ALTERATION OF TESTICULAR RESPONSE TO LONG PHOTOPERIOD BY TRANSIENT EXPOSURE TO SHORT PHOTOPERIOD IN COLLARED LEMMINGS (DICROSTONYX GROENLANDICUS)

The reproductive response of collared lemmings (Dicrostonyx groenlandi cus) to photoperiod is unique for rodents. Whereas most reproductively photoresponsive rodents show maximal gonadal growth when exposed to l ong photoperiod (long day), collared lemmings show delayed maturation when born and maintained under this condition. However, transfer of le mmings from short photoperiod (short day) to long day results in maxim al gonadal growth, indicating that the response to long day depends up on photoperiod history. We hypothesized that the slowing of maturation observed in animals born and maintained on long day reflects an inabi lity to respond fully to long day, resulting from the absence of previ ous exposure to short day. To determine whether young lemmings born in long day are capable of being stimulated by long day, we exposed them at weaning (19 days of age) to 1, 6 or 10 weeks of short day, and the n challenged them with a second exposure to long day. Relative to anim als transferred permanently to short day at weaning, lemmings exposed to 6 weeks of short day showed accelerated gonadal growth after both 5 and 10 weeks of subsequent exposure to long day, and those exposed to 10 weeks of short day had larger testes after 6 weeks of long day. Th us, during transient exposure to short day, the animals acquired sensi tivity to the stimulatory effects of long day. The responses of body m ass, bifid claw width and pelage colour to the photoperiod manipulatio ns did not parallel that of the gonads, indicating independent regulat ion of somatic and reproductive parameters. The unique way in which th e reproductive system of collared lemmings responds to photoperiod may reflect evolution in an environment where the production of offspring during periods of unchanging long day (for example, the Arctic summer ) is not selectively advantageous.