The interaction of photoperiod and testosterone on the development of copulatory behavior in castrated male hamsters

Abstract

Castrated adult male golden hamsters were maintained on either a stimulatory (LD 14:10) or a nonstimulatory (LD 6:18) light cycle for 10 weeks, and then were implanted subcutaneously with empty or testosterone-filled Silastic capsules of various lengths. Animals were tested for copulatory behavior prior to capsule implantation and 10, 20 and 40 days after implantation. Androgen-treated LD 14:10 hamsters showed a higher incidence of ejaculation on the final trial than did similarly treated LD 6:18 animals. When serum androgen levels were maintained at physiological levels (about 3 ng/ml, from a 20 mm capsule), significantly greater numbers of LD 14:10 hamsters intromitted and ejaculated compared to LD 6:18 animals. Examination of the development of copulation over trials revealed an interaction of photoperiod and androgen: LD 14:10 animals showed significant improvement over trials if stimulated with 8,20 or 100 mm long testosterone capsules, while LD 6:18 animals showed increased copulation over trials only if they were implanted with 100 mm capsules. These results indicate that exposure to short days for 10 weeks renders copulatory behavior of the castrate male hamster less responsive to the stimulatory effects of testosterone. This alteration in sensitivity to androgen may be one way in which the photoperiod acts to decrease copulation in seasonally breeding animals.