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

Abstract

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.g., parents with genes for good male fertility traits, such as high sperm speed, would produce primarily sons and fewer daughters because the offspring will inherit advantageous male fertility genes). 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.

Sibling rivalry: Males with more brothers develop larger testes

“…In this study, we found that testis size is not associated with the number of female siblings nor total litter size, suggesting that the critical component of population density that mediates investment in gamete production may be number of potential rivals. Similarly in humans, where intrauterine effects are reduced, men with more brothers produce significantly faster sperm (Mossman et al 2013). Taken together, our results suggest that social conditions during early stages of development, either in utero or during the postpartum period, can have a critical influence on male reproductive potential.…”

Sisters' curse: sexually antagonistic effects constrain the spread of a mitochondrial haplogroup superior in sperm competition

“…Nonetheless, the significance of the sex-biased, mitochondrial selective sieve as a contributor to variation in male fertility and sperm traits remains controversial and inadequately investigated. Several studies have reported marked effects of mitochondrial haplogroup on sperm motility in humans and other taxa [16][17][18][19][20][21], while others have failed to find an association [22,23]. Even less well understood are the contributions of mitochondrial DNA (mtDNA) variation to female re-mating behaviour and sperm competitive ability (but see [24][25][26]).…”

Human semen quality and the secondary sex ratio

“…These findings on curvilinear and average path velocities in seminal plasma and offspring sex were confirmed in an expanded study of 187 men 41. In contrast, although not directly tested for the SSR, in a study of 500 men attending an andrology laboratory in the United Kingdom evaluating sibling sex composition relative to CASA sperm motility measures, men with female-biased siblings had significantly slower sperm than men from male-biased siblings 58. Meanwhile, a Danish study of 15 218 men seeking infertility evaluation reported no evidence for an association between sperm motility and the SSR 39.…”