Science in Mind

The evolution of day care

Mother and baby Black-and-white ruffed lemur, 7 years old and 2 months old.
Mother and baby Black-and-white ruffed lemur, 7 years old and 2 months old.Credit: ©

In the animal kingdom, day care is rare. Humans are among the very few species known to pool child care resources, raising their young with the help of other moms. But a research team from Yale University has discovered that on the island of Madagascar, black-and-white ruffed lemurs will house their young in communal nests—and that this type of shared child care provides clear benefits to their offsprings’ survival.

The finding, reported in the journal Behavioral Ecology and Sociobiology on Tuesday, was the fruit of six years of painstaking labor. Researchers tagged the lemurs, took samples for genetic analysis, and maintained a detailed diary of how moms budgeted their time before and after giving birth.

Although the researchers were dealing with a small community of 22 adult lemurs and their babies, the patterns they observed were clear. The mother lemurs all gave birth to their litters of two to three young in different nests, caring for the babies on their own. But after about two and half weeks, the mothers began to move around to other nests they and others had built—transporting their young, who were still too young and undeveloped to cling to their fur, in their mouths. Of the moms that the researchers followed, three routinely dropped their kids off with other females or baby-sat, two barely ever shared parenting responsibilities, and two who weren’t as closely monitored were often seen sharing nests with others.

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When females were solely responsible for child care, the amount of time they spent eating dropped off, the daily time diaries showed. Once they started sharing child care, however, they spent less time at the nest and more time eating. The researchers believe that likely means they were able to provide higher quality nutrition for their young, which might explain why more young survived once they were in a type of lemur kindergarten. Among single mothers, 3 of 5 infants died, compared with just one of 14 infants among those who shared child care.

“For primates, and especially a lactating mother, that’s really your limiting factor in survival is the time you have [for] feeding” yourself, said Brenda Bradley, a molecular anthropologist at Yale and a co-author of the paper. “The fact females can take turns guarding the nest and spend more time taking care of themselves” appears to benefit the survival of their offspring, too.

The researchers had been particularly curious about the circumstances that would lead to shared child care—would lemurs only co-parent babies who were related to them? To their surprise, the answer was no. Some of the mothers were related, but some weren’t. But the day care arrangements weren’t just random; the mothers would consistently share parenting only with certain mothers.

Why did this behavior evolve in the first place? One clue might be in the environment on Madagascar, which is prone to sudden and severe weather changes. That unpredictability, Bradley said, may have contributed to an unusual boom and bust reproduction pattern, in which the females only reproduce sporadically—and all at the same time. In the six years that post-doctoral researcher Andrea Baden followed these animals, they reproduced only one year—in 2008, and all within a two-week period of one another. It’s possible, Bradley said, that those factors contributed to the evolution of the shared child care behavior.

Perhaps more mystifying, however, is why all moms don’t share child care, since it is clearly beneficial. That’s a question the researchers hope to investigate next.