Known for their slow-moving nature from whence they get their name, sloths have very low metabolic rates that keep them from doing much of anything for most of the day. But do these tree-loving mammals also take the slothful approach to the mating game?
There are six species of living sloths, which are classified as either three-toed sloths or two-toed sloths, and live throughout the jungles of Central and South America.
Despite sharing a name and a similar appearance, the two types of sloths aren't closely related and actually belong to two different taxonomic families. [Photos: Slow-Moving and Ridiculously Cute Sloths]
"A sloth just isn't a sloth," said Jonathan Pauli, a wildlife ecologist at the University of Wisconsin-Madison, who has studied sloths. "Two-toed sloths and three-toed sloths are quite different from each other."
One of the differences between the sloth families is their mating system, Pauli told Live Science.
Three-toed vs. two-toed sloths
Three-toed sloths exhibit strong polygyny and weak female promiscuity, in which certain males are very reproductively successful, mating with many females and fathering the majority of offspring in a population. Two-toed sloths, on the other hand, appear to have weak polygyny and strong female promiscuity.
When the sloths mate also appears to differ between the three-toed and two-toed varieties.
Three-toed sloths have a kind of "birth pulse," Pauli said — females mate during the late summer and early fall, give birth early in the next year and then carry around their baby until the next mating season. "It's fairly fixed on the calendar cycle," he said, adding that two-toed sloths, by comparison, have a "blurred reproductive schedule that happens year-round."
Male two-toed and three-toed sloths are largely solitary creatures that live in defined home ranges, which may overlap some. However, male two-toed sloths' ranges can be up to an order of magnitude larger than their distant three-toed cousins.
Female two- and three-toed sloths also keep their own small home ranges, which often intersect with the home ranges of multiple males at the same time, according to recent research conducted by Pauli and his colleague Zach Peery.
Just how the sloths get down to business within these ranges is not completely understood.
Getting down to business
Reports suggest that females will broadcast their willingness to mate with a kind of high-pitched scream.
"In some localities, local people from rural areas surrounded by large tracts of native forest frequently report hearing the 'whistlelike' call of maned sloths (Bradypus torquatus) in certain times of the year," said biologist and sloth researcher Adriano Chiarello of the University of São Paulo in Brazil. "This call is a loud 'eeeeeh' lasting more or less [about] one second."
One of Chiarello's students once witnessed a male B. variegatus, a three-toed sloth, moving toward a calling female in the forests of Espirito Santo in southeastern Brazil, suggesting the female was actually trying to attract mates, Chiarello said. But what happens when a pair meets up is anybody's guess. "If there is courting behavior, we don't know," he said.
Whatever the case, there does appear to be some kind of sexual selection, with females picking the best males, though what qualities they find desirable is not clear. In a 2012 study in PLOS ONE, for instance, Pauli and Peery reported that a single male somehow sired half of all of the juveniles the researchers tested.
"The females are exhibiting a somewhat strong mate choice," Pauli said. "But what makes those males so sexy to the females?"
Sloth copulation is rarely witnessed in the wild, but researchers have reported seeing it occur both with the male mounting the female from behind and face-to-face. The act is brief, ending in just a few minutes. The male may try to mate with her again a few minutes later.
The pair sometimes remains together, even in the same tree, for more than a day before going about their own business, Chiarello said. And it would appear that males monitor the females in their ranges, and possibly battle with other males if they get too close; but it's unclear if these fights are actually over mates or territory.
"I have seen, however, some old males with scars in their faces or even blind [in] one eye, which could possibly result from fighting against same-sex opponents," Chiarello said.