Testicular size in stallions
Keywords: testicle, stallion, hypoplasiaA stallion with small testicles.
The average size of the equine scrotum when measured transversely in normal stallions, is about 10 cm, each testicle having a transverse diameter of about 5 cm. In general, one would not usually expect to encounter a scrotal with of less than 8 cm. in any “standard-size” breed.
In this case the scrotal width (see red ellipse) was marginally less than 7 cm, surprising small for a “standard-size” stallion. Indeed, it was smaller than for most miniature stallions. Despite this, the stallion had normal libido and produced a normal ejaculate with low but acceptable sperm numbers. In view of his athletic prowess, the stallion continued to be used as stud sire; a factor that often trumps reproductive bench marks in stallions. It was never determined if anabolic steroids had been administered to this stallion; a possible cause of testicular atrophy. In the absence of that information, this was presumed to be a case of testicular hypoplasia rather than atrophy.
In beef bulls, many thousands of measurements of scrotal circumference have been recorded. These have established minimum acceptable sizes for breeding soundness and have also shown clear differences between breeds as well. In stallions however, the association between testicles size and fertility have not been studied as well; probably because of the relatively small numbers of mares bred to each stallion and also, the complication of human intervention in each breeding decision.
In the author’s experience, there is a considerable difference in the size of testicles between breeds with Standardbred and Thoroughbred stallions often having smaller testicles than Warmbloods. However, a recent (2013) literature search did not reveal data to support that statement. Interestingly, Paccamonti D.L. et al 1999 showed that scrotal size in miniature stallions was smaller than in “standard-size” stallions and as one might expect, these stallions produced fewer sperm as well.
1. Thompson, D. L., Pickett, B.W., Squires, E.L. and Amann, R.P. 1979. Testicular measurements and reproductive characteristics in stallions. J. Reprod. Fert. Suppl: 27:13-17
2. Paccamonti D.L. et al 1999 Reproductive parameters of miniature stallions.Theriogenology 51:1343-1349.
