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Sloths, moths and algae: a surprising partnership sheds light on a mystery

Sloths are famous for their exceptionally slow motor skills and petite faces that seem to beam with an almost natural smile. However, less commonly known is the unusual bathroom habit of certain sloth species. While spending the majority of their time in the safety of tree canopies, three-toed sloths regularly place themselves in mortal danger by descending to the forest floor to defecate. For years, scientists have been trying to figure out what is driving this peculiar and risky behavior. Now, as outlined in a recent paper published in Proceedings of the Royal Society, lead author Jonathan Pauli from the University of Wisconsin-Madison believes his team of researchers has found an important clue to this mystery involving an unusual and beneficial relationship among sloths, moths and algae.

Native to the tropics of Central and South America, arboreal sloths spend their lives eating and resting in the forest canopy. Millions of years ago, sloths diverged into two distinct groups: two-toed sloths (Choloepus species) and three-toed sloths (Bradypus species). Sloths, like all arboreal herbivorous mammals, have evolved highly specialized anatomical, physiological and behavioral adaptations to overcome the nutritional and energy limitations of their constrained lifestyles. Their small to mid-sized bodies must be light in order for the tree canopy to support their weight. In addition, both two-toed and three-toed sloths evolved foregut fermenting stomachs; this means that they must chew their highly fibrous plant-based food twice in order to digest it, first when eating the plant and again after the food has fermented in a specialized stomach. But, apart from digestion habits and slow mobility, the two-toed and three-toed sloths lead very different arboreal lifestyles.

A two-toed sloth. Photo by: Jonathan Pauli.
A two-toed sloth. Photo by: Jonathan Pauli.

“What’s interesting is that we have two [groups] of sloths; we have two-toed and three-toed,” Pauli told “They are actually quite different, behaviorally, ecologically. They diverged somewhere between 18-40 million years ago and on an evolutionary time scale, that is something like the difference between a turkey and a chicken. So it shouldn’t be too surprising that their behavioral ecology is quite different as well.”

For starters, the two kinds of sloths have different activity periods. Two-toed sloths are primarily nocturnal whereas three-toed sloths are diurnal, carrying out their activities during the day. Two-toed sloths frequently roost in and travel across multiple trees within their 0.5 square mile (1.4 square kilometer) home range and can live in various habitat types. In contrast, three-toed sloths demonstrate stricter behavior, choosing to roost in only a few trees of specific species, which results in much smaller ranges averaging a tenth the size of those inhabited by their two-toed relatives. Along with occupying different tree habitats, the two species also exhibit very different mating behaviors.

“Three-toed sloths are strongly polygamous, where one male will mate with many females and sire offspring,” Pauli said. “On our study site, one old male was responsible for 50 percent of the juveniles in our population sample. Whereas the two-toed sloths are actually more promiscuous. They demonstrate more of an open mating system between males and females.”

Another notable difference between the two species is their diet and metabolic rate. Two-toed sloths have a very diverse diet consisting of animal matter, fruit and leaves, while three-toed sloths are much pickier eaters and subsist only on the leaves of a few select tree species. The three-toed sloths’ toxic, low-nutrient diet of only tree leaves causes it to have an exceptionally low metabolic rate and the slowest known digestion rate of any mammal. So it is no wonder that these slow moving mammals are called los perezos – the “lazies” in Spanish; their limited diet and slow metabolism leaves them with very little energy to do much else besides resting and eating.

However, despite their limited energy level, three-toed sloths make a weekly and dangerous journey to the forest floor to defecate. At the base of its main tree, the three-toed sloth digs a hole in the ground with its tail, deposits its dung and then covers up its latrine before ascending back into the tree canopy. This peculiar behavior is not only energy intensive for the animal, using an estimated eight percent of its daily energy budget, but also mortally dangerous. Sloths are extremely vulnerable to predation on or near the ground, where more than half of all sloth deaths occur. So why do three-toed sloths risk their lives for their bathroom habits? Scientists have long believed there must be hidden benefits that drive this curious behavior, such as fertilizing a favorite tree or communicating with other sloths. However, Pauli and his team believe the answer could lie within the miniature ecosystems flourishing in the sloths’ fur.

A three-toed sloth. Photo by: Jonathan Pauli.
A three-toed sloth. Photo by: Jonathan Pauli.

The fur of a sloth doubles as a personal, edible garden ecosystem that house a collection of diverse microorganisms, many of which are found nowhere else. A prominent member of this mobile ecosystem is the pyralid moth (Cryptoses species) whose entire life-cycle is dependent on the sloth. When a sloth descends to the forest floor to defecate, a pregnant female moth will leave her host and lay her eggs directly in the sloth’s dung. The larvae develop entirely within the dung, and when they emerge as adults they fly into the canopy to search for mating grounds in sloth fur, thus continuing the moth life-cycle. After observing how the moths benefit from their relationship with sloths, the researchers wondered whether or not the sloths get anything from this association.

To answer this question, Pauli and his team conducted an experiment comparing two species of sloths in Costa Rica: the brown-throated three-toed sloth (Bradypus variegatus), which defecates on the ground, and Hoffmann’s two-toed sloth (Choloepus hoffmanni), which defecates from the tree canopy and occasionally on the ground. They examined samples of sloth fur, collected moths using an “invertebrate vacuum” and analyzed the chemical composition of algae living in the fur. They found all three are interconnected; if an individual sloth has more moths in its fur, it will also have more algae and nitrogen. Three-toed sloths were observed to have more of all three components in their fur than two-toed sloths, which do not venture to the ground as often.

Through the different stages in their life-cycle, pyralid moths may be transporting nutrient-rich waste from sloth dung or perhaps adding nitrogen to sloth fur to replace that which has decomposed. In addition, Pauli believes that characteristics of individual sloth hairs may aid the cultivation process.

“[The mini-ecosystems of two-toed and three-toed sloths] host different organisms, but there is an important commonality,” Pauli said. “[Both species] have this complex micro biota that occurs within their fur. And it seems in part to me, that these hairs either have grooves or cracks that have the capacity to hold water and so they almost create this hydroponic grove for algae to be cultivated and moths which are fertilizing it, all within the complex interaction of micro biota in the fur.”

The cultivation of algae in sloth fur is very important because the researchers found that algal growth is a key source of food for sloths. To check whether or not the algae can be digested by sloths, the scientists mixed sloth fur with bacteria from a cow’s stomach, which has similar properties to a sloth’s stomach, and found the algae to be easily digestible. By analyzing the chemical compounds of the algae, the researchers found it contains the same amount of carbohydrates and protein as the tree leaves that sloths normally eat. In addition, it contains three to five times more fat, providing much-needed supplements to the sloth’s otherwise nutrient-poor diet. The green algal growth is also hypothesized to help sloths blend in with their leafy habitat, giving them additional camouflage protection from predators.

However, there are still some missing pieces to this evolutionary puzzle. Pauli and his team have not yet calculated the amount of energy sloths receive from algal consumption and whether that provides enough extra “fuel” to cover the energy expenditure of traveling to the ground once a week. But this three-way mutualistic relationship among sloths, moths and algae may help further understanding of how these slow, seemingly vulnerable animals have adapted to their environment and persisted for millions of years.

“Centuries ago, the early evolutionary French biologist Buffon said something to the effect, ‘Sloths are one characteristic away from going extinct,’” Pauli said. “Just because they have a unique strategy or because they seem so lethargic and docile, doesn’t mean they are acutely adapted to the system in which they find themselves. Maybe this story, these mutualisms that we see in their fur, [will] help to reveal how well-adapted they truly are to living in their natural ecosystem.”


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