Environmental sensitivity in relation to size and sex in birds: meta-regression analysis.

Studies of animals often report a greater sensitivity of one sex to poor rearing environments. However, it is unclear whether size differences associated with sex, sex itself, or other factors are responsible for differences in performance. While the greater nutritional requirement of the larger sex is a plausible explanation for increased sensitivity (i.e., size-dependent vulnerability), other hypotheses suggest that size-independent traits may have effects on the fitness of offspring (i.e., sex-dependent vulnerability). For example, the heterogametic sex may be more vulnerable to expression of sex-linked recessives in poor environments, or sex-specific phenotypes may have negative effects (e.g., increased testosterone in males). We examined support for these hypotheses through the use of meta-analytic techniques based on the published literature on avian species. Our results revealed small, nonsignificant effects for hypotheses of size- and sex-dependent susceptibilities alone. Application of a multivariate meta-analytic technique (meta-regression) suggests a joint influence of sexual size dimorphism and clutch size in explaining sex-specific patterns of vulnerability. These findings suggest that none of the proposed hypotheses tested here on their own can sufficiently explain the observed patterns and that additional factors must be considered in order to explain the diversity of patterns of sex-specific sensitivity observed in the literature.