Racehorses Sired By Old Stallions Run Slower Than Those With Young Fathers

This is the first study to show that the age of the fathers of thoroughbred racehorses at conception negatively impacts their speed

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Age is just a number, as the old saying goes. Except when it isn’t.

Although parental age effects have been documented in many animals, including in humans and even in plants, our understanding of their prevalence and magnitude is limited in two important ways. First, almost all of these studies only documented offspring survival, finding that offspring of older parents have shorter lifespans. Second, we have a large gap in our knowledge regarding the importance of paternal age effects because most research only focuses on maternal age. (This is for practical reasons: mothers are easier to identify and because mothers provide all or nearly all of the parental care in most mammals.) As the findings show, as a mother ages, the quality and quantity of care she provides her offspring typically declines, and this, in turn, affects offspring development and survival.

But parental age is probably manifested by other molecular mechanisms that may also affect offspring traits. In mammals, it has been shown that overall egg quality may degenerate as the mother ages (ref). Additionally, a variety of epigenetic changes (mediated, for example, by DNA methylation, histone modification and transfer of non-coding RNAs) also appear to increase with advancing maternal age (ref & ref).

But what about racehorses?

“The fact that parental age affects racehorse speed should be of interest to the horseracing industry”, said lead author, evolutionary biologist Patrick Sharman, who is a Postdoctoral Research Associate at the University of Exeter.

Speed is a trait in racehorses that is heavily selected for by horse breeders, so does parental age — especially paternal age — affect thoroughbred running speed?

This was the main question posed by Dr Sharman and his collaborators. They analyzed 906,027 race performances by 101,257 British racehorses between 1996 and 2019. These racehorses included offspring of 41,107 mares and 2,887 stallions.

As expected, the team found a significant effect of maternal age on speed (Figure 1a), with each additional year of a mother’s age at conception decreasing her offspring’s speed by 0.6 inches (or 1.55 cm) per second. This seemingly small margin translates into a one-second difference at the end of a one mile race for offspring of a five-year-old mare versus a fifteen-year-old mare.

But the researchers were surprised to find that the father’s age also played a significant role in his offspring’s speed: for every year increase in a stallion’s age, his offspring slowed by 0.4 inches (or 1.01 cm) per second (Figure 1b).

In a highly competitive sport where even one inch can separate a winner from an also-ran, this growing margin can easily translate into tens of thousands of dollars.

“It is perhaps not surprising that offspring speed declines with increasing maternal age. It is the dams, after all, who care for the foal, first in utero, and then through to around 6 months of age”, Dr Sharman explained in a statement.

“What I find fascinating, though, is that increasing paternal age also causes a significant decline in racehorse speed”, Dr Sharman pondered aloud. “Thoroughbred stallions play no part whatsoever is raising a foal, so what is behind this decline in speed?”

Further research is required to determine the molecular mechanisms that underlie this phenomenon.

“These findings, for a trait of commercial importance, could potentially be used to optimize breeding decisions with respect to both target traits (offspring speed) and financial inputs (e.g. stud fees) within the thoroughbred industry”, the study’s authors write (ref). “Although the present study is clearly not informative for mechanism, we also hope that our findings will stimulate research into the pathways by which parental age influences are transmitted to offspring phenotype.”

“More generally, it adds to an increasing body of evidence which points towards parental ‘state’ at the time of conception having an influence on offspring phenotype”, Dr Sharman pointed out. “This would have implications well beyond racehorses and the horseracing industry.”

Source:

Patrick Sharman, Andrew J. Young and Alastair J. Wilson (2022). Evidence of maternal and paternal age effects on speed in thoroughbred racehorses, Royal Society Open Science 9:220691 | doi:10.1098/rsos.220691

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