Photoperiod History Differentially Impacts Reproduction and Immune Function in Adult Siberian Hamsters

Abstract Seasonal changes in numerous aspects of mammalian immune func-tion arise as a result of the annual variation in environmental day length (pho-toperiod), but it is not known if absolute photoperiod or relative change in photoperiod drives these changes. This experiment tested the hypothesis that an individual’s history of exposure to day length determines immune responses to ambiguous, intermediate-duration day lengths. Immunological (blood leu-kocytes, delayed-type hypersensitivity reactions [DTH]), reproductive, and adrenocortical responses were assessed in adult Siberian hamsters (Phodopus sungorus) that had been raised initially in categorically long (15-h light/day; 15L) or short (9L) photoperiods and were subsequently transferred to 1 of 7 cardinal experimental photoperiods between 9L and 15L, inclusive. Initial pho-toperiod history interacted with contemporary experimental photoperiods to determine reproductive responses: 11L, 12L, and 13L caused gonadal regres-sion in hamsters previously exposed to 15L, but elicited growth in hamsters previously in 9L. In hamsters with a 15L photoperiod history, photoperiods