer in the absence of parental care (‘harm to offspring’ hypothesis; Dale et al. 1996; Listøen et al. 2000) may also affect the cost/benefit balance of optimal parental defence. Theoretical and empirical studies on parental defence of offspring have mainly focused on vertebrates, especially
437 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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birds, mammals and fishes (e.g. Archer 1988; Montgomerie & Weatherhead 1988; Redondo 1989; Clutton-Brock 1991; Maestripieri 1992). Far less is known about parental defence in invertebrates, although defence in crustaceans (Figler et al. 2001) and several species of insects (see Tallamy 1984; Tallamy & Wood 1986; Archer 1988) has been observed. Studies of parental protection in insects mainly concern evolution of parental care (Klemperer 1982; Scott 1990, 1994), especially when defending their offspring against predators and allospecific competitors (Tallamy & Denno 1981; Trumbo 1990; Cocroft 2002). Protection of offspring against conspecific competitors has only rarely been assessed (Hardy & Blackburn 1991; Field & Calbert 1999). In this study we link predictions from parental investment theory and from contest theory (e.g. Maynard Smith 1974; see below) by investigating factors influencing the outcome of contests between females in the context of offspring protection. Our study system is the wasp Goniozus nephantidis Muesebeck (Hymenoptera: Bethylidae), a gregarious ectoparasitoid of larvae of the coconut pest Opisina arenosella Walker (Lepidoptera: Oecophoridae) (Cock & Perera 1987). A female, on encountering a suitable host, stings and paralyses it, laying a clutch of 3e18 eggs (dependant on the size of the host larva; Hardy et al. 1992) about 24 h later. Females guard paralysed hosts during the preoviposition period (Petersen & Hardy 1996) and also after oviposition, while their offspring develop as larvae (Antony & Kurian 1960; Remadevi et al. 1981; Cock & Perera 1987). This guarding behaviour constitutes parental care via defence against superparasitism and multiparasitism (oviposition on the same host by conspecific and allospecific parasitoids, respectively), each of which can considerably reduce offspring survival (Hardy & Blackburn 1991): conspecific females gaining access to a previously parasitized host bearing a clutch of eggs, always commit ovicide by eating the eggs before depositing their own clutch (Hardy & Blackburn 1991). Thus the offspring of only one G. nephantidis female are present on a host at a given time, in contrast to the superparasitism without ovicide observed in many other parasitoid species. Hosts bearing broods at the larval stage of development are rarely superparasitized by conspecific females, and conspecific larvicide has been reported as absent but larvicide by allospecific female parasitoids may occur (Hardy & Blackburn 1991). When a female guarding a host (the ‘owner’) is encountered by a conspecific female (the ‘intruder’), the females usually engage in agonistic behaviour that escalates into brief, but violent, fights with the loser excluded from the vicinity of the host (Petersen & Hardy 1996; Stokkebo & Hardy 2000). Although injuries from such interactions appear to be rare, one instance of fatal fighting has been observed (Humphries et al. 2006). As predicted by game theoretic models (e.g. Maynard Smith 1974; Maynard Smith & Parker 1976; Hammerstein 1981; Enquist & Leimar 1987), the outcomes of G. nephantidis contests occurring during the preoviposition period are influenced by asymmetries between contestant females. These are the differences between females in (1) their ability to acquire and retain the resource (‘Resource-Holding Potential’, RHP) and (2) the value they place on the resource (‘Resource
Value’, RV). Specifically, larger G. nephantidis females are more likely to win contests (body size constituting a resource-uncorrelated component of RHP, Petersen & Hardy 1996). Prior owners are also advantaged, and tend to win against moderately larger intruders (Petersen & Hardy 1996). While owner-advantage could potentially be interpreted as a resource-correlated component of RHP, because of mechanistic advantages (e.g. Stutt & Willmer 1998) or evolved conventions (Maynard Smith 1982), these explanations are unlikely to apply to G. nephantidis: a more probable explanation is that ownership generates an ownereintruder resource value asymmetry because ownership stimulates egg maturation such that owners are more quickly able to exploit the host resource (Stokkebo & Hardy 2000). Goniozus nephantidis contest outcomes also appear to be influenced by RV asymmetries associated with contestant age and host size: in ownereintruder contests, older intruders are more successful, and in ownereowner contests (in which asymmetries in ownership status are absent) owners of the larger (higher quality) host are advantaged (Humphries et al. 2006). Our study primarily involved evaluating the effect of varying the developmental stage (reproductive value) of the brood of offspring that owner females defend during contest interactions, and extended the experiments of Hardy & Blackburn (1991) which did not assess effects of contestant size, egg load or host size (all of which have subsequently been shown to influence contest outcomes, see above). We also assessed two aspects of G. nephantidis behaviour that are likely to underpin an understanding of any changes in contest outcomes as offspring develop: the guarding time investment made by unchallenged mothers and the consequences of intruder take-over for offspring survival to maturity. Prior work has shown that undisturbed females remain with or near paralysed hosts before oviposition (Petersen & Hardy 1996) but there has been no experimental evaluation of the probability and timing of a female choosing to abandon offspring postoviposition.
