Anoestrus in the red kangaroo Megaleia rufa (Desmarest).

Abstract

The red kangaroo, M. rufa, normally has a post-partum oestrus and a quiescent period of lactation as do some other macropod marsupials. The term "quiescent period of lactation" has been defined in this paper as the period of reproductive stasis between post-partum oestrus and the resumption of ovarian activity which follows when the female ceases, or almost ceases to suckle the young. This term is introduced to replace the term "lactation anoestrus". It has been shown that some females failed to undergo post-partum oestrus during prolonged drought, each female having entered anoestrus during the previous pregnancy; such pregnancies were not interrupted, but carried to term, and the young sometimes suckled for 3-4 months. In some other females, anoestrus set in during an unfertilized oestrous cycle. It is thought that anoestrus may result from lack of food because anoestrous females are found only during prolonged drought. The histology of the uteri of lactating anoestrous females has been described, and shown to be indistinguishable from that of the uteri of non-lactating anoestrous females, and very different from that in animals in the quiescent period of lactation. The anoestrous condition appeared in the uterus after the cessation of the luteal phase. Pregnancy is therefore independent of the oestrous cycle in Megaleia when the luteal phase has passed. The onset of anoestrus in Megaleia is compared with that in Elephantulus rupestris jamesoni Chubb, Sorex araneus (L.), Oryctolagus cuniculus (L.), Setonix brachyurus (Quoy & Gaimard), and Potorous tridactjhs (Kerr), all of which usually undergo post-partum oestrus. In Oryctolagus and Elephantulus, anoestrus may lead to the death respectively of the young and foetus. It is suggested that the milder consequences of anoestrus in Megaleia may be of adaptive significance to the species in a semi-arid environment, since the embryos and pouch-young of anoestrous females have a chance to survive drought.