Heart story, or the X-factor theory
Speed and endurance are essential traits that make horse racing an exhilarating event for thousands of spectators. A horse's heart can be likened to a car's engine; while a powerful and robust heart does not guarantee an uninterrupted winning streak, it certainly enhances the likelihood of success. A horse's heart continues to develop until it reaches the age of four. On average, a thoroughbred's heart weighs around 7.9 pounds (3.6 kg), although some horses have been found to possess unusually large hearts. For instance, Eclipse and Phar Lap had hearts weighing 14 pounds (6.4 kg), Sham's heart measured 18 pounds (8.2 kg), and the legendary Secretariat boasted a remarkable heart weight of 22 pounds (10 kg)!
The background to the X-factor theory
Secretariat's remarkable heart gained attention following his passing in October 1989. Dr. Thomas Swerczek, the chief pathologist at the University of Kentucky who conducted the autopsy, was astonished to find not only a healthy and normal heart but one that was extraordinarily large—twice the size of a typical horse's heart. This powerful organ efficiently delivers blood to the muscles, enhancing their oxygen saturation, which in turn boosts strength and endurance. It seems that this unique heart size played a crucial role in enabling Secretariat to win the 1973 Belmont Stakes by an astonishing 31 lengths, setting a new track record in the process.
Journalist Marianne Haun got interested in Secretariat's unusually large heart and conducted an investigation, which resulted in her theory about the X-factor and three books. She began to analyse the known data on horses with large hearts.
Prior to Secretariat, the renowned Australian racehorse Phar Lap (1925-1932) was noted for having a notably large heart. Researchers in Australia investigated this phenomenon and observed a correlation between heart size and a horse's racing success. They proposed that a substantial heart could be linked to gender and is inherited through the maternal line.
The earliest recorded instance of a racehorse with an enlarged heart dates back to the 18th century, involving the famed thoroughbred Eclipse (1764-1789). At that time, it was customary to inter the head, hooves, and heart of distinguished horses. Thus, following Eclipse's death in 1789, a London surgeon performed an autopsy and discovered that Eclipse's heart weighed twice the average for a horse of that era.
After examining these and other cases, researcher Marianne Haun theorised that the large heart represented a genetic mutation originating with Eclipse. This mutation likely began to propagate through the thoroughbred lineage via his daughter, Pocahontas.
Marianne Haun's Theory
Why is it referred to as the X factor, specifically using the letter X instead of another symbol? To grasp this, a basic understanding of genetics is necessary.
Information regarding the body's structure is stored within every cell, found in chromosomes. Most people are now familiar with the double helix structure of DNA. The sex chromosomes determine gender: females possess two X chromosomes, thus categorising them as female, while males have one X and one Y chromosome, with the Y chromosome determining male characteristics.
In reproduction, females produce eggs exclusively carrying the X chromosome, while males generate sperm containing either X or Y chromosomes. Consequently, there is an equal probability—50/50—of conceiving a female (XX) or a male (XY) child.
In relation to large hearts, Marianne Haun conducted extensive research involving volumes of stud books and autopsy findings. It was observed that males with large hearts did not pass this trait to their sons, but instead, it was only transmitted to their daughters. Furthermore, those daughters could then give birth to sons with the same large heart trait, indicating that the characteristic is linked to the female sex through the X chromosome. Hence, the term "X-factor" is used to denote this trait, associating it with female genetics.
The male simply cannot get this trait from his father, it is not on the Y chromosome. But the daughter gets it through the father's X chromosome, and can then pass it on to her children.
Marianne Haun traced the origins of the phenomenon, they all go back to Eclipse.
It is believed that his daughter Pocahontas was homozygous for the "big heart" trait, i.e. information about this was contained in both of her X chromosomes. Therefore, her offspring produce a high frequency of large hearts and victories in races.
Marianne cites cases when a mare with a large heart, a winner of prestigious races, does not produce the same successful offspring. The X-factor theory explains these cases by the fact that the mares are heterozygous for the trait in question, i.e. it is contained in only one X chromosome. If such a mare is bred to an ordinary stallion, the chances of producing strong offspring are practically zero. This applies to the winner of the 1988 Kentucky Derby Winning Colors or the quarter horse mare Dash For Speed, l who won the 1988 All American Derby.
Standardbreds and Quarter Horses
Pocahontas's lineage includes the mare Midnight, who was born in 1865 and is known for her big heart trait. Alongside two other mares, she significantly contributed to the evolution of premier American trotters. Notably, Peace Corps, the outstanding American trotter of 1989, was recognized as the nation’s top two-year-old and three-year-old horse and achieved victory in the Breeders Crown four times.
Additionally, the practice of crossing Quarter Horses with Thoroughbreds has allowed the influence of the X-factor to persist within this breed, tracing back to Everlastin, the daughter of Eclipse. In her observations of leading North American Quarter Horses, Marianne Haun wryly remarks on the recurring patterns in their careers. Following a string of prominent victories, stallions are often retired. Owners unaware of the X-factor anticipate remarkable offspring, hoping for equally exceptional racehorses. However, when those expectations are unmet, the stallion is typically moved to less prominent locations, where, over time, his daughters start producing champions, mirroring their grandfather’s legacy of possessing a big heart.
The Future of the X-Factor Theory
Four lines of thoroughbred horses, descended from Eclipse, are currently holders of the gene for a large heart: the War Admiral, Mahmoud, Blue Larkspur and Princequillo lines. At present, official science can neither confirm the correctness of Marianne Haun's conclusions nor refute them. The theory is considered a pseudo-scientific populist fiction. It is claimed that the horse genome has been studied and the trait of a large heart is not linked to sex chromosomes.
On the other hand, the opposite has been empirically proven. Education, living conditions, the personality of the trainer and other things in combination with the X-factor make a champion out of a horse.
In any case, the topic is complex and requires further detailed study in order to provide recommendations to breeders who are now crossing horses intuitively, taking into account other indicators and not guided by the X-factor.
Perhaps this will create a new Secretariat. It remains to be seen what the horses of the future will be like.