What is the main difference between a donkey and a mule?

The main difference is that a donkey is a distinct species, while a mule is a hybrid offspring of a male donkey and a female horse, making it sterile.

Which is stronger, a donkey or a mule?

A mule is generally stronger and more enduring than a donkey, inheriting the horse's size and athleticism combined with the donkey's toughness and sure-footedness.

Is a mule more expensive to buy than a donkey?

Yes, a mule typically costs more than a donkey because mules are rarer, require a breeding program, and are often prized for their superior working capabilities.

Is it safer for a beginner to handle a donkey or a mule?

A donkey is often safer for a beginner because mules are highly intelligent and can be more stubborn and cunning, requiring an experienced handler to manage them effectively.

Can a donkey and a mule live together in the same pasture?

Yes, a donkey and a mule can live together peacefully, as they share similar social behaviors and dietary needs, though you should always introduce them gradually to prevent conflict.

What is a common beginner mistake when caring for a donkey or a mule?

A common beginner mistake is overfeeding either animal, as both are prone to obesity and laminitis, so they require a strict diet of mostly forage, not rich grains.

Can a donkey and a mule be used interchangeably for riding?

No, they are not interchangeable for riding because mules are typically larger and bred for riding, while donkeys are often too small and better suited for packing or driving.

What is the primary real-world use case for a mule over a donkey?

The primary real-world use case for a mule is for heavy trail riding and packing in rugged terrain, where its strength, endurance, and sure-footedness outperform a donkey.

Can I switch from owning a donkey to owning a mule without changing my setup?

No, you cannot switch without changes because mules generally require larger stalls, heavier-duty fencing, and more robust feed rations than a standard donkey setup provides.

What is a donkey's most defining characteristic compared to a mule?

A donkey's most defining characteristic is its species purity, with long ears, a braying call, and a straight back, whereas a mule has shorter ears, a whinny, and a horse-like body.

Difference Between

Difference Between Donkey and Mule

Written byNex Virox Team

Editorial Team

Reviewed byVarshal Nirbhavane

Senior SEO & Organic Growth Professional · 5+ years

Published: August 26, 202619 min read

Quick answer

The main difference between Donkey and Mule is that a donkey is a distinct species, while a mule is a hybrid offspring. Donkey is a domesticated member of the horse family with 62 chromosomes, while Mule is the sterile cross between a male donkey and a female horse.

Key takeaways

Core distinction: A donkey is a species, while a mule is a hybrid offspring of a male donkey and female horse.
Parentage and fertility: Mules inherit donkey toughness and horse size, but are almost always sterile due to 63 chromosomes.
Physical traits: Donkeys have short upright manes and small hooves, whereas mules show horse-like stature with donkey ears.
Best-fit use: Choose donkeys for rocky terrain and guarding livestock; pick mules for heavy hauling and long-distance packing.
Common mistake: People confuse hinny with mule; a hinny has a horse father and donkey mother, unlike the mule.

