Researchers help trace origin of Madagascar’s mammals
Answer one of natural history’s most intractable questions
The falanouc (Eupleres goudotii), one of Madagascars most enigmatic carnivorans, is a descendent of animals that dispersed from Africa to the island 24 million to 18 million years ago. This nocturnal, solitary animal lives in lowland forests, specializes in eating earthworms and other invertebrate prey, and is endangered.
Photo courtesy of The Field Museum, Neg. CSA77048
All of Madagascars living Carnivora (an order of mammals that includes dogs, cats, bears, hyenas and their relatives) descended from a single species that dispersed from Africa to Madagascar, apparently floating across the ocean barrier aboard wayward vegetation about 24 million to 18 million years ago. Previously, scientists believed that Madagascars seven living species of native Carnivora represented two to four separate lineages, which would have implied that these animals colonized the island independently several times.
The surprising findings will be published in Nature Feb. 13, 2003.
"Our research shows that all the species of Madagascars Carnivora together represent a unique evolutionary branch formed by a significant, one-time event," says co-author John Flynn, MacArthur Curator of fossil mammals at The Field Museum in Chicago. "In fact, all 100 or so known species of terrestrial mammals native to Madagascar, which fall into four orders – carnivorans, lemurs, tenrecs and rodents – can now be explained by only four colonization events."
How and when mammals first populated Madagascar has long remained a mystery due to the lack of fossil evidence from the island, which lies about 240 miles off the east coast of Africa. To overcome this problem, the researchers analyzed genes of Madagascars living species of Carnivora and some of their closest relatives in Africa and Asia.
Specifically, they sequenced the DNA of four genes from 20 different mammals and analyzed the resulting patterns for evidence of which carnivorans are the most closely related, evolutionarily. The researchers also estimated when the animals differentiated from each other by calculating the rate of molecular change for each species and setting these molecular "clocks" according to dates of separation for other mammal lineages that have been established by the fossil record elsewhere around the world.
The results refute two previously accepted models for how mammals colonized Madagascar. One model says mammals were already on Madagascar when the land mass broke away from Africa 165 million years ago. The other suggests they came from Africa via a land bridge, which is supposed to have existed 45 million to 26 million years ago. The new research establishes that Carnivora arrived on the island more recently than either of these models predicted, strongly contradicting both models.
"At long last, statistical methods for estimating divergence ages among organisms are becoming sufficiently sophisticated that we can have confidence in the accuracy of the age estimates," notes Anne Yoder, Associate Professor of evolutionary biology at Yale University, Field Museum research associate, and lead author of the Nature paper.
The study provides further evidence that lemurs also colonized Madagascar in a single over-water event, in this case a much older episode, which the team estimates occurred about 66 million to 62 million years ago. This confirms that crossing a large water barrier, followed by colonization and diversification, has occurred very rarely among land mammals.
One reason Madagascars mammals could have survived a long voyage over the open sea without food or water might be that many of them have the ability to hibernate or maintain a state of torpor for long periods, the authors note.
"This study will shed light on other questions of mammal dispersals elsewhere, such as how monkeys and rodents got from Africa to South America some 35 million or more years ago, when the two continents were separated by an immense water barrier," Flynn says.
Determining how, when, and from where Madagascars unique biota got to the island is "one of the greatest unsolved mysteries of natural history, for the simple reason that the Malagasy fossil record is virtually non-existent for the last 65 million years," says David Krause, Professor of anatomical sciences at State University of New York at Stony Brook. "The discovery of a single African origin of Malagasy carnivorans is stunning and exciting, and a fine example of how scientists have gone the extra mile to devise innovative means to address and solve a previously intractable question."
Study sheds light on Madagascars biodiversity today
The carnivorans living in Madagascar today are commonly known as the fossa (resembling a puma), falanouc, Malagasy striped civet, and four kinds of Malagasy mongooses (resembling a ferret). The study shows that all these animals descended from a mongoose-like animal from Africa and are closely related to true mongooses living in Africa today. African hyenas are the next closest relatives to this group of African mongooses and Madagascar carnivorans.
The scientific classification above the genus level for many of these groups of animals will need to be changed to reflect their new places on the evolutionary tree identified during this study. Flynn, Yoder and Steven Goodman, Nature paper co-author and Field Biologist at the Field Museum, are preparing another paper focusing on these issues.
Madagascar is the size of California and Oregon, combined, making it the worlds fourth largest island. It is especially interesting, scientifically, because some 80% of its plants and animals are not found anywhere else in the world. This is the result of Madagascars isolation from other landmasses for the past 88 million years. Due to its unique history, the island offers a treasure trove of information about evolution, biodiversity and biogeography.
Accordingly, Field Museum researchers have been actively studying the fossil record and modern animal life in Madagascar. Collaborating closely with the Université dAntananarivo, Flynn has led five fossil-collecting expeditions there since 1996, and Goodman has lived there for the past 12 years, conducting extensive biological inventories and publishing widely. His definitive 1,500-page book, The Natural History of Madagascar, co-edited with Jonathan Benstead, will be published later the year by the University of Chicago Press.
Much of the DNA analysis for this study was done at The Field Museums Pritzker Laboratory for Molecular Systematics and Evolution, a world-class lab dedicated to genetic analysis, and understanding and preserving the worlds biodiversity. The lab provides researchers with state-of-the-art equipment in molecular biology, enabling them to pursue genetic studies of evolutionary diversity throughout the tree of life.
The study was supported by grants from the National Science Foundation and conducted with colleagues from Yale University, Northwestern University, and the Muséum National dHistoire Naturelle (Paris). WWF (Antananarivo, Madagascar) and the Université dAntananarivo provided essential assistance and support.
"This study is extremely important for understanding the biogeographic history of Madagascar and the evolution of Carnivora," Goodman says. "It also will inform conservation decisions and could be used to help preserve whats left of Madagascars precious biodiversity."
Greg Borzo | EurekAlert!