The Evolution of Animal Weapons

Abstract

Males in many species invest substantially in structures that are used in combat with rivals over access to females. These weapons can attain extreme proportions and have diversified in form repeatedly. I review empirical literature on the function and evolution of sexually selected weapons to clarify important unanswered questions for future research. Despite their many shapes and sizes, and the multitude of habitats within which they function, animal weapons share many properties: They evolve when males are able to defend spatially restricted critical resources, they are typically the most variable morphological structures of these species, and this variation honestly reflects among-individual differences in body size or quality. What is not clear is how, or why, these weapons diverge in form. The potential for male competition to drive rapid divergence in weapon morphology remains one of the most exciting and understudied topics in sexual selection research today.

Female monopolization mediates the relationship between pre- and postcopulatory sexual traits

“…In contrast, the diversity in weaponry associated with male contest competition is more readily comparable in terms of resource allocation. Second, studies of the evolution of sexually selected traits tend to be strongly biased towards ornaments and female mate choice, with comparative studies on the evolution of weapons lagging far behind 54 .…”

The evolution of genetic and conditional alternative reproductive tactics

“…The key difference is that when males apply fixed tactics, the 'decision' of which tactic to use is made irreversibly at one stage (usually early) in their lifetime. This is exemplified by ARTs in many insects [10,[17][18][19], where males either develop large weapons important in territorial fights or spare this investment [20]. Yet once they are sexually mature they cannot switch tactics.…”

Fighting for mates: the importance of individual size in mating contests in rocky shore littorinids

“…Such a positional advantage of defenders in E. malaccana has also been reported in amphipods (Dick and Elwood 1990) in that defenders, even when disadvantaged by size, were still more likely to win contests because of their precedence in holding the females. Given that females may be limiting in E. malaccana, it is possible that selection may also favour defenders that are tenacious and prepared to invest energy into the contest to defend their resource, resulting in an 'arms-race'-type scenario between the challenger and defender (Emlen 2008).…”