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Grooming and groveling reduces meerkat stress

This week's paper is "Manipulating grooming by decreasing ectoparasite load causes unpredicted changes in antagonism" by Joah Madden and Tim Clutton-Brock, published on-line in Proceedings of the Royal Society.

Social animals often groom each other, removing parasites (lice, ticks, fleas, etc.) from places that may be hard for the beneficiary to reach, while also relaxing them, reducing heart rate and lowering concentrations of stress hormones. This stress-reducing function is sometimes considered more important than the hygenic one, but that made me ask, in an earlier post, why natural selection hasn't made lower (healthier) levels of stress the default. If stress is harmful and grooming isn't always available, why depend on grooming to reduce stress?

Madden and Clutton-Brock tested the interaction among parasites, grooming, and social interactions by chemically treating meerkats to eliminate parasites. Half of the meerkats got treated with an antiparasite chemical while the other half got water as a control, but parasite levels dropped in both. Maybe the chemical rubbed off on nontreated individuals.

With lower parasite loads, they only groomed each other half as much. Self-grooming also decreased. Because the social function of grooming was thought to be important, they expected increased fighting, bullying, etc., but that didn't happen overall. The biggest overall change in social interactions was a near-doubling in "unprompted submissions", basically groveling. Looking more carefully at pairwise interactions, they found some increases in dominance interactions, which were often followed by grooming of the dominant individual by the subordinate. So lower parasite levels led to less grooming, which led to stress, leading to cranky dominants, who were then placated by grooming they didn't really need (at least not for parasites) or general groveling. They concluded that

"for meerkats, grooming primarily serves a hygienic function, and secondarily provides a facultative response to antagonism, functioning to halt persistence or escalation of antagonism. This contrasts with its proposed function suggested for primates as a pre-emptive strategy that inhibits the initiation of antagonism by establishing or reinforcing a social structure."

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