There are many adaptationist hypotheses and non-selectionist accounts for the evolution of play. Some scholars argued that, ‘primary-process play’ originally arose as a by-product of fortuitous conditions, adaptive or not (e.g. high metabolic energy, boredom). Once evolved, these basic playful behavioral elements, characterized by some quirkiness, arbitrariness, redundancy, flexibility, and latent potential, may serve as a built-in reservoir of creative and adaptive variability. As such, they could subsequently be co-opted for beneficial or fitness-enhancing behavioral effects (i.e. exaptations Type 2) during further evolutionary transformation into more elaborate secondary-process and tertiary-process play. We built a theoretical case for (object) play as a co-optable behavioral spandrel, with a potential for exaptations Type 2, and proposed a research design to empirical test whether stone play traditions can be exapted into stone tool use in non-human primates. Our approach is consistent with Gould’s pluralistic perspective on evolutionary theory.

We analysed heterosexual consortships in a free-ranging group of Japanese macaques in which adult females routinely perform female-to-male mounting (FMM). We tested whether FMM is more efficient (i.e., a ‘supernormal courtship’ behavioural pattern) than species-typical female-to-male sexual solicitations (FMSS) at prompting subsequent male-to-female mounts (MFM). In a context of high female-female competition for male mates, we found that (1) FMM functioned to focus the male consort partner’s attention as efficiently as FMSS and prevented him from moving away, and (2) FMM was more efficient than species-typical FMSS at expediting MFM (i.e., the most fitness-enhancing sexual behaviour of a mating sequence). We concluded that FMM could be considered a supernormal courtship behavioural pattern in adult female Japanese macaques. This population-specific sexual adaptation may result from a combination of favourable socio-demographic conditions. This study has implications for the evolutionary history of non-conceptive mounting patterns in Japanese macaques and non-conceptive sexuality in humans.