Thursday, 15 March 2018

Triple Alliance

Brown-throated three-toed sloth (Bradypus variegatus)
Credit: Tauchgurke [Public domain], via Wikimedia Commons
One of the fascinating aspects of evolutionary ecology is how two or more species can become inextricably bound together in an evolutionary alliance of mutual dependency.

One such alliance, actually an alliance between three completely unrelated species, can be found in the Amazon jungle, centred on one of the strangest and more specialised mammals, the three-toed sloth. The three-toed sloth (in fact there are four closely related species, all in the South and Central American jungles) is one of the most slowly moving mammals on Earth, spending almost all its time hanging beneath tree branches high in the canopy, eating leaves or sleeping. However, they descend from the trees to the forest floor once a week to defecate, which they do in a large pile.

Its slow speed, although conserving energy, makes it especially vulnerable to depredation by harpy eagles, jaguars and other predators, so its descent to the forest floor to defecate is all the more puzzling, placing it at additional risk when it could simply defecate in the trees where its faeces could simply drop to the forest floor.

But all this begins to come together and make sense as an alliance with other species.

To aid in camouflage against the green of the forest canopy, the long fur of the sloth is distinctly green, not because of a pigment or iridescence, but because of the presence of algae which coat the hairs in a green film. The fur is also host to a multitude of small moths which don't appear to trouble the sloth. In fact, there must be a benefit to the sloth in having these moths living in its fur because the weekly habit of defecating in a large pile once a week only makes sense as a means to maintain the population of moths. As the sloths defecate, the female moths leave its fur and lay their eggs in the dung, giving their grubs a food supply, so ensuring a new generation of moths.

But the question then is why are these moths beneficial to the sloth? The answer is that they are actually beneficial to the algae which camouflage the sloths because their droppings supply essential nutrients to the algae. And, there is an additional benefit to the sloth in having a healthy population of algae living on its fur. It is actually farming them. By licking its fur, it gains essential nutrients not available in its specialised leaf diet.

So there we have a superb example of a mutually beneficial triple alliance produced by selfish-gene evolution. The alliance involves apparently risky behaviour balanced by ultimate benefit to all the species involved. The genes of three species have formed a mutually beneficial alliance by being selfish.

Of course, like the rest of biology, none of this makes any sense as the product of intelligent design. The entire thing is a solution to the problems of the sloth being slow moving, poorly camouflaged and dependent on a food source missing certain essential nutrients. None of these problems requires a complex alliance to solve and no intelligent designer would have designed problems requiring a solution anyway. Unplanned, mindless, unintelligent evolution on the other hand...

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