It’s traditionally been thought that in mammals only mums can influence the sex of their offspring.

But a new study in wild mice led by Dr Aurelio Malo of Oxford University's Department of Zoology has disclosed that dads can, in fact, influence sex ratios.

The paper is published in the journal Proceedings of the Royal Society B & involves researchers from the UK, Spain & the USA.

Dr Malo said: 'In mammals, theory predicts that offspring sex ratios can only be determined by the mother, as dads have always been thought to inseminate an equal proportion of X & Y sperm, having a random effect on offspring sex that they could not shift from equality, or 50:50.

'Also, mums can influence their offspring in a number of ways from copulation to birth, whereas dads have control over sperm only. This gives mums 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 & quality of her offspring.

'Using a wild rodent model – the white-footed mouse – in lab conditions, we found that there is a relationship between a father's genetic quality & the proportion of sons & daughters he has. We then showed that this relationship is mediated by a trait that is exclusive to the father: the size of the nuclei in their sperm, which reflects the proportion of X to Y sperm. Dads with higher genetic quality produce sperm with smaller head nuclei – a higher proportion of Y sperm – & go on to produce more sons than daughters.

'The implications are important, as we now have the proof that dads matter independently of any maternal effects. Scientists can now improve their predictive models of sex ratios at birth, including not only mums but also dads.'

The researchers also provide an adaptive explanation for why it is in the father's interests to alter the probability of having sons or daughters. According to Dr Malo, one plausible reason is that males of lower genetic quality minimize the cost of having sons, which are more susceptible to the negative effects of inbreeding on fertility, by shifting the sex ratio to daughters, which are more resilient to these negative effects of inbreeding.

Dr Malo added: 'Using a wild species & not a domestic model such as lab mice allows us to extrapolate to other wild species, & to make inferences about adaptation – that is, why natural selection has selected for this ability in dads. These findings are potentially applicable to any other mammalian species, including our own. However, the extent to which we find the effects uncovered here depends very much on the mating systems. For instance, in more monogamous species the expectation that dads would evolve an ability to manipulate sex ratios in their own interests is less clear.

'Predicting sex ratios has great interest for humans, and bioethical implications. In domestic species, such as livestock & pets, the ability to manipulate sex ratios has important economic implications. In endangered species, skewed population sex ratios can push species to the brink of extinction, so breeding programs could pair males & females according to individual attributes that help achieve the rarer sex at birth.

'The long-held expectation that dads would inseminate the same proportion of X & Y sperm generated at meiosis has stopped scientists from exploring paternal effects in other mammals. By showing that dads can adjust sex ratios by varying sperm types, we help open the gates of a new research area of paternal effects on sex ratios. For example, do mums & dads have the same or opposing sex allocation interests? Does this vary across species & contexts?

'In a nutshell, we now know that dads, as well as mums, can alter the sex of their offspring, & that the ability to do so might have evolved through natural selection.'