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Reproductive conflicts and mutilation in queenless Diacamma ants
ANIMAL BEHAVIOUR, 2006, 72, 305e311 doi:10.1016/j.anbehav.2005.10.025
Reproductive conflicts and mutilation in queenless Diacamma ants S EBA STI EN B AR ATT E *, MA TTHEW C OBB † & CHRISTIAN PEETERS *
*Laboratoire d’Ecologie, CNRS and Universite´ Pierre-et-Marie Curie, Paris yFaculty of Life Sciences, University of Manchester (Received 5 April 2005; initial acceptance 1 June 2005; final acceptance 11 October 2005; published online 11 July 2006; MS. number: 8514R)
Reproductive conflicts are particularly intense in queenless ants because colonies are made up of totipotent workers, all potentially able to mate and produce female offspring. After a gamergate (a mated egg-laying worker) dies, aggressive interactions determine her replacement. In Diacamma, the single gamergate systematically mutilates newly emerged workers (callows) and consequently they can never mate. New callows are expected to resist because, once mutilated, they cannot replace the gamergate. In D. ceylonense and D. australe, we studied the behaviour of new callows confronted with dominant individuals of different fertility or age. Faced with a young unmutilated worker (the future gamergate with poorly developed ovaries), callows were aggressive, whereas they did not resist gamergates. We interpret this dichotomy in terms of reproductive differentials between actors and victims. We discuss the selective advantages of mutilation relative to dominance hierarchies which other queenless ants use to regulate monogyny. Mutilation seems to maximize colony productivity by creating irreversibly sterile helpers from newly emerged individuals. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Reproductive conflicts in ants are strongly influenced by the occurrence of two female castes. Queens and workers diverge irreversibly during larval development and workers generally cannot reproduce sexually. However, in about 200 species of queenless ants, colonies consist exclusively of workers with equivalent reproductive potentials (Peeters 1991; Peeters & Ito 2001). Aggressive interactions lead to hierarchies, and one or a few dominant workers mate and lay eggs (e.g. Ito & Higashi 1991; Monnin & Peeters 1999; Gobin et al. 2001; Cuvillier-Hot et al. 2004a). Importantly, subordinate individuals retain the ability to replace the ‘gamergate’ (mated egg-laying worker). A qualitatively different type of behavioural regulation characterizes the genus Diacamma, because the gamergate systematically bites off the ‘gemmae’, a pair of tiny appendages filled with exocrine cells present on the thorax of all emerging workers (Fukumoto et al. 1989; Peeters & Higashi 1989; Peeters & Billen 1991). Once mutilated, Diacamma workers can never mate and are restricted to producing males in colonies without a gamergate (Peeters & Tsuji 1993; Tsuji et al. 1999;
Correspondence: C. Peeters, Laboratoire d’Ecologie, CNRS UMR 7625, Universite´ Pierre-et-Marie Curie, 7 quai Saint Bernard, F-75005 Paris, France (email: [email protected]). 0003e3472/06/$30.00/0
Cuvillier-Hot et al. 2002). Unlike those of other queenless ants, the reproductive conflicts then become analogous to those in species with queen and worker castes. In colonies with a gamergate, mutilation is an almost daily routine and callows (newly emerged workers) are likely to be the gamergate’s daughters. An unstable phase intervenes following two natural events: replacement of a senescent gamergate (life expectancy is less than 2 years, Tsuji et al. 1996; Andre´ et al. 2001) and colony fission. The colonies of all queenless ants reproduce obligatorily by fission (Peeters & Ito 2001), and in monogynous species such as Diacamma, this creates an opportunity for the differentiation of a new gamergate. The details of fission are unknown, but one of the two daughter colonies will inevitably consist of mutilated workers and brood; hence the first cocoon that ecloses yields the future gamergate. This individual immediately begins to mutilate all her sisters that subsequently emerge. Ant callows are usually timid and inactive (Ho¨lldobler & Wilson 1990), but in Diacamma they are aggressive and capable of performing this complex behaviour, sometimes within hours of their own emergence. During the unstable phase, the future gamergate is initially unable to lay eggs (referred to as ‘immature FG’ below), but after about 3 weeks she begins to produce males and starts sexual calling to attract a mate (‘mature FG’; Cuvillier-Hot et al. 2002). She
305 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 72, 2
eventually copulates with a single foreign male. Thus, while the victims of mutilation are always recently emerged callows, the actors can differ markedly in fertility, age and genetic relatedness. To understand the evolution of this irreversible regulation, we determined whether the course of mutilation is influenced by differences in reproductive potential between actors and their victims. A callow confronted with the gamergate may benefit from producing her own offspring (relatedness with sons and daughters is 0.5) instead of rearing brothers and sisters (average relatedness is also 0.5). However, older mutilated sisters should prevent the replacement of their mother (average relatedness with nieces and nephews is 0.375). Similarly, it may be in a callow’s interest to escape mutilation by an immature FG and produce her own daughters, rather than rearing her sister’s. An intermediate situation may occur in colonies with a mature FG, because she is virgin yet has a moderate fertility. Fertility is a crucial parameter affecting the helpers’ inclusive fitness in all social insects, and reliable information about age and fertility is encoded in the cuticular hydrocarbons of D. ceylonense (CuvillierHot et al. 2001, 2002). Callows may thus be able to distinguish between a future gamergate (either immature or mature) and a gamergate, and adjust their behaviour during mutilation accordingly. We introduced new callows into recently orphaned colonies of D. ceylonense and D. australe (belonging to different species-groups; W.L. Brown, personal communication). We compared the course of mutilation by either immature or mature FG with that observed in colonies with a gamergate. We also observed the behaviour of D. ceylonense gamergates faced with surgically mutilated callows and with callows from the nonmutilating ‘nilgiri’ population. Given that the physical act of mutilation separates the conflicts over either male production or female production to a greater extent than in the other queenless ants, we discuss the costs and benefits of its evolution in Diacamma.
METHODS
Housing and Creation of Experimental Groups We collected 16 colonies of D. ceylonense (X SD ¼ 296 92 workers, range 117e448) from south of Bangalore, Karnataka State, India, in April 2001, November 2002 and January 2004 and eight colonies of D. australe in November 1987 from east of Townsville, north Queensland, Australia. In each colony, the wild-caught gamergate, identified by the presence of gemmae, was marked with dots of paint. To prevent natural mutilations, we quickly removed cocoons from the colonies and isolated them in nurseries with about 40 mutilated workers. Both colonies with gamergates and nurseries of each colony were placed in plaster nests covered with glass to allow observation, kept at 25 C under a 12:12 h light:dark cycle and provided with crickets, Tenebrio molitor larvae (mealworms) and honey diluted in water. Cocoons were checked daily and newly emerged callows were isolated.
Callows are easily distinguishable because of their soft cuticle and their light-brown extremities. Of the 964 cocoons of D. ceylonense collected in the field, about one-third (N ¼ 371) eclosed before isolation in the nurseries; around 30% of the remaining cocoons never eclosed, 20% produced males and 50% yielded workers (N ¼ 281 callows). For D. australe, over 300 cocoons eclosed in the laboratory. Several colonies from the nonmutilating population of D. ceylonense, called ‘nilgiri’ (Peeters et al. 1992; Baudry et al. 2003), were also excavated near Bandipur in Karnataka (150 km south of Bangalore) in January 2004. Cocoons of ‘nilgiri’ were placed in ceylonense nurseries, where callows emerged successfully.
Mutilation in Different Social Contexts We compared mutilations by gamergates and future gamergates (immature and mature). To create groups with gamergates, we introduced 0e2-day-old callows (daughters of gamergates) from a nursery to the colony from which they originated (G groups; N ¼ 13 for D. ceylonense). We also introduced ‘nilgiri’ callows (N ¼ 7) and callows with surgically removed gemmae (N ¼ 11) into G groups to observe whether the ceylonense gamergate responded differently to them. Eclosions in nonmanipulated colonies of D. ceylonense and D. australe provided further information about the behaviour of gamergates. To create groups with future gamergates (FG groups), we used orphaned groups of 30e50 mutilated workers; the first emerged callow in these groups retained her gemmae. We then observed mutilations by adding callows (sisters of the future gamergate) from nurseries to the corresponding FG groups: 10 mutilations with 10 immature FG (0e8 days old) and 10 mutilations with 10 different mature FG (39e55 days old). Uncontrolled mutilations also occurred in nurseries whenever more than one callow (2e6) emerged overnight (N ¼ 42). One of the callows started to mutilate the others and we counted how many callows had gemmae the next day (N ¼ 147 callows).
