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Genetic Crossroads

The low DNA diversity is challenging the preservation of the sloth, which is already living in isolation in small sections of the forest

NADIA MORAES - BARROSCommon sloth: populations with distinct genetic lineagesNADIA MORAES - BARROS

Slow moving acrobats of the trees, the likeable sloth, have low genetic diversity. Recent studies, carried out independently by teams from the universities of São Paulo and Minas Gerais, have both arrived at this conclusion. Although it could well be a natural characteristic of these mammals, the reduced variation in DNA could also be yet one more threat to these animals, which have to flee from their natural predators, from illegal hunting and from the shrinkage of their habitat. In general terms, the studies indicated that the prolonged isolation of groups of these animals in small and discontinuous areas of the forest, especially in the perforated Atlantic Rainforest, of which only 7% of the original forest remains, has produced individuals with DNA almost the same in the interior of each population and, at the same time, considerably different from the DNA of members of distant groupings. The evolutionary process makes each region (or state) give origin to animals of a specific and uniquely genetic lineage.

“For example, in the Atlantic Rainforest there is a clear genetic differentiation between animals from the north and those of the south, probably caused by distinct changes in climate and environment that occurred before the colonization of the country”, says geneticist Nadia Moraes-Barros, from the Biosciences Institute of the University of São Paulo (IB/USP), who studied the molecular variability of the sloth of genre Bradypus, with three toes, in a project funded by FAPESP. Within a sloth population there must be lots of relationships between closely related animals, perhaps because of the fact that the grouping apparently is heavily reduced and without communication with individuals from other regions. In an exaggeration, the consanguinity in populations of reduced size could make the species unviable, generating illnesses and infertility.

Each sloth on its own branch
In order to preserve all of the genetic lineages of the sloth, which represent a potential adaptive advantage in the face of new environmental alterations, the researchers do not recommend the mixing of animals coming from different regions, even if they are of the same species. They believe that a recommendation should take into account all of the programs of preservation of these tree dwelling mammals. If possible, each sloth should stay on its own branch. “The idea was that there would be regional centers dedicated to specifically caring for the animals of that place”, says the Colombian biologist Paula Lara-Ruiz, who analyzed the behavior, the physical traits and above all the genetics of the Maned three-toes sloth (Bradypus torquatus), a species found only in Brazil and under threat of extinction, for her master’s degree dissertation at the Catholic Pontifical University of Minas Gerais (PUC/MG).

“To genetically cross very distinct animals could also result in offspring with various problems, such as adaptation to the environment or malformations”, comments the geneticist Fabrício Santos, from the Federal University of Minas Gerais (UFMG), who supervised the genetic analysis of Paula’s work. The studies of the Minas team were able to count upon financial backing from PUC/MG, the National Geographic Society, Flora & Fauna International and the company Aracruz Celulose.

Today there is only one entity dedicated to helping the country’s sloths, the Center for Zoobotanic Rehabilitation Reserve at Ilhéus, in the south of Bahia State, in the middle of an area of Atlantic Rainforest. Linked to the Executive Commission of Planning of Cacao Tree Farming (Ceplac), an organ of the Ministry of Agriculture, the center has been headed since 1992 by the biologist Vera Lucia de Oliveira, who has already won national and international awards for her work with sloths. During its thirteen years of activity, close to 250 animals, 80% of them of species Bradypus Torquatus, have passed through Vera’s hands.

At least seventy of them arrived dying because of wounds or malnutrition, or did not resist in spite of the care given to these animals. The others were treated, readapted to wild life and set free in the Ceplac reserves. Some of them who did not readapt to the forests are maintained at the center itself, where there has been the reproduction in semi-captivity of the Maned three-toed sloth. The Ceplac biologist does not believe that mixing species from different states can produce a negative effect, a controversial statement that is not accepted by many scientists.

“But I’ve never received ant Maned three-toed sloths from other states, at the most common sloths”, explains Vera, who was recently nominated Environmental Secretary for Ilhéus, and who looks after the animals with offspring, carrying them in her lap. A field professional, she has dedicated her life to these likeable mammals and has a friendly relationship, although sometimes difficult, with the biologists from the universities, who study the animals from a greater distance.

Sloths belong to an ancient group of mammals with a placenta found only in the Americas, especially in Central and South America, within the order of Xenarthra, which also includes the armadillos and the tropical anteaters. Among the Xenarthra, the sloths sprung forth approximately 80 million years ago and they were the animals with the greatest diversity of form. Almost one hundred genres have ended up being described. Around 10,000 years ago, at the end of the Pleistocene era, there was a mass extinction of the fauna in the Americas, probably caused by climatic changes. Among the losses, almost all of the types of sloths disappeared, including the giant sloth, which lived on the ground. Only examples adapted to live in the crown of trees survived, today subdivided into two genres: the Bradypus, slothes with three toes, with four species, and the Choloepus, slothes with two toes, with two species. If there is a country with solitary animals, which move only a little and spend  a week without putting one foot on the ground, then that place is Brazil. Of the six species living and known, only one cannot be seen eating leaves while dangling, often with its head down, from the branches of national trees, the very small Bradypus pygmaeus, whose presence is restricted to an island in Panama.

