Threat of abrupt mortality events keeps endangered monkey population at risk, despite decades of growth

Northern muriquis, which live in the Atlantic Forest of Brazil, are much more peaceful and egalitarian compared to other primates. They are also one of the most endangered species of monkey in the world.

Karen Strier, a professor of anthropology at UW-Madison and lead author of the paper, has spent 40 years studying the behavior and ecology of these monkeys in a small, preserved portion of Brazilian forest. She teamed up with Anthony Ives, a professor of ecology and evolution at UW-Madison, who is well versed in modeling demographic changes over time.

“My goal is simply to put into statistics what Karen already knows,” Ives says. “She knows her data so well, and long-term studies with this level of detail for an endangered species are very rare.”

Strier and her team of Brazilian colleagues not only count the number of animals but also track their unique behavior, birth rates, death rates and relationships with one another. They know these animals as individuals, not just data points.

While the species and the land they live on is protected by Brazilian law, the muriquis’ mortality rate rose suddenly in 2016 and has not fallen since. Strier and Ives have found that the animals are still reproducing at a steady rate, pointing to other causes for the population decline.

“Our data imply that there may be some environmental stressors in the habitat such as a decline in forest productivity, which affects food availability, climate stress or predation causing the elevated mortality,” Strier explains.

The study also confirms the benefit of long-term, detailed studies such as this. By 2015, this population of muriquis had grown to a remarkable 356 animals, compared to the roughly 50 animals it consisted of when Strier had begun her data collection in 1982. Ives used data from the first 33 years of Strier’s study to create a model of what the population should look like over the next several decades under the conditions of 2015. It predicted the population would continue to rise exponentially until it reached a carrying capacity of about 500 animals, hovering around that size for the next few decades.

What that model couldn’t predict though, was the two years of drought that began in 2014 or the bout of yellow fever that swept through the population in 2016.

“If you just had data up to 2015, you’d say the population is great!” Ives explains. But luckily, Strier and her team continued to collect data beyond the drought and the yellow fever epidemic, making it possible to document the ongoing impact of habitat changes.

When Ives modeled the population changes, accounting for this dramatic decrease from 2016 to 2022, he found the population’s predicted carrying capacity to be only about 200 animals. And that’s assuming there won’t be another abrupt change to mortality like the one that started in 2016.

With predation and habitat change pinpointed as possible limiting factors for this muriqui population, conservationists are getting a clearer idea of where and how action may need to be taken.

“As almost all previous work from Karen, this will open a new window to understand and improve the design of our [conservation] strategies,” says Leandro Jerusalinsky, the head of the National Center for Research and Conservation of Brazilian Primates, part of the Ministry of Environment in Brazil.

Strier’s data can also be used alongside different models to predict how other populations of muriquis may react to changes in climate and future diseases. Those challenges grow increasingly more likely as climate models predict a warmer, drier world, resulting in increased environmental stress and food scarcity for muriquis and other primates.

Jerusalinsky hopes the data will help them understand what conditions these populations need to survive in the face of a changing world. Eventually it could inform conservation policies that might improve habitat quality management or create habitat corridors between isolated populations of the muriquis.

“Having a person like Karen developing this long-term research and providing these high-quality results is incredible,” Jerusalinsky says. “Even in the desperation we have facing this situation of (the muriquis), this gives us a lot of hope in effectively designing the best strategies possible to try to save this species.”