howler monkey eating in a tree


Two-Toed, Choloepus Hoffmanni; Three-Toed, Bradypus Variegatus – by Jacalyn Giacalone, Ph.D.

Posted in: Mammal Directory, Rainforest Connection Live

Other than howler monkeys, not many mammal species on BCI can live and flourish on a steady diet of tree leaves, but sloths do very well on this food source, provided they are the right kinds of leaves. Tree leaves tend to be coarse and difficult to digest, full of tough cellulose and protected by toxic chemicals produced by trees to ward off their predators. In this war between plants and the animals that would eat them, sloths have succeeded through adaptations that enable them to be leaf-eaters, or folivores. Furthermore, they do their leaf-eating on the crowns of mature trees, only infrequently coming down to the ground, so they are called arboreal folivores. The story of their adaptation to the canopy of the forest and to leafy food supplies is an unusual one.

Sloths have long, shaggy fur that they groom carefully every day. The mother sloth (facing to the left) was clinging to a tree trunk and carrying a baby near her neck (see the small face?).

Two species of sloth are found on BCI, living in the same habitats, and sometimes on the same tree. Both species have shaggy fur that is slightly greenish, long hooklike claws on their front and hind feet, and they usually move very slowly and spend most of their lives hanging upside down. Three-toed sloths, which are very common and may be the most numerous species on the island, have been estimated by Dr. Gene Montgomery to number about 8,000. They can be active both day and night, and tend to stay in the same tree for a day or more and move short distances. They have a smiling face with a dark mask around the eyes, weigh up to about 6 kg, and have a short tail. Adult male three-toes have a large patch of short fur on their back that is golden orange with a dark brown stripe down the middle. The two-toed population may be as high as 1500. These sloths tend to be active at night, changing trees frequently and moving rather long distances in one night. They weigh as much as 8 kg, have two long claws on the front feet, no mask on the face and no tail.

It can be difficult to find sloths on BCI, even though they are numerous, because their camouflage is excellent, and they tend to spend most of their time high in the canopy. Their hairs have tiny grooves in which green algae grow, so they often look like masses of dry vegetation. Whenever possible, they move slowly from tree to tree through touching branches and bridging lianas, rather than come down to the ground. Information about the movements and diets of sloths was learned by capturing sloths and putting collars on them with radio-transmitters. Tracking of sloths revealed that their home ranges on BCI are usually less than 2 ha. Drs. Montgomery and Sunquist reported that even with radio-transmitters on 42 wild sloths, it was difficult to observe them feeding, and very often it was impossible to see the sloths at all, even though their precise location was known. These researchers never did see a two-toed sloth feeding, and compiled only occasional records for three-toed sloths. Many questions remain about the biology of sloths; as these researchers noted, *we do not yet fully understand why each sloth was found where it was on a particular day.*

People used to think that sloths eat only one species of tree leaf, but as a population they feed on a great assortment of trees, using at least 25 tree species on BCI. However, each sloth family has its own few favorite tree species which are found within a small home area, and which are different from the preferences of their neighbors. Mother sloths appear to pass along their food preferences and their knowledge of the home area during a long period when they carry their young around their area. Sloths seem to have a matrilineal (transmitted through the mother) inheritance of home ranges and food preferences.

Sloth digestive processes work very slowly, because their food is difficult to process, and because their body temperatures fluctuate every day. Wild sloths have been found to have deep body temperatures ranging from 28 degrees C to 40 degrees C. They can raise their body temperature by basking in sunlight, but must also move to shade as they become too warm. Thus they practice thermoregulation in much the same way that lizards do. They depend on chemical processes used by bacteria in their gut to digest their food. Since chemical processes slow down as temperatures decrease, the night-time drops in temperature which usually cause a body temperature drop of about 7 degrees C, reduce their rates of digestion. Indeed, the rainy season weather is thought to present problems for sloth digestion, and most dead sloths on BCI are found in the October- December rainy period. Also, few trees have new leaves at this time, which means that although there are many leaves, they are difficult to digest. Sloths may starve at this time of the year, even with full bellies. Their digestion is so slow that it takes several days or longer for food to pass through their gut, and they eliminate waste only about once every 8 days, moving to the ground briefly to defecate. Compared with other mammal species that eat plants, sloths have the slowest rate of passage of food through the gut, a process that usually takes hours. The three-toed sloths use their short tail to dig a hole and then bury their feces, but the two-toeds have no tail and dig no latrine hole. This is a very odd behavior for arboreal animals that otherwise rarely descend to the ground.

