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

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.adult sex ratio ͉ Lacerta vivipara ͉ male behavior ͉ population extinction ͉ sexual coercion

Sex‐biased breeding dispersal is predicted by social environment in birds

“…This hypothesis is supported by experimental studies, which indicate that male aggression toward females increases as a result of increased male–male competition in populations with male‐biased ASRs (Chapman, Arnqvist, Bangham, & Rowe, 2003; Le Galliard, Fitze, Ferrière, & Clobert, 2005), and this may be the main driver of female dispersal in some butterfly populations (Hovestadt & Nieminen, 2009). This process is hypothesized to be the evolutionary outcome of a sexual conflict over mating and reproduction tactics, resulting in adaptations that benefit males (in the short term) but not females (Le Galliard et al., 2005).…”

“…This hypothesis is supported by experimental studies, which indicate that male aggression toward females increases as a result of increased male–male competition in populations with male‐biased ASRs (Chapman, Arnqvist, Bangham, & Rowe, 2003; Le Galliard, Fitze, Ferrière, & Clobert, 2005), and this may be the main driver of female dispersal in some butterfly populations (Hovestadt & Nieminen, 2009). This process is hypothesized to be the evolutionary outcome of a sexual conflict over mating and reproduction tactics, resulting in adaptations that benefit males (in the short term) but not females (Le Galliard et al., 2005). Although sexual coercion is recognized as one of the key forces of sexual selection along with mate choice and mate competition and seems to be widespread in invertebrates, female harassment is known to exist in only a limited set of taxa in birds, for instance in waterbirds (Black, Choudhury, & Owen, 1996; McKinney, 1986) and passerines (Westcott, 1997).…”

“…For instance, Le Galliard et al. (2005) manipulated a population of common lizards ( Lacerta vivipara ) and showed that male mortality and emigration rates were not higher under male‐biased ASR, in contrast to the expectation. Similarly, Trochet et al.…”

Local and neighboring patch conditions alter sex‐specific movement in banana weevils

“…To our knowledge, only two experimental studies, one with the lizard Lacerta vivipara (Le Galliard et al. 2005) and the other with the butterfly Pieris brassicae (Trochet et al. 2013), have focused on the effect of variation in sex ratio on adult movement.…”

“…To our knowledge, only two studies have assessed the movement of adults in response to manipulated sex ratio under seminatural conditions (Le Galliard et al. 2005; Trochet et al. 2013), and the authors of these studies concluded that the adult sex ratio is regulated by a balance between intra‐ and intersexual competition rather than by mate availability as suggested by theory (Perrin and Mazalov 2000).…”