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A marketlike system with variable “prices” determines who gets to handle junior.

“Do my hair before you touch my baby” is the rule among mother vervet monkeys and sooty mangabeys when it comes to sharing their infants with their neighbors.

Like some other primate infants, monkey babies attract crowds of females eager to touch, hold and make silly lip-smacking noises at the little ones, says primatologist Cécile Fruteau of Tilburg University in the Netherlands. Her novel study of infant-touching etiquette in the vervets and mangabeys adds them to the short list of animals known to have “markets” for baby fondling. The moms have to be groomed for a sufficient time before they let the groomer touch the baby.

What makes this exchange a market is the way sufficient grooming time changes with the baby supply, Fruteau and her colleagues explain in a paper now posted online in Animal Behaviour. The price for access to a group’s solitary infant, measured in grooming time for mom, fell when other females gave birth and increased the number of little cuties available for cuddling.

Price is sensitive to other variables as well, says Fruteau, who documented for the first time that age makes a difference in how much grooming a baby can bring to a mom. Newborns earn their mothers the longest grooming sessions. One newborn mangabey, for example, the only baby in its group at the time, earned about 10 minutes of fur cleaning and combing for its mom. In contrast another lone baby didn’t even earn four minutes of grooming once it had reached the advanced age of almost 3 months.

Researchers also found that grooming time correlated with access to vervet babies but not with the amount of fondling time permitted or the degree of familiarity allowed. With enough grooming, moms permitted pretty much any female in their group to at least touch or sniff the baby. But it was mostly females with a history of grooming mom, presumably the well-known and accepted associates, who could actually hold the baby themselves.

“Prices” for a baby encounter also varied with rank, as in other infant-handling markets, Fruteau says. A female ranking lower in the hierarchy of the vervet and mangabey groups had to groom longer for access than a high-ranked monkey did.

Other species where scientists have documented grooming-for-cooing trades include chacma baboons and long-tailed macaques. In spider monkeys the currency is not grooming but hugging moms.

A marmoset system goes in the opposite direction. Moms groom other females that handle infants, but as Fruteau explains, marmosets frequently have twins.

The idea of a market has proved fruitful for studying a wide range of biological exchanges, says study coauthor Ronald Noë of University of Strasbourg in France. Fish eating parasites off other fish in reefs, ants living in specialized plant crannies and chasing away other insects and primates building coalitions all display marketlike qualities.

Comparisons with markets can certainly be useful, says primatologist Rebecca E. Frank of Los Angeles Valley College in Valley Glen, Calif., “but it just leaves some aspects of female exchange unexplained.” In her study of grooming arrangements in olive baboons, about two-thirds of grooming encounters, with or without babies involved, don’t get promptly or obviously reciprocated. These partners appear to have long-term relationships that don’t require immediate settling of accounts.

None of this settles why monkey babies stir up such widespread urges to fondle, Fruteau says. Among the vervets and mangabeys that’s largely a female urge. Males don’t interact much with youngsters until the kids get older.

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