Sociosexual behaviors such as mounting and genital touch have been extensively reported in chimpanzees. However, there are a small number of reported cases of fellatio, with only a few cases between captive juveniles or adult males. In this study, fellatio from a non-estrus adult female to an adult male holding bushpig meat were observed for the first time. After a series of sociosexual behaviors including fellatio, the meat was shared through begging behavior from the female to the male, suggesting that the series of displayed sociosexual behaviors may have occurred to reduce tension, so that the female could then make a smooth transition to begging behaviors. In addition, the fact that fellatio displayed between male and female in a context similar to previously reported cases of fellatio between adult males suggests that the social bond-strengthening effect of fellatio can also function between an adult male and female.

In primates, sociosexual behaviors without mating have been observed in both opposite-sex and same-sex partners. Some of these examples include mounting without penile penetration in the olive baboon (Papio anubis) (Owens 1976) and in the Japanese macaque (Macaca fuscata) (Takahata 1982), and anal penetration between males in the Geoffroy's spider monkey (Ateles geoffroyi) (Busia et al. 2018) and in the snub-nosed monkey (Rhinopithecus roxellana) (Fang et al. 2018).

Although the forms and frequencies of sociosexual behaviors may vary among taxa, they appear to serve social functions. Such behaviors can appear in a variety of contexts, such as mating practices (the Japanese macaque: Gunst et al. 2013), negotiation of dominance relations (the snub-nosed monkey: Huang et al. 2017), and reducing social tension after aggression or during food sharing (the bonobo [Pan paniscus]: de Waal 1990; Hohmann & Fruth, 2000).

Although chimpanzees (Pan troglodytes) engage in sociosexual behavior less frequently than bonobos (Woods & Hare 2011; Gruber & Clay 2016), it has been extensively reported (Crawford 1942; Kutsukake & Castles 2004; also see a review by Sandel & Reddy 2021). The frequently reported behavioral types in chimpanzees are mounting and touching of genitals (including mutual touching). However, there are few reported cases of fellatio, or mouthing of the male penis by another individual. Some acts of fellatio are directed to still immature males as reported by Malick & Savage (1977) and Savage-Rumbaugh & Wilkerson (1978) who noted oral-genital contacts among captive juvenile chimpanzees in the context of play. Nishida (1981) reported that an adult female at Mahale mouthed her weaning male offspring's penis as means of appeasing his frustration. Only two reported cases of fellatio were performed on adult males, and both were by other males to reduce tension, one among captive chimpanzees in Primate Foundation of Arizona in the USA (Shefferly & Fritz 1992) and the other among sanctuary-living chimpanzees in Chimfunshi Wildlife Orphanage in Zambia (Brooker et al. 2020). To the best of my knowledge, there have been no reports of fellatio by an adult female chimpanzee on an adult male.

This study reports the first such case and describes other accompanying sociosexual behaviors, the context, and subsequent meat sharing.

This case was observed between chimpanzees of the M group in the Mahale Mountains National Park, Tanzania, on September 20, 2019 (see Nakamura et al. 2015 for details of the study site). The case was recorded first on a field note and subsequently with a video camera (Panasonic HC-W585M). An adult male named Darwin (DW) and an adult female named Fawn (FW) were the participants in this event. FW had borne an infant that year and thus was non-estrous.

At 08:35 h on September 20, 2019, a large party of the M-group chimpanzees ranged southeast of the area called K1, less than 1 km from the researchers' camp, and DW was followed. At 08:53 h, DW was holding a fresh carcass of a bushpig (Potamochoerus larvatus) piglet. At 08:54 h, after climbing a nearby tree, he started eating meat from the bushpig. At 08:58 h, FW with the infant attached to her belly appeared from an unknown location, climbed the same tree, and sat on a lower branch in front of DW. No other individuals were observed in the vicinity. Because a clear view of the subjects was not possible, at 08:58 h the observation location was changed and video recording begun at 09:01 h (Video 1; hereinafter, times in descriptions from video footage are given in hours:minutes:seconds).

