BackgroundThe evolution of primate sexual swellings and their influence on mating strategies have captivated the interest of biologists for over a century. Across the primate order, variability in the timing of ovulation with respect to females’ sexual swelling patterns differs greatly. Since sexual swellings typically function as signals of female fecundity, the temporal relation between ovulation and sexual swellings can impact the ability of males to pinpoint ovulation and thereby affect male mating strategies. Here, we used endocrine parameters to detect ovulation and examined the temporal relation between the maximum swelling phase (MSP) and ovulation in wild female bonobos (Pan paniscus). Data were collected at the Luikotale field site, Democratic Republic of Congo, spanning 36 months. Observational data from 13 females were used to characterise female swelling cycles (N = 70). Furthermore, we measured urinary oestrone and pregnanediol using liquid chromatography–tandem mass spectrometry, and used pregnanediol to determine the timing of ovulation in 34 cycles (N = 9 females).ResultsWe found that the duration of females’ MSP was highly variable, ranging from 1 to 31 days. Timing of ovulation varied considerably in relation to the onset of the MSP, resulting in a very low day-specific probability of ovulation and fecundity across female cycles. Ovulation occurred during the MSP in only 52.9 % of the analysed swelling cycles, and females showed regular sexual swelling patterns in N = 8 swelling cycles where ovulation did not occur. These findings reveal that sexual swellings of bonobos are less reliable indicators of ovulation compared to other species of primates.ConclusionsFemale bonobos show unusual variability in the duration of the MSP and in the timing of ovulation relative to the sexual swelling signal. These data are important for understanding the evolution of sexual signals, how they influence male and female mating strategies, and how decoupling visual signals of fecundity from the periovulatory period may affect intersexual conflict. By prolonging the period during which males would need to mate guard females to ascertain paternity, the temporal variability of this signal may constrain mate-guarding efforts by male bonobos.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0691-3) contains supplementary material, which is available to authorized users.

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Objectives Female bonobos (Pan paniscus) are characterized as highly affiliative and cooperative, but few studies have quantified the strength and stability of female intra‐sexual relationships or explored how variation in social relationships influences cooperation. We measure female social preferences, identify causes of variation in preferences, and test whether variation in social preferences predicts food sharing or coalitionary support. Methods Data were collected over 3 years from females in the Bompusa community at LuiKotale, DRC. We measured genetic relatedness and constructed social preference indices for party association, proximity, grooming, GG‐rubbing and aggression. We identified preferred social partners based on permutation tests and measured stability using Mantel tests. We used factor analysis to identify inter‐relationships between preference indices and used LMMs to test whether variation in social preferences was explained by relatedness, rank differences, having dependent young or co‐residency time. We used GLMMs to test whether variation in social preferences predicted food sharing or coalitionary support. Results All females had preferred non‐kin partners for proximity, grooming or GG‐rubbing, but only grooming preferences were stable across years. Association indices were higher among lactating females, and aggression was lower among females with longer co‐residency times. The factor analysis identified one factor, representing proximity and GG‐rubbing preferences, labeled behavioral coordination. Dyads with higher levels of behavioral coordination were more likely to share food. Conclusions Female bonobos exhibit stable, differentiated grooming relationships outside of kinship and philopatry. Females also exhibit flexible proximity and GG‐rubbing preferences that may facilitate cooperation with a wider range of social partners.

Referential and iconic gesturing provide a means to flexibly and intentionally share information about specific entities, locations, or goals. The extent to which nonhuman primates use such gestures is therefore of special interest for understanding the evolution of human language. Here, we describe novel observations of wild female bonobos (Pan paniscus) using referential and potentially iconic gestures to initiate genito-genital (GG) rubbing, which serves important functions in reducing social tension and facilitating cooperation. We collected data from a habituated community of bonobos at Luikotale, DRC, and analysed n = 138 independent gesture bouts made by n = 11 females. Gestures were coded in real time or from video. In addition to meeting the criteria for intentionality, in form and function these gestures resemble pointing and pantomime–two hallmarks of human communication–in the ways in which they indicated the relevant body part or action involved in the goal of GG rubbing. Moreover, the gestures led to GG rubbing in 83.3% of gesture bouts, which in turn increased tolerance in feeding contexts between the participants. We discuss how biologically relevant contexts in which individuals are motivated to cooperate may facilitate the emergence of language precursors to enhance communication in wild apes.