Kelsey Neam was strolling through the trees in Costa Rica and looking for sloths when she spotted something unusual. High on a tree branch, a three-toed sloth was eating leaves at an unhurried pace. It seemed oblivious to three brown jays that perched nearby and were watching it intently. Then one jay scooted closer and plunged its beak into the sloth's fur. Neam is a graduate student in ecology at Texas A&M University. She was in the Costa Rican cloud forests to study three-toed sloths (Bradypus variegatus) and where they live. But the interaction she stumbled upon has less to do with where sloths live, and more to do with what lives on them. The brown jay (Psilorhinus morio) seemed to be finding a meal in the sloth's fur. African oxpeckers do the same thing when they perch on the backs of rhinoceroses or zebras and pluck bugs from their skin. Other birds forage for food on the bodies of capybaras, enormous rodents that live in South America. But a brown jay had never before been seen feeding from another animal's fur. How good are the offerings on a sloth? Neam explains that a three-toed sloth's skin is a veritable cafeteria of food options. Fungi and green algae grow on its fur. Certain moth and beetle species reside there too, including pyralid moths. These moths hitch a ride to the forest floor during the sloth's once-a-week climb down from the canopy to defecate. There, the moths lay their eggs in fresh sloth dung. These eggs will hatch into larvae that live and grow inside the dung until they turn into adult moths; then they'll fly back up into the trees to find a new sloth host. When moths die in the miniature jungle of a sloth's fur, its resident fungi break down the moths' corpses. This provides nutrients that in turn feed the green algae growing on the sloth's fur. And three-toed sloths may eat this algae themselves. So while the moths rely on the sloths for their life cycles, the sloths may also rely on the moths. The insects may help fertilize a garden of algae that supplements a sloth's boring diet. Researchers speculated in 2014 that this might be why the sloth bothers to make a dangerous weekly pit stop on the forest floor, helping out its resident moths in the process, instead of just pooping straight off of a tree branch. Neam has observed jays feeding on sloths twice, and a Costa Rican naturalist has seen it at least once too. "My best guess is that this is not a rare occurrence," she says. It's not clear yet how brown jays affect the sloths they're snacking on, Neam says. "It can be difficult to tease apart the nature of species' relationships." If sloths are indifferent to their moth tenants, then brown jays aren't hurting anyone. If brown jays aren't eating moths at all, but instead consume parasites that live on sloth skin—mites, ticks, flies, and others—then the birds could be doing the sloths a favor. On the other hand, if the jays eat moths that the sloth relies on indirectly for its own food, the birds may be a kind of parasite themselves. This would be bad news for any sloths getting a visit from jays. It's just one of many hazards you face when you move through the world so slowly that the animals around you mistake you for a buffet table.
Image: J Yu. Video: Kelsey Neam.
Neam, K. (2015). The odd couple: interactions between a sloth and a brown jay Frontiers in Ecology and the Environment, 13 (3), 170-171 DOI: 10.1890/1540-9295-13.3.170