Trivers suggested that females in relatively poor condition will produce predominately female offspring that will have higher mating success than their male brothers. Such sex ratio bias will evolve by natural selection when (1) the sizes of the offspring are positively correlated with the mother's condition and (2) the likelihood of successful mating of offspring is related to the condition of males but not of females. An alternative hypothesis states that under poor environmental conditions females should produce the sex that requires least investment ('cheaper sex') and thereby maximize the number of offspring. In a range of animal taxa, these two hypotheses predict the same outcome when male individuals are larger on average than females (as in some bird species, for example, blackbirds, grouse, and pheasants); a female-biased sex ratio is expected under poor environmental conditions and male-biased under favorable conditions. Nonetheless, there is little evidence for this hypothesis, especially among unisexual species, where sex is determined purely by genetic mechanism. Among plants, however, the label 'cheaper sex' almost invariably applies to male plants (in case of dioecy) or male functions (in case of hermaphroditism), since the production of pollen and the development of pollen tubes require little allocation of resources in comparison to the production of ovules and seeds.
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