Infanticide: all in the coterie.

Black-tailed prairie dogs are engaging, social creatures that live in large underground communities and use a series of barks to warn neighbors of approaching predators (SN: 1/10/81, p. 29). Researchers have, however, documented a grisly side to their nature: This strain of prairie dog kills a substantial number of its own young, and the culprits are most often females who have recently had a litter and then attack the youngsters of close kin.

"I was flabbergasted at the extent and nature of infanticide [among these prairie dogs]," says biologist John L. Hoogland of the University of Maryland in Frostburg. "My co-workers and I watched them for five years before suspecting what was going on."

It appears, he reports in the Nov. 29 SCIENCE, that infanticide is the major source of juvenile mortality among black-tailed prairie dogs, accounting for the total or partial demise of half of all litters born within a 16-acre colony at Wind Cave National Park in South Dakota. From 1978 to 1984, Hoogland and his assistants detected 73 cases of infanticide. In 40 of the cases, the "marauder" was a nursing female raising a litter of her own. She was usually a close relative -- mother, daughter, sister, aunt, niece or cousin -- of the victims' mother. Genetic relationships have been carefully determined for all young weaned at the colony since 1975.

Other types of infanticide included the killing of abandoned young by members of a coterie (a family group consisting of a male, several females and their young living in the same area) or by outsiders.

Killes sometimes eat parts of their prey, says Hoogland. This was confirmed by an excavation to retrieve several victims. Frequently, he adds, a marauder emerges from a burrow with a bloody face, suggesting that he or she cannibalized a litter.

Infanticide occurs in several groups of mammals where one male mates with a group of females. For example, male lions and langurs (SN: 7/10/82, p. 26) entering a group regularly kill the young fathered by the males they replace. The "payoff" for such males who lose offspring come into estrus and conceive more quickly than those who continue to nurse their young.

In prairie dogs, however, the payoff for male marauders is not so clear. They usually attack litters after nursing has stopped, Hoogland explains, and infanticide does not reduce the time until the next female estrus. Males may be attempting to reduce competition by yearlings in the next breeding season, he suggests.

Female infanticide is even more perplexing, he says. It may be a necessary way to get adequate nutrition while nursing; since coterie members defend against outside attacks, the offspring of kin are more accessible. If natural food supplies are limited, infanticide reduces future competition. Also, mothers whose litters are killed are more likely to help defend remainings litters. Ironically, notes Hoogland, mothers who attack litters of close kin leave their own offspring unguarded.

"I have some dandy hypotheses," he says, "but I can't show with numbers what the payoff is [for marauding prairie dogs]."