METHODS
Rearing Procedure Goniozus nephantidis wasps were reared on larvae of the substitute host Corcyra cephalonica Stainton (Lepidoptera: Pyralidae). Cultures and experiments were carried out in a climate room (12:12 h light:dark cycle at 27 C) with relative humidity maintained by evaporation from a water bath. The host was the same as used in previous studies (Hardy & Blackburn 1991; Petersen & Hardy 1996; Stokkebo & Hardy 2000; Humphries et al. 2006) and the parasitoid strain was obtained from Sreekanth Reddy (University of Agricultural Sciences, Bangalore, India). All culturing procedures were the same as previously described (Stokkebo & Hardy 2000).
Time Invested by Unchallenged Mothers To evaluate how long the mothers stay with their brood when they are unchallenged and free to leave their brood,
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
2e3-day-old females (N ¼ 20) of known weight (to an accuracy of 0.01 mg) were individually provided with hosts weighing 30e40 mg until the hosts were paralysed. They were then placed, with their host, into a one-chamber plastic block covered with a transparent lid which was within a sealed transparent plastic 1-litre container. There was a slot through the plastic block which was left open, allowing females to leave the interior chamber, but not the larger container. This experimental set-up was very similar to that illustrated in Hardy et al. (1999), where it was used to study postemergence dispersal of G. nephantidis offspring. The location of each female was recorded every 12 h until the female died. Locations were classified as (1) in the chamber with the host, (2) in the slot or (3) outside the block (i.e. elsewhere within the 1-litre container). The containers were not moved to avoid disturbing the females. When visible with the naked eye, the developmental stage reached by any offspring was noted.
Maternal Success during Contests To determine the effects of female size, brood stage and host size on contest outcomes, we used 2e3-day-old mated females, kept in isolation in glass vials following emergence as adults to avoid any prior contest experience. After being briefly anaesthetized with carbon dioxide, they were marked on the dorsal surface of the thorax by a dot of red or yellow water-based acrylic paint. A C. cephalonica host larva was introduced into the tube of some of the females; having paralysed the host these females were termed ‘owners’. Hosts were of known weight (mean 33.59 mg) ranging between 20 and 46 mg, but the majority were between 30 and 40 mg. Three categories of owners, defined according to the developmental stage of their brood, were used in contest experiments: (1) owners of paralysed hosts which had not been oviposited on, (2) owners of hosts on which they had laid a clutch of eggs and (3) owners of hosts on which the brood had reached an advanced larval stage. The number of eggs or larvae present on the hosts was counted before the experiments. In contrast, those females that were not given prior access to a host were used as ‘intruders’ in contest experiments. Dyadic contests were set up between females of similar age, bearing different colour (red versus yellow) marks and coming from different broods. Contests were observed in the ‘contest block’ (illustrated and described in Petersen & Hardy 1996; see also Goubault et al. 2006): this apparatus has three chambers interconnected by a slot. Females can pass through the slot when it is not blocked by movable barriers. When contests were set up between two owners, both were placed with their hosts in the central chamber but were initially separated from each other by a barrier. In ownereintruder replicates, intruders were placed in a lateral chamber and owners with their hosts in the central chamber, and kept separate by a barrier. For both types of contests, after a settlement time of 30 min, the barriers were withdrawn allowing both females access to the entire interior of the contest block. Contests were recorded for 90 min by a video camera placed above the central
chamber. Immediately after the end of an observation period, the female in possession of (i.e. in close proximity to) the host(s) was considered as the winner (Petersen & Hardy 1996; Humphries et al. 2006) and the number of any eggs and larvae on the host(s) was checked. Both females were then weighed to an accuracy of 0.01 mg and dissected to determine their egg load (Stokkebo & Hardy 2000). A total of 60 ownereintruder contests were observed, with 20 replicates involving an owner belonging to each of the brood stage categories (see above). There were 60 ownereowner contest replicates: in each case the contestant females belonged to different brood stage categories, with 20 replicates for each combination.
Consequences of Contest Outcome for Offspring Survival We measured the survival of offspring in the presence of their mother or an intruder female. To do so, females of known weight were individually placed with a 30e40 mg host until it was either paralysed or bore eggs or advanced larvae. The host was then removed from the tube and then either replaced with the owner female (control: N ¼ 20 for each of the three host categories) or transferred to a tube with a nonowner female of known weight (N ¼ 20 for each of the three host types). To assess the developmental success until adulthood, the number of eggs and/or larvae on the hosts was checked every day until the emergence of new adults from their cocoons. When hosts bore a clutch of eggs at the time of transfer or replacement, these eggs were individually marked with eosin dye (Hardy & Blackburn 1991): this allowed distinction between eggs laid before and after experimental host handling.
