Howler monkeys fill the forests in which they live with a deep guttural roar. If you've ever heard a group sounding off, you'd be forgiven for thinking the monster from your childhood nightmares was lurking not far away. In a particular patch of Brazilian forest, however, the trademark sound of the primates has been replaced by an eerie silence as the creatures fall prey to an outbreak of yellow fever. While tragic, one researcher says the event could offer valuable insight into simian ecology.

When University of Wisconsin–Madison (UM) professor of anthropology Karen Strier visited a patch of Brazil's Atlantic Forest in January, she found the difference in the sonic landscape startling, considering what she typically heard during her visits there since 1983. "It was just silence, a sense of emptiness," she says. "It was like the energy was sucked out of the universe." According to a UW report, Stier and her colleagues had never seen a disease decimate an entire population of monkeys so fast.

The reserve area Strier is studying is just four square miles large, a result of the way in which the Atlantic Forest has been chopped up into small protected islands of land to create the surrounding farmland.

"I am very surprised at the speed with which the outbreak is advancing through the landscape and by how the virus can jump from one patch of forest to another, even if they are hundreds of meters apart," says Mendes. "It is also surprising that it is spreading across such a large geographic region."

Strier and her Brazilian colleagues will now undertake a count of the remaining monkeys and study how the societal arrangements of the survivors will change with their normal social groups eliminated. "It's like a controlled natural experiment, but one you would never plan to do," Strier says.

"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," she adds. "We are in a position to learn things we never knew before, with all the background information that we have collected."

Monkey in a coal mine?

In addition to looking out for the welfare of the monkeys, the research team may also be able to help local residents who have also seen a spike in yellow fever. The disease can be spread from monkeys to humans and back again through mosquitoes, but the monkeys are more prone to infection, which makes them a bellwether for local outbreaks, akin to a canary in a cold mine, says UW.

"We need to show that they help inform when the virus arrives in a region, because being more sensitive than humans, they die first," says Sérgio Lucena Mendes, a professor of animal biology at the Universidade Federal de Espirito and Strier's collaborator.

As of this month, Brazilian health authorities have clocked over 400 cases of the disease in humans, leading to approximately 150 fatalities. Another 900 possible cases are currently being investigated.

Strier remains hopeful that the howler monkey population could rebound once the disease has run its course based on previous work she did with another monkey species in the same patch of forest known as the northern muriqui. When she arrived in Brazil in 1983 there were 50 of the animals. At the end of last year there were almost 340 in her small study forest, proving that a small population of primates can indeed grow in the region. The muriquis are less susceptible to yellow fever, so their numbers might not be as dramatically impacted as the howlers.

The yellow fever outbreak will also allow Strier and her team to observe how the reduction of one species affects another.

"My happy hypothesis is that the muriquis are out foraging, feasting on all the best fruits and leaves that the howlers used to eat," she says. "Will they eat more of their favorite foods, or travel less? Will their social order change? Will they form smaller groups?" Those are the questions the researcher and her team will try to suss out in the coming months.

Strier's lab has a dedicated project established to monitor and preserve the muriquis, which are considered critically endangered and can only be found on Earth in 10 forests in southeastern Brazil.

The video below shows both howlers and muriquis in their habitats.

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