The temperate arboreal ant Camponotus yamaokai with intranidal mating exhibits extended mating seasons in spring and autumn

Most ant species engage in nuptial flight, a mating behavior that involves flight dispersal. In temperate regions in particular, nuptial flights are highly synchronized, typically occurring within a period of less than one month each year. Some ant species engage in intranidal mating, in which males and alate gynes mate within the nest. For these species, the mating season remains unknown in many cases due to the difficulty of directly observing their mating behavior. In the Japan-endemic, arboreal-nesting ant Camponotus yamaokai, intranidal mating is estimated to occur around May. However, because this estimate is based on limited indirect evidence, it remains unclear whether it accurately reflects the actual mating season. Thus, in this study, we clarified the true mating season of C. yamaokai by dissecting field-collected gynes and conducting rearing experiments. Dissections revealed gynes shortly after mating not only in May, the previously estimated mating period, but also in January, March, April, June, September, and December. In addition, rearing experiments showed that some individuals emerging in August and September mated without overwintering. Therefore, our results suggest that mating occurs asynchronously in C. yamaokai, leading to extended mating seasons in spring and autumn. In ants, intranidal mating is characterized by lower predation risk, greater ease of encountering mates, a reduced likelihood of reproductive interference with closely related species, and a diminished need for inbreeding avoidance compared with species that engage in nuptial flight. These characteristics suggest that selective pressures for mating synchrony are relaxed, leading to the evolution of reproductive asynchrony.