Statistical Analysis Data were analysed using generalized linear modelling with Genstat statistical package (Version 8, VSN International, Hemel Hempstead, U.K.). Our general approach was to use (semi-)parametric analyses in which the assumed distribution of residuals was matched to the data rather than transforming data to fit standard Gaussian assumptions (Wilson & Hardy 2002). We followed backward stepwise procedures, with initial statistical models containing all explanatory variables of interest and the sequence of deletion from the model determined by parameter inspection, with only significant terms reentered to obtain a parsimonious ‘minimal adequate model’ (Crawley 1993; Quinn & Keough 2002; Wilson & Hardy 2002). When explanatory variables are highly mutually correlated, their simultaneous inclusion in a model can lead to interpretational problems because of ‘collinearity’ (Grafen & Hails 2002; Quinn & Keough 2002). We followed Humphries et al. (2006) in, therefore, excluding egg load (highly correlated with female weight) and number of offspring on host (correlated with host weight) from the main analyses (see Results). We used survival analysis (Crawley 1993) to examine the effects of female weight, host weight and number of offspring at the time at which females were first observed
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outside the observation block (see also Hardy et al. 1999). We used a Weibull distribution (suitable for timedependent hazard functions) after checking that this provided a significantly better fit to the data than an exponential distribution assuming a constant hazard (Crawley 1993). Factors influencing contest outcomes (a proportional response variable) were explored using logistic analyses (Hardy & Field 1998) except when initially checking that the colour of the paint mark had no influence on contest outcomes, where binomial tests were used (as in Petersen & Hardy 1996; Stokkebo & Hardy 2000). Log-linear analyses (suitable for ‘small count’ response variables, Crawley 1993) were used to investigate relationships between female egg load and weight, number of offspring and host weight and to compare the number of eggs laid by females on the different host types. The proportions of offspring surviving to adulthood when in the presence of their mother or another female were compared by logistic analyses.
RESULTS
Time Invested by Unchallenged Mothers Females tended to stay on or close to their host until their eggs hatched (Fig. 1). Once eggs hatched, females generally spent more time in the slot of the experimental block: they were usually near to the chamber rather than outside of the block and appeared to be guarding the entrance of the chamber. Females usually left the block about 24 h after their larvae had spun cocoons (Fig. 1). Times at which females were first observed out of the block were not distributed randomly but were timedependent (a Weibull model provided better fit than an Female in the observation block:
exponential model: G1 ¼ 22.35, P < 0.001) with the tendency to leave the block increasing from around day 6 (Fig. 1). The time at which females were first observed outside the block was not influenced by the female’s weight (G1 ¼ 1.78, P > 0.05), the weight of the host (G1 ¼ 1.99, P > 0.05) or the number of adult offspring that were produced from the host (G1 ¼ 1.51, P > 0.05).
Maternal Success During Contests Female characteristics: egg load and brood size Egg loads were highly variable but were positively correlated with female weight (F1,235 ¼ 13.07, P < 0.001, r2 ¼ 0.04) and affected by host type (F3,235 ¼ 26.58, P < 0.001, r2 ¼ 0.24) but the interaction between female weight and host type was not significant. Model simplification by aggregation of host type factor levels (Crawley 1993, page 190) showed that, for a given body size, females bearing a brood of advanced larvae had the largest egg loads (F1,237 ¼ 4.39, P ¼ 0.037) whereas those females bearing a clutch of eggs (i.e. having laid eggs in the last 24 h) had the least (F1,237 ¼ 57.02, P < 0.001). Intruders and females with paralysed hosts had intermediate and similar egg loads (F1,236 ¼ 0.01, P ¼ 0.94). Figure 2 illustrates the minimum adequate model obtained by this analysis. The number of eggs or larvae present on the hosts was influenced by host weight (F1,119 ¼ 22.00, P < 0.001) but not by the offspring developmental stage (i.e. eggs or larvae) (F1,119 ¼ 0.13, P ¼ 0.72).
Contests Ownereintruder contests. To determine the effects of female size, brood stage and host size on contest outcomes, we first considered contest outcome as a binary
With host In slot
Female out of the observation block:
Alive Dead
20
15 Number of females
440
10
5
0
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13
Brood stage
Time(days) Eggs
Larvae
Cocoons
Figure 1. Female location in relation to time and offspring development.
