Behavioral Ecology Vol. 15 No. 2: 187-191
Behavioral Ecology vol. 15 no. 2 © International Society for Behavioral Ecology 2004; all rights reserved
Male dominance and immunocompetence in a field cricket
Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä, Finland
Address correspondence to M. J. Rantala. E-mail: marrant{at}dodomail.jyu.fi.
Received 3 September 2002; revised 4 March 2003; accepted 25 March 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Female preference for dominant males has been found in many species, and it is generally thought that winners of male-male competition are of superior quality. Success in contests probably depends on male condition and overall health. Thus, females could avoid infection and gain genetic benefits in terms of more viable offspring by mating with dominant males. In the present study, we tested whether dominant males of the Mediterranean field cricket, Gryllus bimaculatus, had higher immunocompetence than did their subordinates in experimental trials. We found that dominant males had better immune defense, as indicated by significantly higher encapsulation rate and lytic activity, than did subordinate males of the same size. Dominant males were also more successful in obtaining matings, but this was measured nonindependently of dominance status. Our results suggest that a male's dominance status and success in fights may indicate his immunocompetence to females.
Key words: dominance, female choice, Gryllus bimaculatus, immunocompetence, male-male competition.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Females of many species are known to prefer dominant males as mates, and in some species, females even incite competition between males and then mate with the most dominant male (for review, see Berglund et al., 1996
). A mechanism of female choice can be adaptive if it bestows some benefit on the choosing individual, potentially increasing her reproductive success either directly or indirectly. Success in contests probably depends on male condition and overall health (Borgia, 1979); therefore, females could avoid infection and gain genetic benefits in terms of more viable offspring by mating with dominant males (Berglund et al., 1996; Cox and LeBoeuf, 1977). If only males of relatively high quality are able to bear the costs of dominance (Grafen, 1990; Zahavi, 1975), their position in the hierarchy will reliably reflect male quality (Qvarnström and Forsgren, 1998). One aspect of male quality is the ability to resist parasites and pathogens (Hamilton and Zuk, 1982). It has been shown that parasite infection prevents male mice from becoming behaviorally dominant (Freeland, 1981). However, whether male dominance status reflects ability to resist parasitism is not known.
The term "immunocompetence" refers to the ability of an individual's immune system to resist and control pathogens or parasites. In insects, one of the most informative ways to assay immunocompetence is to measure the cellular encapsulation response to a novel and standardized antigen, such as a nylon monofilament (Koskimäki et al., 2003; Köning and Schmid-Hempel, 1995; Rantala and Kortet, 2003; Rantala et al., 2000, 2002; Ryder and Siva-Jothy, 2000). Encapsulation is a cellular immune response through which insects defend themselves against foreign particles (Salt, 1970). During the encapsulation process, specialized hemocytes recognize invading particles as nonself and cause other hemocytes to aggregate and encapsulate the particle. A cascade of reactions involving the tyrosine-phenoloxidase (PO) pathway causes the melanization of the capsule and the death of the invading particle (Fisher, 1963). PO is a key enzyme in the synthesis of melanin, and the ability to produce melanin is an important aspect of the immune response (Gillespie et al., 1997). In addition, encapsulation response plays a role in defense against viruses (Washburn et al., 1996). The humoral system, on the other hand, is comprised of a myriad of soluble proteins and enzyme cascades, which play important roles in recognizing, signaling, and attacking foreign targets (Leonard et al., 1985) and probably in coordinating the cellular responses (Pech and Strand, 1995). Therefore, in the current study we estimated immunocompetence by measuring the encapsulation rate against a novel antigen and the hemolymph concentration of an antibacterial enzyme, lysozyme.
