Body size, duration of parental care, and the intrinsic rate of natural increase in eutherian and metatherian mammals.

The intrinsic rate of natural increase (r m), conception to weaning time (t cw), age of first reproduction (tmat), and components of fecundity were compared between ecologically similar groups of 42 metatherian (=marsupial) and 42 eutherian mammals. Marsupial t cw s average 50% longer than those of eutherians. Small marsupials (<400 g) mature later, and have lower and r ms than eutherians; large marsupials (>10,000 g) do not mature later but also have lower r ms. At body sizes of 1,000-3,500 g, marsupials and eutherians have similar t mat s and t cw s but marsupials have greater r ms. Marsupials compensate for their longer t cw s by a variety of methods including embryonic diapause, larger litter sizes, and short periods between weaning and maturity. Although the greatest similarities in marsupial and eutherian life histories are at body sizes of 1-5 kg, compensation for long t cw may be seen at any marsupial body size. Other ecological factors not withstanding, marsupial reproduction is neither inherently inferior to that of eutherians nor obviously more advantageous in unpredictable environs.