111 RELATING AMBIENT AND BODY TEMPERATURE IN FREE-RANGING MOOSE: IMPLICATIONS TO HEAT STRESS AND SURVIVAL IN MINNESOTA Michelle Carstensen1, Véronique St-Louis2, and Andrew Tri3 1Wildlife Health Program, Minnesota Department of Natural Resources, Forest Lake, Minnesota, 55025, USA; 2Wildlife Biometrics Unit, Minnesota Department of Natural Resources, Forest Lake, Minnesota, 55025, USA; 3Forest Populations and Research Group, Minnesota Department of Natural Resources, Grand Rapids, Minnesota, 55744, USA. 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 expe- rience heat stress. Our study goals were to measure and explore relationships between ambient tem- perature and body temperature (Tb) of moose and evaluate their potential influence on heat stress and survival. We obtained continuous measurements of internal Tb 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 win- ter (36.3–78.5% of winter days). The percentage of days when a moose exhibited above normal Tb (≥ 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 Tb increased significantly with increasing maximum daily ambient temperature in summer. Predictions from our models suggest that moose in summer may experience elevated Tb, potentially indicative of heat stress, at maximum daily temperatures > 25 °C. We found Tb was most often higher in the evenings and overnight, as 76% of hot Tb occurred between 18:00–6:00 hr. The duration a moose maintained an elevated internal Tb 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 observa- tions 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. ALCES VOL. 59: 111 – 134 (2023) Key Words: Alces alces, ambient temperature, Tb, climate change, heat stress, Minnesota, moose, mortality implant transmitter, summer, survival, thermoregulation, threshold, winter Moose (Alces alces) are an iconic, cold- adapted species associated with boreal forest habitats, and considered threatened by cli- mate change across their range in North America and worldwide (Karns 2007, Dou et al. 2013, Monteith et al. 2015, Weiskopf et al. 2019). Population decline in portions of their southern historic range (Jensen et al. 2018) is associated with numerous climate and non-climate related factors including predation, parasites, forest management, hunting, and habitat loss (Murray et al. 2006, HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 112 2012, Mech and Fieberg 2014, Hasbrouck et al. 2020, Wittische et al. 2021, Marrotte et al. 2022, Peterson et al. 2022). Moose in northwestern Minnesota are nearly extir- pated, and the northeast population has declined by 60% since 2006 (Giudice 2023). Climate change and more explicitly, heat stress, has been implicated (through correla- tion) as a primary cause of the Minnesota moose decline (Murray et al. 2006, Lenarz et al. 2009, 2010). This inference is based upon comparing increasing regional ambient tem- peratures to the upper critical temperature thresholds measured in 2 captive moose (Renecker and Hudson 1986); however, direct extrapolation of these measurements as an operative temperature, thermal stress, or survival threshold of free-ranging moose is overly simplistic. These studies lack any physiological or empirical data to support that heat stress occurs at a level sufficient to evoke an individual mortality response or population decline. Heat stress is a medical term that describes a physiological condition in which both body temperature (Tb) and heart rate increase beyond the normal range for a spe- cies, leading to acute changes in energy bal- ance and metabolism and, if chronic, increased incidence of disease and poor health (Wills 2016, Collier et al. 2017). At normal Tb of 38.4–38.9 °C (Franzmann et al. 1984), moose employ behavioral thermoregulation by using cover and microhabitats to mediate radiant temperature and access/avoid wind and pre- cipitation, and shift activity and energy bud- gets (McGraw et al. 2012, McCann et al. 2016, Street et al. 2016). Although limited in animal sample size and fasted animals, two studies provide upper critical temperature thresholds of moose. Renecker and Hudson (1986) found increased metabolism and pant- ing in moose at -5.1 and -2.2 °C in winter and 14 and 20 °C in summer. McCann et al. (2013) later measured a summer threshold of 17 °C without wind and 24 °C in constant 11.3 mph wind from observations of panting. Again, extrapolating critical temperature thresholds to free-ranging moose across northern latitudes where animals commonly seek thermal refugia in diverse microhabitats is questionable (Lowe et al. 2010, Olson et al. 2014, Street et al. 2015). Recently, Thompson et al. (2020a) measured Tb of semi-captive moose in Alaska to test the critical threshold of 14 °C in summer and found no evidence of a heat stress response. Rather, Tb was most influenced by the interaction of ambient tem- perature and vapor pressure, and daily weather patterns influenced physiological and behavioral responses to dissipate heat. Further, Thompson et al. (2019) found a daily rhythm in summer Tb that ranged 0.9 °C; Tb was concentrated from 37–39 °C year-round. Recent advancements in biotechnology allow repeated measure of Tb in ruminants, including moose (Signer et al. 2010, Herberg et al. 2018, Thompson et al. 2019, Græsli et al. 2020 a, b). Here, we are the first to deploy a rumen bolus, or mortality implant transmitter (MIT), to a large number of free-ranging moose in North America. The bolus was designed originally to detect heart activity and provide instantaneous notification of death through the animal’s paired radio-collar; how- ever, it is also capable of recording Tb. Herberg et al. (2018) determined the device to be highly accurate at estimating core Tb, providing the opportunity to evaluate how wild moose respond to warm environmental conditions. Our goal was to explore the relationship between ambient temperature and Tb of moose with 4 specific objectives: 1) deter- mine if Tb of moose is related to ambient tem- perature, 2) determine if wild moose have elevated Tb when exposed to ambient temperatures above critical temperature thresholds reported in captive moose, 3) determine if moose experience above normal Tb and if so, describe the timing and duration ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 113 of these events, and 4) assess if survival is impacted by heat stress. METHODS Study Area and Moose Capture In 2013–2015, we captured moose (>1 year- old, n = 173) within a 3,733 km2 study area located between 47° 12’N and 47° 95’N lati- tude and 90° 33’W and 91° 72’W longitude in northeastern Minnesota (Fig. 1) as part of a survival and cause-specific