Mules have a centuries-old reputation for outworking horses relative to their size, and the short answer is that much of that reputation holds up, though not quite in the way people assume. A mule is not producing dramatically more muscular force per kilogram than a horse in a controlled lab setting. What makes mules pound-for-pound superior work animals is a package of traits: efficient metabolism, tougher hooves, a higher tolerance for sustained effort under load, and a self-preserving intelligence that keeps them from injuring themselves. The distinction between raw strength and functional strength matters here, and the science on each piece is more interesting than a simple yes-or-no.
What “Pound for Pound” Actually Means in Draft Animals
When people ask whether mules are pound-for-pound stronger, they usually mean something like: if you had a 450-kilogram mule and a 450-kilogram horse, which one could pull more weight, carry a heavier pack, or work longer? The answer depends on which of those tasks you’re measuring. Pure pulling force on flat ground is relatively similar between the two animals at comparable body weights. A study modeling the load-pulling capacity of working equids in Brazil found that the maximum load these animals could pull was roughly twice their mean live weight, and that relationship held across both horses and mules.1Journal of Equine Veterinary Science. Equation for Predicting the Load-Pulling Capacity of Traction Equids That study also found mules were far more prevalent among working cart-pulling animals, making up about three-quarters of the population compared to roughly one-fifth for horses. The preference wasn’t because mules could pull proportionally heavier loads in a single effort. It was because mules could do it more reliably, more efficiently, and for longer.
So the pound-for-pound question splits into two real questions. Can a mule generate more peak force than a horse of the same weight? Probably not by much, if at all. Can a mule do more total work per unit of body mass over a day, a week, or a career? Almost certainly yes. That second question is the one that matters to anyone actually working with these animals, and it’s where mules genuinely shine.
Muscle Fiber Differences Between Mules and Their Parent Species
One way to get at the “stronger pound for pound” question is to look at what’s happening inside the muscles themselves. A 2025 study examined the middle gluteal muscle, one of the largest and most important locomotor muscles, in race mules and Mammoth donkeys. The researchers found that mules had a higher proportion of Type 2A muscle fibers than donkeys, with a mean difference of about nine percentage points.2PubMed Central. Preliminary Evaluation of Muscle Fiber Composition in the Middle Gluteal Muscle in Race Mules and Mammoth Donkeys Type 2A fibers are the fast-twitch variety that can sustain repeated contractions without fatiguing as quickly as the pure power fibers (Type 2B). They’re the endurance-speed fibers, the ones that let an animal maintain a strong pace over distance rather than producing one explosive burst.
The same study found no meaningful difference in the cross-sectional area of individual muscle fibers between the two groups. In other words, the mule’s muscle cells weren’t bigger, they were just distributed differently. More of them belonged to the category that resists fatigue. This is a preliminary finding from a small sample, and comparing mules to donkeys rather than to horses limits how directly we can answer the title question. But it does suggest something interesting about the hybrid: the mule inherits a fiber-type profile that’s geared toward sustained work rather than pure sprint power. That fits neatly with what handlers have observed for centuries.
Why Mule Hooves Are Built for Harder Work
Strength isn’t just about muscles. An animal that goes lame on rough terrain is useless regardless of how much force its legs can produce. This is one area where mules have a clear, well-documented structural advantage over horses. Mule hooves are smaller, longer, and narrower than horse hooves, with a more upright wall angle that gives them better grip on uneven and rocky ground.3Large Animal Review. “Iron and fire”: history and advances of mule shoeing The cartilages supporting the back of the hoof are more developed, and the wall is nearly vertical at the toe, quarters, and heels. The sole retains more moisture in its deeper layers, which keeps it from becoming brittle the way horse hooves can in dry conditions.
The practical result is that mules can work barefoot on terrain that would chip and crack a horse’s hooves. In mountain packing, military supply lines through rocky passes, and trail work in arid environments, this hoof design translates directly into more usable workdays per year. A mule that stays sound on the trail is functionally stronger than a horse that pulls up lame after two days, even if the horse could theoretically carry a heavier load on flat, forgiving ground. This is one of the biggest reasons mules historically dominated pack work in the American West and continue to be preferred for backcountry operations by outfitters and the U.S. Forest Service.
