Metatherian Reproduction: Transitional or Transcending?

Abstract

The major difference between metatherian and eutherian reproductive patterns is the relative apportionment of maternal resources to gestation versus lactation, not the total temporal and energetic investment in each reproductive attempt. Gestation accounts for an average of 12% of the time from conception to weaning in marsupials, whereas in placental mammals it accounts for 56% of this period of development. Parental investment at birth is less in marsupials (litter weight averages 0.09% of female body mass) than in eutherians (average 15.5%). However, total maternal effort is comparable in the two groups, since the litter weight at weaning is similar for metatherians and eutherians of equivalent adult weight. Metabolic rate does not seem to constrain reproductive adaptations. Resorption of postimplantation embryos in placental mammals is not closely linked to environmental conditions. Embryonic resorption, when it does occur, would seem to be more costly and risky than terminating reproduction by cessation of lactation. Metatherian reproduction may confer advantages to the female in unpredictable environments by protecting her ability to reproduce in the future. Metatherian reproduction is best regarded as alternative, not inferior, to eutherian reproduction. In fact, the reproductive mode of marsupials may be more derived than that of eutherians.