Yellow fever has decimated thousands of monkeys in the forests of south-eastern Brazil since 2016. The virus has killed nearly all the population of brown howlers that lived in the RPPN Feliciano Miguel Abdala protected reserve.

Yellow fever virus is carried by mosquitoes and endemic to South America and Africa. Both monkeys and humans are vulnerable to it, but it is the first time that scientists working in the area have seen so many animals die so quickly as a result of the disease.

One of them is Karen Strier, from the University of Wisconsin, who has been studying the critically endangered Muriqui monkey in the reserve since 1983. Muriquis are brown howlers' main competitors.

She now plans to use the experience and the data accumulated over more than three decades to make sense of how many monkeys remain in the forest and how they can be better managed and protected in the near future.

"No one really knows the consequences for the other primates or the forest when nearly the entire population of an abundant species dies from disease in just a few months," Strier points out. "We are in a position to learn things we never knew before, with all the background information that we have collected."

The scientists working in the reserve are not the only one concerned by the virus and what it is doing to monkeys. The latest numbers, published mid-March, suggest that 400 human cases have been diagnosed in the recent outbreak, with 150 people dying. Brazil's Ministry of Health is investigating 900 other potential cases. Health officials now fear that the disease will spread further.

Monkey behaviours

Strier and her Brazilian colleagues will begin a new census of the monkeys in the reserve, comparing it to prior censuses performed in the forest. A priority will be to identify surviving brown howlers, to study how they regroup and restructure their social organisations, as yellow fever has destroyed their social groups.

monkey yellow fever
Muriquis may change their behaviours as a result of brown howlers' deaths

Muriquis, which tend to have a lifespan of 40 years, appear to have been less impacted by the virus. For scientists, it will be an opportunity to see how they develop and thrive in an environment where their main primate competitors for food have disappeared.

"It's like a controlled natural experiment, but one you would never plan to do," Strier explains. "My happy hypothesis is that the muriquis are out foraging, feasting on all the best fruits and leaves that the howlers used to eat. Will they eat more of their favorite foods, or travel less? Will their social order change? Will they form smaller groups?"

She hopes to answer all these questions by spending more time in the reserve in the near future. Based on past research she conducted, she expects to see behaviour changes and social reorganisation among muriquis.