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Effect of body weight, antler length, resource value and experience on fight duration and intensity in fallow deer
ANIMAL BEHAVIOUR, 2004, 68, 213e221 doi:10.1016/j.anbehav.2003.11.005
Effect of body weight, antler length, resource value and experience on fight duration and intensity in fallow deer ´ MH NA LL J. JENNINGS *, M AR TIN P. GA MM ELL†, CA IT RI´ ONA M . C AR LIN ‡ & T HOM AS J. H AY DEN‡ DO
*Department of Psychology, University of York ySchool of Biosciences, University of Nottingham zDepartment of Zoology, National University of Ireland, Dublin (Received 23 April 2003; initial acceptance 13 June 2003; final acceptance 22 November 2003; MS. number: 7695R)
We tested predictions of evolutionary game theory focusing on fight duration and intensity during contests between European fallow deer, Dama dama L. We examined the relation between contest duration and intensity and resource-holding potential (RHP; body weight and antler size), in an effort to reveal the assessment rules used by competing males. We examined other potential determinants of duration and intensity: resource value (the oestrous female) and experience of agonistic interactions. Asymmetry in body weight or antler length of contestants was not correlated with fight duration. Body weight and antler length of the fight winner or loser were also not correlated with fight duration. Neither were the body weight of the heavier or lighter animal or the antler length of the animal that had longer or shorter antlers. A measure of intensity (the jump clash) was positively related to the body weight of the losing animal and the lighter member of the dyad. These results are consistent with the hypothesis that opponents escalate contest intensity based on assessment of their own ability rather than through mutual assessment. There was no evidence that resource value is an important factor in either fight duration or intensity in this population. As the number of fights between pairs of males increased, there was a decrease in fight duration. Fights were longer when at least one member of a competing pair of males had previously experienced a victory. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
A frequently reported aspect of the breeding ecology of many species is the occurrence of intense male competition for access to oestrous or perioestrous females (Alcock 1993). This is particularly true of polygynous species in which a few males may dominate access to females and subsequently achieve most matings (Andersson 1994). Fighting is thought to impose considerable costs, which can result in serious injury or death (Wilkinson & Shank 1976; Silverman & Dunbar 1980; Drews 1996). Therefore, we expect that contestants should scale contest intensity based on an estimate of the costs and fitness benefits that Correspondence: D. J. Jennings, Department of Psychology, University of York, Heslington, York YO10 5DD, U.K. (email: [email protected]). M. P. Gammell is at the School of Biosciences, Division of Agricultural Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, U.K. C. M. Carlin and T. J. Hayden are at the Mammal Research Group, Department of Zoology, National University of Ireland, Dublin, Ireland. 0003e3472/03/$30.00/0
might potentially accrue to them if they won the contest (Maynard Smith 1974). Several factors influence contest duration and intensity, including body condition or body mass and weapon size, resource value and abundance and experience (reviews in Archer 1988; Riechert 1998). When a resource is of equal value to both contestants, contest duration, escalation and outcome may be decided on the basis of asymmetries in resource-holding potential (RHP; Parker 1974; Parker & Rubenstein 1981), which includes body size or mass (reviews in Archer 1988; Riechert 1998) and/or weapon size (review in Andersson 1994). Although most studies have identified body or weapon size asymmetry as a predictor of fight intensity, there are exceptions (e.g. DiMarco & Hanlon 1997; Sneddon et al. 1997; Neat et al. 1998). Within a theoretical framework, such as the sequential assessment game, it is assumed that opponents might assess relative competitive ability based on a quantitative comparison of their own RHP with that of their opponent (Enquist & Leimar 1983;
213 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 68, 1
Enquist et al. 1990). When differences in RHP are large, and when contestants are able to assess these differences, they should resolve a conflict using low-risk display behaviour without escalating to fighting. In contrast, where asymmetries in the relative RHP between opponents are small, fights should be long and intense (Maynard Smith 1982; Riechert 1998). Alternatively, models such as the energetic war of attrition without assessment predict that contest duration is determined by the (RHP-related) maximum cost that the weaker contestant (eventual loser) is willing to endure in an effort to win the contest (MestertonGibbons et al. 1996; Payne & Pagel 1996, 1997). It is possible to differentiate between the sequential assessment game and the energetic war of attrition based on contest duration (Taylor & Elwood 2003). In the SAG, a mutual assessment of opponent RHP hypothesis applies, and the correlation coefficients of winner RHP and loser RHP with contest duration should be approximately equal but of opposite sign. In the eWOA, a maximum cost hypothesis applies, loser RHP should be positively related to contest duration, and there should also be a positive but weaker relation between winner RHP and contest duration. Although Taylor & Elwood (2003) restricted their study to contest duration, it should be possible to extend their analytical strategy beyond duration to other measures of animal contests, such as intensity (Gammell & Hardy 2003). Resource value can influence aspects of contest structure such as intensity and contest duration (Riechert 1998) and may represent the most important nonstrategic variable in fighting behaviour (Enquist & Leimar 1987). Theoretical models predict that the cost incurred by fighting should increase with increasing resource value (Parker 1974; Maynard Smith & Parker 1976; Parker & Rubenstein 1981; Enquist & Leimar 1987). Where ownership of a resource offers a limited fitness advantage to the holder and, consequently, has limited value, escalation in intensity is low (e.g. Davies 1978) compared with a less transient resource that yields a high fitness advantage (e.g. Gilley 2001). The value of a resource to an individual may also vary with physiological state. For instance, increasing the duration of food deprivation can increase the level of escalation in contests (e.g. Hazlett et al. 1975), and where the opportunity to mate is limited, in several species, the presence of a female also serves to increase contest intensity (e.g. Clutton-Brock et al. 1979; Austad 1983; Verrell 1986; Wells 1988; Keeley & Grant 1993). In situations where animals must compete openly for access to a nondivisible resource and resource monopolies do not arise, both contestants should have similar information regarding the properties and hence value of the resource (Enquist & Leimar 1987). Therefore, resource abundance is expected to be negatively related to resource value, because the value of a resource should be greater when the resource is relatively rare. The ability to discriminate between familiar and unfamiliar opponents can reduce the costs of fighting for contestants that have had experience at interacting together (Ydenberg et al. 1988). Fights can occur because there is little or no asymmetry in RHP between two opponents, or because imperfect information exists about the asymmetry
(Maynard Smith & Parker 1976). An increase in competitive encounters with a particular opponent should lead to a corresponding decrease in the amount of error in assessment of opponent RHP (Maynard Smith & Parker 1976; Hammerstein 1981; Hammerstein & Parker 1982) assuming that both individuals recognize that they have fought before and retain information about the fight (Enquist & Leimar 1983). Repeated exposure to an opponent can reduce fight intensity (e.g. Franck & Ribowski 1987; Keeley & Grant 1993; Olsson 1994; Miklo´si et al. 1997; Karavanich & Atema 1998; Lo´pez & Martı´n 2001). Successful fighting experience may also increase the readiness to escalate subsequent encounters where experience may have increased perception of self or individual RHP (Parker 1974; MestertonGibbons & Dugatkin 1995); therefore, penultimate fight experience can also influence the outcome of subsequent contests. Several researchers have reported a winnereloser effect where a win or loss in a prior encounter, regardless of the identity of the opponent, results in a win or loss in subsequent contests (e.g. Beacham & Newman 1987; Otronen 1990; Cloutier & Newberry 2000; Dodson & Schwaab 2001; but see Francis 1983; Thorpe et al. 1995). Using the highly polygynous European fallow deer, Dama dama, we tested predictions concerning effects of body mass, weapon size, resource value and experience on fight duration and intensity. The mating system ranges from leks to nonterritorial systems in different populations (Langbein & Thirgood 1989), and most fighting is localized within the relatively short time span of the annual rut (Jennings 2000). The oestrous female represents a transient resource for which males compete intensively, although most males do not achieve a mating (Moore et al. 1995). There is limited information regarding the frequency of behavioural actions during fighting contests in this species (e.g. Alvarez 1993), although we have failed to find evidence that opponents engage in visual assessment of their opponent’s body mass or antler size (Jennings et al. 2002, 2003). Therefore, in the present study, we addressed the following predictions with regard to fight duration and fight intensity. (1) Using body mass and antler size as indicators of RHP should reveal the assessment process during fights; therefore, following Taylor & Elwood (2003) we predicted that where the relation between winnere loser/largeresmaller contestant and fight duration/intensity is of equal but opposite sign, then opponents are engaged in mutual assessment and the sequential assessment game is therefore the most appropriate gametheoretical model to describe the observed behaviour; or, where there is a positive relation between one contestant’s RHP and a weaker positive relation between the second contestant’s RHP and contest duration/intensity, contestants use self assessment, and the energetic war of attrition is therefore the most appropriate game-theoretical model to describe the observed behaviour. (2) Fight duration and intensity should increase with increasing resource value. (3) The effects of experience should be two-fold. Repeated aggressive encounters with the same opponent should lead to a reduction in fight duration and intensity, and the experience of winning or losing the penultimate fight should affect subsequent fight outcome and intensity, regardless of identity of previous opponent.