Male Mediterranean field crickets, Gryllus bimaculatus, fight over and defend territorial shelters and attract females by calling (Alexander, 1961; Shuvalov and Popov, 1973; Simmons, 1986b). Females are known to choose a mate on the basis of his song (Rantala and Kortet, 2003; Simmons, 1986b). However, because the mating system also involves direct male competition with fierce fighting between males, success in competition can influence a male's mating success (Simmons, 1986a). Although male body weight affects male fighting success, the weight asymmetry of fighting males is not a very reliable predictor of the outcome of fights, because Hofmann and Schildberger (2001) found that heavier animals lost 30% of the encounters even if weight asymmetry was large.
In this study we tested whether male dominance status, that is, success in winning intrasexual fights, could indicate male immunocompetence in the field cricket (G. bimaculatus) and whether dominant males are more successful in obtaining matings in a situation in which both male-male competition and female choice occur.
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METHODS |
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Crickets
The crickets used in this study were from a laboratory stock originating from a commercial supplier (Faunatar, Helsinki) and were maintained at the Experimental Animal Unit at Jyväskylä University. They were maintained at 29 ± 1°C with ad libitum food and water under a 12 h light/12 h dark photoperiod. Experimental crickets were derived from a bulk laboratory stock as larvae and maintained individually (also with ad libitum food and water) in covered plastic containers. Both sexes were physically (but not acoustically) isolated from other individuals, to ensure virginity. Males were 8-days-old on the day of the experiment. Before the experiments, we weighed the fresh body mass of the crickets to the nearest 0.01 g. No cricket was used in more than one experiment.
Encapsulation rate assay
The day before the dominance test, males were chilled on ice for 20 min. Then we inserted a 2-mm-long piece of nylon monofilament (diameter, 0.1 mm) through a puncture in the pleural membrane between the second and third sternite. The males' immune system was allowed to react to this object for 5 h, while crickets were kept individually in plastic vials at constant room temperature (28 ± 1°C). The implant was then removed and dried. The removed monofilament was photographed from three different angles under a light microscope with a digital video recorder. The pictures were then analyzed by using the Image Pro program. The degree of encapsulation was analyzed as gray values of reflecting light from implants. The average gray values of the three video pictures were used in our measure of encapsulation rate. The Image Pro program represented the greatest levels of encapsulation with the smallest numerical values. Therefore, we transformed the data such that the highest numerical values corresponded to the highest encapsulation rate. The transformation was done by subtracting the observed gray values from the control gray value (clear implant). To measure the repeatability of this method, we photographed again 16 randomly chosen implants and analyzed them as above. The repeatability (R) of this method was high (R = 0.997, F15.16 = 778.69, p <.001).
Lysozyme assay
The sample for the lysozyme assay was collected at the same time as the implant for the encapsulation rate measurement was taken out. We collected 10 ml of hemolymph by micropipette from each male from a puncture in the abdomen. The hemolymph was then mixed with 90 ml of phosphate-buffered saline solution (pH 6.4). Samples were immediately frozen at -25°C. We assayed lysozyme activity of hamolymph against Micrococcus luteus turbidimetrically by using a method modified from Ellis (1990; see also Rantala and Kortet, 2003). We mixed 200 ml of 0.35 mg/ml freeze-dried M. luteus buffered (pH 6.4) solution with 90 ml 1:4 buffered hemolymph (pH 6.4) in a plastic multicuvette (Labsystems cliniplate). The mixture absorbance at 492 nm was then measured in 20°C in 1-min intervals for 30 min with a plate reader (Multiskan Plus; Labsystems, Finland). The lytic activity was expressed as total change in absorbance. Because smaller absorbance values indicate higher lytic activity, we transformed the scale oppositely (by multiplying by -1), so that higher values would correspond to higher activity.
Dominance test procedure
Two similar, age- and weight-matched males (98% similarity in fresh weight) and one female were initially placed in a sawdust-covered plastic arena (length, 20 cm; width, 20 cm; depth, 15 cm). The males were marked on the pronotum with enamel paint to allow recognition. Males were placed under glass vials to calm them for 2 min preceding the trial. After removal of the vials, the males usually started fighting immediately. Each trial lasted for 3 min, during which time the male–male contests were videotaped under red-light illumination with a video recorder placed above the arena. The room temperature was kept at 28 ± 1°C. To minimize for possible fluctuations of aggressiveness in a 24-h day, we conducted the experiments between 1600–2100 h. We analyzed the videotapes later to record fight winners and the dominance-indicating behavior of the males. Dominance status of males was easy to observe, because after the fights, the subordinate male showed avoidance behavior toward the dominant male. The investigators analyzing the videotapes did not know the immunocompetence of the males. In total, we performed 25 trials.
