Muse, May/Jun 2004
Link Olson loves tenrecs. A biologist at the University of Alaska Museum, Olson flies to the other side of the world to study these strange animals. Some look like hedgehogs. Others look like moles. Some act like shrews, others like otters. Some talk to each other by rattling the porcupinelike quills that cover their bodies. Some are like bats and use sonar to find their way around in the dark. Others give birth to 32 young at a time.
To Link Olson and other biologists, tenrecs are mysterious creatures. They live only in Madagascar, a Texas-sized island that lies 300 miles off the east coast of Africa. But how did they get to this isolated island? Why aren't they found anywhere else? And once the first tenrecs got settled, how did they evolve into such a stunning variety of different forms? Olson roams the deserts and jungles of Madagascar in search of answers to these tenrec mysteries.
To understand where tenrecs came from, it helps to know something about where Madagascar came from. Some 250 million years ago, an animal could have walked from Madagascar to Antarctica-or to Africa, Asia, India, or South America. That's because all the land on Earth was once fused together into a single supercontinent scientists call Pangea. Gradually, Pangea began to split apart into the separate continents we see today. Madagascar remained joined to India until about 90 million years ago, when the two broke apart, leaving India to drift thousands of miles across the Indian Ocean until it reached its current location.
What does that tell us about tenrecs? Well, tenrecs weren't living on the island when it broke free. As far as scientists can tell, none of the mammals on Madagascar today are closely related to creatures that lived on the island 90 million years ago. The dinosaurs and other creatures that lived there then all became extinct. So they figure that the ancestors of the tenrecs must have come from abroad, after the island formed.
But from where? Scientists' best guess is Africa. First off, Africa is pretty close by. The two bodies of land are separated by the Mozambique Channel and lie only 300 miles apart-about the distance from New York to Pittsburgh. Better still, scientists have discovered that the closest living relatives of most of Madagascar's mammals can be found in Africa. The tenrecs' closest cousins are a bunch of "otter shrews" (which, oddly, are neither otters nor shrews) that live in equatorial Africa. This evidence suggests that some ancient relatives of these African otter shrews made their way to Madagascar and gave rise to the creatures we call tenrecs.
But how did they get there? Tenrecs can't fly, and 300 miles of open ocean is too far for them to swim. One theory involves some stepping stones. Millions of years ago, there might have been islands in the Mozambique Channel that have since sunk below the water's surface. So the ancestors of the tenrecs could have crossed the channel by taking shorter trips, hopping from island to island. Even so, each of those journeys would have been a major challenge for a tiny tenrec.
So the way scientists like Olson see it, tenrecs most likely got to Madagascar by riding on natural rafts. Big floods and storms can rip up entire trees or large clumps of other vegetation and drive them out to sea. Any animal that happens to be seeking shelter on these plants will be carried along with them. Most of the animals don't survive the journey. But on rare occasions an animal lives long enough for its raft to make landfall. Once they arrive, the travelers make themselves at home and can even give rise to new species.
For tenrecs, this journey was an extreme long shot. Scientists like Olson think that it happened only once. In fact, it might have been a single tenrec, a pregnant female clinging to some vegetation, that made it alive. That's the story told by tenrec genes. Olson has been tracing the tenrecs' family tree by looking at their genes, the hereditary material that's passed on from one generation to the next. By comparing the genes from the different types of tenrecs-and there are some 30 species living on Madagascar today-Olson and other scientists have discovered that all of the tenrecs on the island are descended from a common ancestor that lived less than 50 million years ago.
Once it had settled on Madagascar, this rugged ancestor and its offspring spread out and colonized different regions on the island. Some became adapted for digging underground, others for fishing in streams, others for eating termites. As a result, tenrecs evolved into a family of species and can look very different. The smallest living tenrec weighs just two grams-about the same as a couple of raisins-while the largest can reach 2000 grams-about the weight of a two-liter bottle of Coke. (If you weigh 85 pounds, that would be like having a chunky cousin who weighs in at 85,000 pounds- about the same as seven adult elephants!) And scientists are still finding new tenrec species in Madagascar. In some cases they're discovering that tenrecs that look as if they belong to the same species actually belong to two separate ones.
Copyright 2004 Carl Zimmer