Gould, S. J. & Vrba, E. S. Exaptation—a missing term in the science of form. Paleobiology 8, 4–15 (1982).
Google Scholar
Eastwick, P. W. Beyond the Pleistocene: using phylogeny and constraint to inform the evolutionary psychology of human mating. Psychol. Bull. 135, 794–821 (2009).
Google Scholar
Reeve, H. K. & Sherman, P. W. Adaptation and the goals of evolutionary research. Q. Rev. Biol. 68, 1–32 (1993).
Google Scholar
Ventura, R., Majolo, B., Schino, G. & Hardie, S. Differential effects of ambient temperature and humidity on allogrooming, self-grooming, and scratching in wild Japanese macaques. Am. J. Phys. Anthropol. 126, 453–457 (2005).
Google Scholar
Duboscq, J., Romano, V., Sueur, C. & MacIntosh, A. J. J. Scratch that itch: revisiting links between self-directed behaviour and parasitological, social and environmental factors in a free-ranging primate. R. Soc. Open Sci. 3, 160571 (2016).
Google Scholar
Golpanian, R. S., Kim, H. S. & Yosipovitch, G. Effects of stress on itch. Clin. Ther. 42, 745–756 (2020).
Google Scholar
Sanders, K. M. & Akiyama, T. The vicious cycle of itch and anxiety. Neurosci. Biobehav. Rev. 87, 17–26 (2018).
Google Scholar
Lu, J. S. et al. Contagious itch can be induced in humans but not in rodents. Mol. Brain 12, 38 (2019).
Google Scholar
Holle, H., Warne, K., Seth, A. K., Critchley, H. D. & Ward, J. Neural basis of contagious itch and why some people are more prone to it. Proc. Natl. Acad. Sci. U. S. A. 109, 19816–19821 (2012).
Helt, M. S. et al. Contagious itching is heightened in children with autism spectrum disorders. Dev. Sci. 24, e13024 (2021).
Google Scholar
Castles, D. L. & Whiten, A. Post-conflict behaviour of wild Olive baboons. II. Stress and self-directed behaviour. Ethology 104, 148–160 (1998).
Google Scholar
di Polizzi, E., Schino, G., Tiddi, B. & Aureli, F. Scratching as a window into the emotional responses of wild tufted capuchin monkeys. Ethology 118, 1072–1084 (2012).
Google Scholar
Kutsukake, N. & Castles, D. L. Reconciliation and variation in post-conflict stress in Japanese macaques (Macaca fuscata fuscata): testing the integrated hypothesis. Anim. Cogn. 4, 259–268 (2001).
Google Scholar
Castles, D. L., Whiten, A. & Aureli, F. Social anxiety, relationships and self-directed behaviour among wild female Olive baboons. Anim. Behav. 58, 1207–1215 (1999).
Google Scholar
Maestripieri, D. Maternal anxiety in rhesus macaques (Macaca mulatta). Ethology 95, 19–31 (1993).
Google Scholar
Itakura, S. Emotional behavior during the learning of a contingency task in a chimpanzee. Percept. Mot. Skills 76, 563–566 (1993).
Google Scholar
Leavens, D. A., Aureli, F., Hopkins, W. D. & Hyatt, C. W. Effects of cognitive challenge on self-directed behaviors by chimpanzees (Pan troglodytes). Am. J. Primatol. 55, 1–14 (2001).
Google Scholar
Leeds, A. & Lukas, K. E. Experimentally evaluating the function of self-directed behaviour in two adult mandrills (Mandrillus sphinx). Anim. Welf. 27, 81–86 (2018).
Google Scholar
Aureli, F., Van Schaik, C. P. & Van Hooff, J. A. R. A. M. Functional aspects of reconciliation among captive long-tailed macaques (Macaca fascicularis). Am. J. Primatol. 19, 39–51 (1989).
Google Scholar
Palagi, E. & Norscia, I. Scratching around stress: hierarchy and reconciliation make the difference in wild brown lemurs (Eulemur fulvus). Stress 14, 93–97 (2011).
Google Scholar
Barros, M., Boere, V., Huston, J. P. & Tomaz, C. Measuring fear and anxiety in the marmoset (Callithrix penicillata) with a novel predator confrontation model: effects of diazepam. Behav. Brain Res. 108, 205–211 (2000).
Google Scholar
Kato, Y. et al. Vocalizations associated with anxiety and fear in the common marmoset (Callithrix jacchus). Behav. Brain Res. 275, 43–52 (2014).
Google Scholar
Judge, P. G., Griffaton, N. S. & Fincke, A. M. Conflict management by hamadryas baboons (Papio hamadryas hamadryas) during crowding: a tension-reduction strategy. Am. J. Primatol. 68, 993–1006 (2006).