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
Regression Intruder + Paralysed Regression Eggs Regression Larvae
Intruder Paralysed Eggs Larvae 16
14
12
Egg load
10
8
6
4
2
0 0.4
0.6
0.8
1 1.2 Female weight
1.4
1.6
1.8
Figure 2. Relationship between female egg load and weight according to brood stage. The results for intruder females and females defending a paralysed host did not differ significantly and are represented by a single regression line. The observed data points for the four types of females are presented separately to show the variability between the treatments.
response: 1 ¼ intruder won or 0 ¼ intruder lost. We explored the effect of the (i) difference in weight between contestants (intruder weight minus owner weight), (ii) owner’s brood stage (paralysed host, host bearing eggs or host bearing larvae) and (iii) host weight, and their interactions. The probability of the intruder winning was enhanced by being larger than the owner (G1 ¼ 5.94, P ¼ 0.015). The probability of the intruder winning also depended on the stage of the owner’s brood (G2 ¼ 3.28, P ¼ 0.037): the intruders won more often against the owner of a brood of advanced larvae than the other types of owners (with eggs or paralysed host; Fig. 3). Contest outcomes were not affected by the weight of the owner’s host (G1 ¼ 0.80, P ¼ 0.37) or any interaction between the above explanatory variables. Furthermore, the number of offspring did not affect the probability of the intruder winning (simple logistic regression: G1 ¼ 0.04, P ¼ 0.83). Second, to assess the importance of ownership, we redefined the contest outcomes in terms of the colour of the winning female: 1 ¼ red wasp won, 0 ¼ red wasp lost (see Petersen & Hardy 1996; Stokkebo & Hardy 2000). The marking colour was unrelated to contest outcomes (red wasps won 35 of 60 contests, two-tailed binomial test: P ¼ 0.25). A simple logistic analysis using the colour of the winning female as the response variable showed that owners were more likely to win the contests (G1 ¼ 5.65, P ¼ 0.017). A multiple logistic analysis including
ownership, female weight difference and brood stage as explanatory variables confirmed that all these explanatory variables were involved in significant terms (interaction between ownership and female weight: G1 ¼ 4.17, P ¼ 0.041; interaction between ownership and owner’s brood stage: G2 ¼ 3.65, P ¼ 0.026). In three of the 20 replicates involving hosts bearing a brood of larvae, intruders were observed grasping the larvae in their mandibles and pulling them away from the host, which resulted in the death of part or all of the brood. Ownereowner contests. To assess the effect of hosts’, females’ and broods’ properties on contest outcomes, we defined the contest outcomes as a binary response: 1 ¼ female with brood at a later stage won, 0 ¼ female at a later stage lost. We then tested the following explanatory variables: (i) difference in host weight (host weight of the female at the later stage minus host weight of the female at the earlier stage), (ii) difference in female weight (weight of the female at the later stage minus weight of the female at the earlier stage) and (iii) treatment (paralysed host versus host with eggs, host with eggs versus host with larvae, and paralysed host versus host with larvae) and their interactions. Females at a later stage were more likely to win contests when their host was bigger than their opponent’s (G1 ¼ 5.05, P ¼ 0.025, Fig. 4). Brood stage treatment had a marginally nonsignificant effect on contest outcome (G2 ¼ 2.91, P ¼ 0.055): because of the
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ANIMAL BEHAVIOUR, 74, 3
Owner with paralysed host Owner with eggs Owner with larvae
Regression paralysed + eggs Regression larvae
1
0.75 Intruder take over
442
0.50
0.25
0 -0.8
-0.6
-0.4 -0.2 0 0.2 Intruder weight–Owner weight
0.4
0.6
Figure 3. Ownereintruder contests: probability of the intruder winning in relation to differences in female weight and brood stage. Results for the treatments in which an owner defends a paralysed host or a host bearing a clutch of eggs did not differ significantly and are represented by a single regression line. Data points have been vertically displaced from their binary positions to show the numbers of observations.
lack of precision in probability estimates obtained from logistic analyses (e.g. Crawley 1993, page 278) this result may be regarded as equivocal). There were similar likelihoods of a female with host bearing larvae winning against a female with a paralysed host and a female with larvae winning against a female with eggs. When the data from these two categories were pooled, contest outcomes were significantly influenced by brood stage (G1 ¼ 5.28, P ¼ 0.022): females with eggs won more frequently against females with paralysed hosts than females with larvae won against females with paralysed hosts or eggs (Fig. 4). Female size difference did not affect contest outcome (G1 ¼ 0.46, P ¼ 0.495) nor did any of the interactions between fitted main effects. The difference in the number of offspring produced by contestant females did not affect the probability of the female with offspring at the later developmental stage winning (simple logistic analysis: G1 ¼ 0.36, P ¼ 0.55).
