THE HYPOTHALAMIC CIRCUIT MECHANISMS OF SEASONAL HAIR COLOR SWITCHING IN SIBERIAN HAMSTERS (<EM>P. SUNGORUS</EM>)

The ability to adapt to environmental changes is essential for animals inhabiting high-latitude regions. Seasonal pelage color change is a critical adaptive response that facilitates camouflage and enhances survival. To elicit appropriate seasonal phenotypes, a precise mechanism that tracks day length and interprets fluctuating environmental signals is required. Under varying light conditions, the rodent hypothalamus undergoes significant plastic changes at both cellular and network levels. While the endogenous seasonal clock has been studied across mammals, the neural mechanisms that encode light-period information to entrain or drive these circannual physiological changes remain incompletely understood.
In this study, we examine the summer-dark to winter-white hair color transition of Siberian hamsters (Phodopus sungorus) to investigate the integration of day length information and its modulation on seasonal phenotypes. By combining seasonal light paradigms with endocrine profiling and locomotor activity tracking, we demonstrate that day length information is encoded in a circadian activity-dependent manner. Furthermore, using immunohistochemistry and targeted pharmacological ablation, we identify a specific hypothalamic subpopulation of neurons that can tune seasonal dark-to-white hair color transition during a critical seasonal window. These findings provide insight into how hypothalamic neuronal circuitry transforms long-timescale environmental cues into robust physiological adaptations crucial for survival.