Montane and highland elevations (> 2000 m a.s.l.) represent consistently harsh environments for small endotherms because of abiotic stressors such as cold temperatures and hypoxia. These environmental stressors may limit the ability of populations living at these elevations to respond to biotic selection pressures — such as parasites or pathogens — that in other environmental contexts would impose only minimal energetic- and fitness-related costs. To investigate the fitness and aerobic performance related costs of biotic stress on high-elevation adapted populations, we collected tissue and aerobic metrics associated with infections by botfly larvae (Cuterebridae) in deer mice (Peromyscus maniculatus rufinus) living along two elevational transects (2300 – 4400 m) in the Colorado Rockies.
Results/Conclusions
We found no evidence of infection by botfly larvae at elevations above 2400 m. However, below 2400 m, 34% of all deer mice, and 52% of adults sampled were infected. Botfly infections were associated with reduced hematocrit levels (-23%), hemoglobin concentrations (-27%), cold-induced summit metabolic rates (Msum, -19%) and metabolic scope (-26%) compared to uninfected individuals. In contrast to studies at lower elevations, these reductions in aerobic performance appeared to influence fitness, as infected individuals suffered 19 – 34% lower daily survival rates. Our results thus indicate that botfly infections may detrimentally influence the aerobic capabilities and fitness of deer mice living at elevations between 2000 – 2400 m. Therefore, it is likely that the interaction between botflies and small rodents is likely highly context-dependent. These data also suggest that high-elevation populations may be more susceptible to additional biotic selection pressures than lowland conspecifics. More generally, populations inhabiting extreme environments may soon be pushed to face new conditions to which they must rapidly respond, but have little time or capacity to do so.