Table of Contents18 sections

Difference Between Donkey and Mule: Comparison Table

Aspect	Donkey	Mule
Definition	A distinct species, Equus asinus, with 62 chromosomes.	A hybrid offspring of a male donkey and female horse, with 63 chromosomes.
Parentage	Born from two donkey parents of the same species.	Always sterile, produced by crossing a jack with a mare.
Core Mechanism	Reproduces naturally within its own species population.	Cannot reproduce due to an odd chromosome count preventing meiosis.
Ears	Long, thick ears with dark stripes and dark tips.	Longer than a horse's but shorter and finer than a donkey's.
Tail	Short, thin tail with a tuft of hair at the end.	Full, flowing tail inherited from the horse mother, covering the dock.
Body Build	Compact, sturdy frame with a straight back and strong legs.	Larger, more muscular frame combining horse size with donkey endurance.
Head Shape	Large, blocky head with a straight profile and wide muzzle.	Refined head resembling a horse but retaining long donkey ears.
Height Range	Typically 7 to 15 hands, varying widely by breed.	Usually 12 to 17 hands, inheriting height from the mare.
Weight Range	Weighs between 180 and 480 kilograms depending on breed.	Weighs between 270 and 540 kilograms, heavier than either parent.
Strength	Can carry up to 20 percent of its own body weight.	Can carry up to 25 percent of its body weight due to hybrid vigour.
Endurance	Can work 8 to 10 hours daily in harsh, hot terrain.	Combines donkey stamina with horse speed for longer work days.
Speed	Walks at 3 to 4 km/h and trots at 8 to 10 km/h.	Can trot at 12 to 15 km/h and gallop at 40 to 50 km/h.
Intelligence	Highly self-preserving, refuses tasks it deems unsafe.	Learns quickly but displays strong self-preservation instincts.
Temperament	Calm and steady, but can appear stubborn when overloaded.	Generally patient and tolerant, though occasionally wilful.
Trainability	Requires consistent, patient handling to build trust.	Responds well to clear commands but tests inexperienced handlers.
Durability	Thrives on sparse forage and resists many tropical diseases.	Hardier than horses but slightly less resistant than pure donkeys.
Hoof Care	Small, tough hooves that rarely need shoes on soft ground.	Strong hooves but require regular trimming like horse stock.
Lifespan	Lives 25 to 30 years, with some reaching 40 years.	Lives 30 to 40 years, often outliving both parent species.
Diet	Efficient digester needing 30 to 50 percent less feed than a horse.	Requires more feed than a donkey but less than a horse of equal size.
Water Needs	Can survive up to 3 days without water in mild conditions.	Needs water daily but tolerates longer intervals than horses.
Maintenance	Requires minimal grooming and simple shelter in dry climates.	Needs regular grooming, hoof care, and standard equine veterinary checks.
Cost	Purchase price ranges from 200 to 2,000 USD depending on breed.	Generally costs 2 to 4 times more than a comparable donkey.
Use Case	Used for light hauling, guarding livestock, and companion animals.	Used for heavy draft work, packing, riding, and agricultural labour.
Speed of Work	Plods steadily, prioritising endurance over rapid task completion.	Completes tasks faster than donkeys while maintaining consistent output.
Load Capacity	Pulls loads up to 2 times its body weight on level ground.	Pulls loads up to 2.5 times its body weight with greater efficiency.
Climate Tolerance	Adapted to arid, hot deserts and rugged mountain terrain.	Handles heat well but is less suited to extreme cold than donkeys.
Braying	Produces a loud, harsh "hee-haw" call lasting up to 10 seconds.	Makes a distinctive bray that sounds like a horse whinny mixed with a donkey call.
Availability	Widely available globally with an estimated 50 million worldwide.	Less common, requiring deliberate breeding of a jack and a mare.
Sterility	Fully fertile and capable of producing offspring throughout life.	Almost always sterile, with rare exceptions reported in mares.
Best Fit	Ideal for low-cost, low-maintenance work in harsh environments.	Best for demanding tasks needing strength, speed, and hybrid vigour.

What Is Donkey?

Donkey is a domesticated hoofed mammal descended from the African wild ass. It works as a pack animal, carrying loads and people across rough terrain. It exists because humans domesticated it over 5,000 years ago for transport and labor.

Definition of Donkey

Donkey is the domesticated subspecies Equus asinus, a member of the horse family Equidae. It has long ears, a short mane, a dark dorsal stripe, and a tufted tail. It is distinguished from horses by its smaller stature, braying call, and exceptional heat tolerance.

Key Characteristics of Donkey

Characteristic	What It Means in Practice
Long ears	Ears rotate 180 degrees to detect predators and dissipate heat in hot climates.
Sure-footedness	It picks safe footing on narrow mountain trails where horses often slip or refuse.
Heat tolerance	It sweats less than horses and functions well in deserts above 40°C.
Strong digestion	It extracts nutrients from low-quality straw and coarse browse that horses cannot use.
Self-preservation instinct	It refuses to overwork or walk into danger, unlike a horse that flees first.
Braying vocalisation	Its loud, rhythmic call carries over 3 kilometres to signal location or distress.
Dorsal stripe	A dark line along the spine is a primitive marking shared with wild ass ancestors.
Compact hooves	Small, upright hooves grip rocky ground and resist cracking on hard surfaces.
Long lifespan	It commonly lives 30 to 40 years, outlasting most horses by a decade or more.
Low water need	It can lose 30% of body water and rehydrate quickly, surviving arid conditions.

Common Examples of Donkey

Mammoth Jack – a large American breed used to sire mules for farm and draft work.
Poitou Donkey – a French breed with shaggy, corded coat used for heavy mule production.
Miniature Mediterranean – a small breed kept as a companion animal and therapy pet.
Andalusian Donkey – a Spanish breed prized for its strength and calm temperament.
Nubian Wild Ass – a wild ancestor subspecies that survives in small numbers in Sudan.
Somali Wild Ass – a critically endangered ancestor with striped legs, found in Eritrea.
Burro – a feral donkey in the American Southwest, protected by federal law.
Standard Donkey – a general-purpose working animal common in rural India and Mexico.
Zamorano-Leonés – a Spanish breed known for gentle handling and agricultural work.
Baudet du Poitou – a rare French breed with a distinctive unkempt coat and large frame.

Advantages and Limitations of Donkey

Advantages	Limitations
It thrives on poor forage that would