Sex ratio bias, male aggression, and population collapse in lizards

doi:10.1073/pnas.0505172102

Sex ratio bias, male aggression, and population collapse in lizards

^*Laboratoire Fonctionnement et Evolution des Systèmes Ecologiques, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7625, Ecole Normale Supérieure, 46 Rue d'Ulm, 75230 Paris Cedex 05, France; ^†Centre for Ecological and Evolutionary Synthesis, Department of Biology, University of Oslo, P.O. Box 1050, Blindern, 0316 Oslo, Norway; ^¶Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, United Kingdom; and ^∥Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721

Edited by David B. Wake, University of California, Berkeley, CA (received for review June 20, 2005)

Abstract

The adult sex ratio (ASR) is a key parameter of the demography of human and other animal populations, yet the causes of variation in ASR, how individuals respond to this variation, and how their response feeds back into population dynamics remain poorly understood. A prevalent hypothesis is that ASR is regulated by intrasexual competition, which would cause more mortality or emigration in the sex of increasing frequency. Our experimental manipulation of populations of the common lizard (Lacerta vivipara) shows the opposite effect. Male mortality and emigration are not higher under male-biased ASR. Rather, an excess of adult males begets aggression toward adult females, whose survival and fecundity drop, along with their emigration rate. The ensuing prediction that adult male skew should be amplified and total population size should decline is supported by long-term data. Numerical projections show that this amplifying effect causes a major risk of population extinction. In general, such an “evolutionary trap” toward extinction threatens populations in which there is a substantial mating cost for females, and environmental changes or management practices skew the ASR toward males.

Footnotes

↵ § To whom correspondence should addressed. E-mail: j.f.l.galliard{at}bio.uio.no.
↵ ‡ J.-F.L.G. and P.S.F. contributed equally to the work
Author contributions: J.-F.L.G., P.S.F., R.F., and J.C. designed research; J.-F.L.G. and P.S.F. performed research; J.-F.L.G. and P.S.F. analyzed data; and J.-F.L.G., P.S.F., R.F., and J.C. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviation: ASR, adult sex ratio.