Seasonal and circadian patterns of circulating prolactin during lactation and seasonal quiescence in the tammar, Macropus eugenii.

Prolactin was measured in peripheral plasma of female tammars once a week for 14 months and every 2 months it was measured in a subgroup of 7 tammars at hourly intervals for 28 h to determine short-term changes in concentration. During the course of the study 6 females gave birth in January/February and reared their young to weaning in October-November, while another 4 gave birth 1.5-3.0 months later and weaned their young in November-December. Concentrations of prolactin were less than 40 ng/ml in a non-lactating animal and for the first 140 days of lactation in the others. Between 140 and 180 days of lactation the concentrations of prolactin fluctuated between 10 and greater than 100 ng/ml and thereafter remained high until the young vacated the pouch for the last time at about 250 days. After pouch exit the concentrations declined to about 40 ng/ml even though lactation continued for a further 21-35 days. Although the short-term patterns of prolactin concentrations showed no consistent relationship between light-dark phases there was an increase in the magnitude and duration of peaks of plasma prolactin which correlated with the stage of lactation. Removal of pouch young aged between 203 and 242 days for 24 h resulted in a rapid decrease in plasma prolactin concentration which was reversed when the young was returned to the pouch, indicating that the high levels of prolactin in the second half of lactation may be maintained by the sucking stimulus of the pouch young. This period of lactation, therefore, may be equated with lactation in eutherians, but there appears to be no eutherian equivalent to early lactation in the tammar.