Adversity in early life can have lifelong consequences for people’s health, even if their circumstances improve dramatically later in life. Scientists use a cumulative adversity index (CAI), which quantifies measures of adversity such as poverty and stress, to understand health and longevity across a person’s lifespan. This has been useful in identifying specific steps that governments, healthcare providers and families can take to improve people’s lives.
Wild animals can also experience adversity early in life, but the effect this has on their survival and longevity is unknown. While a similar tool could help scientists conserve animal populations by identifying the most influential stressors to mitigate, few populations have been studied for a long enough time to obtain the data needed to develop a CAI for that species.
UCLA biologists are changing that by creating the first cumulative adversity index for yellow-bellied marmots, based on 62 years of continuous data collection at the Rocky Mountain Biological Laboratory in Colorado. This is the second-longest study of individually tagged mammals in the world. The new study, published in Ecology Lettersprovides detailed steps for scientists with large data sets from other species to create their own CAI.
The index they developed identified some predictable but also surprising stressors with significant effects on marmot survival and longevity. For example, it was no surprise that a late start to the growing season reduced survival because marmots must gain weight over the summer for their seven- to eight-month hibernation. But the finding that summer drought had no effect was unexpected. Predation also played a smaller role than anticipated. It’s not surprising that the death of a mother played a major role, but it did so even if it occurred after the pup was weaned. That may be because pups live with their mother for a full year after weaning.
To create the index, PhD student Xochitl Ortiz-Ross selected data from female marmots born after 2001 (when researchers began quantifying physiological stress) that remained at one of the colonies studied until 2019, to ensure an accurate record of their pedigree, age and life experiences. Males typically disperse while females remain in the area where they are born, so biologists can observe females throughout their lives.
This marmot population extends along a 300-meter gradient that divides the population into up-valley and down-valley groups, with different environmental and demographic conditions. Scientists capture individuals from the population every two weeks from spring to late summer, when the marmots are active, and collect behavioral, morphological, and physiological data.
Ortiz-Ross identified the following ecological, demographic, and maternal measures of adversity, all of which can affect a pup’s survival during its first year: late start to the season; summer drought; predation pressure; large litters; male-dominated litters; late weaning; low maternal mass; high maternal stress; and maternal loss. She wanted to find out if these factors had any effect on an individual’s life span beyond the first year.
These variables were fed into computer models that quantified standard, mild, moderate, and acute adversity. All models yielded similar results. Cumulative moderate and acute adversity decreased pup survival probabilities by 30% and 40%, respectively. Pup survival probabilities were significantly higher in the upper valley zone for all models, while maternal loss decreased survival probabilities in all models and by up to 64% in the moderate adversity model. Poor maternal mass decreased survival probabilities by 77% in the moderate adversity model only, while late weaning decreased probabilities by 33% in the standardized and crude models only. Surprisingly, drought increased survival probabilities in all models except the acute adversity model, with the largest effect seen in the moderate adversity model.
The average life expectancy of adults was 3.8 years, but acute ICA tripled the risk of adverse effects on life expectancy.
“We found that a CAI effectively captures short-term survival risk in yellow-bellied marmots, and even over the long term, increased adversity early in life reduced adult lifespan,” Ortiz-Ross said. “The positive effects did not cancel out earlier adverse effects, suggesting that adversity accumulates in marmots and cannot be fully recovered by positive experiences.”
The results supported the hypothesis that a CAI may be a useful tool to assess the impact on long-term survival of multiple stressors in the early life stages of yellow-bellied marmots.
“In terms of biodiversity management, we’re faced with a death by a thousand cuts. We typically study one factor at a time: humans, predators, climate, and so on,” said Daniel Blumstein, co-author and professor of ecology and evolutionary biology. “But these impacts occur together and have a cumulative effect. We need a way to figure out which of these stressors — or what combination — has the biggest cumulative effect, and our research shows that CAI can do that for marmots.”
For example, conservation plans targeting this population of marmots might target the lower valley group, which surprisingly fared slightly worse, and reduce maternal mortality and improve maternal health. But it may not be necessary to aim to reduce predation or counteract the effects of summer drought; these factors did not turn out to be as important as expected.