Stress Is Killing These Teeny Lemurs, and The Story Is In Their Hair

With their small furry bodies and large inquisitive eyes, gray mouse lemurs can seem like a cross between a pug and an alien. In fact, these Madagascar primates share much in common with us. For one, they feel mounting stress as their forest habitat is destroyed—and new research shows how living under constant pressure can hurt their survival.

Mouse lemurs are a subgroup of lemurs that boast the title of smallest primates on Earth. The gray mouse lemur (Microcebus murinus), which measures in at just under a foot from nose to tail and weighs around two ounces, is the largest species within that group. It's currently considered to be a species of "Least Concern" by the International Union for Conservation of Nature's "Red List," but the organization does note that the population of gray mouse lemurs is declining due largely to habitat loss.

Overall, Madagascar's dozens of lemur species have long faced threats from deforestation and hunting by humans. "It's well known that this species is under very high pressure from anthropogenic activities and habitat loss," Josué Rakotoniaina, an ecologist at Germany's Georg-August University of Göttingen, says of his choice to scrutinize these petite primates in particular. "But there was no study of how those human activities can affect these animals ecologically."

Mouse lemurs are proving surprisingly useful to scientists studying human diseases, thanks to their conveniently small size (about double the size of a mouse, with a tail up to twice the length of their body) and genetic similarity to us (they’re primates, like us and unlike mice). In recent years, scientists have found that they make the perfect model for looking at obesity, eye disease and even neurological disorders like Alzheimer’s disease and dementia. 

Rakotoniaina wanted to see how the stress that environmental pressures caused in these lemurs impacted the animals, particularly when it came to their survival and reproduction. Prior research has shown that the hormones released when a person or non-human animal undergoes stress are useful in the short-term for fighting or fleeing from threats, whether from a predator or a street brawl, but physiologically harmful when experienced for long periods. (To be clear, the researchers used “stress” to mean the body’s response to any kind of situation causing hardship, whether it be fear, lack of food or shelter or inability to find a mate.) 

Hormones like cortisol—a steroid found in the blood, saliva, urine, hair and feces of humans and other animals—are often measured by ecologists as a proxy for the health of a group of organisms. But samples from blood or urine capture only the stress levels at a certain point in time for that animal, making it difficult to draw conclusions about the dangerous long-term stress that organism is facing. To get around that issue, Rakotoniaina turned to something most mammals have in abundance: hair.

Hair has many remarkable qualities. For one, as it slowly grows, it preserves traces of an animal’s condition and environment in a timeline that scientists can later interpret, not unlike tree rings or sediment or ice cores. By taking samples of hair from wild gray mouse lemurs that were trapped and released, ecologists can see how the lemurs' cortisol levels have changed over the course of time that those hairs grew, giving a much more complete picture of the long-term stress faced by the animal.

With data from colleagues at the German Primate Center, Rakotoniaina was able to obtain hair samples and keep track of a population of 171 gray mouse lemurs in Madagascar's Kirindy Forest for two years starting in 2012. By connecting the measured cortisol levels to how the lemurs fared during those years, Rakotoniaina and his colleagues found that lemurs showing lower levels of cortisol had an average chance of survival that was 13.9 percent higher than the lemurs with higher cortisol levels, according to their research published today in the journal BMC Ecology.

Though the study did not try to figure out exactly how the stress levels make the lemurs less likely to survive, Rakotoniaina speculates that it could be due to a variety of factors, including stressed lemurs being more vulnerable to disease from weakened immune systems, and less able to react effectively in various life situations that come with normal stress.

For example, a part of the study tracked 48 lemurs during their mating season and found that the stressed lemurs, particularly male ones, had higher chances of dying than the overall average. This was likely because they could not handle the additional stresses of mating on top of survival. Rakotoniaina and his colleagues are planning next to figure out how exactly the stress is hurting these lemurs by more closely tracking their health over time.

With these results, Rakotoniaina sees great use for easily obtained and non-invasive hair samples in studying the health and population dynamics of other mammals or lemur species.

"It's a really huge advance in this field," Rakotoniaina says, noting that this method could be used by conservationists as an accurate barometer for monitoring the animals their tracking and whether their conservation methods are working effectively. 

Michael Romero, a biologist at Tufts University who researches stress physiology, says there haven't been many studies trying to link an animal's response to stress to its survival, and those that have been done haven't had consistent results. 

The study "is an exciting addition to the work on the role of the stress response in helping wild animals survive in their natural habitats," says Romero, who wasn't involved in the study. He sees the new research as a step toward understanding how specific stressful events, especially those caused by humans, can impact an animal's life.

However, Romero does caution that the response to stress that Rakotoniaina's study measured is relatively small. "Whether such a small effect will be a reliable marker is still an open question," he says.

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Ben Panko is a staff writer for Smithsonian.com