We always knew there was something wily about the mongoose. The animal -- famed for its craftiness in fables from Aesop to Kipling -- has stolen the stage once again in Madagascar, where researchers have discovered that all local Carnivorans have roots in the weasel-like creature.
There are plenty of good reasons to be interested in the history of Malagasy (fancy-speak for "from Madagascar") mammals. The island is one of the world's most biodiverse areas, and has been tagged a "conservation hotspot" by scientists and policymakers. When British naturalist Joseph-Philibert Commerson visited the island in 1771, he said, "Here Nature created a special sanctuary wither she seems to have withdrawn to experiment with designs different from those used anywhere else. At every step one finds more remarkable and marvelous forms of life."
Madagascar was once part of the giant supercontinent known as Gondwana, flanked by Africa and Australia. But when the continents began to shift, the island drifted to sea toward its present location in the Indian Ocean. For more than 160 million years, Madagascar has been divorced from other landmasses, and its plants and animals have evolved in isolation. The result is an awesome density of distinctive plants and animals.
The 40 species of lemurs and 50 species of chameleon are some of the island's star species. They, and in fact the majority of Madagascar's flora and fauna, are endemic -- found nowhere else in the world. Of the country's 987 vertebrate species, 771 are unique. These numbers are extraordinary, considering that Madagascar is small, roughly the size of Texas.
John Flynn, a curator of fossil mammals at the Field Museum in Chicago, is among a team of researchers using the sophisticated new kinds of genetic analysis to reveal how -- and when -- Madagascar's mammals got there.
Madagascar's fossil record is fairly rich up to about a thousand years ago, and then again during the Late Cretaceous period. The millions of years in between, however, left little behind -- there were simply not enough rocks forming during these years, says Flynn.
Because the fossil record in Madagascar is so poor, the island has been a subject of contention among evolutionary biologists, who have relied mostly on anatomical comparisons to describe the island's denizens. But now, Flynn says, "DNA studies offer an alternative way to understand the origin and diversity of Madagascar's mammals other than referring to anatomy, which gives ambiguous results."
There are four groups of terrestrial mammals on Madagascar: carnivores, lemurs, hedgehog-like insect eaters called tenrecs, and rodents. Flynn's team focused on the carnivores, which range from the puma-like fossa cat to the slinky, nocturnal falanouc. The researchers had to figure out two things about the different species: how closely related they are to each other, and how old they are -- in evolutionary terms.
To answer the first question, the researchers sequenced DNA from four genes across the group. Dramatic differences between species seemed quite possible, given the striking variation among them. Such a result would have suggested that a few animals arrived separately from mainland Africa, giving rise to different lines of descendants. But the DNA sequences were so similar across the board that a single ancestor species must be responsible for all of the terrestrial mammalian carnivores found on Madagascar today, Flynn says.
How they got there is another matter.
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Sure, swimming from shore to shore isn't the only possibility. The ancestor could have already been on Madagascar when it separated from the African continent 165 millions years ago.
And some scientists believe that a landbridge may have linked the two lands more than 45 to 26 million years ago. In this case, all it would take is a few pioneering mammals making the trek to colonize the island before it broke apart. But if there had been a land bridge, Flynn says, other species probably would have also crossed it.
To settle the matter, the researchers compared the "molecular clocks" -- the rate of evolutionary change in a species -- of current Madagascar inhabitants to similar species in other areas where a stronger fossil record exists.
"There are a certain number of DNA mutations that happen over time," Flynn explained. The rate is different for different species, but by looking at mongoose-like creatures from other areas, the researchers estimated what the rate should be for Malagasy species.
"Then you count up the numbers of mutations to know how long ago the arrival occurred." Carnivores and lemurs arrived at very different times, the researchers found, so their successors can't be explained due to a single common event like a landbridge. What's more, Flynn says, genetic tests show that the Carnivoran mammals of Madagascar are not old enough to have been present on Madagascar before the split with Africa.
That leaves the possibility that a few animals hitched a ride across the channel on driftwood or clumps of seaweed.The African mongoose, the closest relative of the present-day Malagasy carnivores, may have survived the float by going into a hibernation-like state.
"All 100 or so known species of terrestrial mammals native to Madagascar can really be explained by only four colonisation events," Flynn said. "It's nailing down a controversy that has been raging for a very long time."