Monitoring of Behaviours We observed the behaviours of actors and victims until mutilation was complete or until mutilation behaviour had ceased. For D. ceylonense, real-time observations were complemented with recordings made with a wide-angle video camera (Hitachi KP-D51 Color Digital) to monitor general activity around introduced callows. We recorded all aggressive interactions (antennal boxing followed by crouching of the target individual, lunging, biting, sting smearing (Monnin & Peeters 1999; Monnin et al. 2002)), submissive reactions (crouching and avoidance, Peeters & Tsuji 1993) and neutral behaviours (grooming, antennal inspection). Once the callow was quiet and mutilation attempts had begun, a black and white camera (Sony CCD XC-ST70) mounted on a dissection microscope recorded the movement of the mouthparts and antennae around the callow’s gemmae to determine how they were removed. Quantitative data were not obtained for D. australe.
BARATTE ET AL.: REPRODUCTIVE CONFLICTS IN ANTS
RESULTS
Mutilations by a Gamergate In nonmanipulated colonies of both D. ceylonense and D. australe, we regularly observed gamergates standing near and opening cocoons. Soon after emergence, callows were the focus of attention from three to five workers which licked them and removed their larval skin. Workers often held a callow for hours, thereby reducing her ability to move during mutilation. However, we also observed callows free to move in the nest and eventually mutilated by gamergates without the involvement of any workers. Mutilation occurred as quickly as 10 min after emergence (N ¼ 2). When more than one cocoon eclosed within a few hours, several callows were held simultaneously and the gamergate went from one to another. When a callow was transferred to a G group, the gamergate took 6e68 min to identify her (Table 1), depending on whether she was resting or patrolling. As soon as the callow was introduced, 3e15 workers surrounded her (Table 1) and began to lick every part of her body, even though her larval skin had already been removed in the nursery. They did not pay specific attention to the gemmae. In the vicinity of the grooming workers, the gamergate stretched her antennae towards the callow (Fig. 1, ‘identification’), walked closer, pushing aside attending workers, and finally touched the callow’s thorax with her antennal tips. She appeared to become excited and opened her mandibles wide, and repeatedly performed both antennal inspection (antennal tips gently run on the cuticle, Fig. 1a) and antennal boxing (antennal tips rapidly struck the cuticle, Fig. 1b). Each time a gemma was touched by an antenna during inspection, the gamergate scraped the thorax with her mandibles (Fig. 1c). As long as a gemma was present, this behavioural loop helped the gamergate to scrape closer and closer to the target. She eventually succeeded in lifting the gemma from the pleural cavity and then pulled it away. Finally, when both gemmae had been removed, she inspected the callow for several seconds and moved away. She did not show any further interest in the mutilated callows.
Mutilations by a Future Gamergate Both immature and mature FG succeeded in mutilating the introduced callows, removing their gemmae in the
same way as gamergates did (Table 2). None the less, we observed significant differences in the level of aggression of both callows and mutilating ants during identification (KruskaleWallis test: callow’s aggressiveness: H2 ¼ 20.64, P < 0.001; mutilator’s aggressiveness: H2 ¼ 20.54, P < 0.001). A callow was submissive when she detected a gamergate (similar to callows in nonmanipulated colonies; median number of aggressive acts against gamergate ¼ 0; Fig. 2a) and the latter remained nonaggressive (median number of aggressive acts against callows ¼ 0; Fig. 2b). In contrast, a callow was more aggressive when faced with a mature or immature FG (ManneWhitney U tests with sequential Bonferroni’s correction: immature FG versus gamergate: U ¼ 121, N1 ¼ 10, N2 ¼ 13, P ¼ 0.0001; mature FG versus gamergate: U ¼ 127.5, N1¼ 10, N2 ¼ 13, P ¼ 0.0001; mature FG versus immature FG: U ¼ 69, N1 ¼ N2 ¼ 10, P ¼ 0.16; Fig. 2a). A few callows succeeded in escaping from grooming workers, ran after the future gamergate and attacked her. When this happened, the future gamergate lunged back at the callow, attempted sting smearing and a fight ensued. In all cases the future gamergate prevailed but ‘gemmae removal’ was delayed (range 20 mine1 h after the future gamergate identified the callow). Thus, immature FG were more aggressive than gamergates during the initial encounter (U ¼ 124.5, N1 ¼ 10, N2 ¼ 13, P < 0.0001; Fig. 2b). A mature FG showed an intermediate level of aggression when faced with a new callow in comparison with immature FG and gamergates (callow versus immature FG: U ¼ 90, N1 ¼ N2 ¼ 10, P ¼ 0.001; callow versus gamergate: U ¼ 112.5, N1 ¼ 10, N2 ¼ 13, P ¼ 0.002; Fig. 2b). Aggressive resistance from older unmutilated callows was also observed in D. australe, although we did not aim for a quantitative description. The mandibular scrapings by immature FG were also more intense than those of gamergates. Examination of videos indicated that five future gamergates perforated the soft cuticle of callows, releasing drops of haemolymph on the thorax (Fig. 3a), petiole or legs (because the callow’s forelegs often shield the gemmae). When more than one cocoon eclosed during the night in nurseries, we found all but one callow mutilated and also perforated or partially eaten the next day. These mortalities become less frequent as the immature FG grew older (Fig. 3b); 95.1% of fatal mutilations were by 0e3-day-old FG (comparison between ages of immature FG that made fatal versus nonfatal mutilations: ManneWhitney U test; U ¼ 2503, N1 ¼ 66, N2 ¼ 81, P < 0.0001).
Table 1. Behaviour of D. ceylonense gamergates and grooming workers upon introduction of 0e2-day-old callows (C) with no (C0) or two (C2) intact gemmae
Context
Latency to Number of identification Latency to first Latency to remove Latency to remove N grooming workers of callow (min) thorax inspection (min) the first gemma (min) second gemma (min)
Gamergate with C2 13 Gamergate with C0 11
9 (7e15) 7 (3e12)
50 (12e68) 35 (6e73)
5 (0e55) 7 (0e53)
16 (3e75) d
31 (17-72) d
Values are given as median and range. Latencies are from when the callow entered the nest. All pairwise comparisons are not significant (ManneWhitney U test).
307
308
ANIMAL BEHAVIOUR, 72, 2
Each time a gemma is touched by an antenna
Once the callow is quiet
a Antennal inspection
b Antennal boxing
Once both gemmae have been removed
c Scraping with mandibles
A gemma is eventually removed 2 Gemma removal
1 Identification
3 Loss of interest
Figure 1. The three phases of mutilation in Diacamma ceylonense. The callow is shown in the top of the drawings and organs involved in behaviours are highlighted in black (grooming workers around the callow are not represented).
Influence of Gemmae on Mutilation Callows with two gemmae (C2) and no gemma (C0) were equally attractive to workers and both were identified by gamergates (Table 1). However, the sequence of behaviours was significantly modified when the gamergate was faced with C0 or ‘nilgiri’ callows (Table 2). When an introduced callow lacked gemmae, the gamergate did not get excited, but inspected her and rapidly moved away (no antennal boxing, no scraping with mandibles, Table 2); this corresponds to ‘loss of interest’ in Fig. 1. When a ‘nilgiri’ callow was introduced, the ceylonense gamergate identified her, inspected her thorax (Fig. 1a) and performed frequent antennal boxing (Fig. 1b). However, the gamergate did not use her mandibles to scrape the gemmae but licked them with her mouthparts, which was not Table 2. Number of behaviours of D. ceylonense future gamergates (FG) or gamergates upon introduction of 0e2-day-old callows (C) with no (C0) or two (C2) intact gemmae
Context Gamergate with C2 Gamergate with C0 Mature FG with C2 Immature FG with C2 Gamergate with ‘nilgiri’ C2
Scraping Antennal Antennal with Completed N inspection boxing mandibles mutilations 13
13
13
13
13
11
11
0
0
e
10
10
10
10
10
10
10
10
10
10
7
7
7
0
0
Bold numbers are significantly different from those for the gamergate with C2 (pairwise chi-square tests: P < 0.05).
observed with ceylonense callows. The ‘nilgiri’ callows were not mutilated even several days after introduction.
