Beast of Burden

Let’s talk about mules (the animal, not the shoe). The result of breeding a female horse or pony with a male donkey, mules have been used as work animals by humans for thousands of years. Why would we go through the trouble of breeding a hybrid animal (and one that is usually infertile, to boot)? Because mules have something called “hybrid vigor” — they possess the best features of both parents. For example, they have the size of a horse with the surefootedness of a donkey. Mules even exceed the physical capabilities of either of their parents; they have more endurance and can carry more weight than horses or donkeys.


It’s just simple math, really.

All of which begs the question (at least to cognitive scientists), does the hybrid vigor of mules extend to their cognitive abilities?

In order to answer this question, we’ll have to look at studies that test the cognitive abilities of mules, as well as their parent animals (horses and donkeys). If the mules perform better at a particular task than either the horses or donkeys, then we can say that their hybrid vigor includes cognition.

One such study (Proops et al. 2009) looked at the learning abilities of equines (members of the genus equus, in this case mules, donkeys, and ponies). The equines were essentially given a scaled-up version of the food-finding task given to dogs. A piece of carrot was placed in one of two barrels. Instead of having a person point to where the food was located, a picture was placed in front of each barrel. The same picture was always placed in front of the barrel where the food was hidden (the “positive” picture), and a different picture was always placed in front of the empty barrel (the “negative” picture).

Screen Shot 2013-10-09 at 2.59.29 PM

Picture pairs used by Proops et al. (2009). [Originally used by Voith (1975).]

Researchers wanted to see how long it would take the equines to learn the association between the positive picture and the food “baited” barrel. “Learning”, as defined by the researchers, occurred when an equine chose the correct barrel 75% of the time. Once an equine learned the association with one set of pictures, researchers used two new pictures, with one assigned as the positive picture and one as the negative picture. All equines were tested in 25 sessions (12 trials per session), so researchers were interested in how many different picture pairs the different equid species learned in that set amount of time.

On average, the mules learned significantly more picture pairs than either the ponies or the donkeys (the ponies and donkeys were not significantly different). Moreover, as the mules learned more pairs, it took them fewer sessions to learn each new pair (it took 8.83 sessions to learn Pair 1, 7 sessions to learn Pair 2, and only 5.4 sessions to learn Pair 3). This suggests that, in addition to learning the specific picture pairs, the mules were also learning about the task more generally.

These may seem like the same thing, but they’re different in a very important way. The mules’ 75% accuracy in choosing the correct barrel tells us that they learned the association between a specific picture and food. The fact that they took fewer and fewer sessions to learn those specific associations tells us that they may have learned, for example, that the same picture always has the food behind it.

This is because if you know that the same picture always has the food behind it, you won’t waste time checking out what’s behind the other picture. Sure, if you’re given a new pair of pictures, it may take a couple trials to figure out which picture marks the food. But once you’ve found it, you won’t spend as much time going back and forth between the pictures because you know, from previous picture pairs, that the same picture always marks the food.

Cognitive scientists usually consider this ability to generalize to be a better indicator of how good an animal is at learning.

Unfortunately, not enough ponies or donkeys learned enough picture pairs to see if their learning got faster as they learned more pairs, so we can’t compare their performance to mules on that measure. However, the fact that the mules were able to learn more picture pairs than their parent species suggests that their hybrid vigor extends to their cognitive abilities!

In another experiment (Osthaus et al. 2013), mules were found to perform better on a spatial cognition task than horses and donkeys. I won’t go into detail (this post is already too long!), but I definitely recommend reading the paper on it if you’re interested — it’s pretty short and easy to understand.


So the greater variety of genes the mule gets from its parents gives it a physical and cognitive edge over either of its parent species. We see this hybrid vigor in other species, too — for example, mixed-breed dogs tend to be hardier and have fewer health problems than purebred dogs. What are some other examples of hybrid vigor? (Hint: It doesn’t only occur in the animal kingdom!)


Osthaus, Britta, et al. “Spatial cognition and perseveration by horses, donkeys and mules in a simple A-not-B detour task.” Animal cognition 16.2 (2013): 301-305.

Proops, Leanne, Faith Burden, and Britta Osthaus. “Mule cognition: a case of hybrid vigour?.” Animal cognition 12.1 (2009): 75-84.


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