Rainfall Helps Baby Lemurs Survive, Tooth Study Shows

John Roach
for National Geographic News

November 14, 2005

Baby lemurs born to older females with worn-out teeth are likely to
survive, as long as their first few months of life are wet, a new
study suggests.

Patricia Wright, an anthropologist at the State University of New York at Stony Brook, helped discover the surprising link between rainfall and infant lemur survival.

The finding is based on data collected by a team of researchers for a long-term study on how tooth wear affects the ability of lemurs to successfully reproduce.

Wright notes, however, that the rain forests where lemurs make their homes are tending to be drier.

"As we have more deforestation and more fragmentation, it just really means more drying of the rain forest and less rainfall," Wright said. "We've seen that trend over time—we're seeing more erratic rainfall."

Wright is a member of the National Geographic Society's Committee for Research and Exploration, which partially funded the research. She is one of the world's foremost experts on lemurs and a leading advocate for conservation in Madagascar.

Wright and her colleagues report their latest findings in the November 15 issue of the Proceedings of the National Academy of Sciences.

Tooth Map

Lemurs are tree-dwelling, mainly nocturnal primates found only on Madagascar, an island nation located off the east coast of Africa in the Indian Ocean. Most lemur species are endangered.

Primates in general are relatively long-lived and, except for humans, females of most species can reproduce well into old age.

Scientists have theorized, however, that tooth deterioration over time may eventually affect offspring survival.

To test this theory in lemurs, Wright and her colleagues used dental casts and a novel application of Geographic Information System (GIS) technology to document tooth wear in a population of sifaka lemurs (Propithecus edwardsi) over the past 20 years.

GIS is a technology normally used to analyze and map Earth's geographic features.

Peter Ungar, an anthropologist at the University of Arkansas, Fayetteville, is among the pioneers to use GIS for what he calls dental topographic analysis. This technique maps the pointed or rounded part of a tooth's biting surface.

The lemur study is the first time the GIS technique has been applied to link tooth deterioration to reproductive success, Wright said.

In an accompanying commentary in PNAS, Ungar writes that "by combining long-term field studies with dental topographic analyses, [the authors] offer an exciting new way of thinking about [tooth wear], reproductive fitness, and natural selection."

Drying Out

The research team's GIS analysis of molar tooth casts from the lemurs revealed that the sifakas go through three stages of tooth wear.

In the first six years, the animals' low-crowned molars wear away rapidly and reveal so-called compensatory shearing blades. The teeth essentially morph from a single pair of scissors for cutting apart the lemur's leaf-based diet to two pairs of scissors, Wright said.

This second set of shearing blades enables the lemurs to keep ripping apart their food for an additional ten years.

"That hadn't been seen before in primates with low-crowned teeth," Wright said. "No one has documented that the wear itself can help retain the cutting edge for so long."

At 18 years old most lemurs' teeth are close to being worn down to the gums.

But females who have reached this stage are able to keep producing offspring for another decade. The young will survive as long as it rains regularly after they are born.

During dry years old females with worn-out teeth are still able to give birth. But their young seldom survive the dry season months when breast milk is their primary source of nutrition, Wright said.

"The older females who have worn-down teeth … and it's mostly leaves they are eating … they aren't able to shear the leaves. Therefore they aren't able to get nutrients or moisture out of the leaves," she said.

As a result, Wright added, the old females are probably unable to produce high-quality breast milk, if they can produce any milk at all. The infants thus get weaker and eventually die.

Big Red Flag

According to the researchers, their findings raise concern about the impact of changing environmental conditions on already-stressed populations of lemurs.

Females with worn-down teeth accounted for 14 percent of the sifaka population studied. If the trend towards drier dry seasons continues, they'll be unable to raise healthy young, the researchers note.

There are more dry years now than in the past, and the dryness is lasting longer, Wright said. She added that deforestation inside and outside Ranamofana National Park, where the study was conducted, and global warming may be contributing to the drying trend.

Wright now plans to model how the climate around Madagascar may change in the future. If the prognosis is for continued drying, she said, she'll push harder for conservation measures such as reforestation and restoration ecology.

"This little bit of climate change—a reduction in rainfall in certain months—can really have an impact on the survival of these infants," Wright said. "That to me is a big red flag."

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