Swim­ming in the slow lane

Although su­perbly adapted to a se­cret life in the rain­for­est canopy, sloths don’t spend all their lives hang­ing up­side down, as zo­ol­o­gist Becky Cliffe has dis­cov­ered.

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When we think of ocean-go­ing mam­mals we’re likely to imag­ine 15m hump­back whales breach­ing above the waves, or or­cas glid­ing un­der the sur­face, or slinky ot­ters wind­ing their way through kelp forests. What we don’t of­ten think of are sloths. Although a tri­fle un­gainly, a sloth swim­ming in the sea is an amaz­ing sight. When I first saw one, I thought it was seaweed float­ing on the sur­face. It was only on closer in­spec­tion that I re­alised the ‘clump’ was in fact a pygmy three-toed sloth mov­ing steadily through the clear wa­ters of a Pana­ma­nian la­goon.

Su­per­fi­cially, sloths just don’t look like they should be in the water. The soaked fur gave the in­di­vid­ual I saw a bedrag­gled, di­shev­elled look, but in de­meanour it seemed per­fectly calm and con­tent. There­fore, I sup­pressed my nat­u­ral urge to scoop the poor crea­ture up and ‘save’ it from a wa­tery fate. This was clearly a wild an­i­mal in its en­vi­ron­ment and I was the pe­cu­liar, out-of-place hu­man in the scene. Free from the tug of grav­ity and sur­pris­ingly buoy­ant thanks to an over­sized and gassy stom­ach, the sloth bobbed across the sea’s sur­face, pulling it­self along with its fore­limbs in a slow-mo­tion doggy pad­dle.

Sloths are very well adapted for an en­ergy-ef­fi­cient life hang­ing up­side down in the rain­for­est canopy, from

their long gan­gly limbs with elon­gated hook-like fin­gers, to their dense, shaggy coats. How­ever, all six sloth species are ac­tu­ally fan­tas­tic swim­mers, while cer­tainly not win­ning any medals for speed or grace. In­deed, a submerged sloth can move up to three times faster in water than it can move on land.

At first, a sloth in water may seem about as ap­pro­pri­ate as a fish in a tree, and swim­ming a fu­tile abil­ity for an an­i­mal that spends the ma­jor­ity of its time high up in the canopy. But if you’re liv­ing in the rain­forests of Cen­tral and South Amer­ica, swim­ming is an es­sen­tial sur­vival skill. In these re­gions, vast and dy­namic rivers frag­ment the for­est – they can be a pal­try trickle at times but then turn into a rang­ing tor­rent after heavy bouts of rain. Be­ing un­able to leap from branch to branch to tra­verse breaks in the canopy as a monkey would, swim­ming be­comes a sloth’s ef­fec­tive strat­egy to avoid ge­o­graphic iso­la­tion.

Sloth fos­sils tell the story. Mod­ern sloths con­verged into tree-dwellers from two sep­a­rate lin­eages of ground sloths, sep­a­rated by mil­lions of years of evo­lu­tion, and we know these bear- to ele­phant-sized sloths were also ca­pa­ble swim­mers. One genus of fos­sil sloth in par­tic­u­lar, Tha­las­soc­nus, ap­peared not just ca­pa­ble of swim­ming, but was specif­i­cally adapted for a ma­rine life­style. So while mod­ern sloths don’t strike us as par­tic­u­larly adept in the water, they’re in good com­pany within the Xe­narthra group of mam­mals.

Tak­ing a closer look at some mod­ern sloth traits re­veals a num­ber of un­usual fea­tures that fa­cil­i­tate their ap­ti­tude for swim­ming, and that ex­plain their ease in the water. Due to their in­cred­i­bly slow di­ges­tive rate, all sloths boast an enormous multi-cham­bered stom­ach which can ac­count for up to a third of their body weight. This big bag of fer­ment­ing leaves cre­ates a lot of gas and there­fore acts as a gi­ant flota­tion de­vice. Once freed from the need to in­vest ef­fort and en­ergy in stay­ing afloat, the sloth can sim­ply bob along us­ing its long arms to con­trol the di­rec­tion of travel.

Along­side hav­ing a lilo-like stom­ach, three-toed sloths pos­sess nine cer­vi­cal ver­te­brae, two more than any other mam­mal. When in the canopy this ex­tra-long neck pro­vides the an­i­mal with an un­usual abil­ity to turn its head through 270° (in con­trast to our abil­ity to look left and right in a 180° arc), al­low­ing the world to be viewed the right way up de­spite hang­ing up­side down. When swim­ming, this long neck has fur­ther ben­e­fits, al­low­ing the sloth to keep its nose high above the water, much like a swim­ming ele­phant uses its trunk as a snorkel.

