Female chacma baboons modulate their sexual receptivity in response to male intrasexual competition

Research in social mammals has revealed the complexity of strategies females use in response to female-female reproductive competition and sexual conflict. One point at which competition and conflict manifest acutely is during sexual receptivity, indicated by swellings in some primates. Whether females can adjust their sexual receptivity from cycle to cycle to decrease reproductive competition and sexual conflict in response to social pressures has not been tested. As a first step, this study explores whether sexual receptivity duration is predicted social pressures in wild female chacma baboons (Papio ursinus). Given that female baboons face intense reproductive competition and sexual coercion, we predicted that: females could shorten the duration of their sexual receptive period to reduce female-female aggression and male coercion or increase it to access multiple or their preferred male(s). We quantified 157 ovulatory cycles from 46 wild females living in central Namibia recorded over 15 years. We found no support for our hypothesis; however, our analyses revealed a negative correlation between maximal-swelling duration and group size, a proxy of within-group competition. This study provides further evidence that swelling is costly as well as a testable framework for future investigations of ‘cycle length manipulation’.

Methods
Data were collected from identifiable females within two habituated groups, J and L, of a wild chacma baboon population at Tsaobis Leopard Park (22º22´S, 15º 44´E) in central Namibia. Studies on this population, which have been ongoing since 2000, have revealed that the naturally-foraging baboons exhibit low predation risk and high female-female aggression rates that can result in reproductive suppression. Over the course of the study, troop numbers fluctuated around a median of 55 (44–69) in J troop and 52 (21–71) in L troop. 

The data used in this study were collected periodically over 15 years, from 2005 to 2019, during field seasons lasting between 2 and 8 months per annum, usually during the austral winter. During field seasons, troops were visited daily, when possible, with some interruptions for other tasks or unforeseen circumstances. During troop visits, we collected data on troop composition (see below) and individual behaviour ad libitum. 

Individuals’ ages were known from observing births or from patterns of dentition wear following capture. Individuals’ relative ranks were calculated annually from ad libitum recorded dominance interactions in the package Matman 1.1.4 (Noldus Information Technology, 2013). Absolute ranks were converted to a relative scale ranging from 0 (lowest rank) to 1 (highest rank), to control for group size, using the formula 1-((1-r)/(1-n)), with r being individual’s absolute rank ranging from 1 to the total group size n. 

Methods for processing the data: 
Each day that the troop was contacted, a census was completed that recorded the identities of the individuals present and the reproductive states of each adult female. Females’ reproductive states were recorded as: (1) pregnant (determined post hoc based on lack of resumption of swelling, reddening of the paracallosal skin and subsequent birth); (2) lactating (i.e., period following the birth of an infant until cycle resumption); (3) oestrous/swollen (i.e., exhibiting periovulatory swelling of the anogenital region); (4) non-swollen (i.e., deturgescent but not pregnant). For females in oestrus, we also recorded swelling size on a scale from 0.5 (smallest) to 4 (largest), on an absolute (cf. the population) rather than relative (cf. for that particular female) scale.  As mentioned above, baboon troops were not followed daily, and as such there were some missing data during some females’ cycles. We discarded any observations where the start or end of the swelling duration was missing and thus could introduce inaccuracy. 

For females in oestrus, swelling duration was calculated as the number of continuous days that a female was recorded with a swelling of any size. Because different mechanisms could act on the different parts of the cycle, in addition to swelling durations, we calculated the maximal-swelling duration as the number of days the focal female exhibited the largest swelling size during that respective cycle. We excluded from the initial data 8 observations of maximal-swelling duration which were calculated as 0, i.e., observations that showed that females were only swollen, and this resulted in 158 swellings and 158 maximal-swellings from 47 females (median number of swellings / female = 5, range = 1-14). 

We calculated five variables describing aspects of the social environment relevant to our hypotheses: for H1, we calculated (1) the sum of pregnant and lactating females (“PL”) during the time a focal female was either swollen or maximally-swollen. For hypothesis H2a, we measured the number of other females that were (2) not maximally-swollen and (3) maximally-swollen, both when the focal female started swelling. To test hypothesis H2b and part of H2c, we calculated the (4) operational sex ratio (“OSR”) as the ratio of sexually active females (i.e., swollen at any level) to adult males. For H2c, we also calculated the (5) the number of in-group adult males.