mortality study (see Carstensen et al. 2014, 2017). This region has been classified as Northern Superior Upland (Minnesota Department of Natural Resources [MNDNR] 2015) and includes bogs, swamps, lakes, and streams with lowland stands of northern white cedar (Thuja occidentalis), black spruce (Picea mariana), and tamarack (Larix laricina), and upland stands of balsam fir (Abies balsamea), jack pine (Pinus banksiana), white pine (P. stro- bus), and red pine (P. resinosa); trembling aspen (Populus tremuloides), white birch (Betula papyrifera), and conifers are inter- mixed. White-tailed deer (Odocoileus virgin- ianus) occur throughout the study area, with wolves (Canis lupus) and American black bears (Ursus americanus) the primary preda- tors of moose and deer (Fritts and Mech 1981, Nelson and Mech 1986). All moose were fitted with GPS-Iridium satellite collars (Vectronic Aerospace GmbH; Berlin, Germany) to monitor daily survival and record locations at ~ 4-h intervals. The Fig. 1. Capture locations of 41 free-ranging moose implanted with internal temperature loggers from 2013–2017, in northeast Minnesota, and the spatial distribution of 8 Remote Automatic Weather Stations. HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 114 MITs (Vectronic Aerospace GmbH) were placed orally into the reticulum of 41 animals (22 females, 19 males) in 2013 (n = 11 moose), 2014 (n = 13), and 2015 (n = 17) and were programmed to record Tb (0.1 °C accu- racy, Vectronic Aeorspace GmbH) at either 10- or 15-min intervals (Herberg et al. 2018, Minicucci et al. 2018). Age was determined by cementum annuli analysis (Matson’s Lab, Manhattan, Montana) of a lower incisiform canine tooth removed at capture (40 of 41 moose). Age varied from 1 to 16 years old (median = 6 years) within three age classes: young (≤ 3 years, n = 10), prime (4–8 years, n = 20), and old (≥ 9 years, n = 10). All MIT readings obtained 1 week post-capture and 1 week pre-death were excluded from analyses to reduce potential effects of capture stress and mortality on moose Tb (Thompson et al. 2020b). Moose were censored on the date their collar stopped transmitting data, regard- less of their survival state. Weather Data and Seasons Minnesota has a humid continental climate, characterized by hot summers and cold win- ters. Ambient temperature is as low as -51 °C in winter and as high as 45 °C in summer (MNDNR 2024), with daily maximum tem- peratures in summer typically 20 to 30 °C (see RESULTS). The moderating effect of Lake Superior keeps the northeast region relatively cooler in summer and warmer in winter com- pared to most of the state. The MNDNR assesses winter severity (1 November–31 May) with a Winter Severity Index (WSI) cal- culated by accumulating a daily score of 0–2 points: 1 point when daily temperature is ≤ −17.7 °C and 1 point when daily snow depth is ≥38 cm. Maximum WSI values in moose range across 6 winters (2012–13 to 2017–18) varied markedly, ranging from 35–160, 184–245, 54–152, 31–142, 50−159, and 50−179, respectively (MNDNR 2024). We used data from 8 weather stations located in Ely (ELOM5), Fernberg (FRNM5), Grand Marais (KCKC), Isabella (ISAM5), Seagull (SEAM5), Silver Bay (FBFW), Skibo (SKIM5), and Two Harbors (KTWM) (Fig. 1) that were part of the network of Forest Service Remote Automated Weather Stations (RAWS) (Zachariassen et al. 2003) within the study area. All stations operated for the entire study period except SKIM5 which became operable in April 2015. We summarized ambient temperature (measured 2 m above ground), precipitation, and wind speed that each moose potentially experienced at a given point in time by utiliz- ing the 4-h location data. We first generated time series that included the exact times of MIT readings (10- or 15-min resolution) for each animal and merged with the time series of the location data (1- or 4-h intervals). We approximated the location of moose at the time of the MIT readings by linearly interpo- lating the geographic locations (converted from lat-long to UTMs NAD83 Zone 15) between each 1- or 4-h interval. We acknowl- edge that moose do not move in a linearly fashion between time steps; however, this represents an approximation of the general location where a moose was at a given point in time. We assigned the nearest RAWS to each approximate location (at times of MIT recordings) by calculating the minimum Euclidean distance between a moose location and the 8 RAWS available within the study area. We then merged the weather station data to the nearest RAWS time series and used this as our best estimate of the weather conditions a given moose experienced at a given point in space and time. As moose move, the closest weather station may change and thus it was possible to have data from different weather stations for a given moose. The average distance between moose loca- tions and the nearest weather station at any given point in time varied between 5 and 36 ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 115 km for 40 of the moose. We recognize that microhabitats occupied by moose would not necessarily expose moose to wind speed or precipitation measured at the weather sta- tions; however, ambient temperature should be reasonably similar. Four seasons were set annually by aver- aging ambient temperature and snow depth data across the 8 RAWS as follows: fall began at the date of first frost with tempera- tures < 0 °C; winter began with snow depth consistently > 4 inches (10.16 cm); spring began when snow depth was consistently < 4 inches and included parturition; summer began as spring/parturition ended (Table 1). Parturition for females include 21 days pre- and post- of the mean parturition date reported by Severud et al. (2015). Identifying Heat Days and Hot Moose Events We categorized each ambient temperature value as being equal or above (Heat Threshold [HT] = 1) or below (HT = 0) the Renecker and Hudson (1986) thresholds of -2.2 and -5.1 °C in winter and 20 and 14°C in summer for panting (HT_RHp) and increased metabolism (HT_RHm), respec- tively. We also flagged any ambient tem- perature value in the summer that exceeded the thresholds of McCann et al. (2013) of 17 °C (HT_MCm_nowind) and 24 °C (HT_ MCm_wind) for increased metabolism in windless or windy conditions, respectively. We defined Heat Days (HTD) as days where at least one of the ambient tempera- ture values exceeded published thresholds shown to induce panting or increased metab- olism in summer and winter. We coded HTDs according to the same 4 thresholds: the Renecker and Hudson (1986) thresholds for increased metabolism (HTD_RHm) or panting (HTD_RHp) and the McCann