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The reproductive response of xeric and mesic populations of the genus Acomys to photoperiod acclimation was investigated. Populations of Acomys russatus and Acomys cahirinus were used as models for xeric and mesic populations, respectively. Mice (n = 10 of each species) were acclimated to short (10L:14D) and long (14L:10D) photoperiods at the following ambient temperatures: 31 +/- 1 degrees C (A. russatus) and 28 + 1 degrees C ( A. cahirinus) for 52 days. Estrus cycles were monitored by microscopic observation of vaginal epithelial cells three times each week. Three animals from each sex and photoperiod group were sacrificed randomly at the end of acclimation. Wet mass of left testis, uterus and testicular dimensions (for calculation of testis volume) were recorded. The stage of spermatogenesis was determined from histology of seminiferous tubules. Based on the eco-climatic conditions of their natural habitats, we expected A. russatus to be non-responsive to photoperiod and A. cahirinus to be responsive only to long photoperiod. Interestingly, our results showed that only females of A. russatus were unaffected by photoperiod while their male counterparts were unable to maintain testicular activity under a short photoperiod. However, both sexes of A. cahirinus maintained gonadal activity under short and long photoperiod. Our results were compared with those of previous studies and possible ecological explanations were discussed. (c) 2007 Elsevier Ltd. All rights reserved.