Geographic variation in life-history traits: factors shaping energy investment in reproduction in a highland dwelling rodent

Abstract

Variation in life-history traits is shaped by intrinsic and extrinsic factors. Climate influences the availability of energy and nutrients in the environment, and thus animals’ energy balance, their allocation of resources to reproduction and ultimately, fitness. Temperature is reported as a prevailing influence on animals’ investment in reproduction, and diverse hypotheses propose mechanisms by which it occurs. Central and external limits to energy acquisition have been traditionally assumed. Hence, digestive limitations, periods of low food availability or demanding cold conditions would restrict this input. However, under warm conditions endotherms also face the need to dissipate heat from metabolic processes. From this perspective, a high metabolic rate process, such as lactation, would be restricted under increasing ambient temperatures (Ta). Our research addresses the variation in reproductive parameters in Phyllotis xanthopygus, a rodent species broadly distributed in the west highlands and southern parts of South America. Across a pronounced elevation gradient in the central Andes Mountains we found that litter size is comparatively smaller in pregnant females collected at lower elevations. This is congruent with expectations from the heat dissipation limit hypothesis. To disentangle the role of temperature on this pattern, we bred individuals under two thermal treatments. We recorded no changes in the litter size, but pups’ body mass was lower at higher Ta during lactation, also in agreement with decreased metabolism. Offspring size and number are relevant to a species’ fitness. The evidence suggests that in P. xanthopygus these traits vary geographically and are shaped by Ta in the laboratory. A species that evolved in high altitudes, inhabits cold and temperate areas and is sensitive to Ta increases in the laboratory, appears as an excellent candidate to further explore the ecophysiology-thermal landscape interface, crucial to develop accurate predictive models of biodiversity dynamics.