At 09:02:04 h, FW touched the erect penis of DW with her left hand for 3 s. Her face was not visible as her back was turned to the camera, but judging from the direction of the head, it is assumed that her gaze was not on DW's penis but on his hand holding the piglet. DW continued to eat without any significant reaction. At 09:02:07 h, FW mouthed DW's penis for 3 s (Figure 1). She touched DW's penis again for 3 s, this time pinching it with her left thumb and index finger. At 09:02:22 h, FW presented her rump to DW and pressed her non-swollen sexual skin against DW's erect penis for 4 s (Figure 2). No penile penetration occurred. FW reached toward DW's hand holding the piglet at 09:02:27 h, but he moved it away from her. At 09:02:31 h, DW turned his back completely on FW. Then, at 09:02:40 h, FW moved to face DW, and at 09:02:51 h, she reached for his mouth and hand holding the meat while peering into his face. At 09:03:22 h, FW loosely grabbed DW's hand holding the meat. At 09:03:34 h, FW grinned, pant-screamed slightly, and then reached for the meat. FW eventually obtained a small amount of meat which DW had removed from his mouth. DW then climbed further up the tree and left FW, ending the interaction, at 09:03:51 h.

In the present case, a non-estrus female, FW, after several types of sociosexual behaviors, such as touching the penis, fellatio, and genito-genital contact without penile insertion, begged DW for bushpig meat by extending her hand to the mouth of DW. This is the first such report of fellatio between opposite-sex adult chimpanzees and is also the first report of fellatio in the context of food sharing.

Sociosexual behaviors during food sharing have been reported in wild chimpanzees and have been interpreted as actions performed to reduce social tension (Sandel & Reddy 2021). Thus, it can be assumed that the series of sociosexual behaviors in this report also served to reduce tension. Alternatively, such behavior may function as begging, because typical begging behaviors, such as peering into the face and reaching for the mouth, also occurred. Gilby (2006), who studied begging behaviors during meat sharing, classified them according to their intensity, from milder to more intense: (1) sitting close to the meat holder and peering at him, (2) reaching out to the meat holder (no contact), (3) touching the meat holder, and (4) reaching a hand near to the meat holder's mouth and making contact with it. The series of sociosexual behaviors observed in this study, if they were considered begging behaviors, would be classified as category (3). However, several behavioral elements observed in this study were not reported in Gilby's behavioral repertoire of begging. In addition, according to Gilby's definition of harassment during meat-eating, the observed behaviors were not considered harassment, because there was no contact with DW's mouth or hands, nor was his behavior restricted. Although it is difficult to conclude from just one case, the series of displayed sociosexual behaviors may have occurred to reduce tension, so that FW could then make a smooth transition to begging behaviors.

Fellatio is considered rare because males are at high risk of exposing their vital organs to other chimpanzees' mouths. It has been proposed that such high-risk behaviors can be used to test and strengthen alliances between same-sex partners (Kirkpatrick 2000). For chimpanzee males, forming alliances with other males is crucial. Thus, for a rare case of male–male fellatio, such a function of strengthening male–male relationships was emphasized (Brooker et al. 2020). However, as reported in this study, fellatio can occur between a female and a male in the probable context of tension reduction. Thus, the function that is used to test social bonds may also apply to opposite-sex partners.

This report describes a very rare case of heterosexual fellatio observed in the wild. However, it is difficult to know why such behaviors are rare and why this case occurred in this context. More such observations are needed to gain a deeper understanding of sociosexual behaviors as well as the function of fellatio in chimpanzees.

I would like to thank the Tanzania Wildlife Research Institute (TAWIRI), Tanzania Commission for Science and Technology (COSTECH), and Tanzania National Parks (TANAPA) for permission to conduct this research. I am grateful to the Mahale Mountains National Park (MMNP) and Mahale-Gombe Wildlife Research Centre (MGWRC) for their logistic support. I thank all colleagues and local assistants of the Mahale Mountains Chimpanzee Research Project (MMCRP) for their continuous help and support. I would like to thank Editage (www.editage.com) for English language editing. The present study was financially supported by JSPS KAKENHI #2019J23191 and #4903 (JP17H06381).

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Received: 21 January 2022
Accepted: 28 February 2022