Splitting of wheelrunning rhythms by castrated or steroid treated male and female hamsters.

Male and female hamsters, with or without gonadal hormones, were housed in constant light (LL) while wheel running rhythms were recorded. Estradiol benzoate (EB) in Silastic capsules reduced rhythm desynchronies, such as splitting, in ovariectomized animals compared to blank implanted controls. In males, there were no significant effects of testosterone or EB in Silastic implants, castration or sham operation on incidence of rhythm desynchronies. Males generated split rhythms which differed from females in clarity and the angle at which the limbs of the splitting rhythms diverged. Other differences were (a) greater activity onset variability for castrated females with relatively little onset variability for other groups and (b) more running time by EB treated males than any other group. Splitting for all animals occurred with an average latency in LL of 55 +/- 3 days; the period stabilized in 12 +/- 1 days and was 0.2 hr shorter in length. The two limbs of the split rhythm were a mean 181 +/- 5 degrees apart. Induction of splitting by LL is critically discussed with special reference to the two oscillator model of hamster activity and existing evidence for more than two oscillations in wheelrunning activity.