et al. (2013) thresholds with (HTD_MC m_wind) and without wind (HTD_MCm_nowind). Normal Tb of moose ranges from 38.4 to 38.9 °C (101.1–102 °F; Franzmann et al. 1984); therefore, we assumed a moose to be above normal by adding 0.27 °C (0.5 °F) to the high end of the range. This small buffer also accounts for a slight bias of the MIT to be 0.03 °C lower, on average, than vaginal implant-derived Tb of moose (Herberg et al. 2018). We defined a “hot moose day” (HMD) as a day with at least one MIT-derived tem- perature ≥ 39.17 °C. In addition to “flag- ging” days as above or below these thresholds, we also compiled the maximum daily Tb for each moose, as well as the daily number of readings ≥ 39.17 °C. A hot moose Table 1. Seasonal start and end dates from 2013–2017 for moose in northeast Minnesota. Year Winter Start1 Winter End1 Spring Start2 Spring End2 Summer Start3 Summer End3 Fall Start4 Fall End4 2013 study started 4/24/2013 4/25/2013 6/2/2013 6/3/2013 10/14/2013 10/15/2013 12/2/2013 2014 12/3/2013 4/21/2014 4/22/2014 6/6/2014 6/7/2014 10/6/2014 10/7/2014 12/19/2014 2015 12/20/2014 3/10/2015 3/11/2015 6/1/2015 6/2/2015 10/9/2015 10/10/2015 12/18/2015 2016 12/19/2015 3/25/2016 3/26/2016 6/1/2016 6/2/2016 10/20/2016 10/21/2016 11/30/2016 2017 12/1/2016 2/24/2017 2/25/2017 6/2/2017 6/3/2017 10/14/2017 Study ended 1Start and end dates of winter were determined as the dates when the average snow depth across 10 snow depth stations was consistently more than 4 inches. 2Spring starts the first day after the end of winter and ends the last day of parturition (Severud et al. 2015). 3Summer starts the first day after the end of winter and ends a day prior to the start of fall4. 4Start of fall based on the date at which the average temperature across three National Oceanic and Atmospheric Administration (NOAA) weather stations falls below freezing (0°C) and ends a day prior to the start of winter1. HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 116 event (HME) was defined as ≥ 2 consecutive Tb ≥ 39.17 °C. If only one value was above the heat threshold, that HME event consisted of a single observation. We calculated the duration of a HME by subtracting the time- stamps between the start and end of the HME and added a correction factor of 5 or 7.5 min depending on the frequency of MIT- derived readings (e.g., 10- or 15-min inter- vals). Because we did not know exactly when Tb returned to normal within an inter- val, adding a small correction factor ensured that we did not have HME events that last 0 minutes (in the case when it is only 1 event), and accounted for that uncertainty. Although HME is a good indication that Tb was above normal, the type of HME can vary widely in both duration and magnitude (e.g., the number of degrees above normal). Therefore, we categorized each HME into two sets of mutually exclusive categories (moderate vs. severe and acute vs. chronic) to better delineate the variation among HMEs. For magnitude, if an HME had a maximum MIT-derived temperature ≥ 39.17 °C but < 39.44 °C, it was defined as moderate; if the maximum was ≥ 39.44 °C (which added 0.54 °C [1.0 °F] to the high end of the normal range), it was classified as severe. For dura- tion, an HME < 35 min was classified as acute and if > 35 min was classified as chronic. To better understand the time of day moose were most frequently hot in summer, we calculated the distribution of hot Tb across the 24-h day. For each moose, we compiled the total number of Tb readings ≥ 39.17 °C in summer (all years combined) and calculated the percentage that occurred by hour of day. Statistical Analyses We fit a first series of models to assess if there were significant differences in the per- centage of HMDs across seasons, and if these differences varied by sex or age class using mixed-effect models with a random effect of Animal ID to account for the repeated measures of a given moose, and Year to account for annual variation (Table 2). We then fit a second series of models to test the overall effect of ambient temperature on Tb. We fit 4 different mixed-effect models; each contained a ran- dom effect for Year to account for annual variation not captured in the data and Animal ID to account for variation across individu- als. The first two models were linear mixed effect models with maximum daily Tb as measured by the MIT (maxMit) as a response; the explanatory variable was either 1) maximum daily ambient temperature (maxTemp) or 2) a binary variable (1-0) that characterized whether a day was defined as a Heat Day (1-0) based on the different ambi- ent temperature thresholds defined above (HTD_RH m, HTD_RH p, HTD_MCm_wind, HTD_MCm_nowind). The third and fourth models were generalized mixed-effect mod- els (binomial family with logit link function) with a binary response variable (0-1) that represented whether a day was characterized as a HMD for a given moose (Table 2). We included an autoregressive term for the errors (AR1) in the aforementioned models. We tested whether wind speed modified the relationship between maximum daily Tb and ambient temperature in the summer by adding an interaction term for a) maximum daily wind speed (mph), b) mean daily wind speed (mph), or c) whether or not the aver- age daily wind speed was > 11.37 mph, the experimental wind speed of McCann et al. (2013). Moose Survival as a Function of Heat Stress Our objective was to evaluate whether heat stress as we defined above impacted moose survival over the course of a year. By orga- nizing the data of individual moose into ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 117 Table 2. List of fitted regression models including the dependent variable and independent covariates, which season was included, and if a random intercept was included, northeast Minnesota, 2013–2017. Dependent variable Covariate (s) Season (s) Random intercept %HMDs1 Season All Animal ID / Year %HMDs Season * Sex (males) All Animal ID / Year %HMDs Season * Age class (young, prime, old) All Animal ID / Year maxMIT2 Maximum daily ambient temperature (maxTemp) Summer Animal ID / Year maxMIT Heat Day (HTD=1 or 0)6 i) HTD_RHm, ii) HTD_RHp, iii) HTD_MCm_wind, iv) HTD_MCm_nowind Summer Animal ID / Year HMD3 maxTemp7 Summer Animal ID / Year HMD Heat Day (HTD=1 or 0) i) HTD_RHm, ii) HTD_RHp, iii) HTD_MCm_wind, iv) HTD_MCm_nowind Summer Animal ID / Year Moose-year survival8 Mean moderate9 HME events/day All None Moose-year survival8 Mean severe10 HME events/day All None Moose-year survival8 Mean acute11 HME events/day All None Moose-year survival8 Mean chronic12 HME events/day All None Moose-year survival8 Mean moderate HME events/day + Mean acute HME events/day All None Moose-year survival8 Mean severe HME events/day + Mean acute HME events/ day All None Moose-year survival8 Mean moderate HME events/day + Mean chronic HME events/day All None Moose-year survival8 Mean moderate HME events/day + Mean chronic HME events/day All None 1Percentage of days where, for a given moose and within a given season, at least one of the Tb values (as mea- sured with the mortality implant transmitters [MITs]) was above or equal to 39.17°C. 2Maximum daily Tb as measured by the MITs for a given moose. 