Baboons are distinctive primates found across Africa and parts of Arabia, recognized for their complex social structures and unique physical traits. Among their most noticeable features are their prominent hindquarters, which often display a striking red coloration. This characteristic, observed in many baboon species, serves various biological purposes, from practical comfort to intricate social signaling within their troops.
Understanding Ischial Callosities
The “red bottoms” of baboons are specialized anatomical structures known as ischial callosities. These are tough, hairless patches of thickened skin located on their buttocks. Ischial callosities are a common adaptation among Old World monkeys, including baboons, gibbons, and some chimpanzees.
These pads primarily function as padded cushions, allowing baboons to sit comfortably for extended periods on rough surfaces such as tree branches, rocks, or the ground. This specialized design provides stability and support, enabling baboons to rest, sleep, feed, and socialize in an upright position. While both male and female baboons possess these callosities, their appearance can vary significantly, particularly in color and size, especially in females.
The Physiology of Redness
The red coloration observed in baboon ischial callosities, particularly in females, is a result of biological mechanisms driven by hormonal fluctuations. This redness and associated swelling occur due to increased blood flow and fluid retention in the tissues surrounding the genital and anal regions. The skin in these areas has a rich blood supply, which contributes to the visible color changes.
Sex hormones, primarily estrogen, play a significant role in regulating these changes. As a female baboon approaches ovulation during her estrous cycle, estrogen levels surge, leading to the pronounced swelling and reddening of her ischial callosities. This swelling can be substantial, increasing the female’s weight by 14% to 25% during her estrus cycle. Conversely, a decrease in estrogen and a rise in progesterone lead to a reduction in both the swelling and redness. In males, while less dramatic, testosterone levels can also influence coloration and are linked to dominance status.
Communication Through Color
The vibrant coloration and swelling of baboon hindquarters serve as a sophisticated visual signaling system within their social groups. For female baboons, the intensity and size of the red swelling act as a clear advertisement of their fertility and readiness to mate. This “sexual swelling” indicates that the female is in her fertile stage and biologically prepared for pregnancy, with the peak swelling often coinciding with the highest probability of ovulation.
Male baboons, especially higher-ranking individuals, pay close attention to these visual cues to identify fertile females and prioritize mating efforts. A larger, brighter swelling suggests a female is at the peak of her fertility, making her a more desirable mate. While the primary function for females is to signal receptivity and incite male competition, the color can also signal health and vigor. In males, though less overtly displayed through callosities, color changes in other areas, such as the face or scrotum, can signal dominance and health, influencing social hierarchy and reproductive success. The visual communication provided by these colored features is crucial for mate selection and maintaining social order within baboon troops.