Getting to the Bottom of Baboon Bottoms

I have received some interesting reactions when I tell new acquaintances that I study baboons. Recently, someone very enthusiastically asked, “Those are the animals with the HUGE pink butts, right?” They are indeed. And I’m actually quite intrigued by those “pink butts.”


A more technical term for this is “exaggerated sexual swellings.” Around the time of ovulation, females of some Old world monkey and ape species exhibit these swellings. Sexual swellings are not actually swollen butts. Rather, they are swellings of the perineal skin.


The existence of exaggerated sexual swellings is intriguing because they are costly to produce. When maximally swollen, female body weight increased by over 25% in red colobus monkeys (Colobus badius) (Struhsaker 1975). Such a large increase in weight on your backside must be uncomfortable and likely increases the energy costs of locomotion. To add to this, swollen females deal with increased parasites and cuts to the surface of the swelling (Hausfater 1975). Females are dealing with noticeable costs as a result of these sexual swellings. Thus, it is expected that swellings confer significant benefits. What could these benefits be?

hamadryas swelling

The Graded-Signal Hypothesis

Currently, the favored hypothesis regarding the function of exaggerated sexual swellings is the “graded-signal hypothesis” (Nunn 1999). This hypothesis focuses on variation within a female’s reproductive cycle and posits that exaggerated sexual swellings function to both confuse paternity and concentrate the probability of paternity in the highest quality male. Prior to this hypothesis, these two functions were proposed as opposing explanations. They do seem to be contradictory, right? But Nunn (1999) was able to incorporate both ideas into his graded-signal hypothesis.


So how can swellings both concentrate and confuse paternity? They can concentrate paternity in the highest quality male by inciting the most intense competition when the sexual swelling is at its maximum size (when the probability of ovulation is highest), and they can confuse paternity by exhibiting swellings for an extended period of time and mating promiscuously.


This is a remarkable strategy for females! By swelling outside the period of ovulation and mating promiscuously, they create paternity uncertainty. When there is even a slight chance that a male could be the father of an infant, it is unlikely that he would kill it (i.e. commit sexually selected infanticide). Now, these females may be mating promiscuously but at the time of ovulation, they can limit their mating to males of their choice! Thus, females with swellings can receive the benefits of mate choice and paternity uncertainty.


The Reliable-Indicator Hypothesis

Another explanation for the function of exaggerated sexual swellings is the “reliable-indicator hypothesis.” This hypothesis examines variation across females and proposes that swelling size is an honest indicator of the female’s quality as a mate (Pagel 1994).


When we learn about sexual selection, we learn that females should be the more discriminating sex while males should compete for access to females and mate with as many females as possible. Working under this (simplified) framework, why would primate males be choosy about female quality? Pagel (1994) argued that mating can be costly for males, especially in multi-male mating systems in which males compete physically over females. This suggests that males are more selective due to the high mating costs associated with multi-male groups. As a result, it would benefit females to signal their high quality via a nice sexual swelling.

bufallo swelling

These two hypotheses are fundamentally different. The reliable-indicator hypothesis predicts female-female competition and honest signaling to males, whereas the graded-signal hypothesis predicts that females manipulate the information available to males. The graded-signal hypothesis has received substantial theoretical and empirical support. In contrast, the reliable indicator hypothesis has received less consistent support. However, I think the latter provides an interesting perspective and challenges traditional views.


Exaggerated sexual swellings are meant to draw attention to the females exhibiting them. And they do a great job of this! Well…at least humans are drawn to them — both the general public and scientists show an interest in these swellings. But nonhuman primate males seem to be quite interested in them as well! We don’t have all the answers yet though. A combination of theoretical modeling, comparative analyses, experimental studies, and observational studies are necessary to rigorously investigate the adaptive function of exaggerated sexual swellings.



Hausfater, G. 1975. Dominance and reproduction in baboons (Papio cynocephalus): a quantitative analysis. Contributions to Primatology, 7, 1-150.


Nunn, C. 1999. The Evolution of Exaggerated Sexual Swellings in Primates and the Graded-signal Hypothesis. Animal Behaviour, 58.2, 229-46.


Pagel, M. 1994. The Evolution of Conspicuous Oestrous Advertisement in Old World Monkeys. Animal Behaviour, 47.6, 1333-341.


Struhsaker, T. T. 1975. The Red Colobus Monkey. Chicago: Univer- sity of Chicago Press.


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