Performance at High Altitude
Mountain environments test endurance animals in ways that flatland work doesn’t. Thin air means less oxygen per breath, which amplifies any inefficiency in how the body delivers oxygen to working muscles. Two studies looked at how mules and horses compare when exercised at altitude, and the results are surprisingly close, with a few telling differences in the details.
A study evaluating 14 horses and 16 mules during commercial pack station work at altitudes between 2,700 and 4,000 meters found no overall performance differences between the two species.4Equine Veterinary Journal. Performance at high altitude of horses and mules receiving fat supplemented diets Both groups handled the work. The one difference that did emerge was metabolic: after the hardest exercise period, mules had higher concentrations of free fatty acids in their blood than horses did. That’s a sign the mules were burning more fat for fuel during intense work, which is a more efficient energy strategy for sustained effort because fat provides more energy per gram than carbohydrates.
A separate study examining blood chemistry at 3,800 meters found that heart rates were similar between mules and horses at both rest and during exercise.5PubMed. Haematological and respiratory gas changes in horses and mules exercised at altitude (3800 m) However, mules showed lower packed cell volume (a measure of how concentrated the red blood cells are) during exercise, with a roughly 20 percent difference compared to horses. Horses also showed higher levels of a molecule called 2,3-DPG that helps red blood cells release oxygen to tissues. This might seem like a clear win for horses, but the picture is more complex. The mule’s lower reliance on ramping up red blood cell concentration during hard work may reflect a steadier, less reactive cardiovascular response, one that costs less metabolic energy to maintain. The mules were not performing worse; they were getting the same work done with less dramatic physiological swings.
The Physiological Cost of Carrying Heavy Loads
A 2025 study directly measured what happens inside a mule’s body as pack weight increases. Researchers tracked stress hormones, metabolic byproducts, and markers of tissue strain in mules carrying progressively heavier loads. As weight went up, so did cortisol (the primary stress hormone), blood lactate (a sign that muscles are working beyond their aerobic capacity), and rectal temperature.6PubMed Central. Working like a mule? The physiological toll of heavy loads on mules Enzyme markers associated with muscle effort and mild tissue stress also climbed with heavier loads, while an antioxidant enzyme (glutathione peroxidase) declined, suggesting the body’s oxidative defenses were being outpaced.
None of that is surprising on its own. Any animal carrying a heavier load will show higher stress markers. What’s relevant to the strength question is the baseline: the same paper noted that mules have long been considered superior to horses for certain types of work because of their higher endurance capacity, lower feed requirements, and longer working lifespan.6PubMed Central. Working like a mule? The physiological toll of heavy loads on mules A mule can sustain moderate loads day after day with less food input and fewer breakdowns than a horse of similar size. That efficiency is the core of the “pound for pound stronger” claim. It’s not that the mule’s muscles produce more newtons of force per kilogram. It’s that the whole system, muscles, hooves, metabolism, temperament, extracts more useful work from less fuel and lasts longer doing it.
Feed Efficiency and the Economics of Strength
This is one of the least glamorous but most practical dimensions of the pound-for-pound question. Mules are widely recognized in the working-animal world for thriving on less feed than horses performing comparable work. They inherited this from the donkey side of their genetics. Donkeys evolved in arid, food-scarce environments where metabolic thriftiness was a survival advantage, and mules carry a version of that adaptation.
For anyone whose definition of “stronger” includes “produces the most work per unit of food consumed,” mules win decisively. A horse and a mule doing the same trail work over a week will consume noticeably different quantities of hay and grain, with the mule needing less. In remote packing scenarios, where every pound of feed has to be carried in or foraged along the route, this difference compounds. The mule not only carries the load but costs less to keep fueled while doing it. From a working-animal standpoint, this is the most honest version of “pound for pound stronger”: more output per unit of input, sustained over a longer career.
