Let's Talk Primates: New consequence of grooming discovered

Macaca fuscata grooming, Photo: Noneotuho

When primates groom each other, they improve their health and hygiene by removing parasites, dirt, and dead skin. This practice is also known as allogrooming. If a primate lives in a group, you'll likely observe allogrooming. (Note: this is not a term restricted to primates: other animals allogroom as well.)

The benefits of allogrooming extend beyond hygiene though, as evidenced by a study on mangabeys (Cercocebus torquatus lunulatus) that found difficult to reach areas were not groomed as often as one would expect if hygiene were the sole purpose of grooming (Perez and Baro, 1999). The practice also maintains and betters affiliative bonds (Seyfarth and Cheney, 1984; Stammbach and Kummer, 1982). Allogrooming promotes cooperation over food in Japanese macaques (Ventura et al, 2006). It reduces tension in Macaca fascicularis (Schino et al., 2005) and M. mulatta (Aureli, Preston, and de Waal, 1999).

During allogrooming, the primate doing the grooming is obviously focused on another individual, thus it is not entirely surprising that there's a tradeoff between allogrooming and vigilance. Macaque mothers that engage in allogrooming glance at their infants significantly less and those infants are the victim of harassment more often than when mothers are present (Maestripieri, 1993). In general, the costs of allogrooming are less well understood than the benefits. Energetic, cognitive, and other opportunity costs have all been suggested to exist (Russell and Phelps, 2013).

Brown spider monkey, Photo credit: Fir0002

A new study by Rimbach and colleagues describes a previously unknown consequence of allogrooming in the critically endangered brown spider monkey, Ateles hybridus. Studying sixteen individuals, Rimbach and colleagues found that the more connected monkeys had a greater richness of gastrointestinal parasites than monkeys that were less connected. This relationship is based on physical contact, as when the authors examined proximity alone, there was no such relationship. The authors concluded that the largest parasite risk to this particular community of brown spider monkeys is in fact social grooming.

Given that this species is critically endangered due to habit loss and hunting, identifying other potential threats to their health and survival is crucial. I'm not positive what the applications of this new study would be though, as it's impossible to stop primates from grooming each other, thus parasite transmission will continue to occur. If these monkeys aren't negatively affected by their parasites in a significant manner though, the threat posed may be small. One of the parasites found in this study, Strongyloides, has cased moderate to severe disease in grey wooly monkeys though (Lagothrix cana) (Mati et al., 2013), so it seems reasonable to assume a negative effect in brown spider monkeys as well.

Food for thought: How might the adaptive benefits of allogrooming counter the effects of increased gastrointestinal parasites? Is it possible allogrooming no longer acts as a benefit to this particular primate population given their low numbers?

Further reading:
Sciencedaily article
IUCN Redlist page on Brown spider monkeys
What are they picking at?
Baboons groom in the morning to reap benefits in afternoon

Sources:
Aureli F, Preston S.D, de Waal F.B.M. Heart rate responses to social interactions in free-moving rhesus macaques (Macaca mulatta): a pilot study. J. Comp. Psychol. 1999;113:59–65. doi:10.1037/0735-7036.113.1.59

Maestripieri, D. (1993). Vigilance costs of allogrooming in macaque mothers. American Naturalist, 744-753.

Mati, V. L. T., Junior, F. C. F., Pinto, H. A., & de Melo, A. L. (2013). Strongyloides cebus (Nematoda: Strongyloididae) in Lagothrix cana (Primates: Atelidae) from the Brazilian Amazon: aspects of clinical presentation, anatomopathology, treatment, and parasitic biology. The Journal of parasitology, 99(6), 1009-1018.

Pérez Pérez, A., & Josep Veà Baró, J. (1999). Does allogrooming serve a hygienic function in Cercocebus torquatus lunulatus?. American Journal of Primatology, 49(3), 223-242.