The higher proportion of polygynous ant species in northern areas indicates that cold climates influence queen number per colony. It is unclear, however, what ecological and physiological factors facilitate the dominance of polygynous species in cold climates. This is the case in two common arboreal ants in Japan—Camponotus yamaokai and C. nawai—which are quite similar in morphology, but different in social structure and geographical distribution. Polygynous C. yamaokai inhabits colder areas, whereas monogynyous C. nawai inhabits warmer climates. We compared queen survival in both ants at low temperature to evaluate whether interspecific difference in cold tolerance can explain the geographical distribution. We examined the influence of cohabitation with other individuals, as well as individual cold tolerance. Experimental groups with different caste compositions were prepared and maintained under conditions simulating in the laboratory climates of the northern limit of C. nawai. Wintering experiments revealed that C. yamaokai queens survived longer than C. nawai queens under solitary conditions, although half of the queens died in less than a month, even in C. yamaokai. Queens hibernating with workers survived longer than solitary queens, but queen number did not affect queen survival. Cohabitation with workers allowed 80% of C. yamaokai queens to survive more than two months. Under field conditions, monogynous C. nawai foundresses overwinter without workers, whereas new queens of polygynous C. yamaokai always overwinter with many workers. Thus, the geographical distribution of these ants appears to depend on the overwintering behavior of new queens.
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