JENNINGS ET AL.: ASSESSMENT RULES IN DEER FIGHTS
METHODS
Study Site and Population This study was conducted in 1996 and 1997 on a herd of European fallow deer in Phoenix Park, Dublin, Ireland (53(22#N, 6(21#W). The park encloses 709 ha, of which approximately 80% is available to the deer. At the beginning of November 1996, there were 152 fawns and 394 females and 172 males at least 1 year old. At the beginning of November 1997, there were 172 fawns, and 349 females and 197 males at least 1 year old. Most males were tagged (1996: 97%; 1997: 93%R4 years old) and could also be individually recognized by antler shape and coat colour.
Procedures We observed the herd from late August until the end of October on a dawn-to-dusk schedule. We used all-eventrecording procedures (Altmann 1974). All agonistic interactions between male deer at least 4 years old were recorded and screened to investigate the presence or absence of antler contact. Therefore, we recorded two categories of interactions: noncontact displacements, which involved no physical contact between two bucks, and fighting (contact interactions) which involved two bucks locking antlers and engaging in vigorous pushing contests (Clutton-Brock et al. 1982). Fighting increased in frequency as the annual rut approached. The rut began in mid-October in both 1996 and 1997. Fights were recorded on videotape during October, and fight sequences were analysed with the Observer Video Tape Analysis System V. 3.0 (Noldus Information Technology, Wageningen, The Netherlands). In October 1996, we recorded and analysed 100 fights on videotape; 15 of these fights were recorded before we observed any matings in the population (prerut), and the remaining 85 fights were recorded on days when at least one mating was recorded (rut). In October 1997, we recorded 89 fights, 18 of which were recorded before we observed the first mating (prerut), and the remaining 71 fights were recorded during the rut. This gave a total of 189 fights recorded on videotape; of these, 98 fights (51.9%) had a clear winner and loser. We defined fight duration as the period between the times of first and last antler contact (Clutton-Brock & Albon 1979). The jump clash was used as a measure of fight intensity. It is considered the highest-risk behaviour that competing males can use to engage antlers, because of the risk of antler breakage and damage to the skull (Alvarez 1993). A jump clash was recorded when a buck initiated antler contact by jumping at his opponent with his antlers lowered from about 0.5e2.5 m (Alvarez 1993). In all cases, the forelegs of the initiating animal left the ground and the back legs frequently did so.
Experience. We used two measures of experience. (1) Experience at competing with a particular opponent was defined as the number of occasions on which we had recorded two bucks participating in either a low-level displacement or a fight before we recorded them fighting on videotape. (2) Experience of outcome classified a fighting dyad of bucks (on videotape) into one of two categories, based on the results of the penultimate fight in which each dyad member had been involved before being videotaped. The first category included those dyads in which at least one dyad member had won his penultimate fight. The second category included those dyads in which neither dyad member had won his penultimate fight. We removed from analysis situations in which at least one dyad member’s penultimate fight was between him and his current (videotaped) opponent, because outcome would have been constrained (if one animal wins, then his opponent must lose). Resource value was considered to be an index of the abundance of oestrous females in the population, defined as the number of matings observed per day during the rut in October 1996 and 1997. We considered resource value to be highest on days when there were few oestrous females in the population. Therefore, we compared the first 3 days of the rut and the last 3 days of the rut, when there were few receptive females, with the 3 peak days, when many receptive females were present. We used number of matings as the measure of resource value instead of a ratio score of competing males per day to mated females per day, because these two factors were highly related in both years of the study (Spearman rank correlation: 1996: rS ¼ 0:924, N ¼ 16, P ! 0:001; 1997: rS ¼ 0:746, N ¼ 17, P ! 0:001). The rut was defined as the period over which matings were recorded in the population. The rut usually lasted about 2 weeks beginning in the middle of October (1996: 16 October 1997: 15 October). The prerut was defined as the time following cleaning of velvet from the antlers until the first mating was recorded in the herd. Peak day was taken as the day during the rut on which most matings were observed in each year. Backward push (Alvarez 1993): fallow bucks fight by engaging antlers and pushing vigorously. This is often accompanied by wrestling, usually without spatial displacement of either competitor. Many bouts of pushing progress to a phase where one buck rapidly forces his opponent backwards while antlers are still engaged. The point at which the pushing becomes a displacement in favour of one buck (backward push) is taken to represent the termination of energetic investment at a particular point in a bout of fighting by the displaced buck. We calculated the total duration of backward push that each animal in a dyad achieved.
Antler Collection and Measurement Terms and Definitions Further definitions of the terms used in this study are given below.
Antler collection and measurement protocols are described in Jennings et al. (2002). We recorded 39 fights on videotape in 1996 and 1997 where measures of the mean
215
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ANIMAL BEHAVIOUR, 68, 1
antler length (average of both antlers) were available for both members of the fighting dyad. Of the 39 fights, 21 had a clear winner and loser.
5:65 G 0:48 bouts of antler locking per fight, and the majority of fights (N ¼ 145, 76.7%) did not have more than six bouts of antler locking. The average number of jump clashes recorded was 1:9 G 0:26 per fight.
Capture of Bucks and Body Mass Measurement The procedure for selecting bucks for capture and weighing are described in Jennings et al. (2003). Bucks were immobilized using a mixture of etorphine hydrochloride (18e20 mg/kg, C-Vet Veterinary Products, Leyland, U.K.) and xylazine (360e420 mg/kg, Rompun Dry Substance, Bayer, Dublin, Ireland) administered by hypodermic syringe darts fired from a compressed-gas rifle. Bucks were weighed by suspension in a net from a standard Salter balance mounted on a portable tripod. Bucks were revived with a mixture of antagonistic drugs, diprenorphine hydrochloride (Revivon), (24e28 mg/kg, C-Vet Veterinary Products) and antipamozole hydrochloride (50 mg/kg, Antisedan, Pfizer, Dublin, Ireland) in a total volume of less than 2 ml, injected either intravenously or intramuscularly by a hypodermic syringe. We recorded 40 fights on videotape where the body weight of both males was known. Of the 40 fights, 22 had a clear winner and loser.
Ethical Note A qualified veterinarian administered the drugs. In general, animals were not handled for more than 10 min and typically the procedure was completed within 5 min. After reviving drugs had been administered, an observer remained with the animal until it had recovered full mobility and alertness. The behaviour of bucks was monitored for 2e3 days to confirm that they did not suffer increased levels of intraspecific aggression as a result of the procedure.
Statistical Tests Before analyses, we investigated the data to determine their distribution using the KolmogoroveSmirnov onesample test. Standard parametric and nonparametric tests were used based on the distribution of the data. Spearman rank correlation was used to describe the relation between phenotypic characteristics and measures of fight intensity. Descriptive statistics are presented as mean G SE. All P values presented are two tailed and all analyses were performed with SPSS v. 11 (SPSS Inc., Chicago, U.S.A.).
Factors Affecting Fight Duration and Intensity Body weight The range of difference in body weights of opponents was 0e9% of the weight of the lighter buck in the dyad. There was no significant difference in the number of jump clashes given by heavier and lighter bucks (Wilcoxon signed-ranks test: Z ¼ 1:01, N ¼ 39, P ¼ 0:612), and heavier bucks did not achieve significantly longer backward pushes than lighter bucks did (Z ¼ 0:65, N ¼ 40, P ¼ 0:518). There was no greater likelihood that heavier animals (N ¼ 13) would win more fights than lighter animals (N ¼ 9; chi-square test: c21 ¼ 0:73, P ¼ 0:394). We therefore tested the relation between body weight and fight duration and intensity using five measures: difference in body weight of the opponents, body weight of the winner, body weight of the loser, body weight of the heavier buck and body weight of the lighter buck. There was no relation between fight duration and body weight (Table 1). There was a significant positive relation between the losing buck’s body weight and the number of its jump clashes (Table 1, Fig. 1a) and between the body weight of the lighter buck in a dyad and the number of its jump clashes (Table 1, Fig. 1b).
Antler length The difference in antler length ranged from 0.1 to 13.7% of the antler length of the buck with the smaller average antler length. Dyad members with longer antlers did not give significantly more jump clashes than did dyad members with shorter antlers (Wilcoxon signedranks test: Z ¼ 0:11, N ¼ 39), and they were not more likely to beat opponents with smaller antlers (larger: N ¼ 12; smaller: N ¼ 9; chi-square test: c21 ¼ 0:43, P ¼ 0:513). We investigated the relation between antler length and fight duration and intensity using five measures: difference in antler length of the opponents, Table 1. Spearman correlations between fight duration and fight intensity (number of jump clashes) and five measures of body weight: the difference between the weights of the two opponents, the weight of the winner, the weight of the loser, and the weights of the heavier and lighter animal in the dyad Fight duration
Measure
N
Difference between opponents Winner Loser Heavierz Lighterz
40
0.06
22 22 39 39
0.16 0.23 0.15 0.22
Number of jump clashes
RESULTS
Fight Duration and Intensity The mean G SE duration of fights was 97:15 G 8:2 s and the mean duration spent with antlers locked per fight was 50:25 G 4:25 s (N ¼ 189 fights), which was slightly over half (51.7%) of the overall duration of fights. The mean G SE time spent between bouts of antler locking (intralock duration) was 8:16 G 0:86 s. There was an average of
0.05y 0.14 0.63* 0.28 0.32**
*P ! 0:05; **P ! 0:002. yTotal number of jump clashes recorded during the fight. zIn one dyad both bucks were the same weight.
JENNINGS ET AL.: ASSESSMENT RULES IN DEER FIGHTS
relation between the antler length of the buck with the longer antlers and the number of jump clashes (Table 2, Fig. 2).
