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

## TL;DR

Hair loss in rhesus macaques follows seasonal patterns and is worse in pregnant females, with regrowth resuming after giving birth.

## Contribution

This study is the first to document seasonal hair regrowth patterns and their suppression during late-term pregnancy in rhesus macaques.

## Key findings

- Alopecia and hair regrowth in rhesus macaques follow seasonal patterns, with alopecia highest in spring and lowest in late summer.
- Pregnant females, especially in their third trimester, show reduced hair regrowth and higher alopecia levels.
- Hair regrowth resumes postpartum, indicating suppressed regrowth during late-term pregnancy.

## Abstract

Although alopecia is prevalent among captive rhesus macaques, its cause is not well understood. Poor coat quality may raise concerns because it can be a byproduct of conditions such as stress, autoimmune disease, hormonal imbalance, infection, or poor nutrition. Despite lack of consensus as to the cause(s) of alopecia, multiple studies in captive primates have found two commonalities: alopecia fluctuates seasonally, and pregnant females tend to have more alopecia than males or nonpregnant females. Most studies have focused on loss of hair, rather than if and when hair is regrown, but alopecia can result from disruption to any phase of the hair follicle’s cycle of shedding and regrowth. To better understand how season and pregnancy affect the hair follicle cycle and alopecia, we documented the severity of alopecia and the presence of hair regrowth in outdoor group-housed rhesus for one year. We found a seasonal pattern of alopecia and regrowth in all animals, and that females in their third trimester showed less regrowth, which prevented a decrease in alopecia. Regrowth for females resumed on average 1–2 months postpartum. Hair shedding and regrowth follows a seasonal pattern in rhesus, and conditions in late-term pregnancy suppress hair regrowth into early postpartum.

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.