Castrated and Confused

Q. I have a male burro that was gelded about two years ago, when he was approximately 7 months old. He has always been housed in the same corral with two horses (mares) and one jenny (his mother).

Sometimes, the male burro will actively mount the jenny. For example, today I had to go outside and break the two of them up because the male burro was mounting/mating with the jenny. He will not try to mount the two horses, just the jenny. This has happened off and on since before he was gelded.

Is it normal to have a gelding burro, not just mount, but actually try to mate with a jenny? Should I do anything to try to prevent this? Is it possible that he can make the jenny pregnant (not be fully gelded)?

Heather Shatz, via email

A. Castration, which usually reduces male sex drive, does not always eliminate all male-type sexual behavior in both horses and burros/donkeys. In fact, some geldings retain fairly high levels of male sexual interest and response, even when veterinarians confirm that both testicles were removed and there is no evidence of elevated male hormones. In horses, as many as one third of completely castrated geldings will still achieve full erection, mount, insert, thrust, and ejaculate, especially when given pasture free access to females in estrus. In some cultures the belief is that donkey and burro geldings are even more likely than horse geldings to retain significant sexual behavior. I don't know of any data confirming this.

Donkeys' sexual behavior differs quite a bit from that of horses. This would certainly be part of the explanation for your burro's apparent sexual preference for the jenny over the mares. Donkeys are -territorial breeders, where the male guards a territory rather than a harem group. The male gains access to breeding as females wander through his territory. The better the territory, the more likely females will be attracted to it. In donkeys, unlike horses, female-female mounting is also a normal behavior. The females that are in estrus gather together within the territory and mount one another, much like cows do. This serves to attract the jack.

Another common question is whether some of the mounting behavior seen in geldings represents female-female mounting behavior. This is an interesting potential explanation, but again, whether the removal of male hormones by castration results in expression of female-type behavior in donkeys has not been studied, as far as I know.

Regarding your question of possible incomplete castration, whenever a purported gelding shows significant or problematic male sexual behavior, you should consider this possibility. The first step would be to have a veterinarian palpate the scrotal and inguinal area to determine if a testicle that was not present in the scrotum at the time of castration has since dropped. If that is the case, then it would explain the behavior, but keep in mind that the male could be fertile until that testicle is removed.

The next step is have your veterinarian evaluate whether there is an undescended testicle in the abdomen using a challenge test that measures male hormones as an indicator of a retained testicle. If a retained testicle is internal, any sperm produced are not likely to be adequately mature or viable due to the adverse effects of the higher internal body temperature compared to the lower normal temperature of the scrotal position. Even if he or she detects no testicular hormones, your burro might still have a testicle.

Also, abdominal testicles don't always function exactly as would scrotal testicles. The hormone output can be sporadic and less than normal. So you might still want to have a veterinarian (preferably experienced with exploring horses for a retained testicle) perform an internal manual and ultrasound examination of your burro.

If the there is not a retained testicle, there is not much you do about the sexual behavior other than to separate the gelding from the females.

In closing I'll say that with sexual behavior in a gelding, castration history is all too often a complicating question. Especially in this modern era of cell phone cameras, it would be great if it were standard veterinary practice to photo document the one or two removed testicles on the ground beside the castrated horse for inclusion in the medical record.


About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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