110 (a) 108
Resource value
106
There was no significant difference between the prerut and rut for fight duration (ManneWhitney U test: Z ¼ 0:167, N1 ¼ 33, N2 ¼ 156, P ¼ 0:218) or intensity (Z ¼ 1:231, N1 ¼ 33, N2 ¼ 156, P ¼ 0:867). There was no relation between the number of matings recorded on any day during the rut and fight duration (Spearman rank correlation: rS ¼ 0:52, P ¼ 0:521) or fight intensity (rS ¼ 0:36, P ¼ 0:651). The first 3 days, last 3 days and the 3 peak days of the rut did not differ significantly in either fight duration (ANOVA, log-transformed data: F2;89 ¼ 0:21, P ¼ 0:815) or fight intensity (KruskaleWallis test: c22 ¼ 0:602, P ¼ 0:74).
104 102 100
0
1
2
110 (b)
Experience
108 106 104 102 100 98
0
1
2
3
4
5
6
Figure 1. The relation between the number of jump clashes recorded and the body weight (kg) of (a) the loser and (b) the lighter buck in the dyad.
antler length of the winner, antler length of the loser, antler length of the buck with longer antlers and antler length of the buck with shorter antlers. There was no relation between fight duration and antler length for any of the five measures (Table 2). There was a significant Table 2. Spearman correlations between fight duration and fight intensity (number of jump clashes) and five measures of antler size: the difference in antler size of the two opponents, the antler size of the winner, the antler size of the loser, and the antler size of the heavier and lighter animal in the dyad Measure
N
Fight duration
Number of jump clashes
Difference between opponents Winner Loser Longer Shorter
39
0.14
0.3y
21 21 39 39
0.09 0.25 0.09 0.11
0.19 0.21 0.32* 0.09
*P ! 0:04. yTotal number of jump clashes recorded during the fight.
There was no relation between fight duration and the total number of noncontact displacements recorded for a particular dyad before their first videotaped fight (Spearman rank correlation: rS ¼ 0:126, N ¼ 189, P ¼ 0:084). There was a significant negative relation between fight duration and the number of fights recorded for a particular dyad before the dyad was first recorded fighting on videotape (rS ¼ 0:222, N ¼ 189, P ! 0:002, Fig. 3). There was no relation between number of jump clashes and the number of noncontact displacements (rS ¼ 0:046, N ¼ 189, P ¼ 0:531) or fights (rS ¼ 0:004, N ¼ 189, P ¼ 0:959) recorded for a particular dyad before their first videotaped fight in either year. Current fight outcome was related to penultimate fight outcome (chi-square test: c24 ¼ 14:8, P ! 0:005; Table 3). If an individual’s current fight ended in a draw, it was likely that his penultimate fight had also ended in a draw (Table 3). Penultimate fight outcome also influenced the duration of the subsequent fight. When at least one
Antler length of buck with longer antlers (cm)
98
560
540
520
500
480
0
2
4 6 8 10 Number of jump clashes
12
14
Figure 2. The relation between the antler length (cm) of the dyad member with longer antlers and the number of jump clashes recorded during fights.
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ANIMAL BEHAVIOUR, 68, 1
Effects of Body Mass and Antler Length
800
600 Fight duration (s)
218
400
200
0 0
20
10 Number of fights to date
Figure 3. The relation between fight experience and fight duration.
buck in a fighting dyad had won his penultimate fight, fight duration was significantly longer (X G SE ¼ 105:6 G 11:9 s) than when neither buck had won his penultimate fight (94:8 G 14:3 s; log-transformed independent t test: t155 ¼ 2:7, P ! 0:009). There was no significant difference between the number of jump clashes recorded in fights in which at least one buck had won his penultimate fight and fights in which neither buck had won his penultimate fight (ManneWhitney U test: Z ¼ 0:664, N1 ¼ 85, N2 ¼ 103, P ¼ 0:507). DISCUSSION In the present study, we focused on the effects of RHP, resource value and fighting experience on fight duration and intensity between competing fallow deer during the annual breeding season. Investigation of duration and intensity may provide insight into the motivational level of competing males, because it provides an index of the willingness of two animals to persist in a potentially dangerous encounter (Bridge et al. 2000) and might provide information on assessment processes used during escalated contests (Taylor & Elwood 2003).
Table 3. A summary of the outcomes of contestants’ penultimate fights and their current (videotaped) fights, according to whether they had won, lost or drawn their current fights Outcome of current fight Outcome of penultimate fight Won Lost Drawn
Won
Lost
Drawn
31 14 41
27 24 36
38 16 88
Current fights are those fights that were recorded on videotape only. All current fights in which both members of the dyad had fought with each other in their penultimate fights were excluded. One buck was excluded because he was not recorded fighting prior to being recorded on videotape.
Body weight, or disparity in size between opponents, is an important determinant in predicting fight duration (reviews in Archer 1988; Riechert 1998; but see Taylor & Elwood 2003). In general, as disparity in body weight increases, there is a corresponding decrease in fight duration (Riechert 1998). In the present study, possession of larger body weight or longer antlers was not associated with fight success. Taylor & Elwood (2003) found that, by focusing on individual role within fights (winnereloser), it was possible to differentiate between mutual assessment (the sequential assessment game, SAG) and self assessment (the energetic war of attrition, eWOA; see above). In our study, fight duration was not related to asymmetry between contestants in body weight or antler size, nor to the weight or antler size of the lighter or heavier individual. Correlation coefficients were not of equal magnitude but of opposite sign (with the exception of winnereloser body weight, which approached these criteria). These results suggest two possibilities. On the one hand, body weight may not be a correlate of RHP in this species (see also Marden & Rollins 1994). However, an alternative explanation might be based on the bout structure of fallow deer fights. We have found that, at the end of the first bout of fighting, the eventual winner was as likely to terminate the bout as the eventual loser; however, losers had a significantly higher tendency to terminate fighting by the end of the fourth bout (D. J. Jennings, unpublished data). Furthermore, the first bout of fighting was significantly longer than the third or fourth bouts. Current assessment models do not incorporate different decision levels (i.e. ending a bout of fighting versus ending the fight altogether) in their structure, so they have no mechanism for predicting behaviour where multiple decision levels inform each other (D. J. Jennings, unpublished data). The relation between overall contest duration and contestant RHP may be obscured by the bout level nature of these fights, and Taylor & Elwood’s (2003) analytical strategy with regard to fight duration therefore may not be appropriate for fallow deer fights. If the costs of escalated contests increase the risk of injury, then contestants should seek to adopt strategies that limit these risks (Maynard Smith 1982). Losers of fights jump-clash significantly less than do winners (D. J. Jennings, unpublished data). However, in the present study, we found that as the loser’s body weight increased, there was an increasing tendency to use the jump clash to initiate antler contact. This result is consistent with individuals basing escalation decisions on their own ability (Mesterton-Gibbons et al. 1996; Taylor & Elwood 2003). Subordinate animals may seek to offset the relative dominance relationship within dyads by escalating the contest (Dow et al. 1976; Popp 1987; Senar et al. 1992), and the present results show that losing animals increased their use of high-risk strategies as their body mass increased (see also Bridge et al. 2000; Taylor et al. 2001). The jump clash as a measure of intensity is a special case and differs from contest duration in that it is controlled by the initiator, with the costs of the clash spread between
JENNINGS ET AL.: ASSESSMENT RULES IN DEER FIGHTS
the opponents. The individual with the lower RHP does not control the termination of the jump clash, as it might with contest duration. There was a positive relation between the body weight of the lighter member of a dyad and the number of jump clashes recorded; the relation between body weight of the heavier contestant was also positive (of similar sign) but the difference was not significant. This evidence supports the hypothesis that body mass is an important variable in determining the use of high-intensity behaviour (e.g. Turner 1994; but see Hack 1997). Within the analytical framework presented by Taylor & Elwood (2003) the present results suggest that contestants are engaged in a process of self assessment, so the more appropriate model is the eWOA rather than the SAG. An opponent’s RHP is potentially difficult to assess, and body weight may not always be the most reliable indicator of an individual’s competitive ability (Parker & Rubenstein 1981). Some other phenotypic trait, such as weapon size, may be more a more reliable indicator of quality, because it represents the ability to inflict physical damage (Sneddon et al. 1997). Although there is some evidence that weapon size is a predictor of competitive ability (e.g. Barki et al. 1997; Sneddon et al. 1997), the evidence from cervids is less conclusive (the number of antler points is weakly correlated with fighting success in red deer, Cervus elaphus: Clutton-Brock et al. 1979). In this study, there was no relation between fight duration and difference in antler length of the competing males. Furthermore, contest duration was not related to the antler size of the winner or loser, neither was it correlated with the antler length of the dyad member with the longest or shortest antlers. Therefore, we eliminate the SAG as a potential model for fallow deer fights. The results showed a significant relation between antler length and number of jump clashes for the buck with the longer antlers. These results indicate that longer antlers facilitate the use of higher risk tactics during dyadic fighting, although within dyads there was no difference in the number of jump clashes based on antler length (also Schroeder & Huber 2001). These results also support the prediction that contestants base decisions on their own ability and, therefore, they indicate an assessment process compatible with a WOA (Mesterton-Gibbons et al. 1996; Taylor & Elwood 2003).
