Body mass is often viewed as a proxy of past access to resources and of future survival and reproductive success. Links between body mass and survival or reproduction are, however, likely to differ between age classes and sexes. Remarkably, this is rarely taken into account in selection analyses. Selection on body mass is likely to be the primary target accounting for juvenile survival until reproduction but may weaken after recruitment. Males and females also often differ in how they use resources for reproduction and survival. Using a long-term study on yellow-bellied marmots (Marmota flaviventer), we show that body mass was under stabilizing selection in the first years of life, before recruitment, which changed to positive directional selection as age increased and animals matured. We found no evidence that selection across age-classes on body mass differed between sexes. By investigating the link between running speed and body mass, we show that the capacity to escape predators was not consistent across age classes and followed a quadratic relationship at young ages only. Overall, our results indicate that mature age classes exhibit traditional patterns of positive selection on body mass, as expected in a hibernating mammal, but that mass in the first years of life is subject to stabilizing selection which may come from additional predation pressures that negate the benefits of the largest body masses. Our study highlights the importance to disentangle selection pressures on traits across critical age (or life) classes.
Humans currently occupy all continents and by doing so, modify the environment and create novel threats to many species; a phenomenon known as human-induced rapid environmental changes (HIREC). These growing anthropogenic disturbances represent major and relatively new environmental challenges for many animals, and invariably alter selection on traits adapted to previous environments. Those species that survive often have modified their habitat or their phenotype through plasticity or genetic evolution. Based on the most recent advances in this research area, we predict that individuals with highly plastic capacities, those that are generally shy, with high cognitive abilities and stress responses – in other words, individuals displaying a reactive phenotype – would better perform in human-modified landscapes than their counterparts’ proactive phenotypes. Moreover, we hypothesize that when human presence reduces predation, this decouples commonly associated traits resulting in a new range of phenotypes, with individuals characterized by low aggressiveness and physiological stress responses but high boldness, cognitive abilities and plasticity. We coin these individuals as “preactive”, being part proactive and part reactive. While supported by some studies, demonstrating the existence of this new coping style will require additional multivariate studies investigating behavioral and physiological responses to multiple challenges in HIREC impacted species.