The Upper Limits Of Biology

In June 1973, chestnut colt Secretariat ran the Belmont Stakes so fast that no horse has matched him since. In less than two and a half minutes, Secretariat flew along the track, finally pulling away to win by 31 lengths, nearly three quarters of a football field. The other horses were so far behind they weren’t even in the television frame. Fifty years later, billions of dollars have been poured into breeding and training, yet Secretariat’s record still stands.

This is the paradox of the thoroughbred: breeders have spent centuries trying to create a faster horse, but in doing so, they may have bred away the very genetic diversity that would allow the horse to keep improving, or even to remain robust.

[---]

As genetics has played such a large role in breeding horses, it is small wonder that horse breeders are moving with the frontiers of science. The world’s first cloned horse, called Prometea, was born in 2003. Horse cloning is now big business, despite an American Jockey Club ban, as other equine sports, including show jumping and polo, allow clones.

In 2024, the Argentinian company Kheiron announced the production of the first CRISPR-edited horses: Polo Pureza, an award-winning mare, was the foundation for five gene-edited clones with increased explosive speed. According to reports, the company modified the MSTN gene, so that the animals would develop more muscle fibers and potentially run faster as a result. Even so, the prospects for exceeding current records through gene editing are slim. Today’s racehorses are already tightly optimized for speed, balancing strength against the cost of carrying additional muscle mass. Adding more muscle is relatively low-hanging fruit, and if this were the key to unlocking greater speed, selective breeding would likely already have brought it about.

[---]

At the Belmont Stakes in 1973, Secretariat ran in a category of his own. His extraordinary performance seems to have been the result of many unlikely factors aligning. After he died, Secretariat’s heart was reportedly measured at almost 10 kilograms – twice as much as normal. This appears to have been a physiological fluke, however, as it isn’t a trait reliably passed down to offspring. Combined with his unusual stamina, exceptionally powerful legs that allowed him to cover 25 feet per stride, favorable race conditions, and a skilled jockey, this produced a race time that has still never been matched half a century later, despite advances in breeding, training, nutrition, and veterinary care.

Secretariat offers a glimpse of the upper limits of biology. But as racehorses have pushed against that limit to become faster and more powerful, their population has become more fragile as the gene pool has narrowed. Our 4,000-year pursuit of the ultimate horse may not have much further to run.

- More Here