DISCUSSION
Age and Fertility of the Mutilating Ant There was a high level of aggression between future gamergates and callows although mutilation always occurred. In contrast, callows emerging in colonies with a gamergate did not attack her and did not try to escape mutilation. This indicates that a callow is able to adjust her behaviour before being mutilated. When a gamergate (the mother) is present, callow workers have the option to replace her (gaining access to sexual reproduction) or to refrain from reproduction (gaining indirect fitness). In both cases, the callow’s relatedness to offspring will be the same (see Introduction). However, frequent replacements of the gamergate should be counterselected (Monnin et al. 2002). Older workers may not benefit by early replacement of their mother as long as she is fertile because they are more related to their own sisters and brothers (average 0.5) than to the offspring of a sister that mates with a foreign male (0.375). In Dinoponera quadriceps and Streblognathus peetersi, any worker attempting to replace a fertile gamergate is punished by worker policing, induced by the gamergate herself (Monnin et al. 2002; Cuvillier-Hot et al. 2005) or by modifications in their cuticular hydrocarbons linked to the onset of ovarian activity (CuvillierHot et al. 2004b). Similarly to D. quadriceps or S. peetersi, a Diacamma callow that resists mutilation when the highly fertile gamergate is present would soon be policed by her sisters. In D. australe, callows that were experimentally allowed to keep their gemmae for just 1e2 days were aggressive towards future gamergates; this often led to their immobilization induced by sting smearing, which
BARATTE ET AL.: REPRODUCTIVE CONFLICTS IN ANTS
30 (a) 25
20 a 15
10
a
Number of aggressive acts
5 b 0
30
Immature FG N= 10 (b)
Mature FG
G
10
13
a
25
20
15 b
10
5 c 0
Immature FG N=
10
Mature FG
G
10
13
Figure 2. Number of aggressive acts (biting, lunging and sting smearing) during the mutilation of D. ceylonense callows by (a) callows against future gamergates (FG) or gamergates (G) and (b) by FG or G against callows. Results are given as medians and percentiles (10th, 25th, 75th and 90th) of aggressive acts per individual. Different letters above box plots indicate significant differences (pairwise ManneWhitney U tests: P < 0.05).
is equivalent to the worker policing observed in Diacamma sp. from Japan (Kawabata & Tsuji 2005). Unlike the gamergate, an immature FG has only slightly developed ovaries and needs to be aggressive to older nestmates to prevent them from laying male-destined eggs (Cuvillier-Hot et al. 2002). The conflict is even more intense with unmutilated callows because they can replace future gamergates and have a greater interest in becoming gamergates (relatedness with their own daughters and sons is 0.5) than in rearing a future gamergates’s progeny (average relatedness with their nieces and nephews is
0.375). How can the ants assess the fertility or age of gamergates and future gamergates? Cuvillier-Hot et al. (2001) showed that a gamergate’s high fertility is encoded in her cuticular hydrocarbon profile and this information is available to nestmates. In contrast, an immature FG cannot lay eggs yet and lacks an adequate fertility signal. Thus, our data suggest that information about the mutilator’s fertility (and age) can allow a callow to make her decision: resistance (if fertility is low: future gamergate or senescent gamergate) or surrender to avoid policing (if fertility is high). In D. pallidum from Malaysia, mutilated workers are occasionally responsible for removing gemmae (N ¼ 5, Sommer et al. 1993), which was interpreted as worker policing (Monnin & Ratnieks 2001). However, these mutilations are puzzling because they occurred in colonies without a gamergate, and orphaned workers have more interest in having a future gamergate than in mutilating her. In D. australe and D. ceylonense, we never observed grooming workers removing gemmae, and indeed callows could be mutilated without being held captive. Just-emerged callows have an incompletely sclerotized and soft cuticle, but in our study they were generally not injured during mutilation. The only circumstance in which injury happened was when a 0e3-day-old future gamergate removed gemmae. Although mutilation is an innate behaviour (just-emerged callows were able to mutilate), it apparently requires a certain skill and the ability of very young future gamergates to perform mutilation without injury to callows increased with age and/or experience. This may be the result of maturation of the neural circuits involved or learning. All future gamergates that performed their first mutilation at the age of 4 days or older (N ¼ 10, data not shown) did not kill callows; this finding either supports the role of maturation or results from a larger difference in age between actor and victim. The fatal mutilations observed when future gamergates were less than 4 days old is an interesting parallel with the lethal struggles among newly emerged queens in honeybees, Apis mellifera (Visscher 1993).
Communication and Mutilation Mutilation is a systematic process and our results show that it depends on two distinct pieces of information. First, a ‘callow cue’, which we assume to be chemical, allows gamergates and other workers to identify newly emerged individuals. A gemma secretion is not involved in this phase of recognition: workers’ attention continues after mutilation. In other queenless ants where gemmae do not exist, callows are also recognized and attacked by the gamergate and other high-ranking individuals (e.g. Higashi et al. 1994; Monnin & Peeters 1999; CuvillierHot et al. 2004a), because they have a high probability of becoming dominant and eventually mating when the gamergate grows old or if the colony fissions. Variations in cuticular hydrocarbons are probably involved in this recognition since 0e4-day-old D. ceylonense callows have a distinct profile compared to older workers (CuvillierHot et al. 2001). Second, a gemma secretion is detected
309
ANIMAL BEHAVIOUR, 72, 2
(a) Pr
Me
Proportion of surviving callows
310
1
(b)
0.75
0.5
0.25 a 0
a
0 N= 27
a 1 26
a
2 9
3 12
4 20
5 9
Older 44
Age of immature FG (days) Figure 3. Mutilations by very young future gamergates (FG) of D. ceylonense. (a) Example of lethal wound (white arrow) on the thorax of a callow, just next to one gemma (white box); Pr: pronotum; Me: mesonotum. The scale bar represents 1 mm. (b) Proportions of 1-day-old callows that survived mutilation as a function of the age of immature FG (N ¼ 42 FG). For each age, the number of callows is given. Pairwise differences between proportions (chi-square test: P < 0.05) are indicated by different letters.
at close range by antennation and stimulates scraping of the thorax with the mandibles (which eventually leads to gemma removal, Fig. 1). Results from callows with one gemma indicate that the gamergate uses the gemma secretion as an orientation signal to locate and remove the ap¨ lldobler & C. Peeters, pendage (K. Tsuji, S. Baratte, B. Ho unpublished data). An alteration in the gemma secretion probably explains why gamergates lick ‘nilgiri’ gemmae instead of scraping them and why mutilation is not systematic in this population (Ramaswamy et al. 2004).
Mutilation Maximizes Colony Productivity In the queenless ants that use dominance interactions to regulate reproduction, a gamergate that attains a good level of oogenesis (as detected by the fertility signal) becomes much less aggressive (e.g. Monnin & Peeters 1999; Cuvillier-Hot et al. 2004a). Colonies with a fertile egglayer are quiet, and replacement is unlikely to occur before senescence. Our results show that this is true in Diacamma, yet the gamergate spends time and energy mutilating all newly emerged callows. What is the benefit of mutilation for her? In the other queenless ants, hierarchies are clear even in the presence of a gamergate (Dinoponera quadriceps: Monnin & Peeters 1999; Pachycondyla sublaevis: Ito & Higashi 1991; Amblyopone sp.: Ito 1993). High-rankers benefit from competing to replace the gamergate and they spend substantial energy in aggressive behaviours (e.g. Gobin et al. 2003) and also work less than other workers (Cole 1986). These ‘hopeful reproductives’ have a negative cost on colony productivity, which can be crucial when colonies are small (which is the case each time fission occurs) or when hierarchies are long (Monnin & Ratnieks 1999). The uniqueness of Diacamma species is that mutilation prevents workers from being hopeful reproductives. The degeneration of neuronal connections from gemmae to the central nervous system (Gronenberg & Peeters 1993) triggers physiological and/or behavioural changes and could explain why mutilated workers are unable to perform
sexual calling. Therefore, competition among workers disappears (as shown by the absence of a hierarchy) and in ancestral Diacamma populations mutilation could have increased colony productivity. Mutilation optimizes the division of tasks into two behavioural groups of individuals, just like species with castes. In the latter, irreversible differentiation of individuals occurs during larval development, whereas in Diacamma irreversible differentiation is the result of an externally imposed loss of function that produces physiological changes in adults. Selective pressures appear similar in both situations because they relate to a major issue in insect societies: how resources are allocated between sexual reproduction and colony maintenance (Pamilo 1991).
