Factors leading to the evolution and maintenance of a male ornament in territorial species
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Male ornamentation is assumed to have evolved primarily from selection by female mate choice. Yet this is only one possible reason for ornament evolution. Ornaments might also be useful in aggressive competition by improving opponent assessment between males, or they might function to enhance signal detection by making males more conspicuous in the environment. We tested both these ideas in territorial Anolis lizards in which female choice is either absent or secondary to males competing for territories that overlap female home ranges. Male tail crests only evolved in species in which territory neighbors were distant, consistent with the signal detection hypothesis. Once the tail crest had evolved, however, it seems to have become a signal in itself, with variation in the frequency and size of tail crests within species correlating with variables predicted by the aggressive competition hypothesis. Our study presents an apparent example of a male ornament in which the selection pressure leading to variation among species in ornament expression is different from the selection pressure acting on variation within species. The Anolis tail crest is therefore likely to be an exaptation. We caution that conclusions made on the evolution of male ornaments are dependent on the phylogenetic perspective adopted by a study. Studies restricted to single species are useful for identifying selection pressures in contemporary settings (i.e., the current utility of traits), but may lead to erroneous conclusions on the factors that initially lead to the origin of traits.
KeywordsCaribbean anoles Exaptation Visual signal Phylogenetic comparative method Ancestor state reconstruction Sexual selection Natural selection
We thank José Rosado for access to the herpetology collections of the MCZ, Luke Mahler for useful references and help on identifying Cuban species with tail crests, and Jonathan Losos and two anonymous reviewers for insightful comments on a previous version of this manuscript. This work was supported by a US National Science Foundation grant (IOB-0517041/0516998) to TJO, Judy A. Stamps, and Jonathan B. Losos. All data from this publication have been archived in the Dryad Digital Repository (doi: 10.5061/dryad.fg0kq00m). All research described in this article complied with the current laws of the USA and the territory of Puerto Rico under research permits to TJO from the Caribbean National Forest and Departamento de Recursos Naturales y Ambientales de Puerto Rico, and the Animal Use and Care Protocols 05-11652/15243 that were approved on March 24, 2005 and most recently reviewed on March 20, 2009 by the Institutional Animal Care and Use Committee of the University of California, Davis. The authors declare no conflict of interest.
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