Q. I purchased my horse a year ago knowing he had some aggression problems. He is great around people, just not around other horses. Unfortunately, when he attacks, he goes for the throat just behind the jaw line. He refuses to let go at times, causing severe swelling. It's odd he doesn't kick or bite anywhere else. I keep him at a private facility where he is kept alone due to his aggression. Observing him now, he creates these huge poop mounds; his paddock is clean while he goes to the bathroom in only three huge piles.

We have never seen such behavior.  Are these normal signs of a stallion? Could he be a cryptorchid (horse with retained testicular tissue)? He was bought at an auction so I have no history on him. I do know of a pony that was a stallion for years and then gelded.  After being gelded, he got quite aggressive. I don't know where else to look for information on aggressive horses.


A. Thank you for your questions. In your very short note you have raised several interesting topics and good points about gelding behavior.  To address all of them specifically for your horse, I would need a better understanding of the specific details and circumstances. It would also be helpful to have more specific information on the social groupings in which you have seen the behavior occur - for example, to know your horse been with horses before the aggression occurred and how long it continued, and whether the herdmates and the targets of the aggression were mares or geldings. But with the information you have provided, I can make a few comments that might be useful to you and other readers faced with similar problems of aggressive behavior.

It sounds like one of your thoughts is that your horse's aggression represents residual stallion-like behavior. Grasping and holding onto other horses are natural elements of stallion harem formation and maintenance behavior. It is seen in stallions when they are gathering a harem or trying to get a mare to stay with the group. This grasping behavior is also a conspicuous element of play behavior among juveniles, as well as the more serious sparring behavior among bachelor stallions.  The grasp is typically onto the crest of the neck and mane but can be at the throat or jugular area. So if the targets are mares, perhaps your horse's behavior represents stallion-like harem formation behavior. If the targets are other geldings, perhaps it represents inter-male sparring typical of bachelor stallions. Many true geldings retain this and other stallion-like behaviors, so it is difficult to differentiate a retained testicle or other exposure to androgens (male sexual hormones) on the basis of this behavior alone.

You have mentioned another interesting and often misunderstood horse behavior - defecation in three specific places in his paddock. It is a normal stallion behavior to defecate in particular locations, rather than randomly as mares and young males do. Under pasture or natural social conditions, the locations of the stud piles seem to have meaning in terms of marking valuable resources. In wild horses, stud piles often occur in groups of three near water holes or at junctures along routes of travel. Under domestic management, most stallions retain the tendency for this organized elimination-marking behavior. It is one of the joys of mucking a stallion's stall or managing a stallion paddock. Many stallions also urinate in the same place as they defecate.

Many geldings also retain this stallion-like tendency to defecate and urinate in an organized fashion, in one to three places rather than randomly. So, as with the grasping and holding behavior, it is difficult to differentiate a cryptorchid from a completely castrated horse on the basis of whether this behavior is seen.

Having said that, it sounds like you were wondering whether the aggression and elimination-marking behavior are related, and if they might suggest that your horse is a cryptorchid. Many confirmed complete geldings retain stallion-like elimination-marking behavior. So either alone or along with aggressive or sexual stallion-like behavior, it has almost no value in differentiating a cryptorchid from a completely castrated horse.

Decisions to explore the possibility of a remaining testicle or remnant are usually based on the intensity of sexual and inter-male aggressive behavior rather than the number of elements of stallion-like behavior. It's easy to explore whether a purported gelding has remaining testicular tissue. Your veterinarian can do a blood test that involves taking a baseline blood sample, administering hCG (a hormone that will stimulate any testicular tissue to release androgens), then taking a second blood sample.  While it is not 100% accurate in ruling out the presence of remaining testicular tissue, this simple test catches most cases.  And if a hidden testicle or remnant that is producing androgens is found and removed, the problem stallion-like behaviors usually, but not always, subside within a few weeks after surgical removal.

Your question about the possible association of the emergence of aggressive behavior with castration of a mature male is a very interesting concept. I don't know of any research to back this up, but we have seen this phenomenon from time to time.  A colleague draws a parallel between this observation and the human phenomenon affectionately known as "grumpy old man" syndrome. She describes a cluster of short-tempered, mean-spirited behaviors associated with the age-related decline of male hormones in men. I have known some mature stallions for a long time before we experimentally suppressed, eliminated, or manipulated their androgens. For some that had been socially confident and "peacefully dominant" when they had had normal levels of androgens, their behavior actually became more aggressive as androgens diminished.  To us, these animals appeared to become less confident and more dependent upon overt aggression to "prove" themselves to their herdmates.  With low or no androgens they seemed uncharacteristically crabby, similar to a mare protecting her foal.

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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