Knowledge of feeding habits not only contributes information related to the resources that individuals need to survive and breed, but also leads to insights about the interactions between a species and its environment. The optimal foraging theory explains diet selection by several attributes of the trophic scenario such as availability, diversity, quality and predictability of food. Other models propose that digestive tract mass increases under high metabolic demands by cold, affecting food choice and intake. Thus, diet selection emerges as a behavioural trait shaped by intrinsic and extrinsic factors. The goal of our study was to determine variation in the trophic scenario among sites at different elevations, as well as variation in phenotypic traits relevant to the nutritional and energy balance in Phyllotis xanthopygus. This small rodent is widely distributed along the Andes Mountains. We assessed diet selection and digestive tract size in individuals collected at three elevations across its distribution range. Results on dietary proportion of specific trophic categories (green parts, fruits and arthropods) showed that P. xanthopygus alternates between omnivory and granivory/frugivory. Richness, diversity and quality of the available resources evidenced a relatively low-quality trophic scenario at high altitude. Nevertheless, the diets built in by animals from diverse altitudes lacked differentiation in quality or diversity. P. xanthopygus seems to behaviourally compensate environmental variation to cope with nutritional requirements, by changing diet composition and proportion of items included. The resultant uniform diet quality is consistent with the absence of variation in the gastrointestinal tract size. Considering the spatial variability and seasonality of the region, a behavioural response is probably the most convenient strategy to overcome short-term environmental heterogeneity. In a plastic species such as P. xanthopygus, behaviour is a fundamental aspect to take into account by predictive models in the forecasts of climate change effects on biological diversity.

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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.

The study of phenotypic variation within species in response to different environments is a central issue in evolutionary and ecological physiology. Particularly, ambient temperature is one of the most important factors modulating interactions between animals and their environment. Phyllotis xanthopygus, a small Andean rodent, exhibits intraspecific differences along an altitudinal gradient in traits relevant to energy balance that persist after acclimation to common experimental temperatures. Therefore, we aim to explore geographic variations in energetic traits of P. xanthopygus and to assess the contribution of phenotypic plasticity to population differences. We compared metabolic rate and thermal conductance in response to different acclimation temperatures in animals collected at distinct altitudes (F0 generation) and in their offspring, born and raised under common-garden conditions (F1 generation). We found intraspecific differences in resting metabolic rate (RMR) of animals collected at different altitudes that were no longer evident in the F1 generation. Furthermore, although both generations showed the same pattern of RMR flexibility in response to acclimation temperature, its magnitude was lower for the F1 individuals. This suggests that developmental conditions affect the short-term acclimation capacity of this trait during adulthood. On the other hand, thermal conductance (C) showed irreversible plasticity, as animals raised in the laboratory at stable warm conditions had a relatively higher C than the animals from the field, showing no adjustments to thermal acclimation during adulthood in either group. In sum, our results support the hypothesis that the developmental environment shapes energetic traits, emphasizing the relevance of incorporating ontogeny in physiological studies.

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