As Sierra Leone, Forever Young and Mystik Dan thundered towards a three-horse photo finish at the 2024 Kentucky Derby last month, no one could say for sure which horse would win. But it was almost certain that none of the horses would cross the finish line in under two minutes.
In the end, Mystik Dan won America's most famous horse race by just a nose in a time of two minutes 3.34 seconds. It wasn't quite quick enough to break the record set by the legendary Secretariat, who, in 1973, won the race in one minute 59.4 seconds. Only one other horse has ever crossed the Kentucky Derby finish line in under two minutes: Monarchos won the 2001 race in one minute 59.97 seconds.
As Royal Ascot gets under way today, pundits and punters are focused on the "star-studded cast" of runners and riders who will be hoping to make racing history over the next five days, said the event's website. But whether they can break and set any new course records could be down to much more than the months of preparation that have preceded the prestigious event.
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There has been a decades-long debate about whether racehorses have reached their peak speed, with many believing, at least when it comes to Secretariat's record, that his long-standing winning time is unlikely to ever be broken.
What do scientists think?
The jury is out over whether horses have reached their peak speed. In 2008, a study of thoroughbreds, greyhounds and Olympic athletes, published in the Journal of Experimental Biology concluded that there is a physical limit to how fast any animal can run – including humans – and that thoroughbreds may have reached that limit.
However, the study's author also suggested that faster racehorses could theoretically be possible. Thoroughbreds have been selectively bred since the 1700s, with some 95% of present-day animals able to trace their lineage back to a single stallion, the Darley Arabian. This, in effect, has created an "evolutionary bottleneck", with thoroughbreds far less genetically diverse than other breeds of horses, said Quartz. Thoroughbred horses, therefore, may have reached their genetic potential.
But selective breeding that introduced "different equine stock could perhaps yield faster horses", said Mark Denny, the paper's author, adding that the results of his study "do not necessarily address the question of the maximum speed" for horses as a species.
Have horses reached their peak speed?
Several studies have concluded that thoroughbreds may have reached their biological speed limits, but not all scientists agree. A 2015 study from the University of Exeter found that horses had been getting faster – at least when it came to shorter, sprint races.
Researchers thought that previous studies had not accounted for several crucial factors, such as distance, number of runners, age, course and environmental factors. They compiled a massive data set of 616,084 race times run by 70,388 horses between 1850 and 2012 in the UK and found speeds had been improving over time, if unevenly.
In sprint races, the average speed needed to win has increased by about 0.1% each year since 1997. However, speed increases in mid-distance and long-distance races didn't see the same rise.
Study co-author Patrick Sharman told the BBC this could be because horses are being bred for speed rather than endurance. "My hunch is that we are seeing a genetic change, with breeders focusing on speed rather than endurance," he said in 2015. "I don't believe that over the longer distances horses have reached their limit."
In a follow-up study published last year in the journal Heredity, Sharman concluded that improving speeds could be attributed to genetics. Although this was probably "less so than breeders might like", said The Economist.
By linking a large performance database, containing nearly 700,000 race times recorded in Britain between 1995 and 2014, to a family tree of more than 76,000 horses, the study found that speed is "weakly" heritable and that the breed was improving slowly overall.
What about over longer distances?
But questions remain over why there is so little improvement to speeds over longer distances, said Sharman.
"We estimated relatively low heritability for racehorse speed, which goes some way to explaining it, but it would appear there is more to it," he said. "The selection of racehorses for breeding may be less accurate and weaker than generally assumed, particularly over long-distance races, but other factors could be constraining genetic improvement."
In other words, breeders might be "leaving some horsepower in the genetic tank", said The Economist. With breeders still relying on their professional judgement "to some degree" while assessing horses, "less intuitive, more objective statistical techniques" may be the key to unlocking further improvements.
