Anabolic Steroid Effects

A colleague, Larry Soma, VMD, is an anesthesiologist/pharmacologist who conducts research for the Thoroughbred and Standardbred horse racing commissions in Pennsylvania. Basically, his lab develops and validates testing methods for detection of illegal drugs in horses. Recently, Dr. Soma and I crossed paths in the clinic parking lot as we rushed between meetings. He approached me, and with hardly a greeting, he lit off a barrage of questions about the effects of anabolic steroid treatments on a horse's behavior. He asked my opinion on their behavior- modifying capabilities, as one of the commissions' pressing issues involves complaints about steroid-induced changes that threaten the welfare and safety of animals and personnel.

Here are my thoughts:

Over the years working in a reproduction group in a veterinary hospital, I have run into a number of horses that were treated with anabolic steroids or animals that we purposely treated with anabolic steroids. So in a clinical setting, I have had the opportunity to pay pretty close attention to behavior before, during, and after treatment. I have also conducted several research studies in which geldings, mares, or stallions were treated with various anabolic steroids, specifically to study effects on their reproductive behavior and physiology.

In general, whether it is a mare, stallion, or gelding that is being treated with anabolic steroids, the behavior changes are almost always in the way of increased male sexual and aggressive behavior. So what you are likely to notice first are:

  1. More stallionlike vocalization,
  2. Stallionlike enthusiasm and energy for noticing and approaching other horses,
  3. Some of the specific behaviors of sexual investigation and arousal, and
  4. Some of the specific aggressive behaviors more common in stallions.

Horses on anabolic steroids might appear to have forgotten their "manners" and get mouthy, or they are quick to strike or kick. In social interaction with other horses, they often seem especially quick to bite, strike, and kick, as if they have a short fuse for social aggression.

For example, in a herd situation before anabolic steroid treatment, a horse might just turn its butt and threaten to kick at a bothersome herdmate. But when on anabolic steroids, it would more likely go beyond what seems necessary to achieve the goal, and it might enter into a unnecessary double-barreling kicking fit that continues long after the recipient signals submission.

Sometimes people refer to this change brought on in horses by anabolic steroid treatment as being more dominant, but when you think about it, the behavior could also be understood as less confident and more threatened than a truly dominant animal would be. It could be understood as a change in perception of the level of threat, and/or failure to recognize submission. In this regard, the discussion of the adverse effects of anabolic steroid treatment in humans--for example, the tendency for aggressive rage--sure rings true.

The stallionlike behavior induced by anabolic steroid treatment is of a somewhat unique character, compared to ordinary stallion aggression and sexual response in intact males with only their natural hormones. For whatever reason, the aggressive and sexual behavior is often "not quite right," even for a male.

The treated animal might appear to become fixated on certain types of male-type behavior or certain elements within a sequence. An example that always comes to mind is the tendency to have a stronger interest in all the malelike sniffing, investigation, and marking of excreted feces and urine, including the flehmen response (lip curl). In these elimination-marking sequences, the treated animal might just sniff and sniff and sniff, or sniff and flehmen, sniff and flehmen, sniff and flehmen, and not move on to the usual next element of the sequence. Or he might defecate over the feces of another stallion again and again and again, much longer than the usual once or twice.

This is not especially dangerous to anyone, but in my opinion is a clear indication that something in the mechanism controlling behavior sequences is just not right.

Stallions given anabolic steroids will usually show a sudden increase in sexual and aggressive behavior. The aggressive behavior usually emerges before the sexual behavior. A mature, well-behaved stallion will often start to show behavior that appears more youthful and frisky; some people call it "colty." It's not just the level of arousal or energy that leads to rearing and romping, but often of the quirky nippiness typical of colts at puberty.

In my experience, mares on anabolic steroids are even more challenging to safely handle and to understand.

First of all, the colty nipping and the short-fused inappropriate aggression toward people and other horses is one thing in a stallion or a gelding, but in a filly it is often less expected and more erratic than in a treated stallion or gelding. One minute she's a nice, seemingly sweet filly, and the next she lunges at you with bared teeth, seemingly unprovoked.

On lower levels of steroid treatment, fillies and mares might still have their ovarian hormone cycles in the background, while the male-type behaviors pop out here and there. So one minute they are breaking down in estrus, and the next they are nickering like a stallion teasing a mare.

One of the strangest behavior sequences that you can see in horses occurs in most mares treated with low levels of anabolic steroids. It involves the mare responding sexually to herself. It's pretty weird to watch. She might urinate, then respond as a stallion would with sniffing and flehmen, and might even look back at her flank and nicker as a stallion would. These mares often have an anxious and "confused" eye.

On higher levels of treatment, the ovaries can shut down, so you have just stallionlike sexual and aggressive behavior. Very often handlers inadvertently use the masculine pronoun when referring to these fillies or mares.

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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