Effects of Resource Abundance The number of observed fights is often correlated with the number of matings and with mating success (CluttonBrock et al. 1988; Apollonio et al. 1990; Moore et al. 1995; San Jose´ & Braza 1997). Game-theoretical models propose that fight duration and intensity are positively correlated with resource value (Bishop et al. 1978; Hammerstein & Parker 1982; Enquist & Leimar 1987; for evidence see Clutton-Brock et al. 1979; Sigurjonsdottir & Parker 1981; Austad 1983; Verrell 1986). The results of this study failed to support the prediction that an increase in resource value is correlated with an increase in either fight duration or fight intensity. Fights recorded in the prerut in this
population were not significantly shorter than fights recorded during the rut. Furthermore, we expected that fight duration should be related to the relative scarcity of the resource. This was not the case, and in general, there was no relation between the abundance of oestrous females and fight duration or intensity during the rut. However, the breeding system of a species may be an important influence on fight behaviour. Resource ownership, as represented by harem holding, is not observed in this population, and most males move with the doe herd during the day. Mating success in this population is highly skewed and most males do not mate (Moore et al. 1995). In theoretical terms, if the payoff or reward to either or both animal(s) is variable, as it is in our population, then we might expect that the relative abundance of that resource should not influence contest duration or intensity (Maynard Smith 1982); our results support this prediction.
Effects of Experience Non-RHP-related factors such as experience might be important in determining contest outcome and structure (Mesterton-Gibbons et al. 1996). We expected that any information gathered as a result of experience might be used to reduce fight duration and intensity in subsequent encounters (Maynard Smith & Parker 1976; Hammerstein 1981; Hammerstein & Parker 1982; for evidence see Karavanich & Atema 1998; Lo´pez & Martı´n 2001). In such cases, both contestants are expected to benefit from a reduction in the costs associated with fighting (Huntingford & Turner 1987; Archer 1988; Hammerstein 1998). Accordingly, we investigated the amount of agonistic experience between members of particular dyads. There was no relation between fight duration and the number of noncontact interactions. However, there was a significant negative relation between fight duration and the number of fights in which a particular pair of bucks had previously been involved. Retaining information about encounters with individual conspecifics might help reduce fight costs, such as time and energy expenditure, and our results support this prediction. The effects of penultimate fight outcome might be important determinants of subsequent fight outcome, and a winnereloser effect is frequently observed in a variety of species (e.g. Otronen 1990; Cloutier & Newberry 2000; Dodson & Schwaab 2001). We found little evidence for a winnere loser effect when only wins and losses were considered. However, there was a significant effect when drawn fights were included, indicating that when an animal had experience of drawing a fight, there was an increased probability that a subsequent fight would also be drawn. When at least one member of the dyad won his penultimate fight, fights were significantly longer than when neither buck had won. This result has been interpreted as an effect of prior success increasing self perception of individual RHP (Parker 1974; MestertonGibbons & Dugatkin 1995; Dugatkin & Earley 2003). We found no evidence that the outcome of a penultimate fight influenced fight intensity (see also Hsu & Wolf 2001).
219
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Where the population density of competing males is high, as it is in this population, aggressive interactions between males may extract severe costs in terms of energy expended and time. Our measure of fight intensity, but not of fight duration, indicated that individuals might base decisions on estimates of their own ability rather than on a process of mutual assessment. However, the importance of body weight as a correlate of fight duration may be confounded by the bout structure of fallow deer contests, a possibility that remains to be investigated. Although resource value may override other potentially important determinants of fight duration and intensity, such as asymmetry in RHP (Riechert 1998), the present study suggests that in the fallow deer, resource value is of limited importance in influencing fight duration or intensity. However, the experience of fighting with a particular opponent was related to a decrease in fight duration. The ability to recognize opponents, the ‘dear enemy’ effect (Fischer 1954), can assist competing males in reducing costs associated with fighting such as time and energy (see also Lo´pez & Martı´n 2001). Furthermore, we found that the outcome of a prior agonistic encounter also influenced subsequent fight duration. Acknowledgments We thank Du´chas, The Heritage Service and Mr John McCullen, the Superintendent of Phoenix Park, for facilitating access to the deer herd. We express our gratitude to Mr Don Doran (park deerkeeper), the Rangers and staff of Phoenix Park for their support during the field stages of this study. We thank our colleagues in the Mammal Research Group for their help. In particular, we thank Paul Jennings for his assistance in extracting data on fighting from the files and Dr P. Quinlan for advice with statistics. We thank Professor Robert Elwood and an anonymous referee for valuable comments on the manuscript. M.P.G. and C.M.C. received financial support from Enterprise Ireland. References Alcock, J. 1993. Animal Behavior: An Evolutionary Approach. Sunderland, Massachusetts: Sinauer. Altmann, J. 1974. Observational study of behavior: sampling methods. Behaviour, 49, 227e267. Alvarez, F. 1993. Risks of fighting in relation to age and territory holding in fallow deer. Canadian Journal of Zoology, 71, 376e383. Andersson, M. 1994. Sexual Selection. Princeton, New Jersey: Princeton University Press. Apollonio, M., Festa-Bianchet, M., Mari, F. & Riva, M. 1990. Sitespecific asymmetries in male copulatory success in a fallow deer lek. Animal Behaviour, 39, 205e212. Archer, J. 1988. The Behavioural Biology of Aggression. Cambridge: Cambridge University Press. Austad, S. N. 1983. A game theoretical interpretation of male combat in the bowl and doily spider (Frontinella pyramitela). Animal Behaviour, 31, 59e73. Barki, A., Harpaz, S. & Karplus, I. 1997. Contradictory asymmetries in body and weapon size and assessment in fighting male prawns, Macrobrachium rosenbergii. Aggressive Behavior, 23, 81e91.
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Gilley, D. C. 2001. The behavior of honey bees (Apis mellifera ligustica) during queen duels. Ethology, 107, 601e622. Hack, M. A. 1997. Assessment strategies in the contests of male crickets, Acheta domesticus (L.). Animal Behaviour, 53, 733e747. Hammerstein, P. 1981. The role of asymmetries in animal contests. Animal Behaviour, 29, 193e205. Hammerstein, P. 1998. What is evolutionary game theory? In: Game Theory and Animal Behavior (Ed. by L. A. Dugatkin & H. K. Reeve), pp. 3e15. New York: Oxford University Press. Hammerstein, P. & Parker, G. A. 1982. The asymmetric war of attrition. Journal of Theoretical Biology, 96, 647e682. Hazlett, B. A., Rubenstein, D. I. & Ritschoff, D. 1975. Starvation, aggression, and energy reserves in the crayfish Orconectes virilis. Crustaceana, 8, 11e28. Hsu, Y. & Wolf, L. L. 2001. The winner and loser effect: what fighting behaviours are influenced? Animal Behaviour, 61, 777e786. Huntingford, F. A. & Turner, A. K. 1987. Animal Conflict. London: Chapman & Hall. Jennings, D. J. 2000. Contest structure, dyad composition, duration and outcome of fights between male fallow deer (Dama dama L.). Ph.D. thesis, National University of Ireland. Jennings, D. J., Gammell, M. P., Carlin, C. M. & Hayden, T. J. 2002. Does lateral presentation of the palmate antlers during fights by fallow deer (Dama dama) signify dominance or submission? Ethology, 108, 389e401. Jennings, D. J., Gammell, M. P., Carlin, C. M. & Hayden, T. J. 2003. Is the parallel walk between competing male fallow deer (Dama dama) a lateral display of quality? Animal Behaviour, 65, 1005e1012. Karavanich, C. & Atema, J. 1998. Individual recognition and memory in lobster dominance. Animal Behaviour, 56, 1553e1560. Keeley, E. R. & Grant, J. W. A. 1993. Visual information, resource value, and sequential assessment in convict cichlid (Cichlasoma nigrofasciatum) contests. Behavioral Ecology, 4, 345e349. Langbein, J. & Thirgood, S. J. 1989. Variation in mating systems of fallow deer in relation to ecology. Ethology, 83, 195e214. Lo´pez, P. & Martı´n, J. 2001. Fighting rules and rival recognition reduce costs of aggression in male lizards, Podarcis hispanica. Behavioral Ecology and Sociobiology, 49, 111e116. Marden, J. H. & Rollins, R. A. 1994. Assessment of energy reserves by damselflies engaged in aerial contests for mating territories. Animal Behaviour, 48, 1023e1030. 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. & Dugatkin, L. A. 1995. Toward a theory of dominance hierarchies: effects of assessment, group size, and variation in fighting ability. Behavioral Ecology, 6, 416e423. Mesterton-Gibbons, M., Marden, J. H. & Dugatkin, L. A. 1996. On wars of attrition without assessment. Journal of Theoretical Biology, 181, 65e83. ´ ., Haller, J. & Csa´nyi, V. 1997. Learning about the Miklo´si, A opponent during aggressive encounters in paradise fish (Macropodus opercularis L.). Behavioural Processes, 40, 97e105. Moore, N. P., Kelly, P. F. & Hayden, T. J. 1995. Mating strategies and mating success of fallow (Dama dama) bucks in a non-lekking population. Behavioral Ecology and Sociobiology, 36, 91e100. Neat, F. C., Huntingford, F. A. & Beveridge, M. M. C. 1998. Fighting and assessment in male cichlid fish: the effects of asymmetries in gonadal state and body size. Animal Behaviour, 55, 883e891.
