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The sloth’s hidden friend

Mammal's fur is home to a type of symbiotic green algae not found elsewhere in nature

University of HelsinkiTrichophilus welckeri samples: a mysterious symbiosis with slothsUniversity of Helsinki

It is no secret for biologists that the greenish-brown shade of sloths’ thick fur is due to the presence of organisms that contain chlorophyll. Green algae and cyanobacteria (blue algae) hidden in the fur help these slow mammals that live in trees become camouflaged in the woods and to foil their predators. However, researchers had not imagined that this part of sloths’ bodies could be home to such a varied mini-ecosystem. A phylogenetic study of samples of the fur of 71 animals from the six species of sloths that exist found molecular material from 72 different groups of organisms – ranging from spiders, moths, beetles and cockroaches to a large number of microbes. “There were beings that were food producers (algae), consumers (protozoans) and decomposers (fungi),” states the ecologist Adriano Chiarello, from the Pontifical Catholic University of Minas Gerais (PUC-MG), one of the authors of the study, published on March 30 of this year in the journal BMC Evolutionary Biology. “This was unexpected.” Another interesting piece of information was the substantial incidence of a group of green algae of the Trichophilus genus, found in the fur of 73% of the analyzed sloths, regardless of the animals’ geographic origin.

This piece of data lends substance to the idea that there really is an ancient symbiotic relation between sloths and algae. This type of mammal only exists in the tropical forests of Central and South America and the animals analyzed in this study came from four countries: Brazil, French Guiana, Costa Rica and Panama. The researchers believe that a species of green algae, the Trichophilus welckeri, discovered more than 150 years ago, can only be found in sloths’ fur. “The algae was described in 1841 in fur samples of the animals and wasn’t documented ever again in other habitats,” comments Milla Suutari, a Finnish researcher from the University of Helsinki, another author of the study. “It is probably not found in any other environment.” If this hypothesis is correct, it is an alga that developed in parallel with the evolutionary history of these solitary tree climbers, perhaps establishing a close relation with their host par excellence.

Source of nutrients
Sloths’ fur really does seem to be a good habitat for algae. They have grooves and fissures and, unlike the fur of other animals, they absorb water. Besides providing these mammals with a chromatic means of foiling their enemies, the algae may be a small extra source of nutrients that sloths’ skin may absorb by diffusion. Other hypotheses not yet tested have been put forth to explain this close link between the algae and the sloths. For instance, the algae might produce substances that make the fur better suited to the growth of beneficial bacteria. Or they might produce certain types of amino acids that could absorb ultraviolet rays, acting as a sunscreen for the sloths. The algae of the Trichophilus genus perpetuate themselves among the sloths by probably passing from the mothers to their offspring, once the youngsters have reached a few weeks of life, the study suggests. Of the 19 animals that lacked the algae, seven were babies. Perhaps at the time when the fur samples were collected for the study, these young animals had not yet had enough contact with their mothers to acquire the green friend.

There are two sloth genera: Bradypus, which comprises the so-called three-toed sloths, with four species (B. tridactylus, B. torquatus, B. variegatus and B. pygmaeus); and Choloepus, the two-toed sloths, with two species (C. didactylus and C. hoffmanni). The green algae also seem to follow this pattern, as the identified species of Trichophilus in one genus are apparently different from those found in the other genus. Except for the B. pygmaeus sloth, found only on one Panama island, the other five species are found in Brazil. One of them, B. torquatus, popularly known as the maned sloth or ai, is under  threat of extinction and only lives in the Brazilian Mata Atlântica (Atlantic Seaboard Forest). Because it is only found in Brazilian forests, it was the sole Brazilian representative in the study about the algae living in sloths’ fur. Though several types of terrestrial algae were identified in the fur of B. torquatus, samples of the Trichophilus genus were not found. In the C. didactylus, algae of this genus were also not found. However, as there were samples of fur from only two samples of this species, it was impossible to develop a definitive analysis in this case. “We’d now like to study the presence of algae in the B. variegates, the common sloths that are widely distributed in Brazil, including the Mata Atlântica forest and most of the Brazilian Amazon Region,” says Chiarello.

Scientific article
SUUTARI. M. et al. Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri (Chlorophyta, Ulvophyceae). BMC Evolutionary Biology. Published online on 30 May 2010.