Abstract
Sexually selected male weaponry is widespread in nature. Despite being model systems for the study of male aggression in Western science and for cricket fights in Chinese culture, field crickets (Orthoptera, Gryllidae, Gryllinae) are not known to possess sexually dimorphic weaponry. In a wild population of the fall field cricket, Gryllus pennsylvanicus, we report sexual dimorphism in head size as well as the size of mouthparts, both of which are used when aggressive contests between males escalate to physical combat. Male G. pennsylvanicus have larger heads, maxillae and mandibles than females when controlling for pronotum length. We conducted two experiments to test the hypothesis that relatively larger weaponry conveys an advantage to males in aggressive contests. Pairs of males were selected for differences in head size and consequently were different in the size of maxillae and mandibles. In the first experiment, males were closely matched for body size (pronotum length), and in the second, they were matched for body mass. Males with proportionately larger weaponry won more fights and increasing differences in weaponry size between males increased the fighting success of the male with the larger weaponry. This was particularly true when contests escalated to grappling, the most intense level of aggression. However, neither contest duration nor intensity was related to weaponry size as predicted by models of contest settlement. These results are the first evidence that the size of the head capsule and mouthparts are under positive selection via male-male competition in field crickets, and validate 800-year-old Chinese traditional knowledge.
Highlights
Darwin [1] proposed that male weaponry could evolve as a result of aggressive physical competition among males over reproductive access to females
All animals used in the aggressive contests were third generation offspring of adult G. pennsylvanicus caught from the same location during August and September of 2002
Contest duration was positively correlated with contest intensity (Spearman’s rho = 0.333, p = 0.002; Fig. S1). Despite having both shorter hind femora and shorter pronota, wild-caught male G. pennsylvanicus had wider heads, greater maxillae spans, longer maxillae, and longer mandibles than females (Table 1) and were more likely than females to have damaged mouthparts. This pattern of sexual dimorphism is consistent with the hypothesis that male heads, maxillae and mandibles evolved to be larger than females in response to malemale competition since male field crickets use their heads and mouthparts in escalated physical combat with other males [36,41,43,44], whereas females do not [46]
Summary
Darwin [1] proposed that male weaponry could evolve as a result of aggressive physical competition among males over reproductive access to females. The scientific literature abounds with examples of male traits that are used in direct combat between males for mates, including: antlers and horns in ungulates ([2,3], reviewed in [4]), canines in primates [5], avian spurs ([6], reviewed in [4]), heads in lizards [7], chelae in crabs [8], horns and mandibles in beetles ([9,10,11,12], reviewed in [4]), mandibles and maxillae in tree weta [13,14,15,16], forceps in earwigs ([17,18], but see [19,20]), antlers, eyestalks [21] and forelegs in flies [22,23], chelate pedipalps in pseudoscorpions [24], and forelegs in thrips [25] In each of these groups, weaponry is either limited to, or larger in males than females, and after correcting for body size differences between combatants, the male with the relatively larger weapon(s) usually wins in aggressive combat with smaller males All of these models predict that weaponry, as an index of RHP, will be correlated with contest duration
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