A pile of raccoon feces can either be a delicatessen or a death trap. The masked mammals tend to all poop in the same place, and these large mounds of dung contain partially digested seeds that are a treat for some animals, but also contain a roundworm parasite that’s lethal to others. Now, after spending 3 years watching these poop piles, researchers have found that species at risk of dying from the parasite give the latrines a wide berth, whereas those that face only tummy trouble often decide that the allure of seeds is worth it.
The results may not seem surprising, but they shed light on how the avoidance of parasites can shape an entire ecosystem through what some call an “ecology of fear,” says Vanessa Ezenwa, a disease ecologist at the University of Georgia in Athens who was not involved in the research. “Parasites have the potential to be influencing a lot of processes happening in the wild, and that’s a really important thing for us to understand.”
To conduct the study, disease ecologist Sara Weinstein of the University of Utah in Salt Lake City and her team of undergraduates mapped out 50 raccoon latrines in the 60-hectare Coal Oil Point Reserve, a coastal nature preserve in Santa Barbara, California. The mammals weren’t shy about where they pooped: The bases of boulders, the top of fallen logs, and even the summits of prominent hills were popular sites. (It’s thought the latrines may play a role in marking territory.) The team set up camera traps to catch the raccoons in action and monitor how 60 other species responded to the piles.
Rats, which can pick up the raccoon roundworm (Baylisascaris procyonis) but don’t seem badly affected by it, were among the most frequent visitors. They were observed digging through the piles for seeds about once a week. Mice, birds, and rabbits, which can all be killed by the roundworm, stayed away. And animals such as lizards and bobcats, for which raccoon feces is neither food nor a risk, treated the latrines no differently than any other place, the team reports in Oikos. “Animals definitely altered their behavior around these sites,” Weinstein says.
“Avoidance was strongly related to the potential risk of infection,” agrees Janet Koprivnikar, a parasite ecologist at Ryerson University in Toronto, Canada, who was not involved with the research. She says the work is the first study to take such a nuanced look at parasite avoidance across so many species.
Studying parasite avoidance can help scientists understand how disease is helping shape entire landscapes and ecosystems, Weinstein says. For example, some research has shown that invasive fire ants sometimes stay in their nests and avoid foraging to prevent infection by phorid flies, allowing native ants to regain territory. Grazing animals often defecate in certain areas and forage in others, creating a pattern of short and tall grasses that alters local vegetation. Parasite avoidance is also a likely reason why the carcasses of herbivores are rapidly scavenged by other animals, whereas dead carnivores are not and why the latter end up providing more nutrients for invertebrates and vegetation. Carnivores are more likely to harbor parasites that can infect other carnivores. (And cannibalism is especially risky, because a parasite that killed an animal of your own species is more likely to be a threat to you as well.)
There was one other species that strongly avoided the latrines during the study: humans. Raccoon roundworm infection in humans is rare but can be deadly, so the researchers observed the latrines from a distance and wore special shoes which they bagged before going home. “Latrines are not something the average person wants to come into contact with,” Weinstein says. “We didn’t mess with them.”