Hay everyone!

If you thought my last blog was cool, wait until you read this one! Last time, we talked to some experts at Rutgers about their experience with researching horse behavior. Now, we’re kicking it up a notch and hearing from a leading expert in the horse behavior field: Dr. Sue McDonnell. Dr. McDonnell is the founding head of the Equine Behavior Program at the University of Pennsylvania School of Veterinary Medicine. She is incredibly knowledgeable in equine physiology, behavior, and welfare, and she was generous enough to share her expertise with us!


Lord Nelson (LN): What piqued your interest in animal behavior? In other words, why did you choose to study this?

Dr. Sue McDonnell (Dr. M): I’m kind of an accidental behavior person. The short story is that I was working on a master’s in a psychology department, and this was way back when biology departments would have things like ethology, but they didn’t have the type of animal behavior relating to learning and conditioning.

I was working with a professor whose expertise was male sexual behavior, but in things like dimorphic fish and in rats. My next-door neighbor was a veterinarian at New Bolton Center., and his wife and I were sharing babysitting. And I was grumbling to her as I was picking up my daughter, saying that my work was going to be delayed because the rat facility was being renovated, and I could do sexually dimorphic fish, but I had really done all my prep for my thesis in the mammalian model.

Her husband walked in and he, in language I won’t repeat said, “why are you bothering to study anything but stallions?” He had been working all day with a stallion that was having difficulty breeding for no known reason. Nobody talked about behavior, let alone talked about sexual behavior, in any species in much depth. So that’s how I ended up doing my master’s work at New Bolton Center, and then the vet school funded me to go on to do a Ph.D. at New Bolton as well.

I then got funded by NIH for 6 years to use the stallion as a model for men. Because if you think about it, it’s the best animal model in terms of all the features, and the hospital got very fascinated with what else I might be able to do. A big problem in veterinary medicine has long been if you can’t find something that might be the cause of a performance or behavior problem with the current diagnostics that you have, or the owner’s diagnostic dollars, then it was always presumed that it’s just behavior.

So now I still do a lot of stallion work, but most of my everyday work at the hospital is trying to help veterinarians figure out whether the problem is truly behavioral, or whether we need to keep looking, and where we need to look next, to diagnose an underlying physical problem.

LN: Could you tell us a little bit more about the type of research your lab does? For example, I read that you maintain a semi-feral pony herd. What kind of insights have you gained from that research?

Dr. M: We started the semi feral pony herd to look at some questions about how social herd behavior influences male sexual function. There are harem stallions, and there are bachelor stallions. And along the way, even before we had a fully natural herd, we knew that when stallions were exposed to other stallions living in a “bachelor band”, their whole reproductive physiology was down-regulated. They went from full-fledged fighting with one another, as a harem stallion would do to defend his mares, to just playful sparring, and their testicles were smaller. Their sperm production per gram of testis was lower. Their whole endocrinology was down-regulated, and it was a plastic system, so that you could move animals around and put them in different social contexts, and you could drive them into greater fertility.

And I’m sure you can imagine how exciting this was for the horse breeding industry, especially those stallions who need as much sperm as possible. Or on the other hand, stallions who are having difficulty breeding. Many of their behaviors are very similar to what you would see in bachelors.

We started the herd in 1994, and we thought we would do this for a couple of years. It’s still there because it has given us enormous insight, not only into the reproductive behavior, but in all sorts of questions of horse health. These animals are like the poster child for healthiness, they would not keep veterinarians in business. They have no injuries; they have no hoof related problems. They don’t colic. They don’t founder despite the fact that when they’re in domestic management, you can hardly keep them from foundering. They have a very low parasite load, their teeth don’t have problems, they never have any hoof care, yet they have perfect hooves that are perfectly matched for the season and the substrate. That’s a wonderful thing to have in the middle of a veterinary school.

Right now, we’re in the middle of a study. Part of it started in 2016 and another in 2019, where we’re doing some fertility control interventions. For the management of wild horses and burrows, or pretty much any of the wild populations around the world, you really have to maintain a semblance of normal herd behavior. Every act that has been written to protect those animals has language in it saying that you can’t seriously disrupt the behavior. So, this herd is really a good resource. While it’s not completely free, they’re not free to go from Florida to Maine, they are in an enclosure, they’re pretty much unhandled.

Most of their nutrition is pretty natural. In the winter, we do supplement with hay that we spread out so that they have to continue walking as much as they do in the summer while they’re eating, and we can have very good information on their behavior. It would be impossible to actually test these things in terms of the effect on behavior in much detail in a truly free roaming population.

LN: How many ponies do you have? And how big is the enclosure?

Dr. M: Our enclosure is about 40 acres, and we keep it in pretty good Chester County sod conditions. Our highest numbers are in the fall, and as of last week, we have 101 ponies. And they’re little, the weight is okay for that acreage because we’ve got the babies and the juveniles, and then the heaviest harem stallions are just under 500 pounds. Some of the mares in the spring right before foaling are 400 to 450 pounds.

But the average adult animal is around 300 pounds, so they do really well. At times we have worried that the pasture was too lush, but we can’t seem to founder them when they’re out there under natural conditions. Our population control traditionally has been that we take out animals and have students involved in gentling them and introducing them to veterinary procedures. Then the stallions get castrated, and they get rehomed, and we still do that. It’s not quite free-running natural, but we don’t try to influence the sex ratio or we don’t pick the ones that we think are the easiest or the hardest to handle, but nonetheless, we are removing animals.

LN: With this herd, what tools do you use to study them? Is it from a distance, using things like video cameras? How do you collect data?

