738. Death By Drought

Artist's image of a dodo bird

By Sid Perkins, Science Now, March 30, 2012

An extended drought that struck Mauritius about 4200 years ago turned one of the island's few sources of fresh water into a muddy death trap for dodos, giant tortoises, and other wildlife, a new study suggests. The excavations have yielded the fossils of small creatures--including insects, bats, and snails--as well as the pollen and seeds of plants that lived in the area, giving scientists a much more comprehensive look at the dodo's ecosystem.

Mauritius, an island nation in the southwest Indian Ocean about 870 kilometers east of Madagascar, is famed as the home of the dodo, a flightless, turkey-sized relative of pigeons and doves whose name has become synonymous with extinction. Even though dodos died out in the late 1600s, about 80 years after Europeans first colonized the islands, only a few descriptions of the bird exist, and those accounts are often contradictory, says Hanneke Meijer, a vertebrate paleontologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C. From the late 19th century to the mid-20th century, several excavations on the island recovered large amounts of dodo remains, but at the time it wasn't routine to collect information that could provide ecological context.

Since 2005, Meijer and her colleagues have re-excavated portions of a formerly swampy area known as Mare aux Songes ("Pond of Dreams"), one of the sites where many dodo remains were previously unearthed. Thousands of years ago, the area was a small lake--a freshwater oasis in an otherwise dry environment, Meijer says. Along with small fossils such as pollen, seeds, insects, and snails, the team's recent diggings have brought to light a rich layer of fossils of bats, songbirds, dodos, and extinct giant tortoises.

Intriguingly, carbon dating reveals that many of the fossils of the larger creatures, especially the dodos and tortoises, accumulated between about 4235 years and 4100 years ago, the researchers report in the March issue of Naturwissenschaften. This period roughly matches that of a drought known to have struck other regions of the world, including Africa and the Andes in South America, but previously unknown on Mauritius. More than three-fourths of the dodo bones unearthed--235 bones from at least 17 individuals, the researchers estimate--come from the legs and feet. Moreover, Meijer says, all of the bones are well-preserved, showing no signs of being exposed to the elements and no evidence of being gnawed by predators or scavengers.

If the dodos had died of starvation or thirst, the researchers contend, their carcasses would have lain in the open before they were covered by sediment, leaving them exposed to weathering or scavenging. The best explanation for the dodo deaths, they say, is that the hefty, thirsty birds tried to cross the mud flats to reach the drought-shrunken lake and got mired in the muck. While the upper portions of the dodos would have been exposed--and therefore susceptible to decomposition or to scavenging--the legs, already buried, would have been more likely to be preserved intact.

The team's conclusions "seem to be quite reasonable," says Kenneth Campbell, a vertebrate paleontologist and curator of birds at the Natural History Museum of Los Angeles County. "I can see, having struggled to get out of mud myself, how a bird could easily become stuck." One possible scenario, he suggests, is that as dodos crossed mud flats to reach the water's edge, the dry crust atop the mud became increasingly thin, and eventually they broke through the surface like skaters on thin ice.

One curious aspect of the fossil assemblage is that the bones of juvenile dodos are conspicuously absent. This bolsters the case against death by thirst or from poor water quality in the drought-shrunken lake, the researchers say, because juveniles would have been at least as susceptible to those factors. Perhaps the drought-generated mud flats were least exposed during the time of year when young dodos were most common--or hatchling dodos simply weren't heavy enough to break through the thin veneer of crust atop the quagmire beneath.

In a previous analysis, a team including Meijer and another set of colleagues estimated that the Mare aux Songes site, which covers about 2 hectares, holds the remains of some 34,000 dodos and 300,000 giant tortoises that became mired during the more-than-a-century-long Mauritian drought.