Sperm Boys - Sperm speed is associated with sex bias of siblings in a human population

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Recent studies investigating possible causes of male subfertility have largely focused on how lifestyle or environmental factors impact on the process of spermatogenesis. Markedly, fewer studies have investigated those risk factors that result in reduced sperm quality, such as poor sperm motility.

The speed at which sperm swim is a major predictor of fertility and is extremely variable in human populations. It has been hypothesized that offspring sex may be adaptively manipulated to maximize the offspring's reproductive fitness e. Conversely, parents with poor male fertility genes would produce primarily daughters. We tested whether there was an association between how fast a man's sperm swam and the sex bias of his siblings in a sample of men attending clinic for fertility investigations with their partner and with a wide range of semen characteristics, including sperm speed.

We found that the sex bias of a man's siblings is associated with his sperm speed; men with female-biased siblings had significantly slower sperm judged using computer-assisted sperm analysis CASA than men from male-biased sibships.

This observation suggests family composition is an important factor that needs to be considered in future epidemiological and clinical studies of human fertility. In most animal species, sperm motility their ability to swim is vital for fertilisation success and is highly variable within populations. In certain environmental conditions, it may be beneficial to produce mainly males and in alternative environments it may be beneficial to produce mainly females, 3 causing sexual antagonism for offspring gender.

The ability of mammalian mothers to adjust the sex ratio of their offspring is poorly understood although empirical and theoretical studies in other species have shown that: Furthermore, to our knowledge, no study to date has investigated possible associations of an index of reproductive fitness and sibling composition.

One important component of lifetime fitness in humans is fertility.

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