Spare a minute from your frenetically busy day to consider the quite different life of the three-toed sloth.
It’s true that the sloth, which lives in the jungles of Central and South America, would barely prevail a race with a snail. But it’s not a sluggard because it’s lazy. Rather, it has carved out a remarkably ingenious mode of life in the treetops, but one that imposes certain constraints on its speed and energy level.
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.
The probable interplay of these three components has now been worked out by a team of biologists led by Jonathan N. Pauli and M. Zachariah Peery at the University of Wisconsin. Their first step was to ponder a 35-year-old mystery about the behavior of the sloth.
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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.
Its distant evolutionary cousin, the two-toed sloth, stays safely in the canopy, out of the jaguar’s view. The visit to the ground, the researchers concluded, could not be for the tree’s benefit, because the sloth’s dung would not make much difference to its nutrition. Rather, they assumed, it was to favor a critical component of the sloth’s ecosystem, the pyralid moth. The descent to the sloth’s midden affords the pregnant moths in its fleece a chance to lay eggs.
The moths’ caterpillars are coprophagous or, to put it more bluntly, consumers of excrement. They grow to maturity in the sloth’s dung pellets and, on hatching, flutter up to the trees to find a sloth host. Burrowing into its fur, they mostly shed their wings and live there happily for the rest of their days, mating and dying in a safe, protected environment.
Sloths, Moths and Algae
Researchers studying why three-toed sloths would risk their lives to defecate on the forest floor found that the activity helps support a complex and beneficial ecosystem in the sloth’s fur.
Green algae grows on the sloth’s hair, which has tiny cracks that store water. The sloths are thought to eat the nutrient-rich algae to supplement their limited diet of leaves.
Three-toed sloths spend most of their lives in the forest canopy. The sloth’s diet of leaves is hard to digest and low in nutrients, and sloths have the slowest digestion of any mammal.
The sloths descend to the forest floor once a week to defecate. The journey is risky, and uses about 8 percent of the sloth’s daily calories. (Two-toed sloths typically defecate from the canopy instead.)
Adult moths leave the dung pile and fly up to the canopy, in search of sloths and mates. Moths increase the amount of nitrogen in the sloth’s fur, which encourages algae to thrive.
A species of moth lives in the sloth’s fur. Pregnant moths lay eggs in the sloth’s dung pile, where moth larvae will live until they mature.
After they die, their bodies are decomposed by the host of fungi and bacteria in the sloth’s fur. The metabolic products of this decay, especially nitrogen, are the feedstock for the specialist algae that grow in the sloth’s hair shafts. The researchers guessed that the sloths might be eating the algae from their own fur, and that this could be the purpose of the whole system.
Leaves are poor sources of nutrition, and animals that depend on them, like gorillas, often require large guts to hold them all. The sloth, having to climb along thin branches, can’t afford a big gut. It moves slowly because every calorie counts, and it pays to slow down its metabolism. But the invention of giving over its fleece to algae farming would go a long way to solving its problem of limited nutrition.
Dr. Pauli and his colleagues guessed that the sloth might be overcoming the poverty of its leaf diet by eating the algae on its fleece, and that the moths were essential fertilizer for the algae. In their paper they report much evidence in support of their hypothesis. The greater the infestation of moths, the more nitrogen a three-toed sloth carries in its fleece and the greater the amount of algae. An analysis of stomach contents showed the sloths were indeed eating the algae.
Two-toed sloths, which defecate from the trees, also harbor moths though to lesser extent. Still, they seem to be taking advantage of the sloth-moth-algae mutualism without sharing any of the risk. What could be lower on the moral totem pole than a freeloading sloth?
Dr. Pauli said he and Dr. Peery started their sloth project in 2009 on a cacao tree plantation in Costa Rica, with the goal of seeing if the sloths could colonize the plantations when their native forest was destroyed. Studying a sloth’s movements might seem as exciting as watching paint dry but the researchers sidestepped this tedium, Dr. Pauli said, by tracking the sloths with electronic collars.
Genetic engineers sometimes dream of inserting chlorophyll molecules into human skin cells so that people could photosynthesize their own food. The sloth had the idea first, probably millions of years ago.
O.K., back to your harried, fast-paced schedules. But remember the sloth, which has solved all its problems by living in the slow lane.
