In most mammalian species, there is a general principle known as the ‘one-half rule’, in which litter size is half the number of mammaries. However, arboreal squirrels deviate from this rule and small litter size (mainly two to three), whereas ground squirrels adhere to the ‘one-half rule’ in their litter size patterns. In this study, we showed that spring litter sizes of Japanese flying squirrels were typical of arboreal squirrels, while summer litter sizes were comparable to those of ground squirrels, according to the conventional rule. Thus, Japanese flying squirrels have characteristics of both arboreal and ground squirrel species in terms of litter size. This finding provides novel insights that challenge the classical notion that arboreal squirrels have small litter sizes independent of the number of mammaries.