Higham JP et al. (2009). Female reproductive signaling, and male mating behavior, in the olive baboon. Hormones and Behavior. 55: 60-67.

This study assesses the function and importance of female sexual swellings in olive baboons as signals to potential mates by observing the association between the intensity of these swellings and changes in both female and male behavior courtship behavior. The authors collected behavioral data on two troops of olive baboons, a total of 47 individuals, in Nigeria for one year. Fecal samples of cycling females were collected every one to two days and processed in order to quantify relative estrogen and progestogen levels. These hormone levels were then used to estimate ovulation date and a likely fertile period. Simultaneously, size (relative to pelvic width) and color of swelling was measured through digital photographs of the rear and side of females. These measurements of swellings were subsequently divided into three qualitative levels of relative intensity. In addition, 848 focal behavioral observations were made where social and sexual behaviors were recorded and identities of all individuals were known. The observations focused on chasing, aggression, grooming, presentation of sexual swelling to males and copulation behaviors.

The authors found that hormone levels did not correlate with most observed behaviors, but females spent significantly less time presenting the swelling to males when they had higher progestogen and progestogen:estrogen levels. Also, both quantitative measures of swelling intensity (size of swelling) and qualitative measures of intensity (the three classes of relative swelling size) had a significant effect on consortship by adult males; adult males spent more time closely associating or courting females when swellings were most intense. In addition, this increased consortship was correlated with peak ovulation and fertile period. There was a strong relationship between height of swelling and male chasing of females (a possible indicator of the male’s desire to copulate). Also, females with the largest swellings spent more time copulating than other females. From these relationships, the authors conclude that the size of the swelling is a sexual signal used by males to infer when females are most fertile and receptive. Consequently, males can determine precisely when to spend energy in consortships thus enhancing their probability of copulation and paternity of female offspring.

As we learned in class, courtship signals can be affected by the population’s operational sex ratio. Individuals in the population studied are non-seasonal breeders so there are only a few receptive females at any give point in time. As discussed in class, situations like this may result as a consequence of anisogamy because females aren’t receptive to copulation as often as males. Thus, it may be necessary for females to signal their receptivity to competing males and this paper shows that information about receptivity is accomplished by presentation of the sexual swelling, a visual signal to males. Also, we discussed that for mobile species, sperm competition is not the only form of male-male competition, and instead males may also guard or spend more time with females they know to be receptive. This idea is demonstrated here by a strong correlation between swelling size and consortship duration.