Seasonal variation in litter size of Siberian flying squirrels (Pteromys volans)

Litter sizes of rodents may vary seasonally depending on resource availability. The Trivers-Willard hypothesis (TWH) argues that if maternal condition varies between seasons, offspring sex ratio may vary as well. To test these hypotheses, we compared litter size and offspring sex ratio of Siberian flying squirrels (Pteromys volans) between spring and summer in Furano, Hokkaido, Japan. We captured female P. volans and their pups using nest boxes within the University of Tokyo Hokkaido Forest. The proportion of reproductive females was similar in both spring and summer. We observed that mean litter size in summer was significantly larger than in spring, potentially due to increased resource availability for females during summer. However, the overall sex ratio of pups did not differ between seasons, suggesting that P. volans does not conform to the predictions of the TWH. These findings provide valuable insight into the reproductive characteristics of P. volans and will inform future research into seasonal variability in their litter sizes and sex ratios. Litter size and offspring sex ratio of Pteromys volans were compared between 2 reproductive peaks (spring and summer) in Hokkaido, Japan. The mean litter size in summer was significantly larger than in spring. The overall sex ratio of pups did not differ between seasons, not as per predictions of the Trivers-Willard hypothesis.