Early Life Adversity and Survival in Burrowing Mammals

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Research Primer

Background

Yellow-bellied marmots (Marmota flaviventer) are large, burrowing ground squirrels that spend roughly two-thirds of the year hibernating beneath the meadows around Gothic, Colorado. Because pups are born in spring and must gain enough mass to survive their first hibernation by autumn, the months between weaning and the snows of October are a critical window in which experiences can shape an individual's entire life. Researchers studying marmots at RMBL have built one of the longest-running mammal datasets in the world, and over the last decade they have increasingly used it to ask a deceptively simple question: do hard beginnings cast long shadows? This question matters for mountain ecosystems because climate change in the Gunnison Basin is altering snowmelt timing, summer drought, and predator communities, all of which can pile stressors onto young animals during their most sensitive developmental window.

To answer such questions, scientists rely on a few key ideas. Early life adversity refers to any stressful experience between conception and sexual maturity, from a cold late spring to losing a mother, that can leave lasting marks on physiology or behavior. Because young marmots typically face several stressors at once rather than just one, researchers often combine them into a cumulative adversity index, a single score summing how many adverse events an individual experienced. The downstream consequences are then evaluated using survival analysis, statistical methods that estimate the probability an animal lives from one point in time to another (for example, summer survival from emergence to hibernation).

A marmot's life is also shaped by where it lives and who it lives with. Territoriality, the defense of a fixed patch of meadow and its burrows, structures who has access to safe places and good food. Social constraints, the limits that group living places on individual behavior such as where one can forage or whether one can disperse, can amplify or buffer the effects of a rough start. When preferred forage plants wilt under drought or are heavily grazed, marmots may shift to an alternative food source whose palatability, its acceptability based on taste and chemistry, is lower. These shifts in diet and social context provide the mechanistic links between a difficult early summer and an animal's chances of surviving the long winter underground.

Foundational work

The research program rests on decades of natural-history groundwork. Early demographic and behavioral studies showed that marmot societies are organized around matrilines of related females sharing space and defending it from outsiders, with most males never reproducing and a few holding harems (Downhower & Armitage, 1971). Comparative work across burrowing sciurids further argued that sociality itself is a life-history tactic shaped by body-size energetics and the need to accumulate mass for hibernation (Armitage, 1981), and that genetic structure within and among colonies reflects this social substructure (Schwartz & Armitage, 1980). A 32-year demographic synthesis established that females survive better than males after the first year and that survival varies strongly across sites and years (Armitage & Downhower, 1998).

These foundations were extended by studies showing that dispersal, although risky, carries a survival cost only modestly higher than staying home (Van Vuren & Armitage, 1994), and that weather variables can explain up to 58% of variation in demographic rates, largely by affecting the energy available for hibernation and reproduction (Schwartz & Armitage, 2005). Together, this body of work made it clear that early conditions, social position, and environment interact to determine who survives to reproduce.

Key findings

A central result from the long-term study is that climate change has reshaped marmot life history in ways that ripple through survival. Earlier emergence from hibernation and earlier weaning have lengthened the growing season, producing heavier marmots and a sharp increase in population growth rate from roughly 1.02 to 1.18, driven mainly by gains in adult survival (Ozgul et al., 2010). However, these gains are not uniform across seasons: over a 40-year window, summer survival has generally risen while winter survival has declined (Paniw et al., 2020), meaning that what happens to a young marmot during its first summer increasingly determines whether it lives to see another spring.

What happens during that first summer depends heavily on the mother and the social group. Daughters born to older mothers have higher annual and lifetime reproductive success than daughters of young mothers (Kroeger et al., 2020), and supplemental food during the juvenile period raises growth rates and late-season body mass (Allaine et al., 2003), both pointing to early conditions as decisive. Juvenile survival itself is the most variable demographic rate, ranging from 0.15 to 0.89 across years and sites (Ozgul et al., 2006). Recent work using a cumulative adversity index shows that early life adversity interacts with elevation to shape adult stress hormones, with effects differing between low and high sites (Kong et al., 2025), providing direct evidence that hard beginnings can leave physiological fingerprints years later.

Social context modulates these effects in sometimes surprising ways. Strongly affiliative females actually have lower annual reproductive success (Wey & Blumstein, 2012) and strong social bonds are associated with shorter lifespans (Blumstein et al., 2018), suggesting that the social constraints of close ties can be costly. Yet group structure also offers protection: individuals in more cohesive, less fragmentable groups have higher hibernation survival, and multilevel selection acts on group structure at least as strongly as on individual behavior (Philson et al., 2025). Environmental variability further translates into trade-offs, with the conflict between current fecundity and offspring quality expressed most strongly under harsh conditions of low winter snow and hot summers (Bliard et al., 2024).

