Effects of Seasonality and Pregnancy on Hair Loss and Regrowth in Rhesus Macaques.

Several studies have examined the etiology of alopecia, or hair loss, in rhesus macaques. While outcomes differ across studies, some commonalities have emerged. Females, particularly pregnant females, show more alopecia than males, and alopecia follows a seasonal pattern. Much research has explored causes of hair loss; however, alopecia can result from lack of hair growth in addition to hair loss. To better understand how sex, reproductive state, and season affect alopecia, we followed 241 rhesus macaques (Macaca mulatta) in outdoor breeding groups over one year, recording both alopecia severity and presence of hair regrowth. We found that both alopecia and hair regrowth followed a seasonal pattern; alopecia was highest in spring and lowest in late summer, while regrowth started in spring and peaked in late summer. Reproductive state also correlated with both alopecia and hair growth. Females in their third trimester had the highest average level of alopecia and the lowest amount of hair regrowth. Regrowth resumed postpartum, regardless of whether females were rearing an infant. Results indicate that the seasonal pattern of alopecia is due in part to the seasonal limitations on hair regrowth, and that breeding, which also occurs seasonally in rhesus macaques, may further suppress hair regrowth.

Social hair pulling in captive rhesus macaques (Macaca mulatta).

Alopecia is common among captive populations of nonhuman primates. There are many potential causes of alopecia, including physiological conditions such as hormonal imbalance and infection, features of the captive environment such as housing type, ground substrate, and group density, as well as behavioral abnormalities such as self-plucking. A potential behavioral cause of alopecia in group-housed primates is social hair pulling, where one animal pulls hair from a conspecific. While social hair pulling has been conflated with overgrooming in some of the alopecia literature, other authors have categorized it as a form of aggression rather than a form of excessive grooming. In this study, we examined social hair pulling, grooming, and aggression within seven groups of rhesus macaques (Macaca mulatta) (N = 319). We took weekly 30-min behavioral observations on each group for one year to assess the patterns of hair pulling and grooming, which monkeys were receiving and initiating these behaviors, as well as aggression and other behaviors indicating dominance. We also assessed the amount of alopecia on each individual monthly. While grooming tended to be directed "up" the hierarchy (i.e., monkeys were more likely to groom animals of a higher rank than lower rank), most hair pulling was directed "down" the hierarchy. Further, hair pulling seldom co-occurred with aggressive behaviors, suggesting that it was not a form of aggression. Hair pulling also usually resulted in ingestion of the pulled hair. Hair pulling was correlated with alopecia; monkeys who were frequent recipients of hair pulling scored higher on monthly alopecia ratings than those who were less often observed having hair pulled. Our results suggest that social hair pulling is a behavior distinct from either grooming or aggressive behavior, and that it may contribute to alopecia in socially housed macaques.