The cumulative adversity index for individuals measures various forms of hardship, such as poverty and stress, to better understand health and lifespan throughout a person’s life. A similar tool could assist scientists in their efforts to study and conserve animal populations by pinpointing the key stressors that need to be addressed. Biologists have now developed the first cumulative adversity index for yellow-bellied marmots. Their findings reveal that, like humans, early-life adversity has lasting effects and shortens the lifespan of these animals.
Experiencing difficulties early in life can lead to lasting health issues for individuals, even if their situations drastically improve afterward. Scientists have developed a cumulative adversity index (CAI) to quantify hardships, including poverty and stress, to understand health and longevity throughout a person’s life. This index has proved beneficial in identifying specific actions that governments and healthcare providers can take to enhance individuals’ lives.
Wildlife can also face early-life challenges, but the impact on their survival and lifespan remains unclear. A similar tool could aid scientists in conserving animal populations by identifying the most significant stressors to address, but few species have been studied thoroughly enough to obtain the necessary data for a CAI.
Researchers at UCLA are making strides in this area by developing the first cumulative adversity index for yellow-bellied marmots based on 62 years of continuous data collected at the Rocky Mountain Biological Laboratory in Colorado. This study stands as the second-longest observation of individually marked mammals worldwide. The findings, published in Ecology Letters, provide a detailed guide for scientists with extensive datasets for other species to create their own CAI.
The index revealed both expected and unexpected stressors significantly affecting marmot survival and longevity. For instance, it was anticipated that a late-growing season would hinder survival since marmots must build up weight during summer for their 7 to 8 months of hibernation. However, it was surprising to discover that summer drought had no significant effect. Predation also had a lesser impact than expected. Unsurprisingly, the death of a mother had significant repercussions on the pups, even if it occurred after weaning, possibly because pups stay with their mothers for up to a year after being weaned.
To construct the index, doctoral student Xochitl Ortiz-Ross selected data from female marmots born after 2001—when researchers began measuring physiological stress—who stayed in one of the monitored colonies until 2019. This ensures that their pedigree, age, and life experiences were accurately recorded. Females generally remain in their birth area whereas males typically disperse, making it easier for biologists to study females throughout their lives.
This marmot population exists across a 984-foot (300-meter) elevation gradient, separating them into up-valley and down-valley groups with varying environmental and demographic conditions. Scientists trap individuals biweekly from spring through late summer when the marmots are most active, gathering data on their behavior, physical measurements, and physiological health.
Ortiz-Ross identified several ecological, demographic, and maternal factors of adversity that could influence a pup’s survival in its first year: late growing season; summer drought; predation pressure; large litters; male-skewed litters; late weaning; inadequate maternal mass; high maternal stress; and maternal loss. Her goal was to determine whether these factors affected an individual’s lifespan beyond the first year.
These variables were input into computer models to measure standard, mild, moderate, and acute adversities. All models produced similar results. Moderate and acute cumulative adversity reduced pup survival odds by 30% and 40% respectively. Survival rates were notably higher in the up-valley population across all models, while maternal loss decreased survival rates in all instances, reaching a 64% reduction in the moderate adversity model. Poor maternal health lowered survival chances by 77% solely in the moderate adversity model, while late weaning decreased chances by 33% only in the standard and raw models. Surprisingly, drought actually seemed to enhance survival odds in all but the acute adversity model, with the greatest positive effect seen in the moderate adversity model.
On average, adult marmots live for 3.8 years, but acute CAIs significantly increased the risk of decreasing life expectancy.
“Our findings indicate that a CAI is effective in capturing short-term survival risks in yellow-bellied marmots, and an increase in adversity early in life leads to a reduced adult lifespan,” stated Ortiz-Ross. “Positive experiences later in life do not offset earlier hardships, showing that adversity accumulates in marmots and cannot be completely compensated by positive experiences.”
The study supports the idea that a CAI can serve as a valuable instrument to evaluate the long-term survival impacts of various early-life stressors in yellow-bellied marmots.
“In terms of biodiversity management, we face the challenge of multiple stressors all affecting populations simultaneously. We often look at one factor at a time, such as human impact, predators, or climate,” noted co-author Daniel Blumstein, a professor of ecology and evolutionary biology. “However, these factors interact and create cumulative effects. We need a way to identify which stressors—or combinations thereof—have the most substantial cumulative impact, and our research indicates that the CAI can accomplish this for marmots.”
For example, conservation strategies for this marmot population might focus on the down-valley group, which surprisingly showed slightly poorer outcomes, while also addressing maternal mortality and improving maternal health. There may be less urgency to tackle issues related to predation or summer drought since their impacts were not as significant as initially expected.
