Supplementary material from "Biologging in a free-ranging mammal reveals apparent energetic trade-offs among physiological and behavioral components of the acute phase response"

The acute phase response (APR) is an adaptive emergency life-history stage, wherein vertebrates exhibit fever and anorexia to survive an infection. However, induced immune responses are energetically costly and sick animals may reduce physical activity to compensate. Tests of this predicted energetic trade-off in free-ranging animals are rare due to difficulties in measuring individual physiology and behavior under immune challenge in natural settings. However, recent advances in biologging technology now make such studies possible. We surgically implanted heart rate/temperature loggers in free-ranging adult male Arctic ground squirrels, fitted the squirrels with collar-mounted accelerometers and light/temperature loggers, and injected animals with lipopolysaccharide (LPS) to simulate an immune challenge. LPS-injected squirrels exhibited ~1 °C overnight fevers accompanied by slightly elevated (~10 bpm) heart rates; LPS-injected squirrels also spent ~20% less of their time aboveground the following day and reduced overall movement by ~40% compared to saline-injected controls. Thus, we found support for an energetic trade-off between functional immune responses (fever and anorexia) and lethargic sickness behavior within the APR of a free-ranging mammal. Moreover, our results suggest animal-borne devices can play an important role in future studies of vertebrate immunity and disease dynamics.