Abstract
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 body mass and annual survival in yellow‐bellied marmots (Marmota flaviventer), we show that body mass was under stabilizing viability 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 viability 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 viability selection on body mass, as expected in a hibernating mammal, but that mass in the first years of life is subjected 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.
Highlights
Body mass is considered a key life-history trait in many taxa (Roff, 2001) since it reflects past and present access to resources but it is an important determinant of survival and reproductive success (Kingsolver & Pfennig, 2004)
We showed that in yellow-bellied marmot's patterns of viability selection on body mass, a key life-history trait, varied across life stages and that the magnitude of the selective pressure acting on the trait differed across elevations
We showed that the relationship between mass and maximum running speed varied across age classes
Summary
Body mass is considered a key life-history trait in many taxa (Roff, 2001) since it reflects past and present access to resources but it is an important determinant of survival and reproductive success (Kingsolver & Pfennig, 2004). For neonates and juveniles, where the emphasis in the earliest life stages is on rapid growth including fat and somatic gain (Campbell & Dobson, 1992), the expectation is that selection would favor large, fast-growing individuals who are precocial at an earlier age Mass, at these ages, is important for survival, and recruitment into the population, in the following years (Both et al, 1999). Yellow-bellied marmots depend largely on body reserves accumulated during the summer season for survival over the harsh winter season and, as a consequence, they undergo strong circannual fluctuations in mass where they can more than double their mass during the summer (Armitage, 2014) These animals exhibit growth and body mass changes at different life stages, supporting the idea that the selective pressures regulating this trait may be stage-dependent. That age-related variation in selection will be reflected by variation in the relationship between body mass and speed at different life stages, as similar pressures are expected to regulate maximum running speed
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