Monkey Business

Pink, yellow and blue hues paint the early-morning sky over the Gulf of Mexico. Just a few miles inland from the coast in Veracruz, Mexico, three researchers take in the sunrise as they trek through a pasture and along a river, hauling stuffed backpacks. It’s damp, humid and verdant. They pass grazing cattle and hear the calls of distant birds — toucans, parakeets and chachalacas — and listen for the earsplitting cries of mantled howler monkeys.

The team is studying the health of these endangered primates and will trek until they see them. Some days it takes hours romping through the rainforest’s underbrush to find the animals. But today, after an hour of hiking over level grassland, the team reaches a patch of trees where they spot five or so mantled howlers, shiny and black, hanging out in the branches. Then the researchers hunker down for a bit, eat some breakfast and wait for the monkeys to do their business.  

The mantled howlers continue their own morning routine. Eating is their primary waking activity. They munch on leaves, flowers or fruit and then settle down to nap. When they rouse a couple hours later, it’s go time for the scientists.  

“The second they wake up is when they’re going to poop,” says Tabor Whitney ’22 MA, a fourth-year doctoral candidate who is studying biological anthropology at Northwestern. Whitney has been studying these monkeys since 2021, with Northwestern primatologist Katherine Amato. 

Tabor Whitney, pictured here holding a fecal sample, spends two to four months each year doing fieldwork in Mexico. Once she finishes her doctorate, likely in 2025, Whitney hopes to pursue a career in science communication and conservation policy. She’s wanted to work in animal welfare ever since she learned as a child that logging in Colorado, her home state, altered habitats and displaced animals. As an undergraduate at the University of Oregon, she studied the microbiome of colobus monkeys in Ghana and understood that her research could affect conservation efforts. Credit: Diana Christie

The feces may contain clues about the monkeys’ physical condition and could aid Whitney in developing a “health index welfare assessment,” a tool that provides metrics that conservationists can use to make decisions about the endangered creatures. The mantled howler monkeys in this region, which includes the Los Tuxtlas Biosphere Reserve in southeastern Mexico, have lost about 75% of their habitat since the 1960s, as people have converted forests to crop fields, pastures and quarries.  

In the time that she’s been working in Los Tuxtlas (she has visited for several months each year since 2021), Whitney has witnessed changes to the monkeys’ habitat firsthand. She recalls walking through a patch of rainforest cover near an expanding rock quarry and seeing a new break in the forest: The quarry operation had downed trees, splitting 
a forest that had been connected the year prior. She knew human activity was swiftly changing the forest, but to see it with her own eyes was surreal.  

“[The monkeys are] experiencing a major shift in habitat,” Whitney says. They’re living in smaller fragments of forest and with reduced access to the foods they prefer, including the fruits of ficus, cecropia and jobo trees. Climate change is exacerbating the problem by causing shifts in the seasonal availability of various foods. And human activity — including the noise from people and their dogs and machinery — also encroaches on the monkeys’ turf. As their habitats have fragmented and degraded, the primates’ numbers have declined — by an estimated 50% since 1993 — but their populations haven’t been snuffed out just yet.  

Agricultural activities have led to deforestation in the Los Tuxtlas Biosphere Reserve, which includes some of the last remaining pockets of tropical rainforest along the Gulf of Mexico. According to researchers, 14% of the current forest cover in the region will be gone by 2025. Map credit: Michael Newhouse

They’ve survived in part because mantled howler monkeys can adapt their diets to what’s available. They prefer fruit (and play an important role in the ecosystem by spreading seeds) but can get by on leaves, including mature ones that are difficult to digest. However, humans’ influence on the howlers’ habitat could be triggering health problems that are hard to spot and may ultimately affect the subspecies’ ability to survive. Studying the molecules and microbes contained within fecal matter may provide a noninvasive way to snoop on the stress levels, presence of inflammation and overall health of the primates.  

Whitney plans to use the data she collects from the mantled howlers’ feces “to form conservation and management strategies for them.”  

Back in the forest, as the monkeys in the trees above begin to stir, the researchers train their eyes on the primates’ posteriors. When a monkey looks like it’s got a deposit to drop, Whitney or one of her research assistants gets ready to track the trajectory. If the scientists are lucky, the plop of poo lands on a rock or leaf, leaving it uncontaminated by soil, which contains lots of other microbes. To retrieve the sample, the researchers scour the thick vegetation until they find the prize. Then they drop it into a vial of ethanol and log the emitter’s identity. (Researchers can identify particular monkeys based on patches of lighter fur and markings on their paws.) The samples are shipped back to a lab on the Evanston campus, where Whitney and her colleagues will investigate their microbiomes.

Whitney is building on her adviser’s findings with her work on the mantled howler monkeys of Los Tuxtlas. On her most recent visit in fall 2022, Whitney and her team collected samples from about 100 individual monkeys from 10 different forest fragments to study health metrics. So far, the team has documented a dip in the diversity of the monkeys’ gut microbiomes as their habitats’ quality has declined. But it’s not yet clear if and how those changes are linked to poor health.  

In addition to surveying the makeup of the monkeys’ gut microbiomes, Whitney is analyzing the fecal matter for other markers of health. One such marker, lipocalin-2, is associated with intestinal inflammation and found in people who have irritable bowel syndrome. So far, researchers have seen a link between this molecule and inflammation in lab animals, but Amato and Whitney would be the first to find this link in a wild animal population.  

Whitney is also measuring glucocorticoids, a set of steroid hormones. If monkeys have relatively high levels of these molecules across fecal samples at different times, they are likely experiencing chronic stress. Whitney’s co-adviser, Pedro Américo Duarte Dias, a primatologist at Universidad Veracruzana in Xalapa, Mexico, and his colleagues reported an uptick in monkeys’ glucocorticoids, which they linked to high levels of human activity near the animals’ habitat. The bump in glucocorticoids didn’t correspond with other potential explanations, such as aggression against other groups, but seemed to be a result of nearby human activity and its noise.  

If Whitney, Amato and their colleagues can draw out the connections between the mantled howlers’ gut microbiomes and health markers such as inflammation and stress hormones, it may provide a new, noninvasive way to track the monkeys’ health that’s simpler and faster than the current methods. Information on the monkeys’ conservation status is typically gathered via decades-long population surveys. These counts can take 30 to 60 years to show generational differences — a long time to wait in a rapidly changing forest. But fecal samples, which are easy to collect from wild primates, could provide metrics that can be monitored every couple of years.  

“This work has great potential to advance our ability to use the microbiome as a conservation tool,” Amato says, noting that this monitoring methodology could be applied to other species as well. “So many of the world’s primate species — upward of 60% — currently are threatened or endangered.” Trying to manage so many threatened species is like trying to extinguish many fires at once, so the new welfare assessment tool may help conservation groups prioritize the primates facing the greatest risk.  

Whitney is already working on this. She is developing a database for the primatology community to share adaptations to her methods for gauging fecal indicators of health. Because primate species have different diets, the makeup of their feces will vary, with different concentrations of microbes or hormones to measure. A database of methods would foster communication about methodological choices among scientists, Whitney says.