Olsson, M. 1994. Rival recognition affects male contest behavior in sand lizards (Lacerta agilis). Behavioral Ecology and Sociobiology, 35, 249e252. Otronen, M. 1990. The effects of prior experience on the outcome of fights in the burying beetle, Nicrophorus humator. Animal Behaviour, 40, 980e982. Parker, G. A. 1974. Assessment strategy and the evolution of fighting behaviour. Journal of Theoretical Biology, 47, 223e243. Parker, G. A. & Rubenstein, D. I. 1981. Role assessment, reserve strategy, and acquisition of information in asymmetric animal conflicts. Animal Behaviour, 29, 221e240. Payne, R. J. H. & Pagel, M. 1996. Escalation and time costs in displays of endurance. Journal of Theoretical Biology, 183, 185e193. Payne, R. J. H. & Pagel, M. 1997. Why do animals repeat displays? Animal Behaviour, 54, 109e119. Popp, J. W. 1987. Risk and effectiveness in the use of agonistic displays by American goldfinches. Behaviour, 103, 141e156. Riechert, S. E. 1998. Game theory and animal conflict. In: Game Theory and Animal Behavior (Ed. by L. A. Dugatkin & H. K. Reeve), pp. 64e93. New York: Oxford University Press. San Jose´, C. & Braza, F. 1997. Ecological and behavioural variables ˜ ana. Ethology, affecting the fallow deer mating system in Don Ecology and Evolution, 9, 133e148. Schroeder, L. & Huber, R. 2001. Fight strategies differ with size and allometric growth of claws in crayfish, Orconectes rusticus. Behaviour, 138, 1437e1449. Senar, J. C., Camerino, M. & Metcalfe, N. B. 1992. Fighting as a subordinate in finch flocks: escalation is effective but risky. Animal Behaviour, 43, 862e864. Sigurjonsdottir, H. & Parker, G. A. 1981. Dung fly struggles: evidence for assessment strategy. Behavioral Ecology and Sociobiology, 8, 219e230. Silverman, H. B. & Dunbar, M. J. 1980. Aggressive tusk use by the narwhal, Monodon monoceros L. Nature, 284, 57e58. Sneddon, L. U., Huntingford, F. A. & Taylor, A. C. 1997. Weapon size versus body size as a predictor of winning in fights between shore crabs, Carcinus maenas L. Behavioral Ecology and Sociobiology, 41, 237e242. Taylor, P. W. & Elwood, R. W. 2003. The mismeasure of animal contests. Animal Behaviour, 65, 1195e1202. Taylor, P. W., Hasson, O. & Clark, D. L. 2001. Initiation and resolution of jumping spider contests: roles for size, proximity, and early detection of rivals. Behavioral Ecology and Sociobiology, 50, 403e413. Thorpe, K. E., Taylor, A. C. & Huntingford, F. A. 1995. How costly is fighting? Physiological effects of sustained exercise and fighting in swimming crabs, Necora puber (L.) (Brachyura, Portunidae). Animal Behaviour, 50, 1657e1666. Turner, G. F. 1994. The fighting tactics of male mouthbrooding cichlids: the effects of size and residency. Animal Behaviour, 47, 655e662. Verrell, P. A. 1986. Wrestling in the red spotted newt (Notophthalmus viridens): resource value and contestant asymmetry determine contest duration and outcome. Animal Behaviour, 34, 398e402. Wells, M. S. 1988. Effects of body size and resource value on fighting behaviour in a jumping spider. Animal Behaviour, 36, 321e326. Wilkinson, P. F. & Shank, C. C. 1976. Rutting-fight mortality among musk oxen on Banks Island, Northwest Territories, Canada. Animal Behaviour, 24, 756e758. Ydenberg, R. C., Giraldeau, L. A. & Falls, J. B. 1988. Neighbors, strangers and the asymmetric war of attrition. Animal Behaviour, 36, 343e347.
221
Effect of body weight, antler length, resource value and experience on fight duration and intensity in fallow deer ´ MH NA LL J. JENNINGS *, M AR TIN P. GA MM ELL†, CA IT RI´ ONA M . C AR LIN ‡ & T HOM AS J. H AY DEN‡ DO
*Department of Psychology, University of York ySchool of Biosciences, University of Nottingham zDepartment of Zoology, National University of Ireland, Dublin (Received 23 April 2003; initial acceptance 13 June 2003; final acceptance 22 November 2003; MS. number: 7695R)
We tested predictions of evolutionary game theory focusing on fight duration and intensity during contests between European fallow deer, Dama dama L. We examined the relation between contest duration and intensity and resource-holding potential (RHP; body weight and antler size), in an effort to reveal the assessment rules used by competing males. We examined other potential determinants of duration and intensity: resource value (the oestrous female) and experience of agonistic interactions. Asymmetry in body weight or antler length of contestants was not correlated with fight duration. Body weight and antler length of the fight winner or loser were also not correlated with fight duration. Neither were the body weight of the heavier or lighter animal or the antler length of the animal that had longer or shorter antlers. A measure of intensity (the jump clash) was positively related to the body weight of the losing animal and the lighter member of the dyad. These results are consistent with the hypothesis that opponents escalate contest intensity based on assessment of their own ability rather than through mutual assessment. There was no evidence that resource value is an important factor in either fight duration or intensity in this population. As the number of fights between pairs of males increased, there was a decrease in fight duration. Fights were longer when at least one member of a competing pair of males had previously experienced a victory. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
A frequently reported aspect of the breeding ecology of many species is the occurrence of intense male competition for access to oestrous or perioestrous females (Alcock 1993). This is particularly true of polygynous species in which a few males may dominate access to females and subsequently achieve most matings (Andersson 1994). Fighting is thought to impose considerable costs, which can result in serious injury or death (Wilkinson & Shank 1976; Silverman & Dunbar 1980; Drews 1996). Therefore, we expect that contestants should scale contest intensity based on an estimate of the costs and fitness benefits that Correspondence: D. J. Jennings, Department of Psychology, University of York, Heslington, York YO10 5DD, U.K. (email: [email protected]). M. P. Gammell is at the School of Biosciences, Division of Agricultural Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, U.K. C. M. Carlin and T. J. Hayden are at the Mammal Research Group, Department of Zoology, National University of Ireland, Dublin, Ireland. 0003e3472/03/$30.00/0
might potentially accrue to them if they won the contest (Maynard Smith 1974). Several factors influence contest duration and intensity, including body condition or body mass and weapon size, resource value and abundance and experience (reviews in Archer 1988; Riechert 1998). When a resource is of equal value to both contestants, contest duration, escalation and outcome may be decided on the basis of asymmetries in resource-holding potential (RHP; Parker 1974; Parker & Rubenstein 1981), which includes body size or mass (reviews in Archer 1988; Riechert 1998) and/or weapon size (review in Andersson 1994). Although most studies have identified body or weapon size asymmetry as a predictor of fight intensity, there are exceptions (e.g. DiMarco & Hanlon 1997; Sneddon et al. 1997; Neat et al. 1998). Within a theoretical framework, such as the sequential assessment game, it is assumed that opponents might assess relative competitive ability based on a quantitative comparison of their own RHP with that of their opponent (Enquist & Leimar 1983;
213 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
214
ANIMAL BEHAVIOUR, 68, 1
Enquist et al. 1990). When differences in RHP are large, and when contestants are able to assess these differences, they should resolve a conflict using low-risk display behaviour without escalating to fighting. In contrast, where asymmetries in the relative RHP between opponents are small, fights should be long and intense (Maynard Smith 1982; Riechert 1998). Alternatively, models such as the energetic war of attrition without assessment predict that contest duration is determined by the (RHP-related) maximum cost that the weaker contestant (eventual loser) is willing to endure in an effort to win the contest (MestertonGibbons et al. 1996; Payne & Pagel 1996, 1997). It is possible to differentiate between the sequential assessment game and the energetic war of attrition based on contest duration (Taylor & Elwood 2003). In the SAG, a mutual assessment of opponent RHP hypothesis applies, and the correlation coefficients of winner RHP and loser RHP with contest duration should be approximately equal but of opposite sign. In the eWOA, a maximum cost hypothesis applies, loser RHP should be positively related to contest duration, and there should also be a positive but weaker relation between winner RHP and contest duration. Although Taylor & Elwood (2003) restricted their study to contest duration, it should be possible to extend their analytical strategy beyond duration to other measures of animal contests, such as intensity (Gammell & Hardy 2003). Resource value can influence aspects of contest structure such as intensity and contest duration (Riechert 1998) and may represent the most important nonstrategic variable in fighting behaviour (Enquist & Leimar 1987). Theoretical models predict that the cost incurred by fighting should increase with increasing resource value (Parker 1974; Maynard Smith & Parker 1976; Parker & Rubenstein 1981; Enquist & Leimar 1987). Where ownership of a resource offers a limited fitness advantage to the holder and, consequently, has limited value, escalation in intensity is low (e.g. Davies 1978) compared with a less transient resource that yields a high fitness advantage (e.g. Gilley 2001). The value of a resource to an individual may also vary with physiological state. For instance, increasing the duration of food deprivation can increase the level of escalation in contests (e.g. Hazlett et al. 1975), and where the opportunity to mate is limited, in several species, the presence of a female also serves to increase contest intensity (e.g. Clutton-Brock et al. 1979; Austad 1983; Verrell 1986; Wells 1988; Keeley & Grant 1993). In situations where animals must compete openly for access to a nondivisible resource and resource monopolies do not arise, both contestants should have similar information regarding the properties and hence value of the resource (Enquist & Leimar 1987). Therefore, resource abundance is expected to be negatively related to resource value, because the value of a resource should be greater when the resource is relatively rare. The ability to discriminate between familiar and unfamiliar opponents can reduce the costs of fighting for contestants that have had experience at interacting together (Ydenberg et al. 1988). Fights can occur because there is little or no asymmetry in RHP between two opponents, or because imperfect information exists about the asymmetry
