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Evolution

G’day, kangaroos

The oldest relatives of the Australian marsupials may have lived in South America

LEONORA COSTA / UFESCuíca: one of the smallest in BrazilLEONORA COSTA / UFES

Marsupials, like the kangaroo and koala bear, began to diversify in Australia millions of years ago from species of the same group that lived in South America, according to a recent hypothesis that is gaining strength. This approach claims that Brazilian marsupials – the best known being the gambá, cuíca and catita [species of opossum] – form the oldest branch of this group of animals still with living representatives. The strains that lived in Europe or in Asia became extinct (just one species lives in the United States and Canada), with only those in South America and Australia remaining.

This view, reinforced by a study by researchers from the German university of Münster, published in July 2010 in the journal, PLoS Biology, indicates that the branch of marsupials that includes the Brazilian species gave rise to another that today has just one living species, the little monito del monte [little mountain monkey] (Dromiciops gliroides), an animal weighing up to 25 grams that comes from the forests of Chile and Argentina. The small monito may be the most distant living relative of the almost 200 species of Australian marsupials, including the larger varieties of kangaroos that may weigh as much as 70 kg.

“Genetically”, says  Ariovaldo Cruz Neto, a researcher form the Paulista State University  (Unesp) in Rio Claro who is studying these animals in collaboration with Australian colleagues, “the monito shows a greater degree of kinship with the Australian species than with those from South America”. Brazilian marsupials, although older, no longer have any direct kinship with any of those that live in Australia. According to another hypothesis, presented in 2008 at PLoS One by a  group from the Australian University of New South Wales, the species that was the origin of Australian marsupials was different, the  Djarthia murgonensis, which lived 30 million years ago in the east of a super-continent that included the present-day South America and Australia.

Despite the distance the American and Australian marsupials retain other similarities besides the fact that their young are born incomplete, hairless and blind, after a gestation period of Just one or two weeks, and move to the mother’s teats that are normally protected by a pouch (marsupium), where they grow for a further two or three months before seeing the light of day. Cruz Neto and his collaborators from Australia found that the organism of the marsupials of South  America and Australia functions in a very similar way for producing and burning off energy, regardless of the size or type of environment in which they live.

Most of the almost 90 species of marsupials from the Americas weigh between 10 grams and 1 kg, generally live in forests and feed mainly on insects. A kangaroo, on the other hand, may be as big as an adult male, although the smallest of the 21 species of this group weighs 400 grams. In Australia, marsupials live in tunnels, in the desert or in humid forests and, like the American ones, feed mainly on small invertebrates and fruit, although one species prefers nectar and another is a carnivore.

Similar organisms
The researchers examined the metabolism of representatives of two species of cuíca [opossum] from South America, Gracilinanus agilis and Micoureus paraguayanus. The former had an average body temperature of 33.5o Celsius and the latter 33.3o Celsius, at least 2 degrees lower than the average temperature of placental mammals, the group to which we belong. Another measurement was the basal metabolic rate, which indicates the minimum energy level the animal needs to maintain its vital bodily functions. To maintain this rate each of the two species spends 4.8 kilocalories (kcal) and 5.5 kcal a day, respectively.

The body temperature and metabolic rate of the two Brazilian marsupials were close to those of other Australian marsupials that have already been examined. “From the physiological point of view”, says Cruz Neto, “once a marsupial, always a marsupial, despite the millions of years of independent evolution and the differences in diet and habitat of the species that live in South America and Australia”. According to Cruz Neto, it was apparently not selection pressure that led to a modification in the physiological plan that was sufficient for these animals to colonize Australia. “It’s as if the marsupials had a suitcase packed with clothes that allowed them to live in different environments.”

The marsupials from South America, although not as different among themselves as the Australians, exhibit subtle and relevant differences in the size and shape of the skull, jaw, scapula and pelvis that express their feeding habits and the environments in which they live. Diego Astúa, a professor at the Federal University of Pernambuco, compared the skull measurements of 2,932 animals from the didelphid family, which includes most of the Brazilian marsupials, those that live in the Andes and the monito del monte. In a study published in 2010, he showed that half the didelphids had differences in the size and shape of the skull – in all cases the males had larger heads than the females.

The project
The energetics of bats and marsupials: structural bases and the functional meaning of the basal metabolic rate (nº 00/09968-8); Type Young Researcher; Coordinator Ariovaldo Pereira da Cruz Neto – Unesp; Investment R$ 441,455.78

Scientific articles
ASTÚA, D. Cranial sexual dimorphism in New World marsupials and a test of Rensch’s rule in Didelphidae. Journal of Mammalogy. v. 91, n. 4, p. 1011-24. 2010.
COOPER, C.E.; WITHERS, P.C.; CRUZ-NETO, A.P. Metabolic, ventilatory and hygric physiology of a South American marsupial, the long-furred woolly mouse opossum. Journal of Mammalogy. v. 91, p 1-10. 2010.

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