Nature versus Nurture: Are Champion RaceHorses Born Or Made?

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Seattle Slew (1974-2002), the only undefeated horse to win the Triple Crown (1977).

I have been thinking about a paper that was published last week, that analyzed the effects of "nature versus nurture" on the development of a champion racehorse. In short, this paper found that the effects of a horse's pedigree is minor when compared to its environment .. the combined effects of training, diet, choice of races entered, jockey skill and of course, injuries, which are unpredictable. In short, the exorbitant stud fees paid to breed a mare to a prized stallion are not an honest signal of the stallion's genetic quality. However, upon reading this paper, I think the situation is more complicated than how the authors present it.

Evolutionary ecologists Alastair Wilson and Andrew Rambaut at the University of Edinburgh in Scotland compared the stallion nomination fees for 554 stallions currently (or recently) active at stud with the winnings and lifetime earnings of 4,476 of each stallion's male and female ancestors that were foaled in the United Kingdom or America between 1922 and 2003. They found that environmental factors such as training, diet, strategic race entry and the skill of a horse's jockey accounted for 91.5% of the variation in a horse's winnings.

"It seems much more likely that people who can afford to pay high stud fees can also afford to manage and train their horses well," observed Wilson.

According to Wilson and Rambaut's data analysis (figure 1, below), if the predicted progeny lifetime earnings returned some portion of the breeder's investment in the sire's stud fee, the line would slope upwards -- a line with a slope of 1 means that the offspring lifetime earnings return equalled the stallion nomination fee. However, this line is flat -- actually, if you look closely, you will notice that it has a positive slope of 0.02, which corresponds to a 2-cent payback for every dollar spent for stud fees;

However, while it is true that the offspring of expensive stallions tended to win more over their lifetime, Wilson stated, genes played only a small role. In fact, the authors found that 8.5% of a racehorse's success was due to genetics. While a genetic effect of 8.5% sounds trivial to the casual observer, genetics typically accounts for only 1-2% of survival rates in the wild where environmental conditions vary tremendously and where an individual's survival can depend on luck as much as anything else. But because the authors deemed "success" in horse racing as being heavily dependent upon just one factor, winning races, they found that genetics plays a much bigger role in this situation.

"There are good genes out there to be bought but they don't necessarily come with the highest price tag," Wilson pointed out.

Thus, the authors determined that a stallion's stud fees are not an honest signal of that horse's genetic quality if the breeder's objective is to maximize lifetime prize winnings.

Even though this conclusion seems simple enough, the authors neglect to mention quite a few other factors that play a part in breeders' stallion choices. For example; the amount for a newly retired stallion's stud fees are determined by the group of people who purchase lifetime shares in that stallion, and this price is dependent upon the horse's purchase price, and that, in turn, is based on his performance on the racetrack as well as his lifetime earnings and his pedigree. Thus, after a stallion has been siring offspring for five or six years, his stud fee will be adjusted so it reflects his overall quality as a sire of race horses, because several crops of his progeny have provided breeders with examples of their racing abilities. In view of this information, I'll bet the authors will find a much tighter statistical relationship between a stallion's nomination fees and offspring winnings if they limited their dataset to those stallions who have been at stud for at least seven years.

Another consideration is that at least a few stallions have been recognized for their genetic contributions to their daughters, moreso than to their sons. Even though fillies and mares do not typically run in races that have large prizes, their wins are an important indicator of their competitiveness and abilities. Further, mares also provide valuable genes to their offspring so, like stallions, their primary success also lies in the breeding shed. These are examples of several types of less easily analyzed information that a breeder uses when designing pairing between racehorses.

So basically, it is true that a breeder might get what he pays for -- and less -- when breeding his mares, it is more likely that breeders are using several criteria to make their stallion choices other than relying solely on potential progeny lifetime winnings.

This study was published last week in the Royal Society journal Biology Letters.


Alastair J. Wilson and Andrew Rambaut. 2007. Breeding racehorses: what price good genes? Biology Letters (doi:10.1098/rsbl.2007.0588). [PDF]

BBCNews (quotes).

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There's also an issue that the adaptations which contribute to fast racing also produce structurally weaker horses, so horses which could be good earners are more likely to wind up injured and taken out of competition. There's a tradeoff there, and jockey quality figures into how likely a horse is to be in a position to be injured. If the paper models injury as a purely random process, it'll miss real factors.

you're correct, josh. in fact, many breeders will specifically note when the horse has retired sound, without injury, since a fair number of race horses are run until they have an injury, at which point, most of them are retired.

Also, (1) the stud price could be held down by market forces, and (2) the competitive environment isn't static either. Given that human athletes certainly show an overall trend toward better performances over time, I wouldn't be surprised if racehorses did the same.

By David Harmon (not verified) on 27 Dec 2007 #permalink

It could just be that the horses at the top are so close, even the slightest difference is valued. Even if paying extra for the best genes gives you only a horse that can run 2% faster... that could turn a second-place into first.

Racehorse stud fees are based on the combined history of racing success of a stallion and that of his progeny. Although research may show that only 8% of a racehorse's success is based on his genetic make-up, thoroughbred breeders know how indicative a horse's bloodlines are of his racing future. Bloodlines not only determine the conformation of a horse, they help determine the horse's heart. As with humans, attitude is everything, so with the Thoroughbred.