Artist's image of a dodo bird |
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.
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