New Species of Mouse Opossum Discovered in Panama

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A team of researchers from the American Museum of Natural History, Siena College and the Bell Museum at the University of Minnesota, St. Paul, has discovered a new species of the marsupial genus Marmosa living in Panama.

The Adler’s mouse opossum (Marmosa adleri) is much smaller than its sister species (pictured here), the Alston’s mouse opossum (Marmosa alstoni). Image credit: Victor Acosta.

Mouse opossums are relatively small members of the Neotropical marsupial family Didelphidae.

Three genera are known: Marmosa (mouse opossums), Thylamys (fat-tailed mouse opossums) and Tlacuatzin (grayish mouse opossums).

“Most of the Marmosa species are endemic to South America, and biogeographic reconstructions suggest that the genus evolved on that continent while it was still separated from North America by a marine water barrier,” said Dr. Robert Voss, curator of the Mammalogy Department at the American Museum of Natural History, and his colleagues.

“Only a few species of Marmosa now belong to the North American fauna, but these include members of two subgenera.”

“Whereas four North American species belong to the subgenus Exulomarmosa, only one is currently recognized in the subgenus Micoureus.”

The newly-identified species, named the Adler’s mouse opossum (Marmosa adleri), is as among the smallest measured of the subgenus Micoureus.

“In side-by-side morphological comparisons, Marmosa adleri could never be confused with its sister taxon, the Alston’s mouse opossum (Marmosa alstoni),” the researchers said.

“The size difference alone is striking; in fact, there is no overlap among most measurements of these species, and the average body weight of Marmosa adleri (58 g) is less than half that of Marmosa alstoni (137 g).”

Marmosa adleri is also longer tailed (its tail is 161% of its head-and-body length) than Marmosa alstoni.”

“It has proportionately larger ears, its tail has a short furry base, and the unfurred (scaly) part of the tail is almost completely dark, whereas Marmosa alstoni has a long, fluffy-furred tail base, and over 50% of the naked part of the tail is white.”

The Adler’s mouse opossum is only known from Panama, but it is widely distributed within that country, from near the border with Costa Rica to the Colombian frontier, and from near sea level to almost 1,500 m.

Although the new species is not known to occur with other species in the subgenus Micoureus, it may often co-exist with two species of the subgenus Exulomarmosa: the Isthmian mouse opossum (Marmosa isthmica) and the Zeledon’s mouse opossum (Marmosa zeledoni).

“The discovery of this species is highly unusual because I collected it in what may be the most intensively studied area of tropical forest in the world,” said Professor Greg Adler, a researcher at the University of Wisconsin.

“Numerous expeditions collected thousands of specimens there throughout the 20th century and never found this species. It is astounding that it escaped discovery for so long.”

“I’m both flattered and honored to have a new species named after me,” he added.

“It is something of a reward for decades of hard work under difficult conditions in remote tropic forests.”

A paper reporting the discovery was published in the American Museum Novitates.

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Robert S. Voss et al. 2021. A revision of the didelphid marsupial genus Marmosa. Part 4, species of the Alstoni Group (subgenus Micoureus). American Museum Novitates 3983

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