The studies were carried out using the animals’ mitochondrial DNA. Power stations, the mitochondria has a genome of their own, diverse from that existing in the nucleus of the cells. The mitochondrial DNA is transmitted exclusively from the mother to the offspring, without undergoing recombination, and constituted a tool used by molecular biologists to recount the historical evolution of species. As the mutations of this type of genetic material, of maternal lineage, occurs at a rapid and more or less constant rhythm, mathematical models have been created that attempt to establish, in an approximate manner, when two distinct species, or different populations of the same species, had common ancestors.

In an article published in the scientific magazine Genetics and Molecular Biology during 2003, starting from the analysis of the mitochondrial genome, researchers at the Federal University of Para (UFPA) and the State University of Maranhão (UEMA) estimated that the Maned three-toed sloth separated from the common species and from the Pale Throated Three-toes sloth (Bradypus tridactylus) some 7.7 million years ago. The mitochondrial DNA can also be used to measure the degree of genetic variability between the members of a population, and, more recently, even as a means of helping the taxonomists to identify species.

Two-way bridge
The genetic material clearly showed the molecular separation of the populations of slothe with three toes. In an article accepted for publication in the Dutch scientific magazine Genetica, Nadia and her colleagues at the Evolutionary Biology and Vertebrate Conservation Laboratory of USP compared a segment of mitochondrial DNA obtained starting from blood or tissue samples of the two species found in the Atlantic Rainforest, the Maned three-toed sloth, exclusive to the coastline forest, and the Brown Throated Three-toed sloth, whose Brazilian popular name is bicho-preguiça, and whose scientific name is Bradypus variegatus, also present in other ecosystems such as the Amazon basin. The sample was composed of nineteen examples of the first mentioned  species, coming from two distinct regions (the southeast of Bahia and the State of Espírito Santo), and of forty seven (47) of the second species, coming from three distinct areas (the southeast of Bahia, the north of Minas Gerais and São Paulo State).

In a gross manner, the result was always the same: both the Maned three-toed sloth and the common sloth showed distinct genetic lineages, specific to each state. “We inferred that there are two phylogeographic main groups that represent a divergence north and south”, affirmed João Stenghel Morgante, from IB/USP, the project’s coordinator. With the Maned three-toed sloth, the northern group embraces the examples of Bahia and the southern group, the Espirito Santo specimens. For the common sloth, the northern divergence encapsulates the samples from Bahia and Minas and that of the south, the São Paulo territory. “In São Paulo, one of the twenty examples of Bradypus variegatus had different DNA sequencing from the others. That of the others was the same”, says  Nadia.

In another study, as yet unpublished in scientific journals, the IB/USP team compared the diversity of the mitochondrial DNA of the common sloth of three Atlantic Rainforest areas and four from the Amazon basin. Once again the examples of each region affiliated themselves to very distinct genetic groups. The Bradypus variegatus from one geographic zone in particular caught the scientists’ attention. “The differences in the mitochondrial DNA of the sloths living along the north of the river Tocantins, close to the town of Santarém, in the state of Pará, are very large when compared to the common sloth of the other regions”, says Nadia. This genetically very divergent population could belong, in truth, to another known species of the animal, the popular Pale-throated Three-toed sloth (Bradypus tridactylus), which occurs only in a portion of the North Region. Or even to an as yet unknown species of the sloth. As well genetic evidence was collected showing that the Northeast functions as a two way bridge between the genetic groups of the common sloths of the Atlantic Rainforest and the Amazon basin, allowing the passage of individuals from one zone to the other.

The researchers from the states of Minas Gerais, in an independent study from that realized by their USP colleagues, which shortly will be submitted to an international magazine, reached similar conclusions with populations exclusively of the Maned three-toed sloth. They analyzed three strips of mitochondrial DNA from a large sample of Bradypus torquatus: forty four examples, three from Rio de Janeiro, six from Bahia and thirty five (35) from  Espírito Santo. “The sloth populations of one state are isolated from those of another”, says Paula. “There hasn’t been an exchange of genes between them for some time.” Yet again the scientists verified that the DNA of, for example, a specimen from Bahia is almost the same as that of another example from this state, but very distinct from the individuals from both Espírito Santo and Rio de Janeiro.