Acknowledgments We thank Raghavendra Gadagkar and his Bangalore team for their hospitality and for helping us to excavate colonies. We also thank Thibaud Monnin and two anonymous referees for helpful comments on the manuscript. Our research on Indian Diacamma is part of a collaborative project with R. Gadagkar (Indian Institute of Science, Bangalore) and is supported by the CNRS (PICS 1041).
References Andre´, J.-B., Peeters, C. & Doums, C. 2001. Serial polygyny and colony genetic structure in the monogynous queenless ant Diacamma cyaneiventre. Behavioral Ecology and Sociobiology, 50, 72e80. Baudry, E., Peeters, C., Brazier, L., Veuille, M. & Doums, C. 2003. Shift in the behaviours regulating monogyny is associated with high genetic differentiation in the queenless ant Diacamma ceylonense. Insectes Sociaux, 50, 1e8. Cole, B. J. 1986. The social behavior of Leptothorax allardycei (Hymenoptera, Formicidae): time budget and the evolution of worker reproduction. Behavioral Ecology and Sociobiology, 18, 165e173. Cuvillier-Hot, V., Cobb, M., Malosse, C. & Peeters, C. 2001. Sex, age and ovarian activity affect cuticular hydrocarbons in Diacamma ceylonense, a queenless ant. Journal of Insect Physiology, 47, 485e493.
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Cuvillier-Hot, V., Gadagkar, R., Peeters, C. & Cobb, M. 2002. Regulation of reproduction in a queenless ant: aggression, pheromones and reduction in conflict. Proceedings of the Royal Society of London, Series B, 269, 1295e1300. Cuvillier-Hot, V., Lenoir, A., Crewe, R., Malosse, C. & Peeters, C. 2004a. Fertility signalling and reproductive skew in queenless ants. Animal Behaviour, 68, 1209e1219. doi:10.1016/j.anbehav. 2003.11.026. Cuvillier-Hot, V., Lenoir, A. & Peeters, C. 2004b. Reproductive monopoly enforced by sterile police workers in a queenless ant. Behavioral Ecology, 15, 970e975. doi:10.1093/beheco/arh072. Cuvillier-Hot, V., Renault, V. & Peeters, C. 2005. Rapid modification in the olfactory signal of ants following a change in reproductive status. Naturwissenschaften, 92, 73e77. doi:10.1007/s00114004-0588-7. Fukumoto, Y., Abe, T. & Taki, A. 1989. A novel form of colony organization in the ‘queenless’ ant Diacamma rugosum. Physiological Ecology of Japan, 26, 55e61. Gobin, B., Billen, J. & Peeters, C. 2001. Dominance interactions regulate worker mating in the polygynous ponerine ant Gnamptogenys menadensis. Ethology, 107, 495e508. Gobin, B., Heinze, J., Stra¨tz, M. & Roces, F. 2003. The energetic cost of reproductive conflicts in the ant Pachycondyla obscuricornis. Journal of Insect Physiology, 49, 747e752. Gronenberg, W. & Peeters, C. 1993. Central projections of the sensory hairs on the gemma of the ant Diacamma: substrate for behavioral modulation? Cell Tissue Research, 273, 401e415. Higashi, S., Ito, F., Sugiura, N. & Ohkawara, K. 1994. Worker’s age regulates the linear dominance hierarchy in the queenless ponerine ant, Pachycondyla sublaevis (Hymenoptera: Formicidae). Animal Behaviour, 47, 179e184. ¨ lldobler, B. & Wilson, E. O. 1990. The Ants. Cambridge, MassaHo chusetts: Belknap Press. Ito, F. 1993. Social organization in a primitive ponerine ant: queenless reproduction, dominance hierarchy and functional polygyny in Amblyopone sp. (reclinata group) (Hymenoptera: Formicidae: Ponerinae). Journal of Natural History, 27, 1315e1324. Ito, F. & Higashi, S. 1991. A linear dominance hierarchy regulating reproduction and polyethism of the queenless ant Pachycondyla sublaevis. Naturwissenschaften, 78, 80e82. Kawabata, S. & Tsuji, K. 2005. The policing behavior ‘immobilization‘ towards ovary-developed workers in the ant, Diacamma sp. from Japan. Insectes Sociaux, 52, 89e95. Monnin, T. & Peeters, C. 1999. Dominance hierarchy and reproductive conflicts among subordinates in a monogynous queenless ant. Behavioral Ecology, 10, 323e333.
Monnin, T. & Ratnieks, F. L. 1999. Reproduction versus work in queenless ants: when to join a hierarchy of hopeful reproductives? Behavioral Ecology and Sociobiology, 46, 413e422. Monnin, T. & Ratnieks, F. L. 2001. Policing in queenless ponerine ants. Behavioral Ecology and Sociobiology, 50, 97e108. Monnin, T., Ratnieks, F. L., Jones, G. R. & Beard, R. 2002. Pretender punishment induced by chemical signalling in a queenless ant. Nature, 419, 61e65. Pamilo, P. 1991. Evolution of colony characteristics in social insects. I. Sex allocation. American Naturalist, 137, 83e107. Peeters, C. 1991. The occurrence of sexual reproduction among ant workers. Biological Journal of the Linnean Society, 44, 141e152. Peeters, C. & Billen, J. 1991. A novel exocrine gland inside the thoracic appendages (‘gemmae’) of the queenless ant Diacamma australe. Experientia, 47, 229e231. Peeters, C. & Higashi, S. 1989. Reproductive dominance controlled by mutilation in the queenless ant Diacamma australe. Naturwissenschaften, 76, 177e180. Peeters, C. & Ito, F. 2001. Colony dispersal and the evolution of queen morphology in social hymenoptera. Annual Review of Entomology, 46, 601e630. Peeters, C. & Tsuji, K. 1993. Reproductive conflict among ant workers in Diacamma sp. from Japan: dominance and oviposition in the absence of the gamergate. Insectes Sociaux, 40, 119e136. ¨ lldobler, B. 1992. Alternative dominance Peeters, C., Billen, J. & Ho mechanism regulating monogyny in the queenless ant genus Diacamma. Naturwissenschaften, 79, 572e573. Ramaswamy, K., Peeters, C., Yuvana, S. P., Varghese, T., Pradeep, H. D., Dietemann, V., Vedham, K., Cobb, M. & Gadagkar, R. 2004. Social mutilation in the Ponerine ant Diacamma: cues originate in the victims. Insectes Sociaux, 51, 410e413. ¨ lldobler, B. & Rembold, H. 1993. Behavioral and Sommer, K., Ho physiological aspects of reproductive control in a Diacamma species from Malaysia (Formicidae, Ponerinae). Ethology, 94, 162e170. Tsuji, K., Nakata, K. & Heinze, J. 1996. Lifespan and reproduction in a queenless ant. Naturwissenschaften, 83, 577e578. ¨ lldobler, B. 1999. Regulation of worker Tsuji, K., Egashira, K. & Ho reproduction by direct physical contact in the ant Diacamma sp. from Japan. Animal Behaviour, 58, 337e343. Visscher, P. K. 1993. A theoretical analysis of individual interests and intracolony conflict during swarming of honey bee colonies. Journal of Theoretical Biology, 165, 191e212.
