Do monkeys use sex toys? Evidence of stone tool‐assisted masturbation in free‐ranging long‐tailed macaques

Abstract

Recent reports on tool use in nonforaging contexts have led researchers to reconsider the proximate drivers of instrumental object manipulation. In this study, we explore the physiological and behavioral correlates of two stone‐directed and seemingly playful actions, the repetitive tapping and rubbing of stones onto the genital and inguinal area, respectively, that may have been co‐opted into self‐directed tool‐assisted masturbation in long‐tailed macaques (i.e., “Sex Toy” hypothesis). We predicted that genital and inguinal stone‐tapping and rubbing would be more closely temporally associated with physiological responses (e.g., estrus in females, penile erection in males) and behavior patterns (e.g., sexual mounts and other mating interactions) that are sexually motivated than other stone‐directed play. We also predicted that the stones selected to perform genital and inguinal stone‐tapping and rubbing actions would be less variable in number, size, and texture than the stones typically used during other stone‐directed playful actions. Overall, our data partly supported the “Sex Toy” hypothesis indicating that stone‐directed tapping and rubbing onto the genital and inguinal area are sexually motivated behaviors. Our research suggests that instrumental behaviors of questionably adaptive value may be maintained over evolutionary time through pleasurable/self‐rewarding mechanisms, such as those underlying playful and sexual activities.

Stone handling in geladas (Theropithecus gelada): implications for spontaneous drawing-like activity as a playful behavior

“…Together with the self-rewarding nature characterizing SH behavior, past and recent studies have documented just how different SH patterns may actually acquire new functions under different social contexts (Huffman and Quiatt 1986;Leca et al 2008;Tan 2017;Cenni et al 2020Cenni et al , 2022. In long-tailed macaques (Macaca fascicularis), two SH patterns usually characterized as playful actions have been coopted into self-directed tool-assisted masturbating behaviors (Cenni et al 2022). This functional shift has been possible through a process of affordance learning, mediated by the exploration and the discovery of new spatio-temporal relations of the objects (Lockman 2000).…”

Structural analyses in the study of behavior: From rodents to non-human primates

“…Theoretically, this powerful structurefunction interface could be applied to other pairs of behavioral traits that are linked at the proximate levels (i.e., in their developmental trajectories and underlying sensori-motor and cognitive mechanisms) and at the ultimate levels (i.e., in their functional consequences and phylogenetic pathways; Tinbergen, 1963). Among other pairs of behavioral traits that were subjected to differential functional constraints and whose putative mechanistic and evolutionary connections are unravelled by in-depth structural analysis, let us mention object play and tool use (Cenni et al, 2020(Cenni et al, , 2022 as well as female-to-female mounting and female-to-male mounting (Gunst et al 2020(Gunst et al , 2022.…”

“…(Figure 6). Among other examples, lag sequential analyses have been employed to investigate the motivational underpinnings and functional components of specific behavioral elements expressed within sequences of actions performed during playful or sexual activities in free-ranging macaques (Cenni et al, 2022;Gunst et al, 2022aGunst et al, ,b, 2022.…”

Primate Sex and Its Role in Pleasure, Dominance and Communication

“…However, studies on male-male sexual behaviours in Japanese macaques [ 29 ] suggest that these can be considered homosexual behaviours rather than socio-sexual behaviours (i.e., sexual in terms of their superficial form, but enacted to facilitate adaptive social goals) [ 18 ]. Additionally, recent findings on female-male, female-female, and male-female mounting displayed as cultural sexual practices and supernormal courtship behavioural patterns in Japanese macaques [ 36 , 37 ], and on stone tool-assisted masturbation in long-tailed macaques [ 38 ]), emphasize the sexual nature of non-conceptive behaviours, rather than socio-sexual behaviours, in Old World primates.…”