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As forests shrink, mammals are stressed out— with possible fallout for humans

  • Fur samples collected from small rodents and marsupials in the Atlantic Forest of Paraguay showed that the animals in the smaller forest fragments had elevated levels of stress hormones.
  • While small amounts of acute stress can help an animal to get out of a bad situation, prolonged stress levels weaken the immune system, making them more susceptible to disease.
  • Pathogen spillover (from animals to humans) seems more likely in all animals that are crowded together in forest fragments and stressed out, underscoring the importance of the One Health approach, which recognizes the connection between human, environment, and animal health.

Humans aren’t the only ones feeling anxiety about environmental destruction. As forests shrink, small mammals are also feeling stressed, and we can tell by their fur.

A team of scientists collected fur samples from 106 small rodents and marsupials in six forest fragments in the Atlantic Forest of Paraguay. After grinding each fur sample into a powder, the researchers used a bioassay to measure the levels of glucocorticoids (e.g. corticosterone and cortisol), hormones that become elevated with stress.

Animals living in smaller fragments of forest had higher levels of glucocorticoids than those in larger forest patches, according to the study, published in the journal Scientific Reports.

“We suspected that organisms in deforested areas would show higher levels of stress than animals in more pristine forests, and we found evidence that that’s true,” Noé de la Sancha, associate professor of biology at Chicago State University and co-author of the paper, said in a statement.

“Small mammals, primarily rodents and little marsupials, tend to be more stressed out, or show more evidence that they have higher levels of stress hormones, in smaller forest patches than in larger forest patches.”

Pastor E. Perez is holding a mykureí, Marmosa (Micoureus) paraguayana, a small Neotropical marsupial, in Paraguay’s Tapytá Nature Reserve. Photo by Noé U. de la Sancha.

The mammals were caught in both live traps (Sherman traps) and kill traps (Victor traps) and researchers were surprised to find that the type of trap used to capture the animals impacted the hormone levels as well. Fur samples were generally taken from the animals’ hind legs, and they found that, in the live traps, the animals’ urine could penetrate the fur, resulting in elevated levels of glucocorticoid in the sample. This, however, did not affect the result that less forest equals more stress.

Not all stress is bad. Small amounts of stress hormones can give animals the energy to fend off an attack, move to a new location, or address other disturbances in life. Once the threat is managed, the hormone levels return to balance.

“But then these animals are placed into these small fragments of habitat where they’re experiencing elevated stress over prolonged periods, and that can lead to disease and dysregulation of various physiological mechanisms in the body,” David Kabelik, chair of the neuroscience program at Rhodes College and one of the paper’s authors, said in a statement.

This chronic stress, over weeks and months, takes a toll on animals’ immune systems. More disease in animal populations can lead to an increased risk of those diseases moving into humans.

“We have seen many examples in history showing that crowding sick individuals in small spaces can exacerbate disease transmission; and we have also seen many recent examples of pathogens making the jump across the species barrier,” Kabelik told Mongabay.

The forest fragments where the study took place, in Paraguay’s Tapytá Nature Reserve, ranged from 2 to 1,200 hectares (5 to 2,965 acres). After centuries of development, less than one-fourth of the Atlantic Forest, South America’s second-largest rainforest after the Amazon, still remains. As a result, massive amounts of mammals have been lost in the region.

An example of the many forest remnants left in the interior Atlantic Forest in eastern Paraguay, representative of the sites where small mammal samples for this study were captured. Photo by Noé U. de la Sancha.

The effects of environmental degradation like this are likely to be felt by numerous animals, not just the small ones, Kabelik said, and pathogen spillover (from animals to humans) seems more likely in all animals that are crowded together in forest fragments and stressed out, although the study did not directly address this phenomenon.

With more humans living in close proximity to forest fragments worldwide, there is “the potential of hotspots for all sorts of emerging and/or zoonotic diseases,” de la Sancha told Mongabay, and this underscores the importance of the One Health approach, which recognizes the connection between human, environmental, and animal health.

“It is important,” Kabelik said, “to be mindful about the cascade of hidden negative effects on wildlife, and human society, that results from impacting our natural habitats.”


Boyle, S. A., Noé, U., Pérez, P., & Kabelik, D. (2021). Small mammal glucocorticoid concentrations vary with forest fragment size, trap type, and mammal taxa in the Interior Atlantic Forest. Scientific Reports11(1), 1-13. doi:10.1038/s41598-021-81073-2

Banner image of a mouse opossum (Gracilinanus agilis) from a deforested area of the Atlantic Forest, eastern Paraguay. Image © Noé U. de la Sancha (2021).

Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter: @lizkimbrough_

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