Natural regulation of population size in large mammals by means of two new delayed density-dependent fertility mechanisms

It is generally assumed that population size of all wild animal species is regulated through density-dependent mechanisms, but the mechanisms responsible have been difficult to identify for elephants and large whales. We have used information on physiological reproductive mechanisms in humans in a stochastic computer simulation study to explore how known density-dependent fertility mechanisms in humans could regulate population size in a hypothetical large mammal species, assuming no deliberate interference with sexual or reproductive processes. Two physiological reproductive mechanisms in women are dependent on nutrition in utero or early life: age at menarche and post-partum amenorrhea during lactation and were included in the model. If large female mammals in general have physiological reproductive mechanisms similar to human females, strong density-dependence mechanisms will be the result, but with a substantial delay, 20 to 50 years. The model results are discussed in relation to what is known about populations of the Eastern North Pacific gray whales ( Eschrichtius robustus ), Antarctic minke whales ( Balaenoptera bonaerensis ) and elephants ( Loxodonta africana ) in Amboseli National Park, Kenya.