Google Scholar
Neal, S. J. & Caine, N. G. Scratching under positive and negative arousal in common marmosets (Callithrix jacchus). Am. J. Primatol. 78, 216–226 (2016).
Google Scholar
Burghardt, G. M. The Genesis of Animal Play: Testing the Limits. (MIT Press, 2005).
Held, S. D. E. & Špinka, M. Animal play and animal welfare. Anim. Behav. 81, 891–899 (2011).
Google Scholar
Iki, S. & Kutsukake, N. Play face in Japanese macaques reflects the sender’s play motivation. Anim. Cogn. 26, 849–859 (2023).
Google Scholar
Nelson, X. J. et al. Joyful by nature: approaches to investigate the evolution and function of joy in non-human animals. Biol. Rev. 98, 1548–1563 (2023).
Google Scholar
Fagen, R. Animal Play Behavior. (Oxford University Press, 1981).
Iki, S. & Kutsukake, N. Victims of play escalation rank below aggressors in Japanese macaques (Macaca fuscata). Int. J. Primatol. 43, 365–383 (2022).
Google Scholar
Iki, S. & Adachi, I. Fearful snake pictures make monkeys Pessimistic. iScience 26, 107622 (2023).
Google Scholar
Iki, S. & Adachi, I. Affective bodily responses in monkeys predict subsequent pessimism, but not vice versa. Proc. R. Soc. B 292, 20242549 (2025).
Google Scholar
McNaughton, N. & Gray, J. A. Anxiolytic action on the behavioural Inhibition system implies multiple types of arousal contribute to anxiety. J. Affect. Disord. 61, 161–176 (2000).
Google Scholar
Kikuchi, Y. & Noriuchi, M. Power of self-touch: its neural mechanism as a coping strategy in Emotional Engineering, Vol. 7: the Age of Communication. 33–47 (eds Fukuda, S.) (Springer, 2019).
Boehme, R. & Olausson, H. Differentiating self-touch from social touch. Curr. Opin. Behav. Sci. 43, 27–33 (2022).
Google Scholar
Friston, K. J., Daunizeau, J. & Kiebel, S. J. Reinforcement learning or active inference? PLoS One 4, e6421 (2009).
Google Scholar
Clark, A. Whatever next? Predictive brains, situated agents, and the future of cognitive science. Behav. Brain Sci. 36, 181–204 (2013).
Google Scholar
Blakemore, S. J., Wolpert, D. & Frith, C. Why can’t you tickle yourself? Neuroreport 11, R11–R16 (2000).
Kilteni, K., Engeler, P. & Ehrsson, H. H. Efference copy is necessary for the attenuation of self-generated touch. iScience 23, 100843 (2020).
Google Scholar
Gould, S. J. Exaptation: a crucial tool for an evolutionary psychology. J. Soc. Issues 47, 43–65 (1991).
Google Scholar
Nakayama, K. Observing conspecifics scratching induces a contagion of scratching in Japanese monkeys (Macaca fuscata). J. Comp. Psychol. 118, 20–24 (2004).
Google Scholar
Yu, Y. Q., Barry, D. M., Hao, Y., Liu, X. T. & Chen, Z. F. Molecular and neural basis of contagious itch behavior in mice. Science 355, 1072–1076 (2017).
Google Scholar
Laméris, D. W., Berlo, E., Sterck, E. H. M., Bionda, T. & Kret, M. E. Low relationship quality predicts scratch contagion during tense situations in orangutans (Pongo pygmaeus). Am. J. Primatol. 82, e23138 (2020).
Google Scholar
Valdivieso-Cortadella, S., Bernardi-Gómez, C., Aureli, F., Llorente, M. & Amici, F. Yawning and scratching contagion in wild spider monkeys (Ateles geoffroyi). Sci. Rep. 13, 8367 (2023).
Google Scholar
Lemes, W. P. & Amici, F. Contagious yawning and scratching in captive lemurs. Sci. Rep. 14, 26672 (2024).
Google Scholar
Whitehouse, J., Micheletta, J., Kaminski, J. & Waller, B. M. Macaques attend to scratching in others. Anim. Behav. 122, 169–175 (2016).
Google Scholar
Wang, M. Z. & Hayden, B. Y. Monkeys are curious about counterfactual outcomes. Cognition 189, 1–10 (2019).
Google Scholar
Kidd, C., Piantadosi, S. T. & Aslin, R. N. The Goldilocks effect: human infants allocate attention to visual sequences that are neither too simple nor too complex. PLoS One 7, e36399 (2012).
Google Scholar
