Now, a team of scientists from Wisconsin has
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. 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.We'll leave that one until the end.
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.
It's all a nicely spun story, but as in all thing evolutionary there is no exact answer. There is no "purpose" that a particular adaption exists, as if someone thought about it. The real answer is "because it works, for now."
The next important question is how a moth eaten, algae covered beast gets close to so many great chests.
Wombat-socho has the great collection for the week, "Rule 5 Sunday: All Hail The Superb Owl!" up at The Other McCain.
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