(Maynard Smith & Parker 1976). An increase in competitive encounters with a particular opponent should lead to a corresponding decrease in the amount of error in assessment of opponent RHP (Maynard Smith & Parker 1976; Hammerstein 1981; Hammerstein & Parker 1982) assuming that both individuals recognize that they have fought before and retain information about the fight (Enquist & Leimar 1983). Repeated exposure to an opponent can reduce fight intensity (e.g. Franck & Ribowski 1987; Keeley & Grant 1993; Olsson 1994; Miklo´si et al. 1997; Karavanich & Atema 1998; Lo´pez & Martı´n 2001). Successful fighting experience may also increase the readiness to escalate subsequent encounters where experience may have increased perception of self or individual RHP (Parker 1974; MestertonGibbons & Dugatkin 1995); therefore, penultimate fight experience can also influence the outcome of subsequent contests. Several researchers have reported a winnereloser effect where a win or loss in a prior encounter, regardless of the identity of the opponent, results in a win or loss in subsequent contests (e.g. Beacham & Newman 1987; Otronen 1990; Cloutier & Newberry 2000; Dodson & Schwaab 2001; but see Francis 1983; Thorpe et al. 1995). Using the highly polygynous European fallow deer, Dama dama, we tested predictions concerning effects of body mass, weapon size, resource value and experience on fight duration and intensity. The mating system ranges from leks to nonterritorial systems in different populations (Langbein & Thirgood 1989), and most fighting is localized within the relatively short time span of the annual rut (Jennings 2000). The oestrous female represents a transient resource for which males compete intensively, although most males do not achieve a mating (Moore et al. 1995). There is limited information regarding the frequency of behavioural actions during fighting contests in this species (e.g. Alvarez 1993), although we have failed to find evidence that opponents engage in visual assessment of their opponent’s body mass or antler size (Jennings et al. 2002, 2003). Therefore, in the present study, we addressed the following predictions with regard to fight duration and fight intensity. (1) Using body mass and antler size as indicators of RHP should reveal the assessment process during fights; therefore, following Taylor & Elwood (2003) we predicted that where the relation between winnere loser/largeresmaller contestant and fight duration/intensity is of equal but opposite sign, then opponents are engaged in mutual assessment and the sequential assessment game is therefore the most appropriate gametheoretical model to describe the observed behaviour; or, where there is a positive relation between one contestant’s RHP and a weaker positive relation between the second contestant’s RHP and contest duration/intensity, contestants use self assessment, and the energetic war of attrition is therefore the most appropriate game-theoretical model to describe the observed behaviour. (2) Fight duration and intensity should increase with increasing resource value. (3) The effects of experience should be two-fold. Repeated aggressive encounters with the same opponent should lead to a reduction in fight duration and intensity, and the experience of winning or losing the penultimate fight should affect subsequent fight outcome and intensity, regardless of identity of previous opponent.
JENNINGS ET AL.: ASSESSMENT RULES IN DEER FIGHTS
METHODS
Study Site and Population This study was conducted in 1996 and 1997 on a herd of European fallow deer in Phoenix Park, Dublin, Ireland (53(22#N, 6(21#W). The park encloses 709 ha, of which approximately 80% is available to the deer. At the beginning of November 1996, there were 152 fawns and 394 females and 172 males at least 1 year old. At the beginning of November 1997, there were 172 fawns, and 349 females and 197 males at least 1 year old. Most males were tagged (1996: 97%; 1997: 93%R4 years old) and could also be individually recognized by antler shape and coat colour.
Procedures We observed the herd from late August until the end of October on a dawn-to-dusk schedule. We used all-eventrecording procedures (Altmann 1974). All agonistic interactions between male deer at least 4 years old were recorded and screened to investigate the presence or absence of antler contact. Therefore, we recorded two categories of interactions: noncontact displacements, which involved no physical contact between two bucks, and fighting (contact interactions) which involved two bucks locking antlers and engaging in vigorous pushing contests (Clutton-Brock et al. 1982). Fighting increased in frequency as the annual rut approached. The rut began in mid-October in both 1996 and 1997. Fights were recorded on videotape during October, and fight sequences were analysed with the Observer Video Tape Analysis System V. 3.0 (Noldus Information Technology, Wageningen, The Netherlands). In October 1996, we recorded and analysed 100 fights on videotape; 15 of these fights were recorded before we observed any matings in the population (prerut), and the remaining 85 fights were recorded on days when at least one mating was recorded (rut). In October 1997, we recorded 89 fights, 18 of which were recorded before we observed the first mating (prerut), and the remaining 71 fights were recorded during the rut. This gave a total of 189 fights recorded on videotape; of these, 98 fights (51.9%) had a clear winner and loser. We defined fight duration as the period between the times of first and last antler contact (Clutton-Brock & Albon 1979). The jump clash was used as a measure of fight intensity. It is considered the highest-risk behaviour that competing males can use to engage antlers, because of the risk of antler breakage and damage to the skull (Alvarez 1993). A jump clash was recorded when a buck initiated antler contact by jumping at his opponent with his antlers lowered from about 0.5e2.5 m (Alvarez 1993). In all cases, the forelegs of the initiating animal left the ground and the back legs frequently did so.
Experience. We used two measures of experience. (1) Experience at competing with a particular opponent was defined as the number of occasions on which we had recorded two bucks participating in either a low-level displacement or a fight before we recorded them fighting on videotape. (2) Experience of outcome classified a fighting dyad of bucks (on videotape) into one of two categories, based on the results of the penultimate fight in which each dyad member had been involved before being videotaped. The first category included those dyads in which at least one dyad member had won his penultimate fight. The second category included those dyads in which neither dyad member had won his penultimate fight. We removed from analysis situations in which at least one dyad member’s penultimate fight was between him and his current (videotaped) opponent, because outcome would have been constrained (if one animal wins, then his opponent must lose). Resource value was considered to be an index of the abundance of oestrous females in the population, defined as the number of matings observed per day during the rut in October 1996 and 1997. We considered resource value to be highest on days when there were few oestrous females in the population. Therefore, we compared the first 3 days of the rut and the last 3 days of the rut, when there were few receptive females, with the 3 peak days, when many receptive females were present. We used number of matings as the measure of resource value instead of a ratio score of competing males per day to mated females per day, because these two factors were highly related in both years of the study (Spearman rank correlation: 1996: rS ¼ 0:924, N ¼ 16, P ! 0:001; 1997: rS ¼ 0:746, N ¼ 17, P ! 0:001). The rut was defined as the period over which matings were recorded in the population. The rut usually lasted about 2 weeks beginning in the middle of October (1996: 16 October 1997: 15 October). The prerut was defined as the time following cleaning of velvet from the antlers until the first mating was recorded in the herd. Peak day was taken as the day during the rut on which most matings were observed in each year. Backward push (Alvarez 1993): fallow bucks fight by engaging antlers and pushing vigorously. This is often accompanied by wrestling, usually without spatial displacement of either competitor. Many bouts of pushing progress to a phase where one buck rapidly forces his opponent backwards while antlers are still engaged. The point at which the pushing becomes a displacement in favour of one buck (backward push) is taken to represent the termination of energetic investment at a particular point in a bout of fighting by the displaced buck. We calculated the total duration of backward push that each animal in a dyad achieved.
Antler Collection and Measurement Terms and Definitions Further definitions of the terms used in this study are given below.
Antler collection and measurement protocols are described in Jennings et al. (2002). We recorded 39 fights on videotape in 1996 and 1997 where measures of the mean
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antler length (average of both antlers) were available for both members of the fighting dyad. Of the 39 fights, 21 had a clear winner and loser.
5:65 G 0:48 bouts of antler locking per fight, and the majority of fights (N ¼ 145, 76.7%) did not have more than six bouts of antler locking. The average number of jump clashes recorded was 1:9 G 0:26 per fight.
Capture of Bucks and Body Mass Measurement The procedure for selecting bucks for capture and weighing are described in Jennings et al. (2003). Bucks were immobilized using a mixture of etorphine hydrochloride (18e20 mg/kg, C-Vet Veterinary Products, Leyland, U.K.) and xylazine (360e420 mg/kg, Rompun Dry Substance, Bayer, Dublin, Ireland) administered by hypodermic syringe darts fired from a compressed-gas rifle. Bucks were weighed by suspension in a net from a standard Salter balance mounted on a portable tripod. Bucks were revived with a mixture of antagonistic drugs, diprenorphine hydrochloride (Revivon), (24e28 mg/kg, C-Vet Veterinary Products) and antipamozole hydrochloride (50 mg/kg, Antisedan, Pfizer, Dublin, Ireland) in a total volume of less than 2 ml, injected either intravenously or intramuscularly by a hypodermic syringe. We recorded 40 fights on videotape where the body weight of both males was known. Of the 40 fights, 22 had a clear winner and loser.
