Many species produce large numbers of eggs or young, most of whom will not survive until adulthood and provide no care whatsoever.
Among mammals, some provide a degree of care, but nevertheless produce large litters, after a short gestation period and early weaning, and are said to be r-selected. Usually they are shortlived and may live in a difficult environment either because of predators, or more usually because the time periods when conditions are favourable for young are short and unpredictable.
At the opposite extreme are species which are said to be K-selected. Generally the species is larger, gestation may be longer, as may be the period of development and parental care. Usually the environment is predictable. Parents may have more than one batch of young through successive seasons, often having developed ways of surviving the worst conditions.
Each species has a balance between an optimal length of gestation and the period during which care will be necessary. Among the so-called altricial species, shorter gestation produces relatively helpless young. The drain of gestation on the mother's energies will be reduced, but extended postnatal care will be needed. Usually such young are protected in burrows or nests. Other species produce precocial young. Gestation may last longer, but the young are more able to fend for themselves, as for instance young cattle or deer that have to be able to run and walk as soon as possible.
Every mother faces the problem of whether the investment of energy in bearing young will result in success. In evolutionary terms, a mother who died in the attempt, or whose offspring was not able to grow and themselves breed would not pass on her genes to succeeding generations.
Unless she has built up the necessary energy reserves and, as zoo-keepers put it, come into "breeding condition", she is not likely to become estrous. Environmental stress may equally have an effect. Similarly for mothers who provide an extended period of care, it may be better to abandon the young and conserve energy for a possible future brood.
Aquarium keepers are well aware of the need to separate some species of female fish from their eggs. Probably, in the wild, they would swim away after laying. In the aquarium they remain in close proximity. It may be that they do not recognise their eggs for what they are, but simply as food.
However, with species where after-birth care is needed, if a threat is sufficiently serious, such as from a predator, mothers may abandon and even eat their young, on the basis that they represent an investment of energy which can be reabsorbed to provide for a future litter under more propitious circumstances.
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Bland, J., (2002) About Gender: Maternal strategies
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Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
Last amended 15.06.02