It’s been said that there’s an imbalanced focus on ornithology within our site, but we can also claim to have a slightly skewed preference for sloths as well. Whether it’s their permanently gentle grin or their slow, methodical movements we’re not sure, but we know we’re not the only ones who find them fascinating.

Sloths are found in both rainforest and dry tropical forest ecosystems but the biodiversity of their habitat is nothing compared to what they carry around with them in their arboreal lives. A team of biologists from the University of Wisconsin led Jonathan N. Pauli and M. Zachariah Peery has recently tackled a 35-year-old mystery about sloth behavior.

The sloth is not so much an animal as a walking ecosystem. This tightly fitting assemblage consists of a) the sloth, b) a species of moth that lives nowhere but in the sloth’s fleece and c) a dedicated species of algae that grows in special channels in the sloth’s grooved hairs. Groom a three-toed sloth and more than a hundred moths may fly out. When the sloth grooms itself, its fingers move so slowly that the moths have no difficulty keeping ahead of them.

Every week or so, the sloth descends from its favorite tree to defecate. It digs a hole, covers the dung with leaves and, if it’s lucky, climbs back up its tree. The sloth is highly vulnerable on the ground and an easy prey for jaguars in the forest and for coyotes and feral dogs in the chocolate-producing cacao tree plantations that it has learned to colonize. Half of all sloth deaths occur on the ground. The other serious hazard in its life is an aerial predator, the harpy eagle.

Why then does the sloth take such a risk every week? Researchers who first drew attention to this puzzle in 1978 suggested that the sloth was seeking to fertilize its favorite tree. Meanwhile, the algae that gave the sloth’s coat a greenish hue were assumed to provide camouflage.

Writing last week in Proceedings of the Royal Society B, the Wisconsin researchers assembled all these pieces in a different way. They started by trying to understand what would compel the sloth to brave the dangers of a weekly visit to ground zero. Continue reading →