Heart rate biologgers reveal individual-specific energy landscapes for mule deer

An important, yet often overlooked component of animal movement behaviour is the physiological cost incurred by animals when travelling through an environment, i.e., the “energy landscape”. These include the costs related to movement rate and elevation gain, as well as the mediating influence of variation in cover types and substrate. These costs are likely to be mediated by differences among individuals, with body condition and age likely to modify energy expenditure. Using subcutaneous biologgers paired with GPS collars (n = 37), we recorded high-resolution heart rate data from free-ranging mule deer (Odocoileus hemionus) across three study sites in Utah, USA. We generated energy landscapes for mule deer by combining heart rate data with the attributes of movement steps derived from GPS locations to quantify how movement and landscape characteristics influenced heart rate, a proxy for energy expenditure. Overall, heart rates were higher in summer than in winter. Energy landscapes for mule deer were driven by step length and elevation gain, which increased deer heart rates. We also found a pronounced effect of movement state (encamped versus travelling) on heart rates, suggesting that initiating movement resulted in a larger increase in heart rate compared to distance travelled once movement was initiated. Deer in poorer body condition had an increased average heart rate in winter, as did heavier deer and younger deer. We found a novel trade-off in deer step selection, where deer were more likely to take steps that were energetically costly (associated with an elevated heart rate) when those steps had higher forage quality, suggesting a mediating influence of the energy landscape on forage selection by mule deer. Our study highlights the value of quantifying energy landscapes in the field, by revealing how individual energy landscapes can drive movement behaviour decisions and a novel trade-off between energy expenditure and forage quality.