3Binary response variable to categorize a day as Hot Moose Day (1) (i.e., when a moose had at least one MIT value above or equal to 39.17) or not (0). 6Binary explanatory variable to categorize whether or not a day was above one of the heat thresholds (HTD=1) or not (0). Heat thresholds are HT_RHm = -2.2°C in winter and 14°C in summer thresholds for increased metabolism in captive moose (Renecker and Hudson 1986), HT_RHp = -5.1°C in winter and 20°C in summer threshold for panting in captive moose (Renecker and Hudson 1986), HTD_MCm_nowind = 17°C in summer for increased metabolism in captive moose (McCann et al. 2013), and HTD_MCm_wind = 24°C in summer for increased metabolism in captive moose (McCann et al. 2013). 7Maximum daily ambient temperature. 8Interval survival of moose from the first day of summer until the last day of spring (moose-year). 9Moderate hot moose event (HME) = maximum MIT-derived temperature of HME ≥39.17°C but <39.44°C. 10 Severe HME = maximum MIT-derived temperature of HME was ≥39.44°C. 11Acute HME = HME duration <35 minutes 12Chronic HME = HME >35 minutes. HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 118 moose-years (e.g., moose-year 2013 ran from the 1st day of summer [3 June 2013] until the last day of spring [6 June 2014]), we created 63 discrete moose-years of data. We only used data from 3 June 2013 (start of moose-year 2013) through 2 June 2017 (end of moose year 2016). We treated each moose-year independently to evaluate the possible impact of heat stress occurring within a given moose-year. This approach could not account for any potential cumula- tive effects of repetitive heat stress across multiple moose-years. To understand the cumulative heat loads on survival for the entire moose-year, we calculated the average number of HMEs per day. We summed the total number of HMEs of each type (moderate, severe, acute, and chronic) for the entire time a moose was alive within a moose-year, and then divided it by the number of days each moose was alive to account for any mortality. We then compared the average (± 95% bootstrapped confidence intervals) daily rate of the HME types between surviving and dead moose in that moose-year. We used bootstrapped con- fidence intervals because the average HME/ day data were right-skewed. We also fit univariate and bivariate logistic regression models (binomial family with logit link) with survival as a binary response variable (1 for moose that died and 0 for moose that survived the entire moose- year; logit link) and HME type as the predic- tor variables for the full moose-year data. We fit univariate logistic regression models for each HME type (i.e., univariate models) and each possible bivariate model combina- tion of HME type (e.g., moderate-acute, moderate-chronic, etc.; Table 2) as additive (e.g., moderate + acute) and tested for inter- actions between the two variables. We com- pared relative support of the models using AICc (Burnham and Anderson 1998). For each model, we also fit a sex (male or female) and age class (young, prime, or old) ad hoc to determine if any of these variables had an effect on survival after accounting for heat loads. We calculated Nagelkerke’s R2 for each model to understand the variation explained by the heat stress variables included in our models. We visualized results from the best-supported model by back-pre- dicting the survival probabilities against the original range of the explanatory variable (e.g., moderate HMEs/day in the sample = 0.06–2.07). We selected that prediction range to limit our inference to the values observed and account for the full range of values within the dataset. All data visualizations and statistical analyses were conducted in R (R Core Team 2023) with the AICcmodavg (Mazerolle 2023), boot (Canty and Ripley 2024), ggplot2 (Wickham 2016), MASS (Venables and Ripley 2002), MuMIn (Bartoń 2023), and nlme (Pinheiro and Bates 2000, Pinheiro et al. 2023). RESULTS Ambient Temperature Thresholds During the 5 years of our study, summer ambient temperatures were above the RHm and RHp thresholds 86–93% and 61–72% of days, and above the MCm_nowind and MCm_wind thresholds 77–85% and 27–41% of days, respectively (Table 3, Fig. 2a). Similarly, most winter days (38–69%) were above the RHm threshold, while 23–51% of winter days exceeded the RHp threshold (Table 3, Fig. 2b). During these years, moose experienced HTD days above RHm and RMp threshold 81.8–92.5% and 49.3–67.5% of the summer seasons, and 49.2–78.5% and 36.3–60.2% of winter seasons, respectively (Fig. 3). Moose also experienced HTD days above MCm_nowind and MCm_wind thresh- olds 71.9–83.4% and 17.1–37.5% and of ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 119 summer days, respectively (Fig. 3). We found no effect of wind on maximum daily Tb of moose during summer at scales we were able to measure. Hot Moose Days (HMD) Of 41 moose, 3 animals never experienced an MIT-derived temperature reading ≥ 39.17 °C; all other 38 animals experienced at least one HMD and most (i.e., 37 animals) experi- enced an HMD with at least one chronic or one severe event. The range of above normal Tb was 39.17 to 41.46 °C. Moose were most often hot in the evenings and overnight, as 76% of hot Tb occurred between 18:00–6:00 hr (40% between 18:00–24:00 and 36% between 00:00–06:00 hr) in summer (Fig. 4). The number of HMEs varied seasonally, with summers having the most events across all animals (mean = 2,364 events, range = 1,276–3,991 across years; Table 4). The mean duration of HMEs during four summers varied from 16 (SE = 0.5) to 44.4 min (SE = 1.2), ranging from 5 to 1,065 min (Table 4). The percentage of HMDs varied significantly among seasons (p <0.001), with moose exhibiting the highest percent- age of HMDs in the summer (overall mean across all years and individuals = 47% HMDs, SE = 2.6), followed by spring (mean = 8.9%, SE = 1.0), fall (mean = 3.46, SE = 0.8), and winter (mean = 0.7%, SE = 0.6) (Fig. 5). Sex or age class alone did not explain the overall variations in the percent- age of HMDs across the entire dataset, but both