Plasticity of body size and mass in response to environmental changes in a hibernating mammal
Body size and mass are key fitness-related phenotypic traits. Most studies have focused on either size or mass, although one is a monotonic measure resulting from energetic trade-offs during the growth period, whereas the other is a dynamic measure resulting from variable energy storage throughout life. As such, these traits can respond differentially to environmental constraints. The evolutionary mechanisms and ecological factors driving the dynamics of these traits are of particular interest in order to predict organisms’ resilience to environmental changes; however, they remain poorly understood.
Using a 22-year (1991-2013) dataset including 3586 captures of 1329 individuals, we investigated annual changes in body parameters in a hibernating mammal, the Alpine marmot (marmota marmota), in response to vegetation onset, summer aridity and winter conditions (i.e snow depth and temperature). As energetic constraints differ across life stages, we evaluated stage-specific trait variation. For juveniles, response variable was body size and mass at weaning. For subsequent age classes, response variable was body growth and fattening (i.e changes in size and mass from one year to the next). The aim of our study was to (i)quantify temporal trends, (ii)identify the evolutionary and ecological components of phenotypic change, and (iii)determine the environmental drivers of trait variation.
Over the study period, our analyses revealed a significant decrease in juvenile body size (-4.1%), and yearling and subadult body growth (-4.1%). Neither juvenile body mass nor body fattening at older age class changed over time (except for subadults; +4.3%). Fluctuations of both traits were mostly plastic rather than a response to selection. In all age classes, body growth and fattening were both positively associated with vegetation onset. Summer aridity negatively affected the body fattening of juveniles, yearlings and adults. Deep winter snow pack caused slower fattening, possibly due to late snow melt. Surprisingly, winter temperature did not have any effect. Temporal variations in body fattening were mostly explained by vegetation onset (32±14%) followed by summer aridity (15±7%) and snow depth (6±3%).
Our results demonstrate the complex ways in which environmental effects on phenotypic traits vary markedly across age classes. Body growth was only driven by spring conditions, whereas other environmental factors also affected body fattening. This observed phenotypic plasticity may allow alpine marmots to respond fast enough to rapid environmental changes. Our study provides a mechanistic insight into a hibernating mammal’s resilience to climate change, which will in turn allow reliable forecasting of population dynamic responses.