RELATING AMBIENT AND BODY TEMPERATURE IN FREE-RANGING MOOSE: IMPLICATIONS TO HEAT STRESS AND SURVIVAL IN MINNESOTA

Abstract

Climate change, or more explicitly heat stress, has been implicated as a driver of the rapid decline to Minnesota’s moose (Alces alces) population over the past 20 years. While often inferred that moose become heat stressed when ambient temperature exceeds thermal thresholds derived from captive moose, few studies provide physiological data to support that wild moose experience heat stress. Our study goals were to measure and explore relationships between ambient temperature and body temperature (T_b) of moose and evaluate their potential influence on heat stress and survival. We obtained continuous measurements of internal T_b of wild moose (n = 41; 23 females, 18 males) from 2013–2017 with mortality implant transmitters (MIT). We examined how frequently moose experienced ambient temperatures above reported upper critical temperatures (thresholds) in winter and summer that cause increased metabolism and panting. Moose often experienced days when ambient temperature was above all thresholds during summer (49.3–92.5% of summer days) and winter (36.3–78.5% of winter days). The percentage of days when a moose exhibited above normal T_b (≥ 39.17 °C) varied significantly between seasons, with conditions most likely to exceed the thresholds during summer (44–51% of summer days) but not winter. We found maximum daily T_b increased significantly with increasing maximum daily ambient temperature in summer. Predictions from our models suggest that moose in summer may experience elevated T_b, potentially indicative of heat stress, at maximum daily temperatures > 25 °C. We found T_b was most often higher in the evenings and overnight, as 76% of hot T_b occurred between 18:00–6:00 hr. The duration a moose maintained an elevated internal T_b was highly variable (mean = 32 min, range = 5 to 1,065 min). We also found that moose survival was related to the number of hot moose events (HME) they experienced on an annual basis. Moose that died (n = 14) had 2.0–2.8 x higher average HME per day than survivors over the course of a year. Our findings highlight the need for physiological data to support behavioral observations related to how endotherms respond to ambient temperature changes. Presumably, moose adopt behavioral tradeoffs in summer to mitigate heat stress that may reduce overall fitness and survival.