Offspring Survival The probability of the mother’s eggs surviving to adulthood was not affected by host weight (F1,39 ¼ 3.09, P ¼ 0.09) but was considerably reduced when in the presence of a female other than the mother (Table 1). Intruders ate the mother’s eggs before laying their own clutch, causing a significant reduction of egg to larval survivorship (F1,39 ¼ 58.29, P < 0.001). The presence of intruders did not, however, affect survivorship from the larval to the cocoon stage (F1,23 ¼ 0.01, P ¼ 0.94) or the cocoon to adult survivorship
(F1,19 ¼ 0.00, P ¼ 0.99). Clutches of eggs laid by intruders were often smaller than the clutch originally laid by the mother (F1,39 ¼ 5.33, P ¼ 0.027) and intruder clutch size was unrelated to host weight (F1,39 ¼ 0.34, P ¼ 0.56). When offspring had reached the larval stage, the presence of an intruder did not affect their survival to adulthood (Table 1). Contrary to observations during ownereintruder contests, no intruders were seen pulling larvae away from their hosts. Host weight did not influence the probability of larvae surviving to the cocoon stage (F1,39 ¼ 0.01, P ¼ 0.91). The survival to adulthood of eggs laid on paralysed hosts by intruders was similar to that of the owners’ eggs (F1,39 ¼ 0.81, P ¼ 0.37) and was unaffected by host weight (F1,39 0.00, P ¼ 0.98; Tables 1 and 2). Intruder females laid larger clutches on paralysed hosts and hosts that bore eggs than on hosts that bore larvae (Table 2). We detected no influence of host type on the probability of intruders’ eggs surviving to adulthood (Table 2) but note that intruders laid eggs on hosts originally bearing larvae in only two replicates and none of their offspring survived. DISCUSSION As already shown by previous studies of femaleefemale contests in G. nephantidis (Hardy & Blackburn 1991; Petersen & Hardy 1996; Stokkebo & Hardy 2000; Humphries et al. 2006), we found advantages of being larger and of being the owner: owners usually retained their host against intruders, except when intruders were substantially larger. When the ownership asymmetry was absent
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
Eggs vs paralysed
Regression eggs vs paralysed
Larvae vs paralysed
Regression larvae vs paralysed + larvae vs eggs
Larvae vs eggs 1
Later stage female won
0.75
0.50
0.25
0 −12
−10
−8
−6
−4
−2
0
2
4
6
8
10
Later stage female's host weight–earlier stage female's host weight Figure 4. Ownereowner contests: probability of the female which was more advanced with offspring production winning ownereowner contests in relation to differences in host weight and brood stage. Results for treatments where females defend a host bearing larvae against either a female defending a paralysed host or a host bearing a clutch of eggs did not differ significantly and are represented by a single regression line. Data points have been vertically displaced from their binary positions to show numbers of observations.
(ownereowner contests), host size influenced the outcomes: females with larger host usually won (as observed by Humphries et al. 2006). Our results further show that the developmental stage of the brood affects contest outcome. According to the ‘offspring reproductive value’ hypothesis of parental investment theory, older broods should be
Table 1. Percentage of eggs or larvae surviving to adulthood according to which female (mother or intruder) was in the presence of the host Mother’s brood Paralysed host Eggs Larvae
more intensively defended by parents as reproductive value increases with brood age (Montgomerie & Weatherhead 1988; Rytko¨nen et al. 1995). In G. nephantidis, an increase in offspring value would contribute to the resource value (RV) of the host defended by owners and should consequently affect contest outcome. As expected, females
Table 2. Intruder’s clutch size and egg-to-adult survivorship in relation to the stage of the brood on the owner’s host
Female with host
Mean percentage of eggs/larvae surviving to adulthood (N )
Logistic analysis corrected for overdispersion
Owner’s brood
Mother
77.1 (20)
e
Paralysed host Eggs Larvae
Mother Intruder Mother Intruder
57.7 5.5 82.1 64.5
(20) (20) (20) (20)
F1,39¼35.0, P<0.001 F1,39¼2.93, P¼0.095
Average intruder’s clutch size (N )
Mean percentage of intruder’s eggs surviving to adulthood (N )
9.7 (20) 8.6 (20) 0.7 (20) F2,59¼72.70, P<0.001*
59.9 (20) 63.7 (20) 0 (2) F2,41¼2.28, P¼0.12y
*Log-linear analysis corrected for overdispersion. yLogistic analysis corrected for overdispersion.
