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This is the first quantitative study of heterospecific
sexual behavior between a non-human primate and a non-primate
species. We observed multiple occurrences of free-ranging
adolescent female Japanese macaques (Macaca fuscata)
performing mounts and sexual solicitations toward sika deer
(Cervus nippon) at Minoo, central Japan. Our comparative
description of monkey-deer versus monkey-monkey interactions
supported the ‘‘heterospecific sexual behavior’’ hypothesis: the
mounts and demonstrative solicitations performed by adolescent
female Japanese macaques toward sika deer were sexual
in nature. In line with our previous research on the development of
homospecific sexual behavior in immature female Japanese
macaques, this study will allow us to test other hypotheses in
the future, such as the‘‘practice for homospecific sex,’’the‘‘safe
sex,’’the‘‘homospecific sex deprivation,’’the‘‘developmental by-
product,’’and the‘‘cultural heterospecific sex’’hypotheses.
Furtherresearchwill benecessary toascertain whetherthis group-
specific sexual behavior was a short-lived fad or an incipient
cultural phenomenon and may also contribute to better under-
standing the proximate and ultimate causes of reproductive
interference.