While all sloth species share a sim­i­lar fond­ness for water, the enig­matic pygmy three-toed sloths are quite unique as, out­side of the fos­sil record, they are the only species known to swim in salt water. Recog­nised as a dis­tinct species in 2001, these lit­tle sloths are found ex­clu­sively on the small, re­mote Isla Es­cudo de Ver­aguas. Ly­ing 16km off the Caribbean coast of Panama, this hidden world is home to a num­ber of other en­demic species in­clud­ing the soli­tary fruit-eat­ing bat and the Es­cudo hum­ming­bird. On this is­land 3.3km2 of im­pen­e­tra­ble jun­gle are fringed by man­grove forests and pris­tine la­goons, all shel­tered from the un­re­lent­ing force of the Caribbean Sea by a vast coral reef.


Be­ing con­fined on the tiny land­mass since it broke away from the main­land 9,000 years ago has forced pygmy three-toed sloths to de­velop some un­usual adap­ta­tions to is­land life, with per­haps the most ob­vi­ous be­ing their small size. As their name sug­gests, pygmy sloths are 40 per cent smaller than their main­land coun­ter­parts and an ex­cel­lent ex­am­ple of in­su­lar dwarfism. This strange phe­nom­e­non oc­curs when larger species re­act to be­ing iso­lated in a con­fined ecosys­tem by be­com­ing smaller. There can be nu­mer­ous eco­log­i­cal pres­sures re­spon­si­ble for this shrink­ing, such as a re­lease from preda­tory pres­sure, but in the case of the pygmy sloths lim­ited food and re­source avail­abil­ity are thought to be key.

While all three-toed sloths have a pre­dom­i­nantly fo­liv­o­rous (leaf-based) diet and con­sume ma­te­rial with a no­tably low caloric con­tent, pygmy sloths have taken this a step fur­ther. They feed pre­dom­i­nantly, if not ex­clu­sively, in the red man­grove swamps that fringe the is­land. These leaves are nu­tri­tion­ally poor and in­cred­i­bly tough to di­gest com­pared to the trees favoured by the main­land species. So the small size of the pygmy may be due to an even more calo­rie-de­fi­cient diet than a typ­i­cal sloth.

De­spite be­ing one of the most bi­o­log­i­cally pro­duc­tive ecosys­tems on earth, man­groves are an un­usual habi­tat of choice for any mam­mal. Perched pre­car­i­ously be­tween the land and the ocean, these trees are ex­posed to ex­treme vari­a­tions in con­di­tions; from scorching trop­i­cal tem­per­a­tures to suf­fo­cat­ing mud and in­tol­er­a­ble salt lev­els. Ex­actly why this species of three-toed sloths seems to favour the man­groves over the vir­gin rain­for­est cov­er­ing the in­te­rior of the is­land is one of the mys­ter­ies that sci­en­tists are still try­ing to un­ravel.

All sloths utilise cam­ou­flage as their main form of defence. Preda­tors tend to track move­ment, so tak­ing life slowly and al­low­ing green al­gae to grow on their hair are tac­tics that make it frus­trat­ingly dif­fi­cult to spot a sloth in the rain­for­est canopy. While this is an out­stand­ingly suc­cess­ful strat­egy for the sloths, com­bined with the re­mote­ness of the is­land and the dense­ness of the for­est it makes di­rect ob­ser­va­tional re­search par­tic­u­larly dif­fi­cult. As a re­sult, no­body re­ally knows for sure what the pygmy sloths are do­ing, or where they are do­ing it. It is en­tirely pos­si­ble that the sloths have been con­sid­ered by peo­ple to live ex­clu­sively in the man­groves sim­ply be­cause they are easier to spot there.


Another con­se­quence of low ob­ser­va­tions of the an­i­mals is that no­body re­ally knows how many pygmy sloths are on the is­land. Cur­rent data sug­gests there could be fewer than 500 of these lit­tle an­i­mals, but the fig­ure could be as low as 100. They are cur­rently ranked num­ber 16 on the Zoo­log­i­cal So­ci­ety of London (ZSL) EDGE of Ex­is­tence Mam­mals List, and are recog­nised as Evo­lu­tion­ar­ily Dis­tinct and Glob­ally En­dan­gered.

