Reproductive Involution in Male Hamsters Exposed to Naturally Increasing Daylengths after the Winter Solstice

Exposure of adult male hamsters to short (<12.5 hr light/day) but increasing daylengths beginning on December 28 was followed by involution of the testes and accessory sex organs. These changes were accompanied by a drop in pituitary LH and prolactin and by a depression in circulating levels of these hormones. The reproductive organs and hormone levels remained depressed until about May 3 at which time there was evidence of restoration of the weights of the sex organs and a reestablishment of normal hormone levels. By June 1 all reproductive parameters that were measured had returned to preexperiment levels. It is apparent from these results that absolute daylength rather than increasing photoperiod is more important in maintaining the functional capacity of the neuroendocrine-reproductive axis of male Syrian hamsters.