Sloths are adapted to problems of leaf digestion in a variety of ways:

  1. Their stomach has complex pouches that are good for storing bulky food. The pouches separate batches of food that are in different stages of digestion and fermentation by bacteria. Three-toes have more complex stomachs than two-toes. This makes sense because three-toes are obligatory folivores (can live on nothing else), while two-toes can live on various fruits, flowers, and buds, as well as leaves.
  2. Since leaves differ in digestibility, sloths may choose specific types. Active sloths swivel their heads to look around very carefully before deciding which leaves to approach for feeding. Female sloths that are pregnant or carrying a young one often choose to feed on leaves of Lacmellea panamensis, which is one of the most easily digestible to sloths. Pregnant sloths also prefer Cecropia eximia , but mainly in August through December. However, females with young do not very often use Cecropia, which is not as digestible as Lacmellea.
  3. To speed up their life processes, three-toed sloths in particular will hang out in the sun in the morning, exposing their bellies for deep body warming, and will often choose trees to sleep in that will give them good basking sites in the morning.

Sloth mothers give birth to one baby once a year, after a six month pregnancy. The young one is weaned from milk after about six weeks, but continues to ride on the mother for another five months. During this time the young sloth learns from its mother which leaves are to be eaten, where to find shelter, and where edible trees are located. Until six months old, the young one keeps at least one foot on its mother while reaching for leaves to eat, until finally the mother just moves away and keeps on going to another part of her home range.

Two-toed sloths like to sleep in tangles of lianas in trees, as do other arboreal mammals such as squirrels. These tangles provide concealed spaces but also act as early warning systems against approaching predators. A two-toed in a tangle is difficult to see, and then once located, hard to catch, because they wake up when the vines shake and either start to move away or face the opponent, attacking by slashing with claws, grasping and pulling the offender closer to be bitten. Two-toes have two sharp pointed teeth in each of their jaws. These teeth look like, but are not, canines. The upper and lower teeth slide past each other and sharpen the blades so that they are always very sharp. They hiss loudly and make bleating noises when threatened. Three-toes have few flattened teeth and rarely bite, although they will slash out with their front claws if attacked. They are considered more docile than two-toes. However, I have seen a three-way fight among three male three-toes. They first called to each other from about 50 m apart with loud high-pitched screaming sounds (which some describe as shrill whistles). The sloths approached each other at top-speed-for-sloths, and then engaged in slashing, arm-swinging battles. One male fell from the tree to the ground, about 25 m. Stunned, but apparently unharmed, he soon began the process of climbing the nearest sapling and scanning his surroundings, finally reaching for lianas that would get him back up to the canopy.

The anatomical and behavioral differences between these two sloth species support the idea that they descended from two very different sloth lineages, the two-toes being in the same grouping as the extinct giant ground sloths. Researchers speculate that tree-dwelling sloths were on the evolutionary scene before primates appeared and have competed successfully against monkeys that would eat leaves. They achieved their success in excluding all but howler monkeys by 1) being able to use many different tree species, excluding monkeys that might specialize on a few of those species, and 2) evolving a matrilineal social inheritance of tree preference that allows sloths to avoid competing with their sloth neighbors. Indeed, their closest competitors are not mammals, but reptiles: iguanas are arboreal folivores who have generalized food habits. However, iguanas do not have a social inheritance of food preferences.

© Jacalyn Giacalone, Ph.D. 1998