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Using a very simple model, in which the reproductive strategy of an individual is considered to be completely specified by the time he cares for each brood, we find the evolutionarily stable strategies, and characterize them in terms of the parental abilities of males and females. We predict inter alia that: (1) If at any stage the sexes are complementary, either by specialization or by the need for constant attendance with the young, so that two parents are much better than either parent alone, then only at extreme sex ratios will either parent desert. (2) If the sex ratio is unity, then the first parent deserts when both parents are twice as good as the deserted parent alone. (3) Except at extreme sex ratios, if one sex is very much worse than the other at caring for the young, then that sex will desert first. (4) Where the male deserts first, the more heavily the sex ratio is biassed towards males, the less the relative advantage of both parents over the female alone is, at the time of desertion. We conclude with some general points about modelling mate desertion and parental investment. © 1978.

Original publication




Journal article


Animal Behaviour

Publication Date





645 - 652