The X Factor

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Secretariat Secretariat
It’s often been said that some horses have the ‘X Factor’…but what exactly does this mean? Over the years it’s come to represent a champion horse, one with a great heart. Literally!

Interest in the size of a horse’s heart dates back to the great Eclipse, a great-great-grandson of the Darley Arabian, born in England in 1764. He was undefeated, winning all 18 races he started in before retiring to stud, where he sired over 400 foals.

Around 80% of all thoroughbreds today trace their ancestory back to Eclipse. When he died in 1789, tradition of the time was to bury just a horse’s head, heart and hooves but when a surgeon cut the horse open, he found a heart much larger than normal. It weighed in at 6.36kg (14lbs), which was double the weight of an average horse’s heart.

Fast-forward to the 1970’s, where champion American racehorse, Secretariat won the Triple Crown in 1973, the first horse to do so in 25 years. He set multiple track records and won races by many lengths, the most famous being the Belmont Stakes in 1973, when he won by 31 lengths…a feat that remains unchallenged.

Eclipse Eclipse

Secretariat was euthanized in 1989 after battling laminitis and when surgeons performing an autopsy saw his heart, they could only stare in wonder.

”We just stood there in stunned silence,” said Dr. Thomas Swerczek, pathologist at the University of Kentucky. “There were no problems with it. It was just this huge engine.”

Although they didn’t weigh the heart at the time, going on comparisons made to Secretariat’s main rival, a thoroughbred named Sham whose heart weighed 8.18kg at autopsy, it was estimated Secretariat’s heart would have weighed approximately 10kg. Sham lost to Secretariat in all three Triple Crown races.

”I have done thousands of autopsies and had noticed differences in heart size in horses before we did Secretariat,” De. Swerczek said. “I didn’t pay much attention until Secretariat came along, he was completely out of every other horse’s league. It explained what he did in the Belmont Stakes. It would be impossible for a horse with a small heart to do that.”

Pathologists studied Secretariat’s heart in detail but it was anatomically perfect…just huge! It was Secretariat’s autopsy that triggered the search for the X Factor.

Phar Lap Phar Lap's heart

In the 1960’s, Australian researchers were laughed at when they tried to explain the link between heart size and performance. After all, our own Phar Lap had a heart weighing 6.3kg (14lbs), now on permanent display at the National Museum in Canberra. They discovered that large hearts were inherited through female lines, which is how the term, X Factor evolved…it passes through the X chromosome of the mare. It can only be passed to a stallion’s daughters, or via his dam to a colt or filly.

After hours of research tracing thoroughbred lines, the trail for large hearts led to a mare named Pocahontas born in England in 1837 and every large-hearted horse can be traced through the X chromosome back to her. Pocahontas was a descendant of Eclipse, as were Secretariat and Phar Lap. Geneticists believe the large heart trait could be a mutation, so traceable to a single horse. Studies lead back even further to a stallion named Hautboy, who could have inherited the large heart gene from his dam but there the trail goes cold as she is listed only as a ‘Royal Mare’. Eclipse inherited his large heart from his dam, Spilletta, a descendant of Hautboy.

Pocahontas died aged 34 and had many foals for over 20 years.

A racehorse with a large heart can be compared to a V8 turbo car engine racing against horses on 4 cylinder engines…there’s simply no contest! A large heart is the horse’s engine which can pump more oxygen and give him optimum speed, greater endurance and capacity for exercise.

Many famous US racehorses proven to have the X Factor include Man ‘O War, Citation, War Admiral, John Henry, Northern Dancer, Native Dancer and Seattle Slew. How many Australian champion racehorses have/had the X Factor? Probably Makybe Diva for one, and many other thoroughbreds who won multiple staying races, including the Melbourne Cup.

A well known sire of Australian sport horses is Souvenir (1959-1979). When he died, his heart was found to be 6.4kg and as he can trace his lineage to Eclipse and Pocahontas, he definitely had the X Factor, which is why so many of his offspring have gone on to be Olympic and world champions in eventing and showjumping.

Is it possible to specifically breed for large hearts? If a mare is known to carry the X Factor, she has a 50% chance of passing it on to any colt or filly. A book titled The X Factor by Marianna Haun includes pedigrees of many horses known to carry the large heart gene but a copy will set you back between $150-$650.

Researchers have been looking for the genetic marker for the X Factor by using an electrocardiogram to measure heart size, which is what Australian researchers were doing 40 years ago. The results give a ‘heart score’ and after 400 horses were studied, it was found heart size varied greatly. A normal heart score is between 104-116. It’s been estimated Secretariat’s heart score would have been as much as 180 and no other heart has been found to be as large, even in the horse’s progeny.

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