Inferring functional patterns of tool use behavior from the temporal structure of object play sequences in a non-human primate species.

Inferring functional components of behavioral sequences is a crucial but challenging task. A systematic comparison of their temporal structure is a good starting point, based on the postulate that more functional traits are less structurally variable. We studied stone handling behavior (SH) in Balinese long-tailed macaques, a versatile form of stone-directed play. We tested the hypothesis that stones are used by male monkeys to stimulate their genitals in a sexual context (i.e., "sex toy" hypothesis). Specifically, two SH actions (i.e., "tap-on-groin" (TOG) and "rub-on-groin" (ROG), respectively the repetitive tapping and rubbing of a stone onto the genital area) gained functional properties as self-directed tool-assisted masturbation. Owing to the structural organization of playful activities, we predicted that SH sequences without TOG/ROG would exhibit higher levels of variability, repeatability and exaggeration than SH sequences with TOG/ROG. We also predicted that TOG/ROG would occur more often and last longer in SH sequences in which penile erection - a sexually-motivated physiological response in primates - was observed than in SH sequences in which penile erection was not observed. To identify and compare recurring series of SH patterns otherwise undetectable by using conventional quantitative approaches across SH sequences containing TOG/ROG or not, we used a temporal analysis known as "T-pattern detection and analysis" (TPA). Our predictions about variability, exaggeration and temporal association between TOG/ROG in males and penile erection were supported. As expected, SH sequences without TOG/ROG were, on average, more repeatable than SH sequences with TOG/ROG, but the difference was not statistically significant. Overall, the "sex toy" hypothesis was partly supported, and our results suggested that TOG and ROG are two forms of tool-assisted genital stimulation, possibly derived from the playful handling of stones. These findings are consistent with the view that tool use may evolve in stages from initially non-functional object manipulation, such as object play.