Dr. M: When we first started studying them, we were doing a lot of different things. We were getting endocrine samples from them, periodically measuring testicles, all these things that we wanted to know about. We have a handling system built within their enclosure. They pass through this laneway almost daily, and they go family by family. We can actually strategically close the ends of the alleyway and have a family at a time. And then we have a series of little padded sub enclosures. And this is kind of interesting, they’re pretty much wild, but it’s a great demonstration of very simple behavior modification: you can very quickly train stallions to stand and let you measure their testicles and take a blood sample for a little bit of grain.

It’s also lots of observation and it’s over years. We’ve tried to use video and we sometimes do, but we actually do best the old-fashioned way with very well-designed paper and pencil check sheets. Even though there are some programs, we’re still getting our best inter-observer reliability with a live recording, watching a harem at a time.

Probably the biggest contribution of this herd was early, in the first 10 years or so. Based on this herd, we were able to put together the first comprehensive ethogram of any equid. It was quite startling to me that we’ve known horses and dogs for so long, but no one had ever really done this. It’s probably because early ethologists were more interested in wild animals and thought that people really messed up the nature of domestic animals.

We certainly have, but the other beautiful thing is that no matter how much they’ve been domesticated or bred for any specialty or size, one of the amazing things about horses is that when you turn them out and let them interact with natural sexes, not geldings and mares so much, but stallions and mares, they immediately revert to a full complement of the social behaviors that existed before they were domesticated, which is a principle in domestic animals that people look at. The horse is probably the best example of full reversion instantly.

LN: About how many generations do you think you’ve had of these ponies?

Dr. M: Our fillies are maturing a little sooner than the truly wild horse because they are in a better body nutrition state. They’re not starving over the winter. And so most of our fillies are in foal before their 1st birthday, and they’re foaling before their 2nd birthday. So you can figure out the math, but we have had a lot of generations!

Another really cool thing that we figured out, and people were thinking about this but didn’t have the tools or the access to a population to study it, and it’s another ethological principle, is what is the diversity mechanism or the incest avoidance mechanism. We thought we would have to stop this herd because we’d have too much inbreeding. And we hardly have any inbreeding after all these years. And we only started out with 13 males and 13 females. And the principle is that the fillies will accept breeding from the most distantly related male available to them and it’s a female side thing. They breathe in each other’s breath when they first come towards each other, and that seems to be when the “decision” is made.

LN: What would be the mechanism behind that?

Dr. M: There’s been some work that it’s based on the major histocompatibility complex. It’s likely just automatic pheromone-based response. We’ve had a few different people look at the genetics of our herd and that question, and we should have answers within the next couple of years.

LN: So you have no influence on their breeding whatsoever?

Dr. M: No, there’s roughly half males and half females out there now. They’re all intact stallions. And we’ve had some “mistakes” where they did breed with a closer relative than what was available, but that’s usually the fillies’ first breeding while they’re still in their natal band. And then they usually leave sometime with their 2nd or their 3rd foal. When they form a long-term harem relationship, so far, none of them have had any common ancestors.

LN: How long do they usually live?

Dr. M: Typically, the ones that have died naturally in the herd have been in their upper 30s, but ponies live longer than horses generally anyway, if they don’t founder. Those animals that lived that long were all part of the original herd, so they weren’t born there. If we had animals that were born the very first opportunity, which would have been 1995, now they would be 28 years old. So we can’t really say how long they would live.

LN: When you first put the herd together in 1994, what was the general breed of these ponies?

Dr. M: We had one registered Shetland that we knew was registered. The others all came from auctions, and we purposefully looked at their location of origin. When they came to New Holland, they often had been through a few other options on the way. We tried to get paperwork to say that they were from distant locations thinking that hopefully, we got as much genetic diversity to start out with, but they’re all a small Shetland type.

LN: Do you know of any other innovative ways that horse behavior is being studied?

Dr. M: One thing I’m working on is a very applied question to try to use virtual companions for horses. Which could also be a virtual stimulus for animals for reproduction, so virtual stallion teasers for teasing mares for estrus detection and stimulus mares for exciting stallions for semen collection.

We’re also working on a study funded by the Morris Animal Foundation on the effectiveness and adverse side effects of morphine pain control for painful orthopedic horses in the hospital. We’re doing the blind behavior observations. We do a lot of work for the hospital that involves taking a 24-hour video of a horse in a stall, business as usual, but most of the time the horse is just in the stall by itself. And we evaluate that for any sign of discomfort. That’s another ethogram that we published recently that has been kind of a landmark paper that we’re very proud of.

We had had a protocol since 1983 for doing these 24-hour video evaluations for all sorts of types of behavior, but we also noted any discomfort behaviors. This was all just our lab protocol. I started doing it for the hospital, and it’s a kind of a nominal charge to the patient, but whenever there’s any doubt about what might be bothering this horse, they will have us do this. It takes me about an hour to go through it in fast forward, but we sometimes have to stop to figure out what exactly the odd behavior is.

But from that, we recently, I think it was 2021, put together the ethogram. The basic model of an ethogram is to have a word definition of the behavior, line drawing, depictions of the essence of the behavior, and now we can embed a video clip in the published papers online. And so this is a great little training tool for people who might want to do this, and I’ve been trying to convince the larger veterinary practices to hire a young assistant to do this. Our clinicians here sometimes don’t want to even see the horse until we’ve done this so that they can be very efficient at knowing where to look first for a problem.

LN: Thank you Dr. McDonnell! This interview was incredibly informative, and I’m sure the readers will be excited to hear what you had to say.

Dr. M: It was my pleasure, nice to speak with you!


I hope you learned as much as I did from this interview! The herd Dr. McDonnell maintains was super interesting, and I personally can’t wait to find out what else they learn from those ponies. If you want to check out more of the work that Dr. McDonnell and her friends at Penn Vet do, head over to their equine behavior clinic website!

Until next time.

Your friend,

Lord Nelson

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