EVIDENCE FOR A CIRCANNUAL RHYTHM OF REPRODUCTION AND PROLACTIN SECRETION IN A SEASONALLY BREEDING MACROPODID MARSUPIAL, THE BENNETT WALLABY(MACROPUS-RUFOGRISEUS-RUFOGRISEUS)

Two groups of adult female wallabies were maintained in photoperiod-co ntrolled rooms from June 1987 until August 1988. Group SSH was held on summer solstice photoperiods throughout the experiment; group SN was subjected to weekly stepwise simulated natural changes in photoperiod. Plasma melatonin concentrations reflected photoperiod with high conce ntrations during the dark phase in both groups. Group SN wallabies com menced oestrous cyclicity on 21 July (+/- 19 days, n = 6) entered repr oductive quiescence on 14 February (+/- 10 days, n = 5) and recommence d cycling on 8 June (+/- 3 days, n = 4). Group SSH wallabies began cyc ling on 27 July (+/- 9 days, n = 7) at a time that was not significant ly different from that of group SN. Three out of five of group SSH exh ibited a spontaneous period of reproductive quiescence of between 59 a nd 70 days commencing between 3 December and 25 February. There was a highly significant difference between the transient plasma prolactin r esponse to a dopamine antagonist during cycling and quiescent periods in both groups (P < 0.001) such that the response was increased during periods of quiescence. Our data support the hypothesis that prolactin is involved in the control of seasonal quiescence in the female Benne tt's wallaby and demonstrate that spontaneous changes in reproductive state and prolactin can occur when animals are maintained on unchangin g long photoperiods.