Climatic shifts to warmer and often drier conditions are challenging terrestrial species worldwide. These shifts are occurring more rapidly at higher elevations and latitudes, likely causing disproportionate effects to mammalian hibernators there. While there is some information about how these species’ ranges are responding to climatic shifts, we lack an understanding of how climate components are affecting species’ life history variation, which is key to individual success and population-level resilience. We reviewed the literature to identify the direction of life history responses to climate change in mammalian hibernators along three axes: latitudinal, elevational, and temporal. We found 39 studies involving 27 species that reported climate effects on our four target life history traits – phenology, body mass/condition and growth, reproduction, and survival. We found warmer temperatures are advancing hibernator phenology and increasing reproductive success. By contrast, warming and drying trends are having uncertain effects on body condition, and complex effects on survival - depending on season, age class, latitude, and elevation. We found no pattern of significant climate-trait outcomes by duration or decade of study. More research on drought conditions - particularly in relation to resource availability - would help inform hibernator susceptibility to increased drying trends expected to intensify globally. Notably, our results are highly biased towards small mammal hibernators in Northern hemisphere alpine/mountain ecosystems, with few long-term studies conducted on Southern hemisphere hibernators.This review highlights that phenological shifts constitute one of the most obvious consequences of climate change, yet, the timing of life history events (e.g. timing of migration, reproduction, hibernation) remains poorly understood. Further integration of insights from physiologists, evolutionary biologists, and population ecologists working on wild populations will improve our collective understanding of the effects of seasonal climatic shifts on mammalian hibernator life history traits, key drivers of their population-level persistence.

We identified studies that addressed mammalian hibernator responses to anthropogenic climate change by conducting a standardized literature search in the ISI Web of Knowledge. Our final search took place on August 5th 2020 and identified studies published between 1950 and 2020. Searching for relevant studies with combinations of the following keywords - climat* AND hibern*, weather AND hibern* - generated a list of 717 citations (see PRISMA flow diagram presented in fig. 1).      

            We qualitatively disregarded studies that addressed topics that were not in line with our review: those that reported on species that did not truly hibernate (i.e. animals that exhibit torpor bouts < 24 hours, Geiser and Ruf 1995), or species that were not mammals. We further excluded studies where no statistics were reported, for which we could not find an English version, or that did not present sufficient evidence to detect a relationship between life history traits and climate or weather. We accepted experimental work if conducted in natura, but excluded studies that focused on habitat selection processes, activity budgets, and physiological correlates of hibernation, as opposed to the  traits of interest to our study: phenology, body mass/condition and growth, reproduction, and survival. 

            Once a working subset of studies was identified (N=39), we separated results based on the key life history traits of interest (phenology, body mass/condition and growth, reproduction, and survival) and collected the following information for each entry in our dataframe: the species common name, Latin name, Order, study type (qualitative, quantitative, or review), ecosystem (desert, forest, grassland, mountain/alpine, rainforest, savannah, tundra, or other), field site location, latitude, longitude, length of study, beginning year, end year, life history trait(s), season in which trait was measured (summer/active, winter/hibernation), age group measured (pup, yearling, adult), social/reproductive status (e.g. reproductive vs. non-reproductive, subordinate vs. dominant), general climatic driver(s) (i.e. drought index, global index, rain, snow, temperature), specific climatic driver(s) (e.g. July mean maximum temperature), direction of effect(s) (i.e. negative, positive, no effect), statistical method, statistics reported, sample size, and citation information (table S1). Because studies reported a variety of weather and climate variables, we conceptually standardized the results according to four expected future climatic conditions: warmer conditions, drier conditions, a longer growing season, and a shorter winter from earlier snowmelt. For instance, a positive effect of rainfall and negative effect of drought are conceptually the same result, so those would both be categorized here as negative effect of drier conditions.  Each row summarized a unique result linking a specific life history trait and climatic driver(s); hence, studies with multiple results were represented multiple times in our dataframe and in our results.

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