The Role of Temperament in Usable Strength
Mules are famously stubborn, and that reputation is half wrong in an important way. What looks like stubbornness is usually self-preservation. A horse can be pushed to work past the point of injury; it will run on a fractured leg if the rider insists. A mule will stop. It will refuse to cross a bridge it judges unsound, refuse to carry a load it senses is unbalanced, and refuse to keep working when it’s overheated. This behavior drives some handlers crazy, but it’s a significant practical advantage for longevity and total lifetime work output.
An animal that self-regulates its effort stays sound longer. It avoids the catastrophic tendon injuries, hoof cracks, and metabolic crashes that sideline horses mid-career. Over a 25- to 30-year working life (mules commonly outlast horses by five to ten years), that cautious temperament translates into thousands of additional hours of productive work. The mule that refuses to trot down a rocky slope today is the one still packing supplies at age 28.
Where Horses Genuinely Outperform Mules
It would be misleading to frame mules as universally superior. Horses have advantages that matter in specific contexts. For short bursts of maximum speed, a well-bred horse will outrun a mule of similar size. The horse’s cardiovascular system appears to respond more aggressively to intense exercise, as evidenced by the higher packed cell volume and 2,3-DPG levels during hard work at altitude.5PubMed. Haematological and respiratory gas changes in horses and mules exercised at altitude (3800 m) That more reactive physiology is a disadvantage for sustained work but an advantage for explosive effort.
Horses are also more cooperative in team-pulling situations where coordinated bursts of power matter, such as competitive draft-horse pulling. Their willingness to give maximum effort on command (the same trait that makes them more injury-prone) is an asset in events measured in peak force rather than sustained output. And for riding disciplines that require refined responsiveness to leg and rein cues, like dressage or reining, horses have been selectively bred for generations to cooperate in ways that mules, with their independent streak, typically resist.
Why the Phrase “Hybrid Vigor” Only Tells Part of the Story
People often attribute a mule’s toughness to hybrid vigor, the idea that crossbreeding produces offspring that are healthier and hardier than either parent. There’s some truth to this. Mules do tend to be less prone to certain genetic diseases that affect purebred horses and donkeys. But hybrid vigor alone doesn’t explain the specific performance profile mules exhibit. A mule isn’t just a “better horse” or a “better donkey.” It’s a distinct combination of traits drawn from two species that evolved under very different pressures.
From the horse side, the mule gets size, speed, and a cardiovascular system built for athletic performance. From the donkey side, it gets metabolic efficiency, tough hooves, heat tolerance, and a cautious temperament. The combination produces an animal that isn’t the fastest, isn’t the strongest in a single pull, and isn’t the cheapest to buy, but that outperforms both parent species in sustained, real-world work across difficult terrain and harsh conditions. The mule occupies a niche that neither parent fills alone, and “pound for pound stronger” is the folk shorthand for a much more layered reality.
Mule Racing and the Limits of the Comparison
Mule racing exists, particularly in the American South and parts of South America, and it offers one of the few contexts where mules and horses can be compared in something resembling controlled athletic competition. Race mules are typically faster than donkeys but slower than Thoroughbred horses over the same distance. The muscle-fiber data showing elevated Type 2A proportions in race mules suggests their advantage lies in maintaining speed over middle distances rather than in outright sprint capacity.2PubMed Central. Preliminary Evaluation of Muscle Fiber Composition in the Middle Gluteal Muscle in Race Mules and Mammoth Donkeys
Racing also highlights a limitation in the pound-for-pound framing. Mules vary enormously in size depending on the mare used to produce them. A mule out of a draft mare can weigh over 700 kilograms; one out of a pony mare might weigh 250. Comparing “mules” to “horses” as monolithic categories is almost as imprecise as comparing “dogs” to “wolves.” The most honest answer to whether mules are pound-for-pound stronger is that for tasks involving sustained effort, terrain navigation, and metabolic efficiency, a mule of a given weight will typically outwork a horse of the same weight. For tasks involving peak speed, explosive pulling force, or cooperative high-intensity performance, the horse usually has the edge. The mule’s real superpower isn’t raw strength. It’s the ability to deliver consistent, efficient work over a very long life, on less fuel, with fewer injuries, across worse ground.