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with hosts bearing a clutch of eggs were more likely to win against females protecting a paralysed host. However, females with hosts bearing larvae were not advantaged when competing with females with paralysed hosts or hosts with eggs. Indeed, females defending larvae were more likely to be defeated by intruders than were females defending younger broods. Thus, G. nephantidis females appear to protect their older (larval stage) broods less than their younger offspring, even though the reproductive value of older offspring is higher. Such behaviour, where parents take greater risks or make greater efforts to defend younger than older broods, is expected under the ‘harm to offspring’ hypothesis, which was developed for birds (the pied flycatcher Ficedula hypoleuca, Dale et al. 1996). This prediction is based on the observation that older offspring suffer less than younger offspring if unattended (i.e. no incubation, brooding or feeding) because they conserve energy more efficiently (Dale et al. 1996). In G. nephantidis, the analogous explanation is more directly related to offspring survival: successful intruders destroy clutches of eggs but only rarely damage broods of larvae (Hardy & Blackburn 1991; this study). Unattended larvae are consequently less ‘vulnerable’ than unattended eggs. Such vulnerability is, however, unlikely to be related to the probability of G. nephantidis offspring being encountered by intruders and our results cannot be explained by the ‘vulnerability hypothesis’ which was developed for birds on the basis that older chicks call louder, making the nest more conspicuous to predators (Onnebrink & Curio 1991). The avian ‘vulnerability hypothesis’ thus predicts the opposite to what was observed in G. nephantidis: parents should protect more intensively older rather than younger offspring. Results similar to ours have, to our knowledge, only been obtained in the bank vole Clethrionomys glareolus (Koskela et al. 2000). In this mammalian species, maternal defence intensity decreases as offspring age, probably because older pups are less vulnerable to infanticide committed by male adults. It is thought that being bigger and more mobile, older pups are harder to kill than newborn pups (Wolff 1985). In G. nephantidis, conspecifics also kill older larvae less frequently than eggs, but this could be because of greater difficultly of removing larvae from a host and/or to the difference in benefit they gain from infanticide. Indeed, eggs are likely to constitute a source of nutrients since ovicide is committed by ingestion (Hardy & Blackburn 1991; this study). In contrast, when larvae are killed by being pulled away from the host, infanticidal females did not appear to feed on them (M. Goubault, personal observation). Furthermore, infanticide is a way of restoring host quality by eliminating competitors for future offspring. Destroying eggs is likely to result in the acquisition of a host of similar quality to a paralysed host, but once a host bears larvae that have started to feed, less remains of the initial host resource. In the rare instances where the intruder females laid eggs on hosts that initially bore larvae, all their offspring died during the larval stage, probably because of a shortage of host resources. Thus, older (larval) offspring may be attacked less than eggs because they are more difficult to kill and provide no nutritional resource, and
because possession of a partially depleted host is of low benefit to intruders. Despite the reduction in risk from conspecific intruders as the brood develops, G. nephantidis females usually guard their broods until pupation (in common with some bethylid species but in contrast to others which abandon the host soon after oviposition; Mayhew 1997; Takasu & Overholt 1998). Their continued presence can protect larval offspring against predators or heterospecific competitors, such as the braconid wasp Bracon hebetor. Guarding larvae is known to reduce substantially, but not prevent, the detrimental (larvicidal) actions of B. hebetor females (Hardy & Blackburn 1991). Complementary experiments would be needed to test whether offspring defence intensity in G. nephantidis depends upon the species of competitor, as in some other bethylids (Pe´rez-Lachaud et al. 2002; Batchelor et al. 2005). In accord with previous work on G. nephantidis (Stokkebo & Hardy 2000; Humphries et al. 2006), we found a positive correlation between egg load and female weight. We did not detect egg maturation during the preoviposition period (owners with paralysed hosts did not have more eggs than intruders, contra Stokkebo & Hardy 2000). We found that female’s egg loads were higher when broods were at the larval stage than prior to oviposition (immediately after oviposition, egg loads were obviously depleted). These confirm recent egg-load observations (Humphries et al. 2006) and suggest that in nature brood guarding females anticipate future reproductive opportunities (in the laboratory, females are able to produce many broods, Hardy et al. 1992). In semelparous species (one reproductive event per lifetime), the reproductive value of offspring is likely to be higher than under iteroparity and mothers should always guard and fiercely defend their brood. Thus, the higher egg load of females that have already laid, the ability to reproduce repeatedly in the laboratory, the variable investment in brood protection according to the offspring developmental stage and the tendency to cease brood guarding as offspring mature, all countersuggest that G. nephantidis is (effectively) semelparous (Cock & Perera 1987; Hardy et al. 1992) and suggest anticipated future reproduction as an explanation for why G. nephantidis clutch size is smaller than the optimum calculated from laboratory-estimated fitness parameters and the assumption of semelparity (Hardy et al. 1992; see also Petersen & Hardy 1996; Mesterton-Gibbons & Hardy 2004). In conclusion, our results show that G. nephantidis females adjust maternal investment according to changes in both offspring reproductive value and the risk of their brood being destroyed by infanticidal intruders. Acknowledgments This work is an Anglo-French collaboration concerning optimal investment: we hope to have avoided the Concorde fallacy. We thank Tim Batchelor and David Heron for help and discussion, Julietta Marquez for technical assistance, and Sreekanth Reddy for providing G. nephantidis. This research was funded by grant BB/C504778/1 from the Biotechnology and Biological Sciences Research Council, U.K.