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Reproductive conflicts and mutilation in queenless Diacamma ants S EBA STI EN B AR ATT E *, MA TTHEW C OBB † & CHRISTIAN PEETERS *
*Laboratoire d’Ecologie, CNRS and Universite´ Pierre-et-Marie Curie, Paris yFaculty of Life Sciences, University of Manchester (Received 5 April 2005; initial acceptance 1 June 2005; final acceptance 11 October 2005; published online 11 July 2006; MS. number: 8514R)
Reproductive conflicts are particularly intense in queenless ants because colonies are made up of totipotent workers, all potentially able to mate and produce female offspring. After a gamergate (a mated egg-laying worker) dies, aggressive interactions determine her replacement. In Diacamma, the single gamergate systematically mutilates newly emerged workers (callows) and consequently they can never mate. New callows are expected to resist because, once mutilated, they cannot replace the gamergate. In D. ceylonense and D. australe, we studied the behaviour of new callows confronted with dominant individuals of different fertility or age. Faced with a young unmutilated worker (the future gamergate with poorly developed ovaries), callows were aggressive, whereas they did not resist gamergates. We interpret this dichotomy in terms of reproductive differentials between actors and victims. We discuss the selective advantages of mutilation relative to dominance hierarchies which other queenless ants use to regulate monogyny. Mutilation seems to maximize colony productivity by creating irreversibly sterile helpers from newly emerged individuals. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Reproductive conflicts in ants are strongly influenced by the occurrence of two female castes. Queens and workers diverge irreversibly during larval development and workers generally cannot reproduce sexually. However, in about 200 species of queenless ants, colonies consist exclusively of workers with equivalent reproductive potentials (Peeters 1991; Peeters & Ito 2001). Aggressive interactions lead to hierarchies, and one or a few dominant workers mate and lay eggs (e.g. Ito & Higashi 1991; Monnin & Peeters 1999; Gobin et al. 2001; Cuvillier-Hot et al. 2004a). Importantly, subordinate individuals retain the ability to replace the ‘gamergate’ (mated egg-laying worker). A qualitatively different type of behavioural regulation characterizes the genus Diacamma, because the gamergate systematically bites off the ‘gemmae’, a pair of tiny appendages filled with exocrine cells present on the thorax of all emerging workers (Fukumoto et al. 1989; Peeters & Higashi 1989; Peeters & Billen 1991). Once mutilated, Diacamma workers can never mate and are restricted to producing males in colonies without a gamergate (Peeters & Tsuji 1993; Tsuji et al. 1999;
Correspondence: C. Peeters, Laboratoire d’Ecologie, CNRS UMR 7625, Universite´ Pierre-et-Marie Curie, 7 quai Saint Bernard, F-75005 Paris, France (email: [email protected]). 0003e3472/06/$30.00/0
Cuvillier-Hot et al. 2002). Unlike those of other queenless ants, the reproductive conflicts then become analogous to those in species with queen and worker castes. In colonies with a gamergate, mutilation is an almost daily routine and callows (newly emerged workers) are likely to be the gamergate’s daughters. An unstable phase intervenes following two natural events: replacement of a senescent gamergate (life expectancy is less than 2 years, Tsuji et al. 1996; Andre´ et al. 2001) and colony fission. The colonies of all queenless ants reproduce obligatorily by fission (Peeters & Ito 2001), and in monogynous species such as Diacamma, this creates an opportunity for the differentiation of a new gamergate. The details of fission are unknown, but one of the two daughter colonies will inevitably consist of mutilated workers and brood; hence the first cocoon that ecloses yields the future gamergate. This individual immediately begins to mutilate all her sisters that subsequently emerge. Ant callows are usually timid and inactive (Ho¨lldobler & Wilson 1990), but in Diacamma they are aggressive and capable of performing this complex behaviour, sometimes within hours of their own emergence. During the unstable phase, the future gamergate is initially unable to lay eggs (referred to as ‘immature FG’ below), but after about 3 weeks she begins to produce males and starts sexual calling to attract a mate (‘mature FG’; Cuvillier-Hot et al. 2002). She
305 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 72, 2
eventually copulates with a single foreign male. Thus, while the victims of mutilation are always recently emerged callows, the actors can differ markedly in fertility, age and genetic relatedness. To understand the evolution of this irreversible regulation, we determined whether the course of mutilation is influenced by differences in reproductive potential between actors and their victims. A callow confronted with the gamergate may benefit from producing her own offspring (relatedness with sons and daughters is 0.5) instead of rearing brothers and sisters (average relatedness is also 0.5). However, older mutilated sisters should prevent the replacement of their mother (average relatedness with nieces and nephews is 0.375). Similarly, it may be in a callow’s interest to escape mutilation by an immature FG and produce her own daughters, rather than rearing her sister’s. An intermediate situation may occur in colonies with a mature FG, because she is virgin yet has a moderate fertility. Fertility is a crucial parameter affecting the helpers’ inclusive fitness in all social insects, and reliable information about age and fertility is encoded in the cuticular hydrocarbons of D. ceylonense (CuvillierHot et al. 2001, 2002). Callows may thus be able to distinguish between a future gamergate (either immature or mature) and a gamergate, and adjust their behaviour during mutilation accordingly. We introduced new callows into recently orphaned colonies of D. ceylonense and D. australe (belonging to different species-groups; W.L. Brown, personal communication). We compared the course of mutilation by either immature or mature FG with that observed in colonies with a gamergate. We also observed the behaviour of D. ceylonense gamergates faced with surgically mutilated callows and with callows from the nonmutilating ‘nilgiri’ population. Given that the physical act of mutilation separates the conflicts over either male production or female production to a greater extent than in the other queenless ants, we discuss the costs and benefits of its evolution in Diacamma.
METHODS
Housing and Creation of Experimental Groups We collected 16 colonies of D. ceylonense (X SD ¼ 296 92 workers, range 117e448) from south of Bangalore, Karnataka State, India, in April 2001, November 2002 and January 2004 and eight colonies of D. australe in November 1987 from east of Townsville, north Queensland, Australia. In each colony, the wild-caught gamergate, identified by the presence of gemmae, was marked with dots of paint. To prevent natural mutilations, we quickly removed cocoons from the colonies and isolated them in nurseries with about 40 mutilated workers. Both colonies with gamergates and nurseries of each colony were placed in plaster nests covered with glass to allow observation, kept at 25 C under a 12:12 h light:dark cycle and provided with crickets, Tenebrio molitor larvae (mealworms) and honey diluted in water. Cocoons were checked daily and newly emerged callows were isolated.
Callows are easily distinguishable because of their soft cuticle and their light-brown extremities. Of the 964 cocoons of D. ceylonense collected in the field, about one-third (N ¼ 371) eclosed before isolation in the nurseries; around 30% of the remaining cocoons never eclosed, 20% produced males and 50% yielded workers (N ¼ 281 callows). For D. australe, over 300 cocoons eclosed in the laboratory. Several colonies from the nonmutilating population of D. ceylonense, called ‘nilgiri’ (Peeters et al. 1992; Baudry et al. 2003), were also excavated near Bandipur in Karnataka (150 km south of Bangalore) in January 2004. Cocoons of ‘nilgiri’ were placed in ceylonense nurseries, where callows emerged successfully.
Mutilation in Different Social Contexts We compared mutilations by gamergates and future gamergates (immature and mature). To create groups with gamergates, we introduced 0e2-day-old callows (daughters of gamergates) from a nursery to the colony from which they originated (G groups; N ¼ 13 for D. ceylonense). We also introduced ‘nilgiri’ callows (N ¼ 7) and callows with surgically removed gemmae (N ¼ 11) into G groups to observe whether the ceylonense gamergate responded differently to them. Eclosions in nonmanipulated colonies of D. ceylonense and D. australe provided further information about the behaviour of gamergates. To create groups with future gamergates (FG groups), we used orphaned groups of 30e50 mutilated workers; the first emerged callow in these groups retained her gemmae. We then observed mutilations by adding callows (sisters of the future gamergate) from nurseries to the corresponding FG groups: 10 mutilations with 10 immature FG (0e8 days old) and 10 mutilations with 10 different mature FG (39e55 days old). Uncontrolled mutilations also occurred in nurseries whenever more than one callow (2e6) emerged overnight (N ¼ 42). One of the callows started to mutilate the others and we counted how many callows had gemmae the next day (N ¼ 147 callows).
