The seasonal reproductive cycle of a marsupial, Antechinus stuartii: Effects of oral administration of melatonin

Antechinus stuartii is a small marsupial with a brief, highly synchronised mating period believed to be controlled by the rate of change of photoperiod. Two experiments were performed to explore aspects of photoperiodic control of the seasonal cycle. In the first experiment the pineal hormone, melatonin, administered in the drinking water from the winter solstice, changed the normal response of A. stuartii to increasing rate of change of photoperiod. Melatonin administration shifted the induction of estrus in the females from the first week of August (controls) to an earlier time of mid-July and the consequent pouch changes associated with pregnancy and pseudo-pregnancy were also shifted by the same length of time. Post-mating decline and consequent death of males were also accelerated. In the second experiment melatonin was administered from the autumnal equinox, and this experimental protocol resulted in a desynchronisation of reproductive events. Melatonin administration desynchronised the female reproductive cycle, such that the mating period was extended to eight weeks, instead of the two weeks displayed by control females. Pouch changes and birth of young reflected this desynchronisation. Melatonin administration in males resulted in desynchronisation of reproductive parameters. While the normal yearly reproductive cycle was approximated in these males, the high syncronisation of reproductive maturation and male mortality events observed in control males, was not evident in melatonin-treated males. These results indicate that the pineal gland by way of the hormone melatonin is important in the synchronisation of the unusual life history of this marsupial mammal.