Ethical Note A qualified veterinarian administered the drugs. In general, animals were not handled for more than 10 min and typically the procedure was completed within 5 min. After reviving drugs had been administered, an observer remained with the animal until it had recovered full mobility and alertness. The behaviour of bucks was monitored for 2e3 days to confirm that they did not suffer increased levels of intraspecific aggression as a result of the procedure.
Statistical Tests Before analyses, we investigated the data to determine their distribution using the KolmogoroveSmirnov onesample test. Standard parametric and nonparametric tests were used based on the distribution of the data. Spearman rank correlation was used to describe the relation between phenotypic characteristics and measures of fight intensity. Descriptive statistics are presented as mean G SE. All P values presented are two tailed and all analyses were performed with SPSS v. 11 (SPSS Inc., Chicago, U.S.A.).
Factors Affecting Fight Duration and Intensity Body weight The range of difference in body weights of opponents was 0e9% of the weight of the lighter buck in the dyad. There was no significant difference in the number of jump clashes given by heavier and lighter bucks (Wilcoxon signed-ranks test: Z ¼ 1:01, N ¼ 39, P ¼ 0:612), and heavier bucks did not achieve significantly longer backward pushes than lighter bucks did (Z ¼ 0:65, N ¼ 40, P ¼ 0:518). There was no greater likelihood that heavier animals (N ¼ 13) would win more fights than lighter animals (N ¼ 9; chi-square test: c21 ¼ 0:73, P ¼ 0:394). We therefore tested the relation between body weight and fight duration and intensity using five measures: difference in body weight of the opponents, body weight of the winner, body weight of the loser, body weight of the heavier buck and body weight of the lighter buck. There was no relation between fight duration and body weight (Table 1). There was a significant positive relation between the losing buck’s body weight and the number of its jump clashes (Table 1, Fig. 1a) and between the body weight of the lighter buck in a dyad and the number of its jump clashes (Table 1, Fig. 1b).
Antler length The difference in antler length ranged from 0.1 to 13.7% of the antler length of the buck with the smaller average antler length. Dyad members with longer antlers did not give significantly more jump clashes than did dyad members with shorter antlers (Wilcoxon signedranks test: Z ¼ 0:11, N ¼ 39), and they were not more likely to beat opponents with smaller antlers (larger: N ¼ 12; smaller: N ¼ 9; chi-square test: c21 ¼ 0:43, P ¼ 0:513). We investigated the relation between antler length and fight duration and intensity using five measures: difference in antler length of the opponents, Table 1. Spearman correlations between fight duration and fight intensity (number of jump clashes) and five measures of body weight: the difference between the weights of the two opponents, the weight of the winner, the weight of the loser, and the weights of the heavier and lighter animal in the dyad Fight duration
Measure
N
Difference between opponents Winner Loser Heavierz Lighterz
40
0.06
22 22 39 39
0.16 0.23 0.15 0.22
Number of jump clashes
RESULTS
Fight Duration and Intensity The mean G SE duration of fights was 97:15 G 8:2 s and the mean duration spent with antlers locked per fight was 50:25 G 4:25 s (N ¼ 189 fights), which was slightly over half (51.7%) of the overall duration of fights. The mean G SE time spent between bouts of antler locking (intralock duration) was 8:16 G 0:86 s. There was an average of
0.05y 0.14 0.63* 0.28 0.32**
*P ! 0:05; **P ! 0:002. yTotal number of jump clashes recorded during the fight. zIn one dyad both bucks were the same weight.
JENNINGS ET AL.: ASSESSMENT RULES IN DEER FIGHTS
relation between the antler length of the buck with the longer antlers and the number of jump clashes (Table 2, Fig. 2).
110 (a) 108
Resource value
106
There was no significant difference between the prerut and rut for fight duration (ManneWhitney U test: Z ¼ 0:167, N1 ¼ 33, N2 ¼ 156, P ¼ 0:218) or intensity (Z ¼ 1:231, N1 ¼ 33, N2 ¼ 156, P ¼ 0:867). There was no relation between the number of matings recorded on any day during the rut and fight duration (Spearman rank correlation: rS ¼ 0:52, P ¼ 0:521) or fight intensity (rS ¼ 0:36, P ¼ 0:651). The first 3 days, last 3 days and the 3 peak days of the rut did not differ significantly in either fight duration (ANOVA, log-transformed data: F2;89 ¼ 0:21, P ¼ 0:815) or fight intensity (KruskaleWallis test: c22 ¼ 0:602, P ¼ 0:74).
104 102 100
0
1
2
110 (b)
Experience
108 106 104 102 100 98
0
1
2
3
4
5
6
Figure 1. The relation between the number of jump clashes recorded and the body weight (kg) of (a) the loser and (b) the lighter buck in the dyad.
antler length of the winner, antler length of the loser, antler length of the buck with longer antlers and antler length of the buck with shorter antlers. There was no relation between fight duration and antler length for any of the five measures (Table 2). There was a significant Table 2. Spearman correlations between fight duration and fight intensity (number of jump clashes) and five measures of antler size: the difference in antler size of the two opponents, the antler size of the winner, the antler size of the loser, and the antler size of the heavier and lighter animal in the dyad Measure
N
Fight duration
Number of jump clashes
Difference between opponents Winner Loser Longer Shorter
39
0.14
0.3y
21 21 39 39
0.09 0.25 0.09 0.11
0.19 0.21 0.32* 0.09
*P ! 0:04. yTotal number of jump clashes recorded during the fight.
There was no relation between fight duration and the total number of noncontact displacements recorded for a particular dyad before their first videotaped fight (Spearman rank correlation: rS ¼ 0:126, N ¼ 189, P ¼ 0:084). There was a significant negative relation between fight duration and the number of fights recorded for a particular dyad before the dyad was first recorded fighting on videotape (rS ¼ 0:222, N ¼ 189, P ! 0:002, Fig. 3). There was no relation between number of jump clashes and the number of noncontact displacements (rS ¼ 0:046, N ¼ 189, P ¼ 0:531) or fights (rS ¼ 0:004, N ¼ 189, P ¼ 0:959) recorded for a particular dyad before their first videotaped fight in either year. Current fight outcome was related to penultimate fight outcome (chi-square test: c24 ¼ 14:8, P ! 0:005; Table 3). If an individual’s current fight ended in a draw, it was likely that his penultimate fight had also ended in a draw (Table 3). Penultimate fight outcome also influenced the duration of the subsequent fight. When at least one
Antler length of buck with longer antlers (cm)
98
560
540
520
500
480
0
2
4 6 8 10 Number of jump clashes
12
14
Figure 2. The relation between the antler length (cm) of the dyad member with longer antlers and the number of jump clashes recorded during fights.
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Effects of Body Mass and Antler Length
800
600 Fight duration (s)
218
400
200
0 0
20
10 Number of fights to date
Figure 3. The relation between fight experience and fight duration.
buck in a fighting dyad had won his penultimate fight, fight duration was significantly longer (X G SE ¼ 105:6 G 11:9 s) than when neither buck had won his penultimate fight (94:8 G 14:3 s; log-transformed independent t test: t155 ¼ 2:7, P ! 0:009). There was no significant difference between the number of jump clashes recorded in fights in which at least one buck had won his penultimate fight and fights in which neither buck had won his penultimate fight (ManneWhitney U test: Z ¼ 0:664, N1 ¼ 85, N2 ¼ 103, P ¼ 0:507). DISCUSSION In the present study, we focused on the effects of RHP, resource value and fighting experience on fight duration and intensity between competing fallow deer during the annual breeding season. Investigation of duration and intensity may provide insight into the motivational level of competing males, because it provides an index of the willingness of two animals to persist in a potentially dangerous encounter (Bridge et al. 2000) and might provide information on assessment processes used during escalated contests (Taylor & Elwood 2003).
Table 3. A summary of the outcomes of contestants’ penultimate fights and their current (videotaped) fights, according to whether they had won, lost or drawn their current fights Outcome of current fight Outcome of penultimate fight Won Lost Drawn
Won
Lost
Drawn
31 14 41
27 24 36
38 16 88
Current fights are those fights that were recorded on videotape only. All current fights in which both members of the dyad had fought with each other in their penultimate fights were excluded. One buck was excluded because he was not recorded fighting prior to being recorded on videotape.