Monitoring of Behaviours We observed the behaviours of actors and victims until mutilation was complete or until mutilation behaviour had ceased. For D. ceylonense, real-time observations were complemented with recordings made with a wide-angle video camera (Hitachi KP-D51 Color Digital) to monitor general activity around introduced callows. We recorded all aggressive interactions (antennal boxing followed by crouching of the target individual, lunging, biting, sting smearing (Monnin & Peeters 1999; Monnin et al. 2002)), submissive reactions (crouching and avoidance, Peeters & Tsuji 1993) and neutral behaviours (grooming, antennal inspection). Once the callow was quiet and mutilation attempts had begun, a black and white camera (Sony CCD XC-ST70) mounted on a dissection microscope recorded the movement of the mouthparts and antennae around the callow’s gemmae to determine how they were removed. Quantitative data were not obtained for D. australe.
BARATTE ET AL.: REPRODUCTIVE CONFLICTS IN ANTS
RESULTS
Mutilations by a Gamergate In nonmanipulated colonies of both D. ceylonense and D. australe, we regularly observed gamergates standing near and opening cocoons. Soon after emergence, callows were the focus of attention from three to five workers which licked them and removed their larval skin. Workers often held a callow for hours, thereby reducing her ability to move during mutilation. However, we also observed callows free to move in the nest and eventually mutilated by gamergates without the involvement of any workers. Mutilation occurred as quickly as 10 min after emergence (N ¼ 2). When more than one cocoon eclosed within a few hours, several callows were held simultaneously and the gamergate went from one to another. When a callow was transferred to a G group, the gamergate took 6e68 min to identify her (Table 1), depending on whether she was resting or patrolling. As soon as the callow was introduced, 3e15 workers surrounded her (Table 1) and began to lick every part of her body, even though her larval skin had already been removed in the nursery. They did not pay specific attention to the gemmae. In the vicinity of the grooming workers, the gamergate stretched her antennae towards the callow (Fig. 1, ‘identification’), walked closer, pushing aside attending workers, and finally touched the callow’s thorax with her antennal tips. She appeared to become excited and opened her mandibles wide, and repeatedly performed both antennal inspection (antennal tips gently run on the cuticle, Fig. 1a) and antennal boxing (antennal tips rapidly struck the cuticle, Fig. 1b). Each time a gemma was touched by an antenna during inspection, the gamergate scraped the thorax with her mandibles (Fig. 1c). As long as a gemma was present, this behavioural loop helped the gamergate to scrape closer and closer to the target. She eventually succeeded in lifting the gemma from the pleural cavity and then pulled it away. Finally, when both gemmae had been removed, she inspected the callow for several seconds and moved away. She did not show any further interest in the mutilated callows.
Mutilations by a Future Gamergate Both immature and mature FG succeeded in mutilating the introduced callows, removing their gemmae in the
same way as gamergates did (Table 2). None the less, we observed significant differences in the level of aggression of both callows and mutilating ants during identification (KruskaleWallis test: callow’s aggressiveness: H2 ¼ 20.64, P < 0.001; mutilator’s aggressiveness: H2 ¼ 20.54, P < 0.001). A callow was submissive when she detected a gamergate (similar to callows in nonmanipulated colonies; median number of aggressive acts against gamergate ¼ 0; Fig. 2a) and the latter remained nonaggressive (median number of aggressive acts against callows ¼ 0; Fig. 2b). In contrast, a callow was more aggressive when faced with a mature or immature FG (ManneWhitney U tests with sequential Bonferroni’s correction: immature FG versus gamergate: U ¼ 121, N1 ¼ 10, N2 ¼ 13, P ¼ 0.0001; mature FG versus gamergate: U ¼ 127.5, N1¼ 10, N2 ¼ 13, P ¼ 0.0001; mature FG versus immature FG: U ¼ 69, N1 ¼ N2 ¼ 10, P ¼ 0.16; Fig. 2a). A few callows succeeded in escaping from grooming workers, ran after the future gamergate and attacked her. When this happened, the future gamergate lunged back at the callow, attempted sting smearing and a fight ensued. In all cases the future gamergate prevailed but ‘gemmae removal’ was delayed (range 20 mine1 h after the future gamergate identified the callow). Thus, immature FG were more aggressive than gamergates during the initial encounter (U ¼ 124.5, N1 ¼ 10, N2 ¼ 13, P < 0.0001; Fig. 2b). A mature FG showed an intermediate level of aggression when faced with a new callow in comparison with immature FG and gamergates (callow versus immature FG: U ¼ 90, N1 ¼ N2 ¼ 10, P ¼ 0.001; callow versus gamergate: U ¼ 112.5, N1 ¼ 10, N2 ¼ 13, P ¼ 0.002; Fig. 2b). Aggressive resistance from older unmutilated callows was also observed in D. australe, although we did not aim for a quantitative description. The mandibular scrapings by immature FG were also more intense than those of gamergates. Examination of videos indicated that five future gamergates perforated the soft cuticle of callows, releasing drops of haemolymph on the thorax (Fig. 3a), petiole or legs (because the callow’s forelegs often shield the gemmae). When more than one cocoon eclosed during the night in nurseries, we found all but one callow mutilated and also perforated or partially eaten the next day. These mortalities become less frequent as the immature FG grew older (Fig. 3b); 95.1% of fatal mutilations were by 0e3-day-old FG (comparison between ages of immature FG that made fatal versus nonfatal mutilations: ManneWhitney U test; U ¼ 2503, N1 ¼ 66, N2 ¼ 81, P < 0.0001).
Table 1. Behaviour of D. ceylonense gamergates and grooming workers upon introduction of 0e2-day-old callows (C) with no (C0) or two (C2) intact gemmae
Context
Latency to Number of identification Latency to first Latency to remove Latency to remove N grooming workers of callow (min) thorax inspection (min) the first gemma (min) second gemma (min)
Gamergate with C2 13 Gamergate with C0 11
9 (7e15) 7 (3e12)
50 (12e68) 35 (6e73)
5 (0e55) 7 (0e53)
16 (3e75) d
31 (17-72) d
Values are given as median and range. Latencies are from when the callow entered the nest. All pairwise comparisons are not significant (ManneWhitney U test).
307
308
ANIMAL BEHAVIOUR, 72, 2
Each time a gemma is touched by an antenna
Once the callow is quiet
a Antennal inspection
b Antennal boxing
Once both gemmae have been removed
c Scraping with mandibles
A gemma is eventually removed 2 Gemma removal
1 Identification
3 Loss of interest
Figure 1. The three phases of mutilation in Diacamma ceylonense. The callow is shown in the top of the drawings and organs involved in behaviours are highlighted in black (grooming workers around the callow are not represented).
Influence of Gemmae on Mutilation Callows with two gemmae (C2) and no gemma (C0) were equally attractive to workers and both were identified by gamergates (Table 1). However, the sequence of behaviours was significantly modified when the gamergate was faced with C0 or ‘nilgiri’ callows (Table 2). When an introduced callow lacked gemmae, the gamergate did not get excited, but inspected her and rapidly moved away (no antennal boxing, no scraping with mandibles, Table 2); this corresponds to ‘loss of interest’ in Fig. 1. When a ‘nilgiri’ callow was introduced, the ceylonense gamergate identified her, inspected her thorax (Fig. 1a) and performed frequent antennal boxing (Fig. 1b). However, the gamergate did not use her mandibles to scrape the gemmae but licked them with her mouthparts, which was not Table 2. Number of behaviours of D. ceylonense future gamergates (FG) or gamergates upon introduction of 0e2-day-old callows (C) with no (C0) or two (C2) intact gemmae
Context Gamergate with C2 Gamergate with C0 Mature FG with C2 Immature FG with C2 Gamergate with ‘nilgiri’ C2
Scraping Antennal Antennal with Completed N inspection boxing mandibles mutilations 13
13
13
13
13
11
11
0
0
e
10
10
10
10
10
10
10
10
10
10
7
7
7
0
0
Bold numbers are significantly different from those for the gamergate with C2 (pairwise chi-square tests: P < 0.05).
observed with ceylonense callows. The ‘nilgiri’ callows were not mutilated even several days after introduction.
