Environmental Uncertainty and the Parental Strategies of Marsupials and Placentals

Abstract

Reproductive effort, defined as the cost in energy spent and risks taken in reproduction, has two components: parental effort and mating effort. Environmental uncertainties influencing juvenile survivorship affect the temporal pattern of parental effort: when the parent cannot decrease offspring mortality by increased investment, it must minimize the effort invested in offspring which die. Environmental uncertainty responsible for juvenile mortalities before the termination of parental care causes juveniles to have low reproductive values and is responsible for the apparently widespread tendency toward low early parental effort. Several predictions about the pattern of expenditure of parental effort result, although at present the empirical bases for testing these predictions fully are inadequate, and comparisons are largely inferential. A comparison of marsupial and placental parental strategies as a test of these predictions suggests that marsupials have long been under selection imposed by frequent loss of offspring or conditions in which termination of offspring was frequently favored.