Early life experiences have a disproportionate impact on individuals’ fitness, but most research has focused on the effects of single experiences, or stressors, often under controlled conditions. Protecting natural populations that must contend with co-occurring stressors requires a better understanding of how multiple early-life stressors affect the health and ecology of natural systems. However, the complexity of such research has limited its advancement. To address this challenge, human studies adopted cumulative risk models that predict adult health risk based on early adversity exposure. We propose a novel framework on how to adapt such models to assess the health and ecology of natural populations. In this framework we detail how and when to develop various types of cumulative early adversity (CEA) indices for wild populations. We then use a case study to demonstrate that such indices can predict pup survival and adult longevity in a wild population of yellow-bellied marmots (Marmota flaviventer). Our results highlight that CEA indices yield unique insights and improve model fit. With this framework we hope to spur further investigations on the impact of cumulative adversity in natural systems, which is critical to inform conservation and management in the Anthropocene.

With global climates changing rapidly, animals must adapt to new environmental conditions with altered weather and phenology. Key to adapting to these new conditions is adjusting the timing of reproduction to maximize fitness. Using a long-term dataset on a wild population of yellow-bellied marmots (Marmota flaviventer) at the Rocky Mountain Biological Laboratory (RMBL), we investigated how the timing of reproduction changed with changing spring conditions over the past 50 years. Marmots are hibernators with a four-month active season. It is thus crucial to reproduce early enough in the season to have time to prepare for hibernation, but not too early so as snow cover prevents access to food. Importantly, climate change in this area has increased spring temperatures by 5 oC and decreased spring snowpack by 50 cm over the past 50 years. We evaluated how female marmots adjust the timing of their reproduction in response to the changing conditions and estimated the importance of both genetic variance and plasticity in the variation in this timing. We showed that, within a year, the timing of reproduction is not as tightly linked to the date a female emerges from hibernation as previously thought. We reported a positive effect of spring snowpack but not of spring temperature on the timing of reproduction. We found inter-individual variation in the timing of reproduction, including low heritability, but not in its response to changing spring conditions. There was directional selection for earlier pup emergence date since it increased the number and proportion of pups surviving their first winter. Taken together, the timing of marmot reproduction might evolve via natural selection, however, plastic changes will also be extremely important as long as plasticity is not limited. Further, future studies on the marmots should not operate under the assumption that females reproduce immediately following their emergence.

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.