Body weight, or disparity in size between opponents, is an important determinant in predicting fight duration (reviews in Archer 1988; Riechert 1998; but see Taylor & Elwood 2003). In general, as disparity in body weight increases, there is a corresponding decrease in fight duration (Riechert 1998). In the present study, possession of larger body weight or longer antlers was not associated with fight success. Taylor & Elwood (2003) found that, by focusing on individual role within fights (winnereloser), it was possible to differentiate between mutual assessment (the sequential assessment game, SAG) and self assessment (the energetic war of attrition, eWOA; see above). In our study, fight duration was not related to asymmetry between contestants in body weight or antler size, nor to the weight or antler size of the lighter or heavier individual. Correlation coefficients were not of equal magnitude but of opposite sign (with the exception of winnereloser body weight, which approached these criteria). These results suggest two possibilities. On the one hand, body weight may not be a correlate of RHP in this species (see also Marden & Rollins 1994). However, an alternative explanation might be based on the bout structure of fallow deer fights. We have found that, at the end of the first bout of fighting, the eventual winner was as likely to terminate the bout as the eventual loser; however, losers had a significantly higher tendency to terminate fighting by the end of the fourth bout (D. J. Jennings, unpublished data). Furthermore, the first bout of fighting was significantly longer than the third or fourth bouts. Current assessment models do not incorporate different decision levels (i.e. ending a bout of fighting versus ending the fight altogether) in their structure, so they have no mechanism for predicting behaviour where multiple decision levels inform each other (D. J. Jennings, unpublished data). The relation between overall contest duration and contestant RHP may be obscured by the bout level nature of these fights, and Taylor & Elwood’s (2003) analytical strategy with regard to fight duration therefore may not be appropriate for fallow deer fights. If the costs of escalated contests increase the risk of injury, then contestants should seek to adopt strategies that limit these risks (Maynard Smith 1982). Losers of fights jump-clash significantly less than do winners (D. J. Jennings, unpublished data). However, in the present study, we found that as the loser’s body weight increased, there was an increasing tendency to use the jump clash to initiate antler contact. This result is consistent with individuals basing escalation decisions on their own ability (Mesterton-Gibbons et al. 1996; Taylor & Elwood 2003). Subordinate animals may seek to offset the relative dominance relationship within dyads by escalating the contest (Dow et al. 1976; Popp 1987; Senar et al. 1992), and the present results show that losing animals increased their use of high-risk strategies as their body mass increased (see also Bridge et al. 2000; Taylor et al. 2001). The jump clash as a measure of intensity is a special case and differs from contest duration in that it is controlled by the initiator, with the costs of the clash spread between
JENNINGS ET AL.: ASSESSMENT RULES IN DEER FIGHTS
the opponents. The individual with the lower RHP does not control the termination of the jump clash, as it might with contest duration. There was a positive relation between the body weight of the lighter member of a dyad and the number of jump clashes recorded; the relation between body weight of the heavier contestant was also positive (of similar sign) but the difference was not significant. This evidence supports the hypothesis that body mass is an important variable in determining the use of high-intensity behaviour (e.g. Turner 1994; but see Hack 1997). Within the analytical framework presented by Taylor & Elwood (2003) the present results suggest that contestants are engaged in a process of self assessment, so the more appropriate model is the eWOA rather than the SAG. An opponent’s RHP is potentially difficult to assess, and body weight may not always be the most reliable indicator of an individual’s competitive ability (Parker & Rubenstein 1981). Some other phenotypic trait, such as weapon size, may be more a more reliable indicator of quality, because it represents the ability to inflict physical damage (Sneddon et al. 1997). Although there is some evidence that weapon size is a predictor of competitive ability (e.g. Barki et al. 1997; Sneddon et al. 1997), the evidence from cervids is less conclusive (the number of antler points is weakly correlated with fighting success in red deer, Cervus elaphus: Clutton-Brock et al. 1979). In this study, there was no relation between fight duration and difference in antler length of the competing males. Furthermore, contest duration was not related to the antler size of the winner or loser, neither was it correlated with the antler length of the dyad member with the longest or shortest antlers. Therefore, we eliminate the SAG as a potential model for fallow deer fights. The results showed a significant relation between antler length and number of jump clashes for the buck with the longer antlers. These results indicate that longer antlers facilitate the use of higher risk tactics during dyadic fighting, although within dyads there was no difference in the number of jump clashes based on antler length (also Schroeder & Huber 2001). These results also support the prediction that contestants base decisions on their own ability and, therefore, they indicate an assessment process compatible with a WOA (Mesterton-Gibbons et al. 1996; Taylor & Elwood 2003).
Effects of Resource Abundance The number of observed fights is often correlated with the number of matings and with mating success (CluttonBrock et al. 1988; Apollonio et al. 1990; Moore et al. 1995; San Jose´ & Braza 1997). Game-theoretical models propose that fight duration and intensity are positively correlated with resource value (Bishop et al. 1978; Hammerstein & Parker 1982; Enquist & Leimar 1987; for evidence see Clutton-Brock et al. 1979; Sigurjonsdottir & Parker 1981; Austad 1983; Verrell 1986). The results of this study failed to support the prediction that an increase in resource value is correlated with an increase in either fight duration or fight intensity. Fights recorded in the prerut in this
population were not significantly shorter than fights recorded during the rut. Furthermore, we expected that fight duration should be related to the relative scarcity of the resource. This was not the case, and in general, there was no relation between the abundance of oestrous females and fight duration or intensity during the rut. However, the breeding system of a species may be an important influence on fight behaviour. Resource ownership, as represented by harem holding, is not observed in this population, and most males move with the doe herd during the day. Mating success in this population is highly skewed and most males do not mate (Moore et al. 1995). In theoretical terms, if the payoff or reward to either or both animal(s) is variable, as it is in our population, then we might expect that the relative abundance of that resource should not influence contest duration or intensity (Maynard Smith 1982); our results support this prediction.
Effects of Experience Non-RHP-related factors such as experience might be important in determining contest outcome and structure (Mesterton-Gibbons et al. 1996). We expected that any information gathered as a result of experience might be used to reduce fight duration and intensity in subsequent encounters (Maynard Smith & Parker 1976; Hammerstein 1981; Hammerstein & Parker 1982; for evidence see Karavanich & Atema 1998; Lo´pez & Martı´n 2001). In such cases, both contestants are expected to benefit from a reduction in the costs associated with fighting (Huntingford & Turner 1987; Archer 1988; Hammerstein 1998). Accordingly, we investigated the amount of agonistic experience between members of particular dyads. There was no relation between fight duration and the number of noncontact interactions. However, there was a significant negative relation between fight duration and the number of fights in which a particular pair of bucks had previously been involved. Retaining information about encounters with individual conspecifics might help reduce fight costs, such as time and energy expenditure, and our results support this prediction. The effects of penultimate fight outcome might be important determinants of subsequent fight outcome, and a winnereloser effect is frequently observed in a variety of species (e.g. Otronen 1990; Cloutier & Newberry 2000; Dodson & Schwaab 2001). We found little evidence for a winnere loser effect when only wins and losses were considered. However, there was a significant effect when drawn fights were included, indicating that when an animal had experience of drawing a fight, there was an increased probability that a subsequent fight would also be drawn. When at least one member of the dyad won his penultimate fight, fights were significantly longer than when neither buck had won. This result has been interpreted as an effect of prior success increasing self perception of individual RHP (Parker 1974; MestertonGibbons & Dugatkin 1995; Dugatkin & Earley 2003). We found no evidence that the outcome of a penultimate fight influenced fight intensity (see also Hsu & Wolf 2001).
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Where the population density of competing males is high, as it is in this population, aggressive interactions between males may extract severe costs in terms of energy expended and time. Our measure of fight intensity, but not of fight duration, indicated that individuals might base decisions on estimates of their own ability rather than on a process of mutual assessment. However, the importance of body weight as a correlate of fight duration may be confounded by the bout structure of fallow deer contests, a possibility that remains to be investigated. Although resource value may override other potentially important determinants of fight duration and intensity, such as asymmetry in RHP (Riechert 1998), the present study suggests that in the fallow deer, resource value is of limited importance in influencing fight duration or intensity. However, the experience of fighting with a particular opponent was related to a decrease in fight duration. The ability to recognize opponents, the ‘dear enemy’ effect (Fischer 1954), can assist competing males in reducing costs associated with fighting such as time and energy (see also Lo´pez & Martı´n 2001). Furthermore, we found that the outcome of a prior agonistic encounter also influenced subsequent fight duration. Acknowledgments We thank Du´chas, The Heritage Service and Mr John McCullen, the Superintendent of Phoenix Park, for facilitating access to the deer herd. We express our gratitude to Mr Don Doran (park deerkeeper), the Rangers and staff of Phoenix Park for their support during the field stages of this study. We thank our colleagues in the Mammal Research Group for their help. In particular, we thank Paul Jennings for his assistance in extracting data on fighting from the files and Dr P. Quinlan for advice with statistics. We thank Professor Robert Elwood and an anonymous referee for valuable comments on the manuscript. M.P.G. and C.M.C. received financial support from Enterprise Ireland. References Alcock, J. 1993. Animal Behavior: An Evolutionary Approach. Sunderland, Massachusetts: Sinauer. Altmann, J. 1974. Observational study of behavior: sampling methods. Behaviour, 49, 227e267. Alvarez, F. 1993. Risks of fighting in relation to age and territory holding in fallow deer. Canadian Journal of Zoology, 71, 376e383. Andersson, M. 1994. Sexual Selection. Princeton, New Jersey: Princeton University Press. Apollonio, M., Festa-Bianchet, M., Mari, F. & Riva, M. 1990. Sitespecific asymmetries in male copulatory success in a fallow deer lek. Animal Behaviour, 39, 205e212. Archer, J. 1988. The Behavioural Biology of Aggression. Cambridge: Cambridge University Press. Austad, S. N. 1983. A game theoretical interpretation of male combat in the bowl and doily spider (Frontinella pyramitela). Animal Behaviour, 31, 59e73. Barki, A., Harpaz, S. & Karplus, I. 1997. Contradictory asymmetries in body and weapon size and assessment in fighting male prawns, Macrobrachium rosenbergii. Aggressive Behavior, 23, 81e91.
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