Sloths, moths, algae may live in three-way benefit pact

Insects and green slime may justify slow mammal’s risky descent from trees

MAMMAL’S LITTLE HELPERS  To make a living as one of the rare mammals that eats only tree leaves, a brown-throated three-toed sloth may be getting some help from mating moths and sloth-specialist algae.

Christian Mehlführer/Wikimedia Commons

A tree-dwelling sloth’s climb down to ground level for its weekly bathroom break may not be pointless daintiness.

The trip is risky and (for a sloth) energetically expensive. Yet the effort could be the sloth’s contribution to a mutually beneficial, three-way partnership.  The sloth’s trips seem to encourage moths that mate in the mammal’s fur and algae that thrive on moth detritus, suggests Jonathan Pauli of the University of Wisconsin-Madison.

The payoff for the sloth, after a Rube Goldberg-like string of actions by the other partners, could be rich blooms of nutritious algae to eat off its own fur, Pauli and his colleagues propose January 22 in the Proceedings of the Royal Society B.

“Sloths are bizarre,” Pauli says with evident enthusiasm. Why three-toed sloths go to the trouble to clamber down to the forest floor to scrape out a latrine depression about once every 8 days has been just one question about their lives.

Musing about what might drive tree descent, Pauli and other researchers compared a tree-descending species with a less fastidious, two-toed sloth. At a study site in Costa Rica, researchers vacuumed the fur of strict tree-descenders, the brown-throated three-toed sloths (Bradypus variegatus) and of Hoffmann’s two-toed sloths, which sometimes climb down and sometimes don’t bother to. Compared with their tree-pooping cousins, the three-toed sloths harbored more than four  times as much moth mass in their fur. Washings from their fur samples had more nitrogen available to nourish algae — and more algae.

The scenario Pauli and colleagues put together starts with Cryptoses moths gathering in sloth fur to choose mates. When the sloth climbs to the ground, female moths rush to lay eggs in fresh sloth dung, the preferred baby food for the larvae.

The idea that the moths benefit from hanging around sloths is “not at all new,” says Adriano Garcia Chiarello, a conservation biologist at the University of São Paulo. What is novel, he says, is the suggestion that harboring abundant moths eventually provides a benefit for the sloths.

Moths may boost sources of nitrogen in the fur, Pauli and his colleagues say. Newly mature moths landing on sloth fur may bring with them nitrogen-spiked traces of their dung-rich childhoods. And when adult moths die after their mating frenzy, their decomposition may release nitrogen into the fur.

Moths could thus fertilize growth of the Trichophilus green algae that favor sloth fur. Lab tests found the algae should be highly digestible and up to five times richer in quick-energy lipids than sloths’ usual diet of leaves.  And the research team found algal cells in samples from three-toed sloth forestomachs. Just what percentage of the diet comes from the algae remains to be seen, Pauli says.

Chiarello, however, says that in far more than 1,000 hours that he and his students have watched sloths in the wild, no one has seen sloths eating algae. “I don’t remember ever seeing a sloth lick its fur,” he says.

The paper may be overlooking some other players, speculate Jaanika Blomster and Milla Suutari of the University of Helsinki. They’ve described a wide range of tiny organisms that survive sloth grooming. It may be “more like a mini-ecosystem occurring in sloth fur, rather than just a two- or three-way symbiosis,” Blomster says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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