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

Jean François Le Galliard, Patrick S. Fitze, Régis Ferrière, Jean Clobert

Research output: Contribution to journalArticlepeer-review

313 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)18231-18236
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number50
StatePublished - Dec 13 2005


  • Adult sex ratio
  • Lacerta vivipara
  • Male behavior
  • Population extinction
  • Sexual coercion

ASJC Scopus subject areas

  • General


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