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.