GOUBAULT ET AL.: PARASITOID MATERNAL DEFENCE
References Antony, J. & Kurian, C. 1960. Studies on the habits and life history of Perisierola nephantidis Muesebeck. Indian Coconut Journal, 13, 145e153. Archer, J. 1988. The Behavioural Biology of Aggression. In: (Ed. by G. Barlow, P. P. G. Bateson & R. W. Oppenheim), Cambridge: Cambridge University Press. Batchelor, T. P., Hardy, I. C. W., Barrera, J. F. & Pe´rez-Lachaud, G. 2005. Insect gladiators. II: competitive interactions within and between bethylid parasitoid species of the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae). Biological Control, 33, 194e202. Carlisle, T. R. 1985. Parental response to brood size in a cichlid fish. Animal Behaviour, 33, 234e238. Clutton-Brock, T. H. 1991. The Evolution of Parental Care. In: (Ed. by J. R. Krebs & T. H. Clutton-Brock), Princeton, New Jersey: Princeton University Press. Clutton-Brock, T. & Godfray, C. 1991. Parental investment. In: Behavioural Ecology. 3rd edn (Ed. by J. R. Krebs & N. B. Davies), pp. 234e262. Oxford: Blackwell Scientific. Cock, M. J. W. & Perera, P. A. C. R. 1987. Biological control of Opisina arenosella Walker (Lepidoptera, Oecophoridae). Biocontrol News and Information, 8, 283e310. Cocroft, R. B. 2002. Antipredator defense as a limited resource: unequal predation risk in broods of an insect with maternal care. Behavioral Ecology, 13, 125e133. Crawley, M. J. 1993. GLIM for Ecologists. Oxford: Blackwell Scientific. Dale, S., Gustavsen, R. & Slagsvold, T. 1996. Risk taking during parental care: a test of three hypotheses applied to the pied flycatcher. Behavioral Ecology and Sociobiology, 39, 31e42. Dawkins, R. & Carlisle, T. R. 1976. Parental investment, mate desertion and a fallacy. Nature, 262, 131e133. Enquist, M. & Leimar, O. 1987. Evolution of fighting behaviour: the effect of variation in resource value. Journal of Theoretical Biology, 127, 187e206. Field, S. A. & Calbert, G. 1999. Don’t count your eggs before they’re parasitized: contest resolution and the tradeoffs during patch defense in a parasitoid wasp. Behavioral Ecology, 10, 122e127. Figler, M. H., Blank, G. S. & Peeke, H. V. S. 2001. Maternal territoriality as an offspring defense strategy in red swamp crayfish (Procambarus clarkii, Girard). Aggressive Behavior, 27, 391e403. Goubault, M., Batchelor, T. P., Linforth, R. S. T., Taylor, R. J. & Hardy, I. C. W. 2006. Volatile emission by contest losers revealed by real-time chemical analysis. Proceedings of the Royal Society of London, Series B, 273, 2853e2859. Grafen, A. & Hails, R. 2002. Modern Statistics for the Life Sciences. Oxford: Oxford University Press. Hammerstein, P. 1981. The role of asymmetries in animal contests. Animal Behaviour, 29, 193e205. Hardy, I. C. W. & Blackburn, T. M. 1991. Brood guarding in a bethylid wasp. Ecological Entomology, 16, 55e62. Hardy, I. C. W. & Field, S. A. 1998. Logistic analysis of animal contests. Animal Behaviour, 56, 787e792. Hardy, I. C. W., Griffiths, N. T. & Godfray, H. C. J. 1992. Clutch size in a parasitoid wasp: a manipulation experiment. Journal of Animal Ecology, 61, 121e129. Hardy, I. C. W., Pedersen, J. B., Sejr, M. K. & Linderoth, U. H. 1999. Local mating, dispersal and sex ratio in a gregarious parasitoid wasp. Ethology, 105, 57e72. Humphries, E. L., Hebblethwaite, A. J., Batchelor, T. P. & Hardy, I. C. W. 2006. The importance of valuing resources: host weight and contender age as determinants of parasitoid wasp contest outcomes. Animal Behaviour, 72, 891e898.
