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Humans are weird mammals. We have enormous brains, we live long lives, and we constantly walk upright. But perhaps one of our most unique features is our hairlessness. No other mammals, aside from subterranean naked mole-rats and marine-living species, exhibit this trait. Generally, we think that mammals with missing hair are sick, like a dog that has mange.

When I first started my study of captive bonobos at the Columbus Zoo and Aquarium years ago, I didn’t really pay attention to the fact that many of my subjects were missing patches of hair or were almost completely hairless. I had known these animals before studying them and their appearance didn’t phase me. It wasn’t until I repeatedly overheard comments from zoo visitors and had many conversations with my undergraduate advisor that I decided this was worth investigating.

At this point, I should note that the cause of this hair loss was not a mystery. They were plucking it. One hair at a time.

This hair plucking behavior doesn’t occur in wild bonobos or wild populations of other primates. But it does happen in captivity. It’s best documented in lab-housed macaques and zoo-housed great apes (orangutans, gorillas, and chimpanzees). Macaques that were observed to pluck their own hair were thought to be stressed. When macaques plucked other individuals, they directed this aggressive behavior toward lower ranking individuals.

I wanted to better understand this fascinating behavior. In our first study, I observed the Columbus Zoo bonobos for 128 hours and recorded every instance of hair plucking. We had a number of interesting findings:

·      Hair plucking rarely occurred outside of grooming bouts

·      Social plucking (plucking another bonobo) was not aggressive

·      21% of grooming bouts involved at least once instance of hair plucking

·      Wild-born and infant bonobos were never observed to hair pluck

We constructed a plucking “score” for each bonobo per day by dividing the number of grooming bouts that involved plucking by the total number of grooming bouts that day.

 ·      Females and males were not different, but subadult males plucked more than adults

·      Plucking occurred more when bonobos were housed indoors (but so did grooming)

·      Zoo visitor crowd size did not affect plucking rates

We were a bit surprised by these results. First, hair plucking was relatively common in this group. Second, not every individual hair plucked. Third, some individual factors (male age) and location (indoor vs outdoor) resulted in varied hair plucking.

Next, we decided to test the idea that hair plucking was associated with high levels of stress. We measured the level of cortisol, a stress hormone, in each bonobo over the course of the same summer we collected behavioral data. Then, we examined the relationship between the number of grooming bouts with plucking and stress levels for each bonobo. We analyzed self plucking and social plucking separately because they may have a different relationship to stress.

We found that social plucking was not related to stress. However, self plucking was related to stress in females but not males (see figure below). This supports the idea that hair plucking is stress induced but only for females and only for self plucking. Clearly, a lot of hair plucking also happens independently of stress.

Like any good research project, we ended with more questions than answers. In a later post, I’ll provide an update on we’ve learned about bonobo hair plucking since these first two studies.

Brand CM, Boose KJ, Squires EC, Marchant LF, White FJ, Meinelt A, Snodgrass JJ. 2016. Hair plucking, stress, and urinary cortisol among captive bonobos (Pan paniscus). Zoo Biology. 35: 415-422.

Brand CM, Marchant LF. 2015. Hair plucking in captive bonobos (Pan paniscus). Applied Animal Behaviour Science. 171: 192-196.



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