What Are They Picking At?

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The Question: What exactly are the things that apes groom out of each other’s fur? Ticks? Fleas? Dirt?

Submitted by: Thomas

Green Monkeys grooming

The Short Answer: You’re on track. Apes and monkeys (and lots of other animals as well) clean each other of dead skin, dirt, bits of leaves and other debris, and especially parasites like ticks, fleas, and lice. One study of Japanese macaques (Macaca fuscata, also called snow monkeys), found that overwhelmingly, what the monkeys were picking off each other were lice and lice eggs, which the cleaners then generally ate. Mmmm …

To see Japanese macaques grooming each other, click here.

More Information: When animals groom each other, it’s called “allogrooming” (autogrooming is cleaning your own fur…or washing your car). There are two theories about why animals engage in allogrooming, and as is often the case when two theories continue to compete for decades, both are probably true. The first idea is that it’s just about parasite removal and hygiene. Animals reciprocate in cleaning each other and removing blood suckers like ticks, fleas, and lice. The other theory is that allogrooming is primarily a social thing, a kind of snuggling with your buddy … or someone you want to be your buddy … with parasite removal as a side benefit.

Testing whether one of these two theories or the other is dominant in any one species isn’t easy, but sometimes it can be done. For example, one study with white-crowned mangabeys (Cercocebus torquatus) meticulously recorded what part of the body the monkeys allogroomed. The scientists reasoned that if allogrooming were primarily about social interaction, the monkeys wouldn’t necessarily favor the hard-to-groom parts of the body. What they found was that, in this species at least, they did favor parts of each other’s bodies that were hard for an individual to reach herself.

Other researchers have tried to predict how the behavior of primates who are allogrooming for social benefit would differ from those who allogroom simply for hygiene’s sake. For example, the purely hygiene theory would predict that the social status of allogroomers shouldn’t matter. Everyone has lice, so everyone should groom everyone else equally. But if allogrooming is primarily a social behavior, you’d predict the monkeys might try to win social favor, which means preferentially grooming those above you on the social hierarchy. You might even expect the monkeys to compete to see who gets to groom the higher-ups. And in some species, that’s exactly what the research has found.

It seems likely then, that the behavior is important in both ways, as hygiene and as social bonding, with the primary benefit varying from one species, and probably one situation, to the next.

Too Much Information: Just the other day, I went hiking with my wife and shortly after arriving home, I discovered I had a tick on my stomach. I was able to remove that one myself (no, I didn’t eat it … although I was tempted), but then my wife and I examined the parts of each other we couldn’t easily reach or see to make sure neither of us had any more ticks. There we were, a couple of great apes allogrooming … right there in my own house!

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Schino, G., and F. Aureli. “The relative roles of kinship and reciprocity in explaining primate altruism.” Ecology letters 13.1 (2010):45-50.

Gabriele Schino. “Grooming and agonistic support: a meta-analysis of primate reciprocal altruism.” Behavioral Ecology 18(1) (2007):115-120.

Madden, J., and T. Clutton Brock. “Manipulating grooming by decreasing ectoparasite load causes unpredicted changes in antagonism.” Proceedings of the Royal Society B: Biological Sciences 276.1660 (2009):1263-1268.

Perez, A., and J. Vea. “Allogrooming behavior in Cercocebus torquatus: The case for the hygienic functional hypothesis.” Primates 41.2 (2000):199-207.


Schino, G. “Grooming, competition and social rank among female primates: a meta-analysis.” Animal behaviour 62(2001):265-271.

Cite this article as: Pelletier, TC. (April 12, 2011). What Are They Picking At? Retrieved from https://askanaturalist.com/what-are-they-picking-at/ on March 27, 2017.

2 thoughts on “What Are They Picking At?”

  1. Thomas emailed me this great followup question:

    “What about grooming for the sake of getting food, or is the nutritional value of lice too low for that to be a primary motivation?”

    There probably is very good nutrition in lice and ticks. And there has been speculation by biologists that if the nutrition gained by allogrooming is significant, it would help solve the other big puzzle of allogrooming, which is why do animals help each other in what seems to be an altruistic way. If animals are gaining significant nutrition when allogrooming, then it lessens the altruistic nature of the behavior and replaces it at least partially with self interest.

    However, aside from speculation about that, I can’t find any published research where anyone has actually tried to measure the amount of nutrition gained in allogrooming. My own speculation is that while lice and ticks are probably nutritious (and tasty), the amount of them is not enough to amount to a significant addition to nutrition.

  2. By the way, louse eggs are also called nits, so this activity is also “nitpicking”.


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