Jaroensutasinee, M. & Jaroensutasinee, K. 2003. Type of intruder and reproductive phase influence male territorial defence in wildcaught Siamese fighting fish. Behavioural Processes, 64, 23e29. Jenssen, T. A., Marcellini, D. L., Buhlmann, K. A. & Goforth, P. H. 1989. Differential infanticide by adult curly-tailed lizards, Leiocephalus schreibersi. Animal Behaviour, 38, 1054e1061. Klemperer, H. G. 1982. Parental behaviour in Copris lunaris (Coleoptera, Sacrabaeidae): care and defence of brood balls and nest. Ecological Entomology, 7, 155e167. Koskela, E., Juutistenaho, P., Mappes, T. & Oksanen, T. A. 2000. Offspring defence in relation to litter size and age: experiment in the bank vole Clethrionomys glareolus. Evolutionary Ecology, 14, 99e109. Listøen, C., Karlsen, R. F. & Slagsvold, T. 2000. Risk taking during parental care: a test of the harm-to-offspring hypothesis. Behavioral Ecology, 11, 40e43. Maestripieri, D. 1992. Functional aspects of maternal aggression in mammals. Canadian Journal of Zoology, 70, 1069e1077. Mayhew, P. J. 1997. Fitness consequences of ovicide in a parasitoid wasp. Entomologia Experimentalis et Applicata, 84, 115e126. Maynard Smith, J. 1974. The theory of games and the evolution of animal conflicts. Journal of Theoretical Biology, 47, 209e221. Maynard Smith, J. 1982. Evolution and the Theory of Games. Cambridge: Cambridge University Press. Maynard Smith, J. & Parker, G. A. 1976. The logic of asymmetric contests. Animal Behaviour, 24, 159e175. Mesterton-Gibbons, M. & Hardy, I. C. W. 2004. The influence of contests on optimal clutch size: a game-theoretic model. Proceedings of the Royal Society of London, Series B, 271, 971e978. Montgomerie, R. D. & Weatherhead, P. J. 1988. Risks and rewards of nest defence by parent birds. Quarterly Review of Biology, 63, 167e187. Onnebrink, H. & Curio, E. 1991. Brood defense and age of young: a test of the vulnerability hypothesis. Behavioral Ecology and Sociobiology, 29, 61e68. Pe´rez-Lachaud, G., Hardy, I. C. W. & Lachaud, J. P. 2002. Insect gladiators: competitive interactions between three species of bethylid wasps attacking the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae). Biological Control, 25, 231e238. Petersen, G. & Hardy, I. C. W. 1996. The importance of being larger: parasitoid intruder-owner contests and their implications for clutch size. Animal Behaviour, 51, 1363e1373. Quinn, G. P. & Keough, M. J. 2002. Experimental Design and Data Analysis for Biologists. Cambridge: Cambridge University Press. Redondo, T. 1989. Avian nest defence: theoretical models and evidence. Behaviour, 111, 161e195. Remadevi, O. K., Mohamed, U. V. K. & Abdurahiman, U. C. 1981. Some aspects of the biology of Parasierola nephantidis Muesebeck (Hymenoptera, Bethylidae), a larval parasitoid of Nephantis serinopa Meyrick (Lepidoptera, Xylorictidae). Polskie Pismo Entomologiczne, 51, 597e604. Ricklefs, R. E. 1969. An analysis of nesting mortality in birds. Smithsonian Contributions to Zoology, 9, 1e48. ¨ nen, S., Orell, M. & Koivula, K. 1995. Pseudo concorde falRytko lacy in the willow tit? Animal Behaviour, 49, 1017e1028. Scott, M. P. 1990. Brood guarding and the evolution of male parental care in burying beetles. Behavioral Ecology and Sociobiology, 26, 31e39. Scott, M. P. 1994. The benefit of parental assistance in intra- and interspecific competition for the burying beetle, Nicrophorus defodiens. Ethology, Ecology and Evolution, 6, 537e543. Stokkebo, S. & Hardy, I. C. W. 2000. The importance of being gravid: egg load and contest outcome in a parasitoid wasp. Animal Behaviour, 59, 1111e1118.
445
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ANIMAL BEHAVIOUR, 74, 3
Stutt, A. D. & Willmer, P. 1998. Territorial defence in speckled wood butterflies: do the hottest males always win? Animal Behaviour, 55, 1341e1347. Takasu, K. & Overholt, W. A. 1998. Brood guarding behavior and life history characteristics of Goniozus indicus Ashmead (Hymenoptera: Bethylidae), a larval ectoparasitoid of lepidopteran stemborers. Applied Entomology and Zoology, 33, 121e126. Tallamy, D. W. 1984. Insect parental care. BioScience, 34, 20e24. Tallamy, D. W. & Denno, R. F. 1981. Maternal care in Gargaphia solani (Hemiptera: Tingitidae). Animal Behaviour, 29, 771e778. Tallamy, D. W. & Wood, T. K. 1986. Convergence patterns in subsocial insects. Annual Review of Entomology, 31, 369e390.
Trivers, R. L. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man 1871e1971 (Ed. by B. Campbell), pp. 136e179. Chicago, Illinois: Aldine. Trumbo, S. T. 1990. Interference competition among burying beetles (Silphidae, Nicrophorus). Ecological Entomology, 15, 347e355. Wilson, K. & Hardy, I. C. W. 2002. Statistical analysis of sex ratios: an introduction. In: Sex Ratios: Concepts and Research Methods (Ed. by I. C. W. Hardy), pp. 48e92. Cambridge: Cambridge University Press. Wolff, J. O. 1985. Maternal aggression as a deterrent to infanticide in Peromyscus leucopus and Peromyscus maniculatus. Animal Behaviour, 33, 117e123. Wolff, J. O. & Peterson, J. A. 1998. An offspring-defense hypothesis for territoriality in female mammals. Ethology, Ecology and Evolution, 10, 227e239.