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Meerkats Become Fat Cats in Large Cooperatives

By John Roach
for National Geographic News
October 1, 2001
 
The fat cats in meerkat society are the ones that thrive on the backs of
others. Researchers have found that the larger their social cooperatives
the more they are able to spread the duties of rearing their young and
standing guard against predators—giving individuals greater
opportunities to look for food.

The foot-long (30-centimeter) mongoose that dwells in the Kalahari Desert of southern Africa and dines on everything from scorpions and grasshoppers to small reptiles and birds lives by a philosophy of sharing and caring. And the hallmark of a well-organized group of meerkats is a marked increase in the weight of the individuals.



"Heavier juveniles are likely to be dominant to lighter ones and so may get priority access to better feeding sites," said Tim Clutton-Brock, an animal ecologist at the University of Cambridge in England.

"Additionally," he added, "heavier meerkats may be able to take greater risks in digging deeper holes in search of larger prey instead of spending more time feeding on smaller prey items on the surface."

The secret to healthy heft lies in the inner workings of the mob, according to research co-authored by Clutton-Brock in the September 28 issue of Science. The work was supported by the National Geographic Society's Committee for Research and Exploration. Meerkats live in groups—called mobs—of two to 30 individuals. Researchers have long known that larger mobs tend to be the most successful in terms of breeding and fitness. However, the mechanisms that make life in big mobs superior to life in small mobs have remained uncertain.

The study by Clutton-Brock and his colleagues shows that a large mob full of helpers who feed and protect the young not only increases the daily weight gain and survival of the young, but also enables the helpers to gain and maintain heft themselves.

This mutual weight-gain benefit of shared rearing of meerkat pups helps explain why breeding success increases with group size in several cooperative mammals, according to the research.

"Our work shows that individuals that are heavier at the end of the pup feeding are more successful foragers when they are subsequently feeding independently," said Clutton-Brock. "In many mammals, feeding success at this stage is critically important to later survival."

Meerkat Group Dynamics



About 80 percent of the offspring in any given mob are the product of a single dominant male and female, though all mob members actively assist in feeding the young and guarding the mob from predators.

To determine the dynamics that make larger mobs more successful, Clutton-Brock and his colleagues manipulated the helper-to-pup ratio of wild Kalahari meerkat mobs that had been habituated to human presence.

The mobs under study lay claim to patches of land on a ranch in South Africa 17 miles (27 kilometers) south of the border with Botswana. Clutton-Brock and his colleagues began studying the meerkats there in April 1993 and today the meerkats are very accustomed to human presence.

The habituation of the meerkats allowed the researchers to temporarily remove pups from or add pups to the mobs and weigh the same individuals several times each day, which gave the researchers an accurate measure of the meerkats feeding success and weight gain. "This has never been previously possible with a wild animal," said Clutton-Brock.

The research shows that reducing the ratio of helpers to pups' by adding additional pups to the mob reduces the daily weight gain of the pups and the helpers, which reduces pup survival rates and decreases their foraging abilities in later life.

In mobs that have a low helper-to-pup ratio, the helpers are unable to sufficiently respond to all the pups' yelps for food, nor do the helpers have enough food to maintain their own weight, the researchers note.

In mobs where helper-to-pup ratios are higher, helpers are able to both provide sufficient food to the young and maintain their own food intake and weight gain, which makes keeping a high helper-to-pup ratio in meerkat mobs advantageous for both helpers and pups.

"In colonies that are a little too small there are not enough [meerkats] to do the work," said Pam Wallberg, director of the Fellow Earthlings Wildlife Center, a captive meerkat facility in Morongo Valley, California. "They need a breeding pair, baby-sitters, a sentry…"

"Fifteen to 30 is pretty good," she said. "Less than five is too narrow of a number to do all the jobs effectively. Some jobs will be neglected, say guard duty, and they will have predator problems."




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