Primates basically invented "You scratch my back and I'll scratch yours." Baboons, for example, trade grooming for favors from other troop members. Social relationships are important to the monkeys. But it seems they put more effort into certain relationships depending on the time of day: in the morning, lower-ranking baboons invest more energy in grooming animals who can make the rest of their day go smoothly. Chacma baboons help each other out in several ways, says Claudia Sick, a biologist at the University of Copenhagen. They groom each other, which keeps them clean, relieves stress, and strengthens their social bonds. They also trade grooming for favors like access to infants (everyone in the troop enjoys hanging out with the babies), mating, and maybe even backup in fights. We can think of these favors like commodities that the monkeys exchange in a "biological market," Sick says. Another favor baboons trade for grooming is access to food. If you've recently groomed a high-ranking baboon, she's more likely to let you share the patch of land where she's foraging. But the value of this commodity changes over the course of the day, Sick says. If you can "purchase" food-foraging credit early in the morning, you might have more chances to eat throughout the day. But if it's evening, and you've already eaten, getting permission to eat from dominant baboons isn't as important. Because the value of food access changes over the day, Sick guessed that baboons might put more effort into grooming higher-ranking individuals—the ones who control food access–earlier in the day. To find out, she and her colleagues spent about six months following two baboon troops through a park in Namibia. Not counting the (much passed-around) babies, the troops had two dozen and three dozen baboons, respectively. By the end of the experiment the researchers had observed more than 1,600 grooming sessions. Within each troop, Sick explains, there's a ladder of hierarchy stretching from top to bottom. Each individual baboon has its own spot on the ladder, which means every other baboon is either dominant or subordinate to it. Spying on the troops' grooming sessions, she saw that subordinates preferred grooming higher-ranked baboons before midday

. On average, that means a baboon grooming a superior in the morning will choose an individual one extra step higher on the ladder. Baboons are flexible and strategic about their social behavior, Sick says. All of their back-scratching (literal and not) helps them maintain their bonds with each other over the long term. But in the earlier part of the day, they're more focused on short-term gains: I scratch your back now, and you let me eat today. Although the study wasn't about humans, we're social primates too. Can we learn anything from the baboons? "I think it is safe to say that humans also have social strategies that work over many different time scales," Sick says. "Sometimes we think in short-term benefits when interacting with other people." But we also "spend a lot of time on establishing new friendships and maintaining existing ones," she says— "well, at least some people."

Image: Two chacma baboons grooming in Tsaobis Leopard Park in central Namibia. Claudia Sick/Tsaobis Baboon Project, Zoological Society of London.

Sick, C., Carter, A., Marshall, H., Knapp, L., Dabelsteen, T., & Cowlishaw, G. (2014). Evidence for varying social strategies across the day in chacma baboons Biology Letters, 10 (7), 20140249-20140249 DOI: 10.1098/rsbl.2014.0249