When it comes to sharing, brains are overrated. A new experiment shows that the common marmoset, a relatively primitive monkey, is willing to give food to unrelated marmosets even when there's no chance of having the favor returned. Scientists had speculated that such charitable behavior stemmed from more advanced minds, but this new research suggests that other factors drove the evolution of altruism.
Food sharing is often observed in the animal kingdom, but it usually results from self-interest or coercion. One recent study, for example, found that chimps would gladly help others gain access to a room full of food but only if the others asked for it by banging on the door (ScienceNOW , 25 June). Rats also share food but only after having been the recipient of food charity earlier (ScienceNOW , 5 July). Although both kinds of animals showed signs of altruistic behavior, they lacked "true" altruism.
To see whether marmosets are more selfless, a team of researchers led by anthropologist Judith Burkhart of the University of Zurich in Switzerland placed two of the monkeys in adjacent cages. The "donor" marmoset could reach one of two trays on a platform outside its cage. On each tray sat two dishes--one with a tasty cricket, the other without. When the donor monkey pulled a tray close, one dish came to it, while the second slid within reach of the "recipient" monkey next door. The researchers found that when another monkey was present, the donor was more than 20% more likely to pull the tray containing food to its counterpart. The donor was never rewarded for its good deed and knew it couldn't score a cricket by pulling the tray, but that didn't matter. It seems the marmoset simply felt the urge to feed a stranger.
Burkhart believes the marmosets' "spontaneous concern about the welfare of others" evolved not because of brainpower but because it helped the species survive as a "cooperative" breeder. Unlike chimpanzees, marmosets enlist help in child rearing. It is not uncommon for grandparent, aunt, or uncle marmosets to be involved in raising offspring, notes Burkhart, a social structure that the monkey shares with humans. The team presents its findings online this week in the Proceedings of the National Academy of Sciences.
Charles Snowdon, a psychologist at the University of Wisconsin, Madison, is working on a similar experiment with cotton-top tamarins, another monkey that breeds cooperatively. He says he's "excited" about Burkhart's paper and that it confirms some of his predictions about altruistic behavior in cooperatively breeding primates. Although Snowdon cautions that chimps and marmosets have many more differences than mere breeding style, he says that this research "suggests that social structure and social organization might be more important for organizing altruistic behavior than brain size."