Of whales and women

Population scientists talk of two general strategies for achieving reproductive fitness among animals.

These two strategies are mutually exclusive. They mean that all-out reproduction (just having as many offspring as possible) is not always the best way for a population to grow reliably and for a species to flourish in the long-term.

r-selection

The first reproductive strategy, r-selection, is contingent largely on the rate of reproduction -- how rapidly a species can produce offspring.

Key features are rapid sexual maturity, superabundant production of eggs by the female, low survival rates of offspring, and a short life for the parent.

r-selection is typically favored by (or forced on by nature) smaller species, whose numbers wax and wane dramatically with changes in the environment.

When the environment turns bad, numbers decline drastically. When it improves again, an explosive increase in numbers is needed for the species to exploit the opportunity quickly.

Reproductive growth is highly leveraged ... but adult life is brittle and short. Think of insects, butterflies and all sorts of other small animals.

The mother may not survive more than a few hours after laying eggs. Relatively speaking, small mammals such as mice (and the Australian native tree-living marsupial Antichinus) show features of r-selection too, such as large litters, as well as short times to maturity and a short adult life, often with death after reproduction.

The father may not survive more than a few hours after copulation, all energy having been given to producing sperm.

K-selection

The second strategy, K-selection, shows sexual maturity gained slowly, limited production of eggs, high survival rates of (usually single) offspring, and a longer life for the parent, generally with several episodes of gestation -- and invariably with a premium placed on parental care of the young. This strategy witnesses competition at the longer term capacity (k) of the environment.

It means that the offspring must reach first the size, then the sexual maturity, to reproduce, and thus for their parents' genetic potential to be finessed.

In general, the larger the animal, the more it will rely on a K-selective strategy for reproductive success: whales and elephants both have long gestation periods (15 months for sperm whales, 22 months for elephants), long times from birth to adolescence and sexual maturity, and 3- to 5-year intervals between births, to protect the parent and the existing offspring from the ravages frequent pregnancy otherwise would have on the mother.

Our species, Homo sapiens, has become even more K-selected than the world's largest mammals, despite our being physically much smaller. This is because the time that our young need to grow to be independent has, for different purposes, become the same.

Our gestation periods are shorter, at nine months, so that our large-headed babies don't get stuck during birth too often. So human infants are much more dependent at birth than whale calves or elephant calves are, with large brains anticipating the learning that will follow.

Add to this the time taken for the human brain to mature and be educated and we have in our species the longest times of childhood dependency and maternal vulnerability seen on the planet.

Yet reproduction takes its toll on humans also. There is an ever-present risk of death in childbirth, especially from bleeding, and in all but the most well-fed societies aging occurs quickly during the child-bearing years.

Correspondingly, in natural circumstances, graced by few trappings of civilization, the interval between human births -- the "birth interval" -- is about the same as the three years or more found among whales and elephants.

A lengthening birth interval for our prehistorically evolving hominoid ancestors depended on various natural ways -- various biological devices -- for spacing pregnancies. We need to understand the importance of long birth intervals and the nature of nature's devices for achieving these long intervals if we're to comprehend what today we call infertility.