Effects of photoperiod on the 24-hour melatonin profiles of the Bennett's wallaby (Macropus rufogriseus rufogriseus).

The Bennett's wallaby is a seasonal breeder in which photoperiod is an important proximate factor involved in regulating the timing of the breeding season. The present study was undertaken in order to describe the melatonin profiles of female wallabies under natural photoperiod and to examine the effects of abrupt changes in day length on the melatonin profile. Under artificial and natural photoperiods, plasma melatonin concentrations were low or nondetectable during the day but increased after the onset of darkness to values of up to 50 pg/ml. Under natural photoperiod, melatonin profiles were obtained from animals on December 17, April 3, June 26, and October 7. The duration of the nocturnal melatonin rise was positively correlated with the duration of the night, and there was no significant effect of time of year on the amplitude of the nocturnal melatonin peak. In a further study, 12 animals were placed in 9L:15D and after 7 days, one-half were transferred to 18.5L:5.5D for 6 weeks. Before and after each change of photoperiod, melatonin profiles were obtained from both groups. As demonstrated for animals on natural photoperiod, duration but not amplitude of the nocturnal melatonin rise was affected by day length. It is noteworthy that two of the 17 animals used in this study did not show evidence of a nocturnal rise in melatonin levels.