DISCUSSION
Age and Fertility of the Mutilating Ant There was a high level of aggression between future gamergates and callows although mutilation always occurred. In contrast, callows emerging in colonies with a gamergate did not attack her and did not try to escape mutilation. This indicates that a callow is able to adjust her behaviour before being mutilated. When a gamergate (the mother) is present, callow workers have the option to replace her (gaining access to sexual reproduction) or to refrain from reproduction (gaining indirect fitness). In both cases, the callow’s relatedness to offspring will be the same (see Introduction). However, frequent replacements of the gamergate should be counterselected (Monnin et al. 2002). Older workers may not benefit by early replacement of their mother as long as she is fertile because they are more related to their own sisters and brothers (average 0.5) than to the offspring of a sister that mates with a foreign male (0.375). In Dinoponera quadriceps and Streblognathus peetersi, any worker attempting to replace a fertile gamergate is punished by worker policing, induced by the gamergate herself (Monnin et al. 2002; Cuvillier-Hot et al. 2005) or by modifications in their cuticular hydrocarbons linked to the onset of ovarian activity (CuvillierHot et al. 2004b). Similarly to D. quadriceps or S. peetersi, a Diacamma callow that resists mutilation when the highly fertile gamergate is present would soon be policed by her sisters. In D. australe, callows that were experimentally allowed to keep their gemmae for just 1e2 days were aggressive towards future gamergates; this often led to their immobilization induced by sting smearing, which
BARATTE ET AL.: REPRODUCTIVE CONFLICTS IN ANTS
30 (a) 25
20 a 15
10
a
Number of aggressive acts
5 b 0
30
Immature FG N= 10 (b)
Mature FG
G
10
13
a
25
20
15 b
10
5 c 0
Immature FG N=
10
Mature FG
G
10
13
Figure 2. Number of aggressive acts (biting, lunging and sting smearing) during the mutilation of D. ceylonense callows by (a) callows against future gamergates (FG) or gamergates (G) and (b) by FG or G against callows. Results are given as medians and percentiles (10th, 25th, 75th and 90th) of aggressive acts per individual. Different letters above box plots indicate significant differences (pairwise ManneWhitney U tests: P < 0.05).
is equivalent to the worker policing observed in Diacamma sp. from Japan (Kawabata & Tsuji 2005). Unlike the gamergate, an immature FG has only slightly developed ovaries and needs to be aggressive to older nestmates to prevent them from laying male-destined eggs (Cuvillier-Hot et al. 2002). The conflict is even more intense with unmutilated callows because they can replace future gamergates and have a greater interest in becoming gamergates (relatedness with their own daughters and sons is 0.5) than in rearing a future gamergates’s progeny (average relatedness with their nieces and nephews is
0.375). How can the ants assess the fertility or age of gamergates and future gamergates? Cuvillier-Hot et al. (2001) showed that a gamergate’s high fertility is encoded in her cuticular hydrocarbon profile and this information is available to nestmates. In contrast, an immature FG cannot lay eggs yet and lacks an adequate fertility signal. Thus, our data suggest that information about the mutilator’s fertility (and age) can allow a callow to make her decision: resistance (if fertility is low: future gamergate or senescent gamergate) or surrender to avoid policing (if fertility is high). In D. pallidum from Malaysia, mutilated workers are occasionally responsible for removing gemmae (N ¼ 5, Sommer et al. 1993), which was interpreted as worker policing (Monnin & Ratnieks 2001). However, these mutilations are puzzling because they occurred in colonies without a gamergate, and orphaned workers have more interest in having a future gamergate than in mutilating her. In D. australe and D. ceylonense, we never observed grooming workers removing gemmae, and indeed callows could be mutilated without being held captive. Just-emerged callows have an incompletely sclerotized and soft cuticle, but in our study they were generally not injured during mutilation. The only circumstance in which injury happened was when a 0e3-day-old future gamergate removed gemmae. Although mutilation is an innate behaviour (just-emerged callows were able to mutilate), it apparently requires a certain skill and the ability of very young future gamergates to perform mutilation without injury to callows increased with age and/or experience. This may be the result of maturation of the neural circuits involved or learning. All future gamergates that performed their first mutilation at the age of 4 days or older (N ¼ 10, data not shown) did not kill callows; this finding either supports the role of maturation or results from a larger difference in age between actor and victim. The fatal mutilations observed when future gamergates were less than 4 days old is an interesting parallel with the lethal struggles among newly emerged queens in honeybees, Apis mellifera (Visscher 1993).
Communication and Mutilation Mutilation is a systematic process and our results show that it depends on two distinct pieces of information. First, a ‘callow cue’, which we assume to be chemical, allows gamergates and other workers to identify newly emerged individuals. A gemma secretion is not involved in this phase of recognition: workers’ attention continues after mutilation. In other queenless ants where gemmae do not exist, callows are also recognized and attacked by the gamergate and other high-ranking individuals (e.g. Higashi et al. 1994; Monnin & Peeters 1999; CuvillierHot et al. 2004a), because they have a high probability of becoming dominant and eventually mating when the gamergate grows old or if the colony fissions. Variations in cuticular hydrocarbons are probably involved in this recognition since 0e4-day-old D. ceylonense callows have a distinct profile compared to older workers (CuvillierHot et al. 2001). Second, a gemma secretion is detected
309
ANIMAL BEHAVIOUR, 72, 2
(a) Pr
Me
Proportion of surviving callows
310
1
(b)
0.75
0.5
0.25 a 0
a
0 N= 27
a 1 26
a
2 9
3 12
4 20
5 9
Older 44
Age of immature FG (days) Figure 3. Mutilations by very young future gamergates (FG) of D. ceylonense. (a) Example of lethal wound (white arrow) on the thorax of a callow, just next to one gemma (white box); Pr: pronotum; Me: mesonotum. The scale bar represents 1 mm. (b) Proportions of 1-day-old callows that survived mutilation as a function of the age of immature FG (N ¼ 42 FG). For each age, the number of callows is given. Pairwise differences between proportions (chi-square test: P < 0.05) are indicated by different letters.
at close range by antennation and stimulates scraping of the thorax with the mandibles (which eventually leads to gemma removal, Fig. 1). Results from callows with one gemma indicate that the gamergate uses the gemma secretion as an orientation signal to locate and remove the ap¨ lldobler & C. Peeters, pendage (K. Tsuji, S. Baratte, B. Ho unpublished data). An alteration in the gemma secretion probably explains why gamergates lick ‘nilgiri’ gemmae instead of scraping them and why mutilation is not systematic in this population (Ramaswamy et al. 2004).
Mutilation Maximizes Colony Productivity In the queenless ants that use dominance interactions to regulate reproduction, a gamergate that attains a good level of oogenesis (as detected by the fertility signal) becomes much less aggressive (e.g. Monnin & Peeters 1999; Cuvillier-Hot et al. 2004a). Colonies with a fertile egglayer are quiet, and replacement is unlikely to occur before senescence. Our results show that this is true in Diacamma, yet the gamergate spends time and energy mutilating all newly emerged callows. What is the benefit of mutilation for her? In the other queenless ants, hierarchies are clear even in the presence of a gamergate (Dinoponera quadriceps: Monnin & Peeters 1999; Pachycondyla sublaevis: Ito & Higashi 1991; Amblyopone sp.: Ito 1993). High-rankers benefit from competing to replace the gamergate and they spend substantial energy in aggressive behaviours (e.g. Gobin et al. 2003) and also work less than other workers (Cole 1986). These ‘hopeful reproductives’ have a negative cost on colony productivity, which can be crucial when colonies are small (which is the case each time fission occurs) or when hierarchies are long (Monnin & Ratnieks 1999). The uniqueness of Diacamma species is that mutilation prevents workers from being hopeful reproductives. The degeneration of neuronal connections from gemmae to the central nervous system (Gronenberg & Peeters 1993) triggers physiological and/or behavioural changes and could explain why mutilated workers are unable to perform
sexual calling. Therefore, competition among workers disappears (as shown by the absence of a hierarchy) and in ancestral Diacamma populations mutilation could have increased colony productivity. Mutilation optimizes the division of tasks into two behavioural groups of individuals, just like species with castes. In the latter, irreversible differentiation of individuals occurs during larval development, whereas in Diacamma irreversible differentiation is the result of an externally imposed loss of function that produces physiological changes in adults. Selective pressures appear similar in both situations because they relate to a major issue in insect societies: how resources are allocated between sexual reproduction and colony maintenance (Pamilo 1991).