sex and age class significantly influ- enced the differences in the percentage of HMDs across seasons (p = 0.0019 and p <0.0001 for the interaction between season and sex, and season and age class, respec- tively). Sex was only significant in summer, with males experiencing a higher percentage of HMD than females (p <0.001). Young moose were also more likely (p <0.001) to Table 3. Seasonal variations in ambient temperature compiled across eight Remote Automatic Weather Stations (RAWS) in northeast Minnesota, 2013–2017, and the percentage of days that exceeding published ambient temperature thresholds for heat stress in moose (Alces alces), called a Heat Day (HTD). Season Year Avg % HTD RHm1 Avg % HTD RHp2 Avg% HTD MCm_ Nowind3 Avg% HTD MCm_ Wind4 Max (°C) Min (°C) Summer 2013 91.57 65.07 84.26 31.59 33.33 -4 2014 86 61.1 77.04 26.64 35 -3 2015 92.58 72.45 85.08 40.96 34 -6.11 2016 90.42 67.45 81.83 34.85 33.33 -5 2017 90.09 62.56 77.47 33.77 33 -5 Winter 2012–13 68.79 51 14 -38.33 2013–14 38.09 30.27 17.78 -40.56 2014–15 39.96 23.36 17 -38.89 2015–16 67.6 47.05 25 -34.44 2016–17 55.38 40.98 17.22 -38.33 1HT_RHm = -2.2°C in winter and 14°C in summer thresholds for increased metabolism in captive moose (Renecker and Hudson 1986). 2HT_RHp = -5.1°C in winter and 20°C in summer threshold for panting in captive moose (Renecker and Hudson 1986). 3HTD_MCm_nowind = 17°C in summer for increased metabolism in captive moose (McCann et al. 2013). 4HTD_MCm_wind = 24°C in summer for increased metabolism in captive moose (McCann et al. 2013). HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 120 Fig. 2. Seasonal variability in maximum daily temperature (°C) across A) summer and B) winter, averaged across eight Remote Automatic Weather Stations located throughout the moose range in northeast Minnesota, 2013–2017. The horizontal lines correspond to previously published ambient temperature thresholds shown to induce heat stress in captive moose, including: (MCm_wind (increased metabolism with a constant wind of 11.37 mph) and MCm_nowind (increased metabolism under no wind), McCann et al. (2013); RHm (increased metabolism) and RHp (panting), Renecker and Hudson (1986). ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 121 experience a HMD in summer compared to prime-aged and old animals. Ambient Temperature Effects On average, maximum daily Tb increased (Intercept = 38.81 (95% CI: 38.75–38.88), slope = 0.016 (95% CI = 0.014–0.018), p <0.001) with increasing maximum daily ambient temperature in summer; however, substantial individual variation existed in the population trend (Conditional R2 = 0.19) (Fig. 6). Based on a 95% confidence interval, our fitted model predicts a moose will experi- ence above normal Tb or HME at least once on days when the maximum daily ambient temperatures is ≥ 25 °C in summer (Fig. 6). The probability of a HMD also increased with increasing ambient temperature (p <0.001, Conditional R2 = 0.24) in summer, and a moose was more likely (p <0.001) to experi- ence a HMD on a day defined as a HTD with odds ratios varying between 1.79 and 2.76 (Conditional R2 = 0.15–0.16). We were unable to fit a model of HMD as a function of HTD in winter due to the rarity of HMDs. Heat Stress Impact on Moose Survival Of the total number of HMEs observed throughout the study (n = 10,755), the major- ity were of acute duration (77%) and moder- ate magnitude (73% Chronic (23%) and severe HMEs (27%) occurred less frequently and primarily in the summer season. We found considerable variation among individ- uals in the average daily rate of HMEs in a moose-year. Beyond this variable HME Fig. 3. Average (95%CI) percentage of days moose experienced ambient temperatures above previously published thresholds (defined as a heat day [HTD]) shown to induce heat stress, during summer and winter seasons, 2013–2017, northeast Minnesota. A moose experienced a HTD if the maximum daily ambient temperature recorded at the weather station nearest its location was above one of the published heat thresholds that shown to induce increased metabolism (metabol) or panting: MC_wind (increased metabolism with a constant wind of 11.37 mph) and MC.nowind (increased metabolism under no wind) (McCann et al. 2013), RH (Renecker and Hudson 1986). HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 122 data, only a small proportion of moose died (14 deaths in 63 moose-years of data or 3 per year); most deaths (8) occurred in summer, 4 in winter, and 1 each in fall and spring. The proximate cause of death was determined in a companion project of cause-specific mor- tality and included infection (n = 5), wolf predation (n = 4), parasites (n = 3), vehicle collision (n = 1), and undetermined (n = 1) (Carstensen et al. 2017) For the full moose-year data, moose that died had 2.0–2.8 times higher average HMEs per day (mean daily rate ranged from 0.32– 0.96 among different HME types; Table 5) than surviving moose for all HME types (mean daily rate ranged from 0.16–0.46 among different HME types; Table 5); how- ever, the associated confidence intervals were quite wide. By moose-year, moose that died had 1.3–4.9 times more HMEs per day on average (mean daily HME rates ranged from 0.10–1.63 among different HME types; Table 6) than moose that survived (mean daily HME rates ranged from 0.04–0.54 among different HME types; Table 6); how- ever, only a few moose-year/HME type combination pairs were statistically different because the confidence intervals overlapped in most cases (Table 6). Neither sex or age class were statistically significant (p > 0.05) relative to survival in the regression models and were dropped from the final models. We also found no interac- tions between any of the parameter combina- tions in the bivariate models, and dropped the interaction terms and fit simpler, additive models. Our model selection results indicated the average number of moderate HMEs per Fig. 4. Hourly distribution (%) of individual moose Tb ≥39.17 °C during summers, 2013–2017, northeast Minnesota. A smooth regression line (blue) is included to show the general pattern with 95% confidence intervals (grey). ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 123 day was a significant variable. It showed up in the top three model results (all within 2.16 ΔAICc) which carried 73% of the cumulative weight (Table 7). Of the bivariate models, the magnitude variables seemed to rise to the top, but under closer inspection these are likely less informative parameters due to the signif- icant amount of variation in HMEs in the dataset (Arnold 2010). Of the top three mod- els, only the univariate moderate model had a significant slope parameter (β = 2.31, 95% BCI = 0.80–3.84) and was most parsimonious (Fig. 7). There was consider- able variation in the top model (univariate moderate model) and very little of the overall variation in moose survival was explained by just the heat parameter (Nagelkerke’s r2 = 0.18). In that model, for an average increase of 1 acute HME per day, the odds of a moose dying increased by 9.99 (95% CI: 1.98– 38.27). Because the daily means were rela- tively low and an increase of 1 moderate unit per day would be rather large, we divided the regression parameter and bootstrapped Table 4. Occurrence and duration of above normal Tb (≥39.17°C) in wild moose (Alces alces) in northeast Minnesota, 2013–2017. We defined days with Tb ≥39.17 °C as a Hot Moose Day (HMD) and their occurrence as a Hot Moose Event (HME). Season Year N1 N HME2 Mean # days3 Mean % (SE) HMD4 N moose (%) HMD5 Mean # (SE) HME Mean duration (min) (SE) HME Range of duration (min) of HME Spring 2013 13 66 37.6 8.0 (1.7) 12 (92) 5.1 (1.0) 21.0 (2.5) 5 – 98 2014 18 266 43.7 11.0 (1.9) 16 (89) 14.8 (4.1) 20.1 (1.7) 5 – 173 2015 25 311 77.9 8.0 (1.8) 21 (84) 12.4 (3.8) 38.5 (4.1) 5 – 745 2016 13 174 64.6 11.4 (2.4) 13 (100) 13.4 (3.5) 44.4 (6.2) 5 – 796 2017 5 1 28.4 0.3 (0.3) 1 (20) 0.2 (0.2) 5 5 – 5 Summer 2013 12 1276 115.1 43.6 (5.6) 12 (100) 106.3 (15.4) 16 (0.5) 5 – 128 2014 18 2641 99.8 44.7 (4.6) 17 (94) 146.7 (27.6) 17.3 (0.4) 5 – 245 2015 23 3991 120.1 50.4 (4.4) 23 (100) 173.5 (26.5) 44.4 (1.2) 5 – 1065 2016 11 1550 120.8 49.7 (5.9) 11(100) 140.9 (20.0) 37.4 (1.3) 5 – 645 2017 1 0 11.0 0.0 0 0.0 Fall 2013 10 2 42.0 0.2 (0.2) 1 (10) 0.2 (0.2) 5 (0) 5 – 5 2014 14 134 67.7 2.3 (1.0) 8 (57) 9.6 (6.0) 18.5 (1.8) 5 – 113 2015 19 234 67.8 6.2 (1.7) 14 (74) 12.3 (5.7) 85.3 (8.6) 5 – 775 2016 7 9 41.0 2.8 (1.4) 4 (57) 1.3 (0.6) 20.6 (5.0) 5 – 45 Winter 2012–13 14 17 73.9 0.4 (0.2) 3 (21) 1.2 (1.1) 19.8 (3.9) 8 – 53 2013–14 21 71 90.0 0.7 (0.5) 5 (24) 3.4 (2.4) 22.5 (2.5) 5 – 95 2014–15 30 5 36.0 1.2 (0.8) 3 (10) 0.2 (0.1) 15.1 (6.6) 5 – 38 2015–16 18 5 82.0 0.3 (0.2) 3 (17) 0.3 (0.2) 55 (38.7) 5 – 205 2016–17 7 2 79.3 0.4 (0.4) 1 (14) 0.3 (0.3) 5 (0) 5 – 5 1Number of moose being monitored during each season and year. 2Total number of Hot Moose Events (HME) 3Average number of days N moose were monitored in a given season and year. 4Average percentage of Hot Moose Days (HMD) moose experienced across all moose in a given season and year. 5Total number (N moose) and percentage of moose that had at least one HMD in a given season and year. HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 124 confidence intervals by 7 to mimic the effects of increasing a moderate HME per week on survival. On average, the odds of a moose dying were 1.42 times higher (95% CI: 0.28– 5.47) for each increase of 1 moderate HME per week. DISCUSSION We found moose in Minnesota are more likely to experience heat stress during summer than winter. In contrast, Lenarz et al. (2009) hypothesized that ambient temperatures above the critical threshold of -5 °C in January caused heat-stress and lower probability of survival. However, when exposed to ambient temperatures exceeding this threshold for over half of the winter seasons, Tb was rarely elevated in our study moose; conversely, HME and HMD events were common in summer. While we found moose were more likely to be hot when ambient temperature was above the two published summer thresh- olds (Renecker and Hudson 1986, McCann et al. 2013), this was likely less related to a bio- logical relevance of the static thresholds and more likely attributed to moose exposed to heat days nearly the entirety of the summer seasons (77.0–92.6% and 61.1–72.5% of the time above thresholds for increased metabo- lism and panting, respectively). The signifi- cant relationship between the hottest summer days and an HME is critically important to consider because climate models predict the average summer temperature will increase 5–6°C across Minnesota by the end of the 21st century (MNDNR 2016). Fig. 5. Maximum daily Tb of moose as a function of maximum daily ambient temperature they experienced across all seasons, 2013–2017, northeast Minnesota. The red line indicates the Tb at which moose may begin to experience heat stress symptoms (39.17 °C). The black lines represent the predicted regression line (95%CI) from the modeled relationship of maximum daily Tb versus maximum daily temperature with animal ID and year as random intercepts. ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 125 Our model predicts that moose may expe- rience heat stress symptoms (as defined by ele- vated Tb) in summer when daily ambient temperature is ≥ 25 °C, a threshold higher than reported by Renecker and Hudson (1986) and McCann et al. (2013). Interestingly, the highest Table 5. Average number of daily hot moose events (± 95% bootstrapped confidence intervals) for moose (Alces alces) that died or survived for a moose-year (1st day of summer-last day of spring) in northeast Minnesota, 2013–2017. Moose that survived (n = 49) Moose that died (n = 14) Type of hot moose events X̅/day (bootstrapped CI) X̅/day (bootstrapped CI) Acute* 0.46 (0.37–0.54) 0.95 (0.56–1.35) Chronic 0.12 (0.09–0.16) 0.33 (0.15–0.57) Moderate* 0.42 (0.35–0.50) 0.96 (0.61–1.35) Severe 0.16 (0.13–0.19) 0.32 (0.18–0.46) All types combined* 0.58 (0.48-0.68) 1.28 (0.82-1.79) *denotes statistical significance at the α=0.05 level. Fig. 6. Maximum daily Tb of moose as a function of the maximum daily ambient temperature they experienced during summers 2013–2017, northeast Minnesota. The red line indicates the Tb at which moose may begin to experience heat stress symptoms (39.17 °C). The black lines represent the predicted regression line (95%CI) from the modeled relationship of maximum daily Tb versus maximum daily temperature with animal ID and year as random intercepts. The blue vertical line represents the ambient temperature (25 °C) above which our regression model predicts that a moose will begin experiencing heat stress symptoms. HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 126 Table 7. Model selection results for the effect of average daily number of hot moose event types within entire moose year (1st day of summer to last day of spring) on annual moose (Alces alces) survival in northeast Minnesota, 2013–2017. We calculated Nagelkerke’s R2 values for the univariate models and Nagelkerke’s adjusted R2 values for the bivariate models. Model K AICc Δ AICc AICc Weight Cumulative weight -log- likelihood Nagelkerke’s R2 Moderate 2 58.22 0.00 0.42 0.42 -27.01 0.18 Moderate-Acute 3 59.98 1.76 0.17 0.59 -26.79 0.28 