Repeatability and reproductive consequences of boldness in female gray seals

Wild animals show consistent individual variation in behavior across time and/or contexts, now referred to as animal personality. While this variability may have important ecological and evolutionary implications, how and why variation in animal personality is maintained in a natural population remains unclear. In this study, we assessed the influence of environmental and biological sources of variation on behavioral responses measured along the shy-bold continuum in a long-lived, iteroparous marine mammal, the gray seal (Halichoerus grypus). Between 2008 and 2016, 469 females from the Sable Island, Nova Scotia breeding colony of gray seals were given a boldness score in response to a human approach, designed to stimulate maternal defense of offspring. Using generalized linear mixed-effects models (GLMM) in a Bayesian framework, we show that boldness is highly repeatable between and within years. There were age differences in boldness, with younger females being less bold than older, more experienced females providing some support for the life history trade-off hypothesis. We further used GLMMs to assess sources of variation on offspring weaning mass. We found that young females that were bolder produced heavier pups than shyer counterparts, and that pups produced by bolder females were on average ~ 2 kg heavier than pups of shy females. These results provide further evidence that personality influences life history strategies, and illustrates the evolutionary potential of animal personality in response to selection. Significance statement Consistent individual differences in behavior influence various aspects of ecology including species interactions, species distributions, and life history strategies. However, how and why this individual variation is maintained in a natural population remains uncertain. In this study, we assessed the influence of boldness, specifically maternal defense of offspring, on a component of reproductive success in a long-lived marine mammal. We showed highly repeatable behavioral differences, and found that boldness varied with age, with younger individuals being less bold than older individuals. Younger individuals that were bolder produced heavier offspring than shyer counterparts. Our study contributes to an under-represented group of animals, wild marine mammals, in the personality literature, and further prompts the investigation into the proximate and ultimate factors influencing personality in an ecologically important marine predator.