Identifying the ecological factors that shape the evolution of life histories has been a topic of great interest and research for several decades. Populations of the same species can express important differences in life history traits, and such variability is often apparent when comparing populations along an elevational gradient, where significant differences in phenology, reproduction, survival, and growth exist. Higher elevations are typically characterized by a shorter growing season, lower temperatures, and limited resources. High-elevation individuals must allocate limited resources among growth, immune function, and reproduction, ultimately shaping a different life history strategy than that of populations at lower elevations. However, trade-offs between physiology and life history traits have rarely been studied along elevational gradients. To address this key knowledge gap, we quantified trade-offs between body mass dynamics, reproduction, and immunocompetence in two populations of Uinta ground squirrels (Urocitellus armatus, UGS) that experience different phenologies along a 600 m. elevation gradient. We recorded phenology, body mass, and reproductive condition; blood samples were collected and analyzed using a bacterial killing assay to assess innate immune function throughout the active season.
Higher elevation (HE) yearlings and adults emerged from hibernation at lower mass than lower elevation (LE) individuals, but compensated by increasing their investment in growth, entering estivation at the same mass as LE individuals. This compensatory growth mechanism came at a cost, as immunocompetence decreased more rapidly throughout the season in individuals with a higher growth rate, indicating a trade-off between growth and immunity induced by a shorter season and limited access to resources. HE yearling females were less likely to reproduce than LE yearling females, also suggesting a trade-off between growth and reproduction. Interestingly, LE yearling females gained less mass, possibly due to the energetic cost of reproduction. Although all juveniles experienced similar growth rates and immunocompetence over the season. HE juveniles emerged from their natal burrows at a lower mass than LE individuals, and remained lighter through estivation. Further research will help determine whether growth rates are plastic between years, since the length of the active season varies annually with snowpack. Such trade-offs could ultimately affect survival and population growth, and thereby dictate the ability of UGS populations to adapt to an increasingly variable climate.