The Evolution of Animal Weapons Douglas J. Emlen Division of Biological Sciences, The University of Montana, Missoula, Montana 59812; email:
[email protected] Annu. Rev. Ecol. Evol. Syst. 2008. 39:387-413 Key Words First published online as a Review in Advance on animal diversity, sexual selection, male competition, horns, antlers, tusks September 2, 2008 The Annual Review of Ecology, Evolution, and Abstract Systematics is online at ecolsys.annualreviews.org Males in many species invest substantially in structures that are used in com- This article's doi: bat with rivals over access to females. These weapons can attain extreme 10.1146/annurev.ecolsys.39.110707.173 502 proportions and have diversified in form repeatedly. I review empirical lit- Copyright © 2008 by Annual Reviews. erature on the function and evolution of sexually selected weapons to clarify All rights reserved important unanswered questions for future research. Despite their many 1543-592X/08/1201-0387$20.00 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 hon- estly 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. 3*7 INTRODUCTION Sexual selection is credited with the evolution of nature's most extravagant structures, and these include showy male adornments that are attractive to females (ornaments) and an arsenal of outgrowths that function in male-male combat (weapons) (Darwin 1871).