Moderate-Chronic 3 60.38 2.16 0.14 0.73 -26.98 0.28 Acute 2 60.91 2.69 0.11 0.84 -28.36 0.15 Severe 2 62.39 4.17 0.05 0.89 -29.10 0.13 Chronic 2 62.95 4.73 0.04 0.93 -29.38 0.12 Severe-Acute 3 63.12 4.90 0.04 0.97 -28.36 0.22 Severe-Chronic 3 63.43 5.21 0.03 1.00 -28.51 0.21 Table 6. Average number of daily hot moose events (± 95% bootstrapped confidence intervals; BCI) for moose (Alces alces) that died or survived for each moose-year (1st day of summer-last day of spring) in northeast Minnesota, 2013–2017. Surviving moose (n = 49) Moose that died (n = 14) Type of HME Moose year N Mean number HME/day 95% Lower BCI 95% Upper BCI N Mean number HME/day 95% Lower BCI 95% Upper BCI Acute 2013 9 0.29 0.22 0.37 3 1.01 0.19 2.24 2014 14 0.54 0.40 0.71 3 1.31 0.12 2.30 2015 17 0.46 0.33 0.62 6 0.62 0.32 0.97 2016 9 0.48 0.29 0.71 2 1.29 0.81 1.77 Chronic 2013 9 0.04 0.03 0.06 3 0.22 0.05 0.49 2014 14 0.08 0.05 0.13 3 0.10 0.03 0.16 2015 17 0.17 0.09 0.26 6 0.33 0.08 0.70 2016 9 0.17 0.12 0.23 2 0.86 0.69 1.03 Moderate 2013 9 0.25 0.19 0.33 3 0.91 0.16 2.04 2014 14 0.42 0.29 0.57 3 1.06 0.13 1.82 2015 17 0.47 0.34 0.62 6 0.71 0.32 1.24 2016 9 0.49 0.31 0.70 2 1.63 1.19 2.07 Severe 2013 9 0.08 0.04 0.11 3 0.33 0.07 0.69 2014 14 0.20 0.13 0.27 3 0.35 0.02 0.63 2015 17 0.17 0.12 0.23 6 0.24 0.09 0.45 2016 9 0.16 0.11 0.22 2 0.52 0.31 0.73 ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 127 McCann et al. (2013) threshold (24 °C) was measured in constant wind speed of 11.3 mph and was 7 °C higher than in windless condi- tions. We were unable to account for the effect of wind on our individual animals but recog- nize that wind speed varies spatially and by elevation, cover type, and time-of-day (McCann et al. 2016), and often aids thermo- regulation. Environmental variation, common thermoregulatory behaviors of moose, and the large variation in individual response (Fig. 6) all refute the use of a static threshold of ambi- ent temperature as predictive of heat stress in moose. As Mitchell et al. (2018) and Thompson et al. (2019) cautioned, use of such thresholds may be misleading and are not intended for direct field application (Pekins 2020). The diurnal pattern of Tb, with evening peaks and dissipation of excess heat over- night, was similar to trends reported for ruminants in northern latitudes (Signer et al. 2011, Thompson et al. 2019, Græsli et al. 2022), as well as desert ungulates (Fuller et al. 1999, Mitchel et al. 1997, Maloney et al. 2002). However, in our study the magnitude and duration of daily elevation of Tb were unique. Semi-captive Alaskan moose exhib- ited a daily rhythm (range = 0.9 °C) in core Tb in summer without predicted heat stress, with Tb peaking at 21:00 hr (Thompson et al. 2019). Our moose peaked somewhat later in the day (24:00 hr) and were slower to dissi- pate excess heat, as 36% of elevated Tb occurred from 0:00 to 6:00 hr. However, because daylight and time-of-day affect for- aging activity that elevates Tb, the disparate daylight conditions in these two regions confound direct comparisons, and we could not account for daily foraging activity and movement that influence daily patterns of Tb in these analyses. Possibly, physiological ability to mitigate heat stress differs in moose living at the southern extent of their bioclimatic range and more northern populations. The gut flora of ruminants is heat sensitive and does not survive long durations of temperatures > 40 °C (Hungate 1966). Adverse effects of heat stress in the rumen of cattle include lower gut motility, rumination, and depressed appetite, effectively reducing intake and digestibility of forage (Yakav et al. 2013). Beale et al. (2018) hypothesized that elevated summer body temperatures that reduce forage digestibility could negatively impact metabolism, energetics, and ulti- mately survival. It is possible that heat loads we observed in some individual moose during summer are reducing digestibility of even high-quality forage, which can nega- tively impact metabolism, energetics and ultimately survival (Beale et al. 2018). Overall, however, the average summer HME (i.e., 32 min) was less than acute by our defi- nition (35 min), and of moderate magnitude (< 39.44 °C), although a few individual HMEs extended several hours (longest = 17.7 h). The lack of chronic and severe HMEs is perhaps unsurprising given the environmental variation, behavioral choices, Fig. 7. Effect of average daily number of moderate HMEs within entire moose year (1st day of summer to last day of spring) on annual moose (Alces alces) survival probability in Minnesota, 2013–2017. 1.00 0.75 0.50 0.25 0.00 Cumulative number of moderate HMEs/days in the sample Pr ob ab ili ty o f d ea th 0.0 0.5 1.0 1.5 2.0 HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 128 and mobility of moose to maintain Tb within a mostly 2-degree C range, as documented in Alaska (Thompson et al. 2019). The maximum Tb we recorded was 41.5 °C and higher than that (40.8 °C) measured with a MIT in a wild moose hunted with dogs in Sweden (Græsli et al. (2020b). While unable to account for frequency and duration of predator encounters experienced by moose in our study, it is likely that certain HMEs reflected predator interactions. Wolves were abundant in our study area and accounted for approximately a third of adult moose deaths (Carstensen et al. 2017). For example, one moose that was killed by wolves in summer had a rapid increase in Tb from 38.6 to 41.3 °C coincident with a 618 m movement just prior to its death (Carstensen, unpublished data), indicating that a wolf encounter can induce acute and severe increase in Tb. Further, such increases in Tb might adversely affect survival due to cytotoxicity as damage to mammalian cells from hyperthermia begins in a relatively short period of elevated Tb (> 40–41°C). The extent of damage is dependent on exposure time and other stress factors but might result in organ failure and death (Lepock 2003, Tansey and Johnson 2015). If hunting moose with dogs can cause a notable stress event despite increased resting times, and possibly cause adverse effects to reproduction and body condition (Græsli et al. 2020b), it's plausible that a similar response might occur from extended chases in moose-wolf encounters. We found a relationship between HME and survival, as moose that died had 2.0–2.8 times higher average HMEs per day over the course of a year, and dead moose had higher mean HMEs of all types compared to survi- vors. However, our data were highly vari- able, the sample of dead moose was small and similar annually (n = 2–3, except in 2016), and moderate and acute HMEs were predominant (>73%); chronic (2,492) and severe (2,857) HMEs accounted for only 23–27% of HMEs. Interestingly, we found that despite the low daily rate, having more moderate HMEs may be more problematic for moose than extreme HMEs alone. For each increase of 1 moderate HME per week, the odds of a moose dying increased (0.28 to 5.47 times). While arid-zone ungulates are well adapted to withstand hyperthermic events, such as employing selective brain cooling to conserve body water (Mitchell et al. 2002), moose are maladapted to increasing environmental temperatures (especially hotter summers) given their large body size and insulative winter pelage (Kelsall and Telfer 1974). Perhaps the fre- quent occurrence of HMEs in summers, although mostly moderate and acute events, has a cumulative survival effect. Our moose- year survival period included the possible direct effect of HMEs within the current summer and later in post-summer seasons (indirect effect). While most deaths occurred during summer (8 of 14), we cannot know if heat stress played a direct role or contributed to stressors (e.g., high parasite loads, poor nutrition, injury) associated with these mortalities. The leading proximate cause of mortal- ity in this study was infection (36%) that is potentially linked to immunosuppression, co-infection with pathogens or parasites, poor body condition, and attributes of chronic heat stress (Jolles et al. 2015). Murray et al. (2006) found a negative associ- ation between moose population change and summer temperature in northwest Minnesota, speculating that heat stress was linked to poor body condition, higher mortality from parasitism and infection, and population decline. Interestingly, the moose in our study with the longest duration (17 h) of a HME in summer was a 6-year old emaciated bull that died mid-winter from a Pasteurella sp. ALCES VOL. 59, 2023 HEAT STRESS IN MINNESOTA MOOSE 129 sepsis infection, in concert with severe liver fluke (Fascioloides magna)-induced hepati- tis, pleuropneumonia, and Echinococcus sp. cysts in the lungs. The stress of high heat loads in summer may have affected foraging activity and body condition of this animal by exacerbating the adverse effects of an already high parasite load; however, the cause of the extended HME is unknown. The complexity of teasing out ultimate from proximate causes of mortality must be emphasized as we consider the role that heat stress may play either directly or indirectly in predisposing individual moose to predation risk, parasite exposure, poor body condition, and reduced fitness in context with the larger population. For example, our study population had an average pregnancy rate of 83% and a 30% twinning rate in prime-aged females (Severud et al. 2019) that reflects adequate nutritional condition and good productivity. Yet, this population has declined markedly since 2005 with health-related causes driv- ing the majority of adult moose deaths (Carstensen et al. 2017) and predation sup- pressing calf survival (Severud et al. 2015). Numerous studies in North America and Scandinavia reported behavioral responses of wild moose to perceived thermal stress in summer, typically through reduced move- ment and activity patterns, and selection for thermal shelter (Dussault et al. 2004, Broders et al. 2012, van Beest and Milner 2013, Melin et al. 2014, Street et al. 2015, Ditmer et al. 2017, Montgomery et al. 2019); conversely, others found minimal behavioral response at high ambient temperatures perceived to induce heat stress, and questioned whether heat stress in moose was a population-level concern along their southern border (Lowe et al. 2010, Murray et al. 2012). Common to all these previous studies was the lack of con- current physiological data to accompany their field observations due to the challenges of obtaining continuous physiological measurements such as core Tb in free-ranging animals. It is possible that the lack of behav- ioral response by Ontario moose (Lowe et al. 2010) reflected high quality habitat with ample thermal shelter near abundant forage, allowing moose to manage potential thermal stress without making fitness tradeoffs. In Ontario, moose had a high availability of dense mixed-wood stands that simultane- ously provided foraging opportunities and thermal cover (Street et al. 2016), whereas Minnesota’s moose habitat was dominated by deciduous cover with a more open can- opy, potentially leaving them more vulnera- ble to thermal stress. That said, moose remain highly mobile and are overall habitat gener- alists, utilizing a wide variety of forage spe- cies to their meet nutritional needs. As concern for climate-driven changes to forest composition heightens, the importance of identifying moose habitat prescriptions that maximize both juxtaposition and availability of forage and cover schemes will become central to future moose habitat management. Here we have demonstrated that ele- vated Tb in moose is a physiological response to high ambient temperatures in summer, and may influence survival. Future research will explore relationships between the MIT- derived Tb and behaviors of moose, includ- ing seasonal and daily habitat use, activity, and movement to identify and assess the role and availability of thermal refugia in the recovery of moose in northeast Minnesota. The influence of climate on moose is often indirect as well, including shifts in forage composition and quality (Soja et al. 2007), reduced calf recruitment (Holmes et al. 2021), range expansion of pathogens and parasites (Repel 2011, Feldman et al. 2017), and increased competition with white-tailed deer (Weiskopf et al. 2019). As global warm- ing heightens, improved understanding of all these relationships and their influence on nutritional condition and productivity of HEAT STRESS IN MINNESOTA MOOSE ALCES VOL. 59, 2023 130 moose will be foundational to future management. ACKNOWLEDGEMENTS We thank the Environment and Natural Resources Trust Fund and the Minnesota Department of Natural Resources for funding this study. We thank all the wildlife staff from the Minnesota Department of Natural Resources, Fond du Lac Band of Lake Superior Chippewa, and 1854 Treaty Authority that assisted with moose captures and mortal- ity investigation. We thank L. Minicucci, A. Herberg, D. Thompson, and J. Crouse for assistance in MIT deployment and calibration. We thank A. Wuenschmann and A. Armien for assistance with diagnostic investigations to determine causes of death in our moose. LITERATURE CITED AckermAn, T. N. 1987. Moose response to summer heat on Isle Royale. M. S. 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