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201221Jul07:58

Hoffmann’s two-​toed sloth: a lazy mat­ing mammal

Infor­ma­tion
pub­lished 21 July 2012 | mod­i­fied 25 July 2012
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Hoffmann’s two-​toed sloth males do defend ter­ri­to­ries from rivals, but their sloth­ful natures mean they aren’t much good at hold­ing onto females. Why is it that they treat their sex­ual part­ners this way?

Slow, so slow

Hoffmann two-toed slothAll sloths have a rep­u­ta­tion for being lazy. This is some­times exag­ger­ated — they don’t sleep much more than humans do — but basi­cally cor­rect. Sloths have unusu­ally low meta­bolic rates and spend hours each day doing noth­ing. Hoffmann’s two-​toed sloth is a case in point. It spends the day hang­ing upside-​down from branches high in trees, often hid­den away within tan­gles of vines. Dur­ing the night the sloths move around and feed, often for 7 or even 11 hours. But they’re not exactly ath­letes, mov­ing along branches at just 14 cen­time­tres per second.

They are also com­pletely and utterly anti­so­cial. Unless they’re mat­ing or car­ing for a young­ster, you hardly ever see more than one sloth in a tree. That sug­gests male sloths might be ter­ri­to­r­ial, con­trol­ling patches of for­est and pre­vent­ing other males from enter­ing. Con­trol­ling ter­ri­tory is one way of con­trol­ling access to females. But Zachariah Peery of the Uni­ver­sity of Wisconsin-​Madison won­dered how good these sloth­ful males would be at monop­o­lis­ing females.

Every breath you take

With Jonathan Pauli, Peery mon­i­tored 152 sloths in Costa Rica from 2010 to 2011 (results pub­lished recently in Ani­mal Behav­iour). Most males had small home ranges of less than 0.2 square kilo­me­tres. There was some small over­lap around the edges of the ranges but, in gen­eral, males kept out of each other’s territories.

That wasn’t true of the females though. On aver­age there were 3.2 adult females within the range of each male, and 60 per cent of females lived in the ranges of more than one male — some­times as many as four. So it didn’t look like the males were monop­o­lis­ing the females, and that was con­firmed by pater­nity tests. Females and their off­spring were some­times found liv­ing in the range of a male other than the father, so males aren’t con­trol­ling the females in their ter­ri­to­ries. Or if they are, the males haven’t really mas­tered the skill.

The slow-​moving nature of sloths may be largely respon­si­ble for the males’ casual atti­tude, but Peery points out another pos­si­ble expla­na­tion. Males tend to set up their ter­ri­to­ries close to their birth­place, often within a kilo­me­tre, so neigh­bour­ing males were often closely related.

As a result, males can actu­ally ben­e­fit their genetic legacy by allow­ing neigh­bours to sneak in the odd mat­ing. By doing so, they are help­ing rel­a­tives to pass on their genes — many of which they will share. If that’s true, care­less lazi­ness is a way to help your fam­ily prosper.(Another exam­ple which would fit nicely into Richard Dawkins’ book The Self­ish Gene, –Moos-​)

The above news item is reprinted from mate­ri­als avail­able at New­Sci­en­tist. Orig­i­nal text may be edited for con­tent and length.

(Source: New­Sci­en­tist Zoolog­ger, 20.07.2012)

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