How­ever, new re­search be­ing car­ried out by ZSL’s EDGE of Ex­is­tence Pro­gramme is be­gin­ning to shed light on these small mam­mals. The project in­volves the first long-term study into the dis­tri­bu­tion, move­ment pat­terns, and habi­tat use of the pygmy sloths. Twice a year a team of conservati­onists visit the is­land and walk tran­sects through the man­groves and dense in­te­rior for­est, count­ing sloths. By com­plet­ing these reg­u­lar sur­veys the team is able to mon­i­tor any changes in the over­all size of the pop­u­la­tion as well as map the dis­tri­bu­tion of sloths. Through this work, the first pygmy sloth liv­ing deep in the in­te­rior for­est has been dis­cov­ered, adding ev­i­dence to the the­ory that the an­i­mals may not be en­tirely re­liant on the man­groves.

The team has also tagged a num­ber of pygmy sloths with very high fre­quency (VHF) ra­dio­track­ing col­lars and hope to em­ploy a col­lec­tion of minia­ture GPS log­gers at­tached via back­pack-style har­nesses. Un­like col­lars you might have seen de­ployed on larger mam­mals, these har­nesses are built specif­i­cally for the sloths so they don’t ham­per their range of move­ment or add ex­ces­sive weight that would force the an­i­mals to burn more calo­ries while climb­ing. From the re­sul­tant data, re­searchers will be able to see ex­actly where each an­i­mal trav­els and how they are util­is­ing each type of habi­tat. In­for­ma­tion like this is es­sen­tial for the long-term pro­tec­tion of pygmy sloths. Without know­ing the par­tic­u­lar habi­tat re­quire­ments of a species it is very dif­fi­cult to de­velop ef­fec­tive con­ser­va­tion strate­gies that tar­get those areas.


Ecosys­tems ex­ist in a del­i­cate bal­ance. Un­for­tu­nately, it does not take much to tip the scale and the pres­sures of an ever-ex­pand­ing hu­man pop­u­la­tion are be­gin­ning to take their toll. Although the is­land of Es­cudo de Ver­aguas is tech­ni­cally un­in­hab­ited, fish­er­men fre­quent the area and sea­sonal set­tle­ments are be­com­ing more preva­lent. As a re­sult, log­ging is a rapidly grow­ing threat as peo­ple use in­creas­ing amounts of man­grove tim­ber for fire­wood and con­struc­tion. Fur­ther­more, there is a wor­ry­ing in­crease in the num­ber of tourists il­le­gally vis­it­ing Es­cudo, and the is­land is dif­fi­cult to po­lice. Tourists, how­ever well mean­ing, in­evitably dis­turb the wildlife and add more pres­sure to a frag­ile ecosys­tem. While many wildlife-watch­ing ac­tiv­i­ties take rea­son­able steps to pre­vent bring­ing harm or dis­tur­bance to the an­i­mals of in­ter­est, we are learn­ing more and more that there is no way to visit the land of the swim­ming sloth without hav­ing an im­pact.

In an at­tempt to min­imise the ef­fects of hu­man en­croach­ment on the pygmy sloth’s habi­tat, there are nu­mer­ous on-go­ing projects aimed at im­prov­ing en­vi­ron­men­tal ed­u­ca­tion in the lo­cal com­mu­ni­ties and work­ing with au­thor­i­ties to de­velop con­ser­va­tion so­lu­tions. This ap­proach, com­bined with sci­en­tific re­search and an in­creased aware­ness of the na­ture of the pygmy sloth, pro­vides hope for the fu­ture of this en­chant­ing is­land.

Sloths may lack the majesty we as­so­ciate with other, more charis­matic mam­mals, but they are fas­ci­nat­ing crea­tures. Care­ful re­search will con­tinue to un­ravel their mys­ter­ies so that no more of their kind en­ter the fos­sil record.

Above: sloth hair grows in the op­po­site di­rec­tion to most an­i­mals so water runs away from the skin when they are up­side down. Be­low: sloths move slowly to con­serve en­ergy.

Above left: Becky Cliffe ( left) and Sarah Kennedy take hair sam­ples from a pygmy three-toed sloth. Above right: the Crit­i­cally En­dan­gered species has a tan-coloured face with a dis­tinc­tive dark band across the fore­head.

Above: Pygmy three-toed sloths are ar­bo­real fo­li­vores that eat the leaves of a va­ri­ety of trees. Right: The species has evolved a much smaller body size com­pared to its main­land rel­a­tives.

A pygmy three-toed sloth swims in a man­grove for­est. The species can only be found on the tiny is­land of Es­cudo de Ver­aguas, Panama.

Right: fe­male pygmy three-toed sloths may carry their ba­bies for six to 12 months. Be­low: sloths use their fore­limbs to move through the water.

Becky Cliffe re­leases a sloth that is wear­ing a ‘back­pack’ in a sanc­tu­ary.

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