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Since the earliest days of horse racing enthusiasts have talked about horses with great hearts. Horses who have stood out dramatically from their fellows – who run not only faster, but harder, who never give up until the finish line. It did not refer to a clear-cut physical characteristic, but to a quality of spirit. Imagine then, the surprise when the autopsy on the English racehorse Eclipse – one of the most famous horses of all time – showed him to have a heart more than twice the normal size. An average racehorse of that era had a heart weighing around six pounds; Eclipse’s heart weighed fourteen.

Two hundred years later when the Triple Crown winner Secretariat was autopsied, his heart was estimated at an astonishing twenty-two pounds. For comparison the average modern Thoroughbred has a heart weighing eight and a half pounds. Indeed, many of the greatest racehorses of all time: Man o’War, Phar Lap, War Admiral, Citation, and Seattle Slew, among others, are believed to have substantially larger than normal hearts. Great-hearted has ceased to be a metaphor, and become a physiological fact – and one which may explain some of the mysteries of Thoroughbred breeding.

As early as the 1960’s, Australian researchers were beginning to study the hearts of Thoroughbreds, and to correlate heart size with performance. They found a strong, positive correlation. They also began to believe that there was a single gene, probably sex-linked, that produced horses with unusually large hearts. However, when they tried to relate their conclusions to US veterinarians, they were ignored or ridiculed because they had no idea where the trait had originated, nor could they point out with any reliability which living horses were likely to have large hearts.

Things have changed a little since then. For one thing, many more Thoroughbreds (as well as Standardbreds and Quarter Horses, which also show the trait) have been found to have large hearts, and their pedigrees have been examined in detail to trace back the common ancestry. For another, there is now a means of estimating the heart size of a living horse. Dr. James Steel, an Australian researcher created the Heart Score, a scale that correlates heart weight, stroke volume, cardiac output, and aerobic power. Using the score,


researchers can determine with some degree of certainty whether a living horse has a small, normal, or large heart. Larger scores correlate with larger hearts with the break points between normal and large hearts considered to be 117 for females and 120 for males. Using the data from known large hearted horses, and from over 400 horses measured by a team at the University of Kentucky, the large hearted gene has been tracked through the female lines all the way back to the daughters of Eclipse. Those daughters and their descendants were shipped all over the world, spreading the large hearted X-chromosome throughout the Thoroughbred world. Eclipse himself may be the originator of the large hearted gene in the Thoroughbred line.

The X-linking of the large-hearted gene also explains one of the most frustrating facets of Thoroughbred breeding. Over and over again, wonderful racehorses would fail to sire wonderful racers. In particular, many sires were known for throwing good daughters, but only mediocre sons. Since a colt only receives one X-chromosome, and that from his dam, a large hearted stallion would have no particular advantage in his sons. His daughters, on the other hand, would always inherit his large hearted gene. A mare with a large hearted sire might, or might not, have a large heart herself, depending on which X-chromosome she expresses, but she will carry the gene, and may pass it on to her offspring. So ironically, the stallions known for being great sires were those whose owners chose the mares carefully, with the stallion receiving credit for the mare’s contribution.

Large hearts can potentially have a tremendous impact on earnings. In a study of Standardbreds (trotters) by Nielsen and Vibe-Petersen of the Royal Veterinary and Agricultural University of Copenhagen, the 41 stallions with heart scores above 115 earned more than twice the amount of the 81 stallions with heart scores under 115. One can only assume that a study of Thoroughbreds would likely show similar results.

Obviously the large heart is not everything in a racehorse. Poor condition, unsound legs, or a bad temperament can undo any racer, large hearted or not. But all other things being equal, a large heart can make the difference between a good horse and a great one. With all the new information on the advantages of the large heart, and the new pedigree and inheritance information, the discussions of racehorses with great heart is likely to take on a whole new meaning.

Further Reading:
AQJ article on large hearted inheritance