Oestrus in the lactating red kangaroo.

Abstract

The reproductive condition of 18 female red kangaroos, Megaleia rufa (Desmarest), collected in central Australia and found to be suckling sizeable pouch young (average age about 54 days), is described. They were either approaching, undergoing, or had just undergone oestrus. None had a mature corpus luteum of lactation in either ovary. No similar case was found in many hundreds of females suckling sizeable pouch young which had a mature corpus luteum of lactation in one of their ovaries. It is concluded that the presence of a functional corpus luteum of lactation or of pregnancy in the ovary may be the only inhibitor of ovulation in the mature Megaleia, provided that environmental conditions are suitable. Some kangaroos during severe drought miss the customary oestrus following birth of a young because the pregnant females do not undergo the reproductive changes necessary for it. Despite this disruption of the reproductive cycle the young are born and suckled normally, but the females cannot, therefore, have a corpus luteum of lactation. The 18 females described in this paper must have belonged to this class of animals during the drought, their condition having been brought on presumably by poor nutrition. The unusual features of reproduction in Megaleia are hard to reconcile with the assumptions that pregnancy in marsupials depends on the hormones of the normal oestrous cycle, and that reproduction in marsupials is controlled by the same kinds of hormones found in eutherian mammals; doubt is, therefore, thrown on the validity of these assumptions. Megaleia has a remarkable ability to care for its young during severe drought, and a method of overcoming the effects of the drought on its reproductive cycle when the drought breaks, with the result that the production of young is not impaired. Both these features must aid the chance of the species to survive in the arid parts of Australia.