Does (co-)variation between boldness and growth differ between sexes? A nuanced and powerful evaluation of the pace of life hypothesis.

A major goal of behavioral ecology is identifying sources of among-individual variation in behavior. Theory suggests that life history traits and personality should be correlated at the individual level, but despite much attention, it has little support. A potentially crucial aspect influencing life history and personality associations that has been overlooked is sex. Males and females of many species differ in energy requirements, energy allocation, and behavior, suggesting that sex may affect correlations between life history and personality differently; if so, not accounting for sex differences may obscure relationships. We address this possibility by intensive longitudinal sampling of boldness and growth in 66 male and 92 female pillbugs (Armadillidium vulgare) (total N = 158) using novel statistical methods to estimate sex-specific (co-)variances in a single parsimonious model. We found that females and males had similar mean boldness, similar among-individual variation in boldness and in growth rate, but females grew faster than males. We found support for a positive correlation between boldness and growth in males but no correlation in females. Differences in personality-life-history correlations between females and males may be linked to sex-specific reproductive strategies. Given the ubiquity of sex differences in life history and behavior, this sex-specific approach to studying trait integration is likely to provide novel insights in many systems that may have gone unnoticed.