Male dominance and mating success
To test whether dominant males have higher mating success, we conducted another experiment with another set of crickets following the method used in Wedell and Tregenza (1999). Before the experiment, both males were exposed to virgin females that were tied to prevent mating, until both males had begun courtship singing, thus indicating they had a spermatophore ready. To test whether females prefer dominant males, we released two weight-matched males into an arena containing a 10-day-old virgin female. The experimental crickets were observed, and their actions and behavioral patterns were recorded on videotape for 10 min. To prevent male spermatophore release, we halted matings by hand immediately when the female climbed onto the male's back. According to our observations, male crickets may stop their courting behavior after spermatophore release. We measured the female preference by using counts of the number of times that the female mounted each male. Dominance status of males was defined similarly as in the first experiment. Likewise, also in this set of trials, after the fights, the subordinate male showed clear avoidance behavior toward the dominant male. Male mating success here was not measured independent of his dominance status, but it is worth noting that the female cricket can not be raped, and in all the cases, she made her choice freely. In total, we performed 16 trials.
Statistical analyses were performed using SPSS Advanced Statistics 7.5, 1997.
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RESULTS |
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Immunocompetence and dominance
Males engaged in one to five clear fights to determine their dominance hierarchy. After the fights, one of the males always achieved dominance status, and after that, he was superior in all further encounters. Dominant males had stronger encapsulation responses than did their subordinates in 20 of the 25 trials (Wilcoxon signed-rank test: Z = -2.70, N = 25, p =.007) (Figure 1a). Furthermore, dominant males had higher lytic activity than did their subordinates in 16 of the 25 trials (Wilcoxon signed-rank test: Z = -2.09, N = 25, p =.037) (Figure 1b). Because the male pairs were weight matched, dominant males did not differ in weight from their subordinates (Wilcoxon signed-rank test: Z = -0.65 N = 25, p =.514). There was a slight positive correlation between encapsulation rate and lytic activity (Pearson r =.291, N = 50, p =.040). Surprisingly, there was no correlation between male weight and encapsulation rate (Pearson r =.068, N = 50, p =.638) or lytic activity (Pearson r =.133, N = 50, p =.357). Likewise, when dominance groups were analyzed separately, no significant correlations between weight and immune parameters could be found (dominant males: encapsulation, Pearson r = -.176, N = 25, p =.400; lytic activity, Pearson r =.104, N = 25, p =.622; subordinate males: encapsulation, Pearson r =.361, N = 25, p =.077; lytic activity, Pearson r =.214, N = 25, p =.303).
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Male dominance and mating success
Similarly to what was found in the first set of trials, after the first fourth of the 10-min observation period, one of the males always achieved dominance status. Dominant males were more successful to obtain matings in 14 of the 16 trials (mean counts of the times when the female mounted the male were 6.45 and 0.675, for the dominant and subordinate males, respectively; Wilcoxon signed-rank test, Z = -3.24, N = 16, p =.001).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Dominant males had stronger encapsulation responses than did their subordinates. Furthermore, dominant males also had higher lytic activity than did their subordinates. One explanation for our results might be that both immune function and dominance status are condition dependent. Unfortunately, the experimental design used in the present study did not involve varying conditions or an investigation of the effects of this on dominance and immune defense. However, the energetic expenditure needed for producing and maintaining components of the immune system has been suggested to have a major effect on condition, thus creating a link between immune system and condition dependent traits (Wedekind, 1992
; Wedekind and Folstad, 1994). A trade-off is expected such that resources are used up by both sexual advertisement and the immune system (see Kotiaho, 2001). Therefore, only individuals in good condition can mount a strong immune defense and produce an extravagant sexual ornament or behavior (Sheldon and Verhulst, 1996; Westneat and Birkhead, 1998). On the other hand, dominant males may be forced into more frequent antagonistic interaction and be more exposed to wounds and pathogenic infection (see Andersson, 1994). Thus, in general, dominant males may be selected to allocate more energy to immune function.
