Adaptive divergence in reproductive seasonality and underlying physiological features fit <i>Rattus norvegicus</i> to live as opportunistic breeders

How organisms respond to complex environments is one of the unsolved problems in ecology. Life history patterns of a species provide essential information on how different populations may respond and adapt to environmental changes. Compared to typical seasonal breeders, which have limited distributions, the worldwide distribution of brown rats (Rattus norvegicus) across highly complex and divergent habitats suggests they exhibit exceptional adaptiveness. However, the difference in physiological mechanisms by which brown rats respond and adapt to markedly different environments is seldom investigated. Here, we reveal a significant divergence in reproductive seasonality and environmental responses between two brown rat subspecies: one subspecies, R. n. caraco, lives in the temperate zone, and another subspecies, R. n. norvegicus, lives in the subtropical region. Although R. n. caraco displayed a significantly higher reproductive seasonality than R. n. norvegicus, both subspecies adapted to sub-optimal breeding conditions mainly by regulating the seminal vesicle rather than testis development. Especially in responding to severe winter conditions in high-latitude regions, bodyweight-dependent recovery of testicular development in adults enables R. n. caraco to initiate reproduction more rapidly when conditions are suited. These findings elucidate a regulatory process of how brown rats live as opportunistic breeders by benefiting from enhanced semen production.