Embryonic diapause in marsupials.

Amongst the marsupials embryonic diapause has been regarded as a characteristic of the Family Macropodidae, since it has been described in all but one of the 20 or so kangaroos and wallabies examined. Diapause has not been demonstrated unequivocally in other marsupials, although the non-macropodids Cercartetus and Acrobates have uncertain status in this regard. Recently, however, diapause has been described in the non-macropodid Tarsipes. Diapause in macropodid marsupials may be obligate or facultative. The predominant pattern is one related to the suckling stimulus. Most species show a post-partum oestrus, although in the swamp wallaby (Wallabia bicolor), at least, fertilization of the ovum which is to become the delayed embryo precedes the birth of the previous conceptus. In a few species, the suckling stimulus may postpone the initiation of the pro-oestrous phase, with diapause occurring after fertilization during an oestrous cycle in the late stages of pouch suckling. In the majority of species, however, pregnancy does not prevent ovulation, but the corpus luteum resulting from pre- or post-partum ovulation is held quiescent by the suckling stimulus. In the tammar wallaby, Macropus eugenii, the control of diapause is well understood, but there is considerable variation in the nature and occurrence of diapause even amongst closely related species. The almost universal distribution of diapause among macropodids and the recent descriptions of the probably occurrence of diapause in other marsupial families suggest that the phenomenon may be widespread in marsupials. On the basis of recent work, three groups of marsupial reproductive patterns are suggested: monotocous or polytocous polyoestrous marsupials with no diapause; monotocous, polyoestrous marsupials with diapause, and polytocous, polyoestrous marsupials with diapause. Groups 2 and 3 appear to have evolved independently from Group 1.