Acknowledgments We thank Raghavendra Gadagkar and his Bangalore team for their hospitality and for helping us to excavate colonies. We also thank Thibaud Monnin and two anonymous referees for helpful comments on the manuscript. Our research on Indian Diacamma is part of a collaborative project with R. Gadagkar (Indian Institute of Science, Bangalore) and is supported by the CNRS (PICS 1041).
References Andre´, J.-B., Peeters, C. & Doums, C. 2001. Serial polygyny and colony genetic structure in the monogynous queenless ant Diacamma cyaneiventre. Behavioral Ecology and Sociobiology, 50, 72e80. Baudry, E., Peeters, C., Brazier, L., Veuille, M. & Doums, C. 2003. Shift in the behaviours regulating monogyny is associated with high genetic differentiation in the queenless ant Diacamma ceylonense. Insectes Sociaux, 50, 1e8. Cole, B. J. 1986. The social behavior of Leptothorax allardycei (Hymenoptera, Formicidae): time budget and the evolution of worker reproduction. Behavioral Ecology and Sociobiology, 18, 165e173. Cuvillier-Hot, V., Cobb, M., Malosse, C. & Peeters, C. 2001. Sex, age and ovarian activity affect cuticular hydrocarbons in Diacamma ceylonense, a queenless ant. Journal of Insect Physiology, 47, 485e493.
BARATTE ET AL.: REPRODUCTIVE CONFLICTS IN ANTS
Cuvillier-Hot, V., Gadagkar, R., Peeters, C. & Cobb, M. 2002. Regulation of reproduction in a queenless ant: aggression, pheromones and reduction in conflict. Proceedings of the Royal Society of London, Series B, 269, 1295e1300. Cuvillier-Hot, V., Lenoir, A., Crewe, R., Malosse, C. & Peeters, C. 2004a. Fertility signalling and reproductive skew in queenless ants. Animal Behaviour, 68, 1209e1219. doi:10.1016/j.anbehav. 2003.11.026. Cuvillier-Hot, V., Lenoir, A. & Peeters, C. 2004b. Reproductive monopoly enforced by sterile police workers in a queenless ant. Behavioral Ecology, 15, 970e975. doi:10.1093/beheco/arh072. Cuvillier-Hot, V., Renault, V. & Peeters, C. 2005. Rapid modification in the olfactory signal of ants following a change in reproductive status. Naturwissenschaften, 92, 73e77. doi:10.1007/s00114004-0588-7. Fukumoto, Y., Abe, T. & Taki, A. 1989. A novel form of colony organization in the ‘queenless’ ant Diacamma rugosum. Physiological Ecology of Japan, 26, 55e61. Gobin, B., Billen, J. & Peeters, C. 2001. Dominance interactions regulate worker mating in the polygynous ponerine ant Gnamptogenys menadensis. Ethology, 107, 495e508. Gobin, B., Heinze, J., Stra¨tz, M. & Roces, F. 2003. The energetic cost of reproductive conflicts in the ant Pachycondyla obscuricornis. Journal of Insect Physiology, 49, 747e752. Gronenberg, W. & Peeters, C. 1993. Central projections of the sensory hairs on the gemma of the ant Diacamma: substrate for behavioral modulation? Cell Tissue Research, 273, 401e415. Higashi, S., Ito, F., Sugiura, N. & Ohkawara, K. 1994. Worker’s age regulates the linear dominance hierarchy in the queenless ponerine ant, Pachycondyla sublaevis (Hymenoptera: Formicidae). Animal Behaviour, 47, 179e184. ¨ lldobler, B. & Wilson, E. O. 1990. The Ants. Cambridge, MassaHo chusetts: Belknap Press. Ito, F. 1993. Social organization in a primitive ponerine ant: queenless reproduction, dominance hierarchy and functional polygyny in Amblyopone sp. (reclinata group) (Hymenoptera: Formicidae: Ponerinae). Journal of Natural History, 27, 1315e1324. Ito, F. & Higashi, S. 1991. A linear dominance hierarchy regulating reproduction and polyethism of the queenless ant Pachycondyla sublaevis. Naturwissenschaften, 78, 80e82. Kawabata, S. & Tsuji, K. 2005. The policing behavior ‘immobilization‘ towards ovary-developed workers in the ant, Diacamma sp. from Japan. Insectes Sociaux, 52, 89e95. Monnin, T. & Peeters, C. 1999. Dominance hierarchy and reproductive conflicts among subordinates in a monogynous queenless ant. Behavioral Ecology, 10, 323e333.
Monnin, T. & Ratnieks, F. L. 1999. Reproduction versus work in queenless ants: when to join a hierarchy of hopeful reproductives? Behavioral Ecology and Sociobiology, 46, 413e422. Monnin, T. & Ratnieks, F. L. 2001. Policing in queenless ponerine ants. Behavioral Ecology and Sociobiology, 50, 97e108. Monnin, T., Ratnieks, F. L., Jones, G. R. & Beard, R. 2002. Pretender punishment induced by chemical signalling in a queenless ant. Nature, 419, 61e65. Pamilo, P. 1991. Evolution of colony characteristics in social insects. I. Sex allocation. American Naturalist, 137, 83e107. Peeters, C. 1991. The occurrence of sexual reproduction among ant workers. Biological Journal of the Linnean Society, 44, 141e152. Peeters, C. & Billen, J. 1991. A novel exocrine gland inside the thoracic appendages (‘gemmae’) of the queenless ant Diacamma australe. Experientia, 47, 229e231. Peeters, C. & Higashi, S. 1989. Reproductive dominance controlled by mutilation in the queenless ant Diacamma australe. Naturwissenschaften, 76, 177e180. Peeters, C. & Ito, F. 2001. Colony dispersal and the evolution of queen morphology in social hymenoptera. Annual Review of Entomology, 46, 601e630. Peeters, C. & Tsuji, K. 1993. Reproductive conflict among ant workers in Diacamma sp. from Japan: dominance and oviposition in the absence of the gamergate. Insectes Sociaux, 40, 119e136. ¨ lldobler, B. 1992. Alternative dominance Peeters, C., Billen, J. & Ho mechanism regulating monogyny in the queenless ant genus Diacamma. Naturwissenschaften, 79, 572e573. Ramaswamy, K., Peeters, C., Yuvana, S. P., Varghese, T., Pradeep, H. D., Dietemann, V., Vedham, K., Cobb, M. & Gadagkar, R. 2004. Social mutilation in the Ponerine ant Diacamma: cues originate in the victims. Insectes Sociaux, 51, 410e413. ¨ lldobler, B. & Rembold, H. 1993. Behavioral and Sommer, K., Ho physiological aspects of reproductive control in a Diacamma species from Malaysia (Formicidae, Ponerinae). Ethology, 94, 162e170. Tsuji, K., Nakata, K. & Heinze, J. 1996. Lifespan and reproduction in a queenless ant. Naturwissenschaften, 83, 577e578. ¨ lldobler, B. 1999. Regulation of worker Tsuji, K., Egashira, K. & Ho reproduction by direct physical contact in the ant Diacamma sp. from Japan. Animal Behaviour, 58, 337e343. Visscher, P. K. 1993. A theoretical analysis of individual interests and intracolony conflict during swarming of honey bee colonies. Journal of Theoretical Biology, 165, 191e212.
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