Endocrinology of Pregnancy, Parturition and Lactation in Marsupials

Abstract

The earliest mammals were all small, nocturnal insectivorous creatures, and it appears that the basic mode of mammalian reproduction remained unchanged from the time of the origin of mammals in the late Triassic (about 200 million years ago) until the dispersal of the Gondwanan continents in the middle Cretaceous (95-110 million years ago). This meant that there was no land faunal communication between Africa and the Antarctic-Australia-India complex except via South America after the earliest Cretaceous (Lillegraven et al., 1979). The major adaptive radiations of the advanced therian mammals occurred during the middle Cretaceous, so that by the late Cretaceous (75-80 million years ago) the Eutheria and Metatheria were already distinct and largely geographically isolated (Archer, 1984; Tyndale-Biscoe and Renfree, 1987). Whilst marsupials are true mammals in every sense, and in endocrine terms have the same range of hormones, these hormones are sometimes used in different ways (Amoroso et al., 1980) - for example, prolactin inhibits the marsupial corpus luteum, whereas it is luteotrophic in some eutherian mammals. This communality of hormones but diversity of actions between the mammalian groups clearly demonstrates the validity of Medawar’s (1953) aphorism that it is not hormones that have evolved, but rather the uses to which they are put.