Dominant Meerkats Commit Infanticide, Force Bereaved Mothers to Play Wet-Nurse

Mother meerkats whose babies are killed by the dominant female in the group have been observed acting as wet-nurses for the alpha mothers, which researchers say is a form of "rent" paid by the subordinate meerkats that allows them to remain within the community.

After collecting 15 years worth of data among 40 social groups of meerkats, the researchers, writing in the journal Animal Behavior, present what they say is the most comprehensive evidence to date as to why wet-nursing -- also called allolactation -- is preformed by certain members of animal groups, suggesting that the practice is most often done by mothers who have lost litters to infanticide or who have recently returned to the community group following prior eviction.

"Breeding opportunities are monopolized by a single behaviorally dominant female in meerkat groups," said Kirsty MacLeod of the University of Cambridge's Department of Zoology. "She maintains this position through suppressing breeding attempts by other females -- either through evicting them or killing their pups -- and these subordinate females are then also more likely to wet-nurse the dominant female's pups. This suggests to us that infanticide by the dominant female might have two evolutionary advantages for her -- she reduces competition for care for her own pups, and is more likely to secure allolactation for her litter."

MacLeod added that wet-nursing by formerly evicted meerkats "may be a way of 'paying rent' to be allowed back into the group without receiving further aggression."

This "rent-paying" system has also been observed in some bird species, but until now has only been seen in one other mammal species: the naked mole rat.

To conduct their survey, the researchers created a long-term database and recorded pregnancies, lactation periods and other life-history details.

"Now that we have a clearer idea of which females are more likely to invest energy in this highly cooperative behavior," MacLeod said, "our next step is to figure out what benefits each party is getting from this. We know that lactation is costly, so it's likely that if additional females also provide milk, those costs should go down. We'll know that soon.

"These results, however, hint at what the benefits might be for subordinate allolactators. Because subordinate females were more likely to allolactate if they are related to the litter's mother, this suggests that they may gain an indirect benefit from the activity. Evictees from the group suffer considerable stress, weight loss and reduced survival. If contributing to the maternal cares of another's offspring allowed renewed access to the social group, or to remain in the group once following infanticide, there would be an incentive to 'pay-to-stay'."