We found a slight positive correlation between the encapsulation rate and lytic activity in the present study. A positive correlation between the two immune traits has been suggested to be in agreement with the general condition-dependent trade-off between the immune system and other traits (see Kurtz and Sauer, 1999).
We also found that dominant males were more successful in obtaining matings than their subordinates. In the present study, male mating success was not measured independently of his dominance status. Nevertheless, female crickets cannot be coerced into mating because they must actively mount the male and allow him to attach a small spermatophore to the female (Wedell and Tregenza, 1999). Thus, our results indicate that female crickets prefer dominant males as mates. This is consistent with studies of house crickets, Acheta domesticus, which showed that females preferred the calling song of dominant males (Crankshaw, 1979). A number of studies have shown that variation in encapsulation ability and hemocyte load can be heritable in insects (see Carton and Boulétreau, 1985; Carton et al., 1992; Fellowes et al., 1998; Kraaijeveld and Godfray, 1997; Kraaijeveld et al., 2001; Ryder and Siva-Jothy, 2001). Furthermore, Kurtz and Sauer (1999) detected heritable variation in lytic activity and phagocytosis ability. Thus, females may increase the parasite resistance of their offspring by preferring dominant males. This might be consistent with previous findings in G. bimaculatus by Wedell and Tregenza (1999), who found that successful fathers sire successful sons.
Many theoretical and empirical works have shown that male sexual traits may convey reliable information about male parasite resistance ability to females (see Hamilton and Zuk, 1982; Møller, 1988, 1990; Taskinen and Kortet 2002). This is suggested to be the result of a trade-off between sexual trait expression and immune function (Folstad and Karter, 1992; Sheldon and Verhulst, 1996). For example, if resources must be diverted away from the immune system to maximize expression of a trait, males may suffer increased susceptibility to pathogenic infections (Folstad and Karter, 1992; Sheldon and Verhulst, 1996). Recent studies in insects support the idea that secondary sexual characters indicate male immunocompetence to females (Rantala et al., 2000, 2002; Ryder and Siva-Jothy, 2000; Siva-Jothy, 2000). Our study suggests that male dominance status may also be equivalent to sexual ornament for a choosy female as an indicator of a male's immunocompetence status.
In previous studies with G. bimaculatus, body size has been shown to have important implications for male competitive ability and female mate selectivity (Simmons, 1986a,b). However, in the present study we did not found any correlation between any immune parameter and male size.
In conclusion, the present study supports several aspects of the immunocompetence handicap hypothesis (Folstad and Karter, 1992) and suggests that success in male–male fights gives reliable information to females about male quality. Thus, females may choose superior sires simply by watching the outcome of male–male competition.
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ACKNOWLEDGEMENTS |
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We thank Tarmo Ketola, Janne Kilpimaa, Emily Knott, Janne Kotiaho, Seppo Kuukasjärvi, Leena Lindström, John Loehr, Johanna Mappes, Jukka Suhonen, Jouni Taskinen, and all other round-table members of the Department of Biology at the University of Jyväskylä, who gave fruitful comments on the manuscript. Special thanks to J. Lampela, S. Koistinen, J. Tuusa, J. Valkonen, and S. Väänänen for their assistance in the laboratory. This study was supported by the Academy of Finland under the Finnish Centre of Excellence Program during 2000–2005 (Project: 44878). We also thank the Emil Aaltonen Foundation for a grant to R.K. Experimental Animal Unit of the University of Jyväskylä provided the facilities for the study.
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