Moose of the Snowy Range are at the southern extent of the species’ distribution, and may be more vulnerable to heat stress when compared with those of more northerly populations because of limitations in habitats that provided needed thermal refuge. Knowing the heat thresholds for Shiras moose (the subspecies that occur in the Snowy Range) and the habitats that can help mitigate the effects of heat stress is critically important for assessing the habitats needed to support healthy moose populations.

Assessed thermal qualities of bed sites: We used GPS-collar data to identify used bed site locations of moose and characterized the micro-climate by setting up mini-weather stations at each site. We then compared daily temperatures, shade, soil moisture, and wind speeds of the used bed sites with unused bed sites to evaluate if moose were selecting for certain characteristics that made used locations cooler than others.

Portable weather station used to measure the micro-climate of bed sites

Mapped thermal landscape: Using remotely-sensed data of habitat characteristics within home ranges and micro-climate data collected from bed sites, we mapped the thermal landscape to identify areas that may serve as thermal refuge for moose.

Moose physiology: We modeled the physiological responses of Shiras moose linking measurements of body measurements, pelt color, hair length, etc. with changes in metabolism.

Thresholds of heat stress: We integrated our assessment of moose physiology with the qualities of the thermal landscape to determine how characteristics of an animal’s environment influenced metabolic demands. Linking the attributes of an individual animal with those of the environment allowed us to determine temperature thresholds at which an animal would undergo heat stress relative to the cooling properties of their immediate surroundings.

Overall, moose selected for bed sites with cooler temperatures with moist or wet ground or with standing water. Moose avoided bedding down in dry, open, and sparsely vegetated (e.g., dry meadows, fire-affected areas, and sagebrush). On warm days and during the warmest part of the day moose selected strongly for wet ground and selection for standing water was through the roof on warm days! In contrast, the abundant shade offered by the forest was not sufficient to keep moose from overheating during the summer.

Selection of bed-site characteristics. Coefficients (dots) to the right of the doted line indicate characteristics moose selected for, while left of the dotted line indicates characteristics moose avoided. Moose strongly selected for sights with standing water on hot summer days.

Overwhelmingly, wet ground was most effective at reducing heat stress, even when compared with canopy cover. On a summer day, moose could overheat at temperatures as low as 54°F (!!) when bedded on dry ground with no shade. In contrast, with wet ground and 100% shade, moose were not at risk of overheating until temperatures exceeded 82°F—which is still a fairly common temperature of summers in the Snowy Range.

Thresholds at which a moose would be expected to experience heat stress during the day in various habitat conditions: shaded and wet, shaded and dry, sunny and wet, sunny and dry. Moose in sunny and wet habitats would likely experience heat stress during most of the day (9am-5pm).

Beetle-kill following the mountain pine beetle epidemic of the last decade has substantially changed the structure of conifer forests, including those of the Snowy Range region. The decreased canopy cover resulting from beetle-killed trees falling can increase sunlight needed to grow vegetation of the understory (i.e., moose food) but likely reduces potential thermal refuge for moose seeking shade during the heat of day. Understanding how habitats with substantial beetle kill affect the thermal landscape for moose is needed for management of this valued species in the face of climate change and alterations to historic habitat structures.

We GPS-collared 30 moose starting in 2015 and evaluated how moose were selecting for various habitats of the Snow Range including intact and beetle-killed conifer forests.

In general, moose avoided conifer forests, regardless of the amount of beetle-killed trees within the forest. Instead, they selected for willow complexes and aspen forest. These findings suggest that moose may not actually be all that affected by beetle kill in the Snowy Range, at least at this stage in the structure of downed trees.

Despite the recent colonization of moose to the Snowy Range in the 1980’s, the current status of the population is relatively unknown, but there have been growing concerns that the population may be in decline. Moose populations are often limited by habitat, because similar to other ungulates, moose commonly rely on fat stores gained from available food to finance survival and reproduction. Thus, evaluation of the relationship between nutritional condition (i.e., fat stores) and fitness (i.e., reproductive success and survival) of individual animals is often needed to understand the status of populations. Yet, the role that fat plays in population dynamics of Shiras moose (the subspecies that occupies the southern extent of the species’ distribution) is not well understood.

Adult survival: We monitored survival of adult moose via GPS collars.

 Reproductive status: Each spring (March) we used ultrasonography to evaluate pregnancy. Each autumn (December), we then evaluated whether a moose successfully reared a calf over the summer.

 Nutritional condition: We recaptured GPS-collared moose every autumn (December) and spring (March) to measure fat reserves as they changed across seasons.

Adult survival, overall, was high—>93% in summer and winter. Survival was most influenced by nutritional condition and age, in that animals in poorer condition and of an older age were more likely to die, when compared with younger, fatter animals. The cause of death of most animals was independent of disease or predators, which can have an effect on moose populations in other areas of the North American.

Similar to survival, the probability of both pregnancy and successful rearing of young was greater for younger and fatter animals, when compared with skinnier, older animals.

Pregnancy in moose was strongly influenced by the amount of fat (ingest-free body fat; IFBFat %) an animal had at the onset of winter and the probability of a moose with 4% body fat was less than 50%.