The low genetic diversity is a worrying situation, but has not come as a total surprise in the case of the Maned three-toed sloth, whose registered trade mark is a band of dark hairs around its neck. In the end one is dealing with a presumably rare animal, perhaps with few living individuals, although nobody knows how many there are for certain, which lives practically camouflaged in the crown of trees, some twenty to thirty meters from the ground, in a heavily delimited area of the Atlantic Rainforest, between the south of Sergipe and the north of Rio de Janeiro and Minas Gerais.

Perhaps it is even more shocking to discover that the common sloth, relatively abundant and much easier to be found, confronts the same genetic bottleneck. The populations of Bradypus variegatus, with their typical dark markings around their eyes that look like glasses, are present in a strip of land that is much broader than that occupied by the selective Bradypus torquatus. On the continent, their vast habitat stretches from Honduras, in Central America, until the north of Argentina. On national territory, the common sloth can be found from north to south, in almost all of the states. Even urban city parks have the common sloth.

It would be simplistic and over alarming to compare the small genetic diversity of the common sloth and that of the Maned three-toed sloth to a time bomb programmed by biology since – sooner or later – they will be eliminated from the face of the earth. Little variability in the sequencing that makes up the DNA of a species does not necessarily mean to say only a few years of life. Sometimes, low genetic disparity is evidence that the group of animals has passed or is in the middle of an evolutionary process given the name of population bottleneck or founder effect.

Due to some environmental changes, such as excessive climate warming or a reduction in their natural habitat, a dramatic reduction in the original number of members of a population occurs. In this case, a part of the total genetic diversity of the species is lost. If there is not, within a short space of time, another major environmental change, these remaining members, in spite of being few, and sometimes with reduced genetic diversity, can be successful in re-establishing the specie’s population. Within an optimistic scenario, the number of individuals of this population will slowly come back to grow and possibly its genetic variability as well. This is a happy ending for an animal that, at a given moment in its history, is encountering a population bottleneck. “We don’t discard the hypothesis that the founder effect has occurred in populations of the Maned three-toed sloth in every State, but we need further studies to confirm or to discard this idea”, explains Fabrício.

A fundamental question that neither the study of Bradypus torquatus or Bradypus  variegatus have yet managed to respond to with certainty is if the low genetic variability is the product only of the life style and habitat of the sloths, with historical roots, or has it derived from recent pressures?

Human action, the cutting down of the native vegetation, especially the Atlantic Rainforest, is bad news for the sloths. These animals, which usually move about over an area of five hectares, have fewer and fewer portions of the forest at their disposition. The problem is that as yet there are no molecular markers capable of measuring the recent impact of civilization upon these animals. To collect more DNA samples of sloths, of more species and from varied localities, and to study other strips of the genome of these animals, could well help to clear up this question.

Also, it is indispensable to deepen the studies concerning their habitat and the physical characteristics of the five sloth species that exist in Brazil. There is not, for example, any information about the population sizes of these animals. The Maned three-toed sloth, which in its adult phase measures and weighs a little more (60 to 80 centimeters and 5 to 7 kilograms) than the common sloth, is only considered to be in extinction due to the exiguity and the fragmentation of the area in which it is found in the Atlantic Rainforest. “It is very difficult to find this species”, details Paula Lara, who, persistent, managed to capture four dozen Bradypus torquatus. Even the notion of a sloth population is something diffuse. How many individuals make up a population for this animal? Ten? One hundred? Nobody knows for certain.

The sloth is a solitary animal, of nocturnal and day activity, which sleeps half of the day. It comes down from the tree basically to defecate and urinate, once per week on average. The male goes about unaccompanied and the mother carries her young up to between six and eight months. Afterwards she sets the young loose in her territory. Happily, some pioneering work, such as that of Adriano Chiarello, from PUC/MG and Paula’s supervisor, who since 1994 has been observing with the help of radio transmitters, the movements of certain Maned three-toed sloths on a reserve in Espírito Santo, has produced new information concerning the conservation of these tree mammals. “An animal, rehabilitated and reintroduced into the forest must be monitored for at least twelve months after is release”, says the ecologist. “Only after this period of time can we affirm that this process of adaptation to the locality has been a success.”

The Projects
1.
Genetic variability, Evolutionary standards, philogeography and the conservation of neo-tropical vertebrates (00/13213-2); Modality: Regular Line of Research Assistance; Coordinator: João Stenghel Morgante – IB/USP; Investment: R$ 552,504.74 (FAPESP).
2. Translocation as a tool for the preservation of the Maned three-toed sloth, Bradypus torquatus; Coordinator: Adriano Chiarello – PUC/MG; Investment: US$ 8,680.00 (National Geographic Society and Flora & Fauna International).
3. Genetic Diversity and philogeography of the Maned three-toed sloth, Bradypus torquatus (Xenarthra, Bradypodidae); Coordinators: Adriano Chiarello – PUC-Minas Fabrício R. dos Santos – UFMG; Investment: R$ 17,000.00 (Research Incentive of PUC-Minas)

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