Current frontier

Early work in the 1990s and 2000s established the demographic and behavioral baseline. Studies since 2020 have shifted toward mechanism and heritability, asking not only whether early adversity matters but how it is transmitted across generations. New quantitative-genetic analyses show that fearfulness, measured as flight initiation distance, is heritable in the wild (Scurka et al., 2025), as are the propensity to give alarm calls (Blumstein et al., 2025) and the acoustic noisiness of those calls (Blumstein et al., 2025). At the same time, researchers are dissecting how environment continually reshapes behavior: rainfall in the prior week increases foraging vigilance (Bobb et al., 2025), warmer temperatures reduce fearfulness of approaching humans (Sanchez et al., 2025), and lifetime social variation, although biologically plausible, appears to have little effect on lifetime reproductive success (Blakely, 2025).

The frontier is moving toward integration. Studies are linking immune state, gut microbiome diversity, social network position, and group structure to fitness simultaneously, with hints that more social individuals harbor less diverse microbiomes (Sarkar et al., 2023) and that group-level social phenotypes carry their own selection pressures (Philson et al., 2025). Marmots themselves are emerging as a model for understanding incipient society formation (Blumstein, 2025), with their facultative sociality making them especially informative for testing how environmental change reshapes group living.

Open questions

Several questions stand out for the next decade. First, through what physiological pathways, hormonal, microbial, or epigenetic, does early life adversity translate into reduced winter survival, and can these pathways be measured non-invasively in wild marmots? Second, as snowpack declines and summers warm, will the recent gains in summer survival continue to offset declining winter survival, or will cumulative adversity in early life eventually outpace the benefits of longer growing seasons? Third, given that strong social bonds can be costly while cohesive group structure is protective, what configurations of sociality will be favored under future climates in the Gunnison Basin? Answering these questions will require continued long-term monitoring at RMBL, integration of behavioral, demographic, genetic, and microbial data, and close collaboration with land managers concerned about how mountain mammals will fare in a rapidly changing landscape.

References

Allaine, D., et al. (2003). The effect of food supplementation on juvenile growth and survival in Marmota flaviventris. →

Armitage, K. B. (1981). Sociality as a life-history tactic of ground squirrels. Oecologia. →

Armitage, K. B., Downhower, J. F. (1998). A 32-year demography of yellow-bellied marmots (Marmota flaviventris). →

Blakely, B. (2025). Is social plasticity good? Does lifetime social variation enhance LRS and longevity in yellow-bellied marmots? →

Bliard, L., et al. (2024). Detecting context dependence in the expression of life history trade-offs. Journal of Animal Ecology. →

Blumstein, D. T. (2025). Society formation and maintenance in yellow-bellied marmots. Animal Behaviour. →

Blumstein, D. T., et al. (2018). Strong social relationships are associated with decreased longevity in a facultatively social mammal. →

Blumstein, D. T., et al. (2025). Is the propensity to alarm-call heritable and related across multiple contexts? Animal Behaviour. →

Blumstein, D. T., et al. (2025). The sound of fear is heritable. Current Zoology. →

Bobb, K., et al. (2025). Does rainfall or temperature influence antipredator vigilance in a hibernating mammal? Behavioral Ecology. →

Downhower, J. F., Armitage, K. B. (1971). The yellow-bellied marmot and the evolution of polygamy. American Naturalist. →

Kong, J., et al. (2025). Is early life adversity associated with adult stress in a wild rodent? Ecology and Evolution. →

Kroeger, S. B., et al. (2020). Older mothers produce more successful daughters. →

Ozgul, A., et al. (2006). Spatiotemporal variation in survival rates: implications for population dynamics of yellow-bellied marmots. →

Ozgul, A., et al. (2010). Coupled dynamics of body mass and population growth in response to environmental change. Nature. →

Paniw, M., et al. (2020). Contrasting effects of climate change on seasonal survival of a hibernating mammal. →

Philson, C. S., et al. (2025). Multilevel selection on individual and group social behaviour in the wild. Proceedings of the Royal Society B. →

Sanchez, A., et al. (2025). Climatic variation and risk assessment in a highly seasonal mammal. Current Zoology. →

Sarkar, A., et al. (2023). The social microbiome: gut microbiome diversity and abundance are negatively associated with sociality in a wild mammal. →

Schwartz, O. A., Armitage, K. B. (1980). Genetic variation in social mammals: the marmot model. Science. →

Schwartz, O. A., Armitage, K. B. (2005). Weather influences on demography of the yellow-bellied marmot (Marmota flaviventris). →

Scurka, M., et al. (2025). The heritability of fear: decomposing sources of variation in marmot flight initiation distance. Animal Behaviour. →

Van Vuren, D., Armitage, K. B. (1994). Survival of dispersing and philopatric yellow-bellied marmots: what is the cost of dispersal? Oikos. →

Wey, T. W., Blumstein, D. T. (2012). Social attributes and associated performance measures in marmots. →

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