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But a new study in wild mice led by Dr Aurelio Malo of Oxford University's Department of Zoology has shown that fathers can, in fact, influence sex ratios.
The paper is published in the journal Proceedings of the Royal Society B and involves researchers from the UK, Spain and the USA.
Dr Malo said: 'In mammals, theory predicts that offspring sex ratios can only be determined by the mother, as fathers have always been thought to inseminate an equal proportion of X and Y sperm, having a random effect on offspring sex that they could not shift from equality, or 50:50.
'Also, mothers can influence their offspring in a number of ways from copulation to birth, whereas fathers have control over sperm only. This gives mothers more scope to alter the sex ratio of their offspring. The physical costs of gestation are obviously higher for the mother, so it's in her own interests from an evolutionary point of view to invest her resources wisely in terms of the sex, size and quality of her offspring.