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Biology of Reproduction, Vol 13, 329-339, Copyright © 1975 by Society for the Study of Reproduction
1 Department of Zoology, University of Texas, Austin, Texas 78712 The mechanism of discrimination of daylength and the effect of light on the reproductive
system were investigated in male golden hamsters (Mesocricetus auratus). The photoperiodic
response of hamsters is not determined by the absolute amount of light or of darkness (on each
day) or the ratio of light:dark. Rather, a circadian rhythm of photosensitivity determines whether
light will stimulate the hypothalamo-hypophysio-testicular axis. Hamsters were exposed to cycles
of 6 h of light combined with varying durations of darkness to generate period lengths of 24, 36,
48 and 60 h. Six h of light presented daily (LD 6:18) or every 2 days (LD 6:42) were not capable
of maintaining the testes and seminal vesicles in a mature state. On the other hand, 6 h of light
presented every 1 Essentially the same pattern of response was observed when hamsters with regressed testes and
seminal vesicles were exposed to the same four light cycles. Considerable increase in organ weights
(recrudescence) occurred in animals exposed to LD 6:30 and LD 6:54 while the organ weights of
those exposed to LD 6:18 and LD 6:42 increased much less (LD 6:18) or not at all (LD 6:42). The photoperiodic response of the reproductive system during two widely separated phases of
the annual testicular cycle in the hamster depends primarily on the position of light relative to the
circadian system.
days (LD 6:30) or every 2 days (LD 6:54) were interpreted as "long days"
by hamsters, and testes, and seminal vesicles were maintained at relatively high levels for up to 89
days.
Accepted on June 16, 1975
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