BBC Wildlife Magazine
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.
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.