Shortened non-breeding period and extended reproductive activity in male wild mice during a context of increased temperature

Abstract This study investigated seasonal changes in the reproductive cycle of wild large Japanese field mice ( Apodemus speciosus ) in Niigata, Japan, within a context of increased temperature. From April 2024 to February 2025, monthly sampling showed that spermatogenesis was only briefly suppressed in November and December, with recovery observed by January. Histological analysis of testicular tissue revealed that the non-breeding period was shortened compared to previously reported patterns (Akiyama et al. 2015. Effect of seasonal changes on testicular morphology and the expression of circadian clock genes in Japanese large field mice ( Apodemus speciosus ). J. Biol. Regul. Homeost. Agents 29: 589–600), and reproductive activity was extended into months that had previously been considered non-breeding. These observations were made during 2024, which recorded the highest average annual temperature in Japan’s history, suggesting that reproductive timing in A. speciosus may be influenced by recent warming.