Sexual selection and sex ratios in Anolis lizards

Abstract

Biased sex ratios, or unequal numbers of males and females in a population, can alter the intensity of sexual selection by enhancing competition for mates, and thus may affect the evolution of reproductive strategies. Studies of sex ratio variation across a clade provide an opportunity to examine the morphological or behavioral consequences of different levels of sexual selection. We examined sex ratio variation, using phylogenetic comparative methods, across a diverse clade of terrestrial vertebrates, the Anolis lizards. Across a sample of 14 species in 21 localities, we found remarkable bidirectional variation in sex ratios across the Anolis clade: males are more common in some populations and females in others. However, we find no evidence that sex ratio bias is associated with sexual size dimorphism, a proxy for sexual selection. Nor do we find an association of sex ratio bias with ecological niches (i.e. ecomorphs), which vary in sexual selection pressures and mating systems. The observed inter-specific variation in sex ratio suggests that a balance of different, possibly opposing, factors, including sex-specific dispersal and mortality rates, may play a role in determining population sex ratio.

Intensity of male‐male competition predicts morph diversity in a color polymorphic lizard

“…We found substantial variation in adult sex ratio among populations, in agreement with findings for other lizards (e.g., Schoener and Schoener ; Muralidhar and Johnson ; for lacertids see Capula et al. ; Galán ; Le Galliard et al.…”

“…We found substantial variation in adult sex ratio among populations, in agreement with findings for other lizards (e.g., Schoener and Schoener 1980;Muralidhar and Johnson 2017; for lacertids see Capula et al 2002;Galán 2004;Le Galliard et al 2005) and other taxa (Cockburn et al 1985;Donald 2007;Székely et al 2014). Although only a small portion of populations diverge significantly from an unbiased sex ratio (likely due to the small sample sizes), even nonsignificant deviations from the predicted 1:1 sex ratio may reflect a biologically meaningful phenomenon (see Donald, 2007;Muralidhar and Johnson 2017). The morph composition also varies among populations, with about half the sampled populations showing different numbers of morphs for males and females.…”

“…). Although only a small portion of populations diverge significantly from an unbiased sex ratio (likely due to the small sample sizes), even nonsignificant deviations from the predicted 1:1 sex ratio may reflect a biologically meaningful phenomenon (see Donald, ; Muralidhar and Johnson ). The morph composition also varies among populations, with about half the sampled populations showing different numbers of morphs for males and females.…”

It’s Not Easy Being Green: Behavior, Morphology, and Population Structure in Urban and Natural Populations of Green Anole (Anolis carolinensis) Lizards

“…For example, Schoener and Schoener (1980) found that sex ratios and population densities differed among populations with different habitat types in four species of Bahamian anoles. Muralidhar and Johnson (2017) reported similar intraspecific variation in sex ratio in some species of Caribbean anoles. Furthermore, Schoener and Schoener (1980) also present a model predicting that numbers of females should vary more than numbers of males within populations over time, such that denser populations should have greater numbers of females.…”

“…Following a recent study of sex ratios in anoles by Muralidhar and Johnson (2017) I calculated sex ratio as the proportion of males among all adults in the population. I calculated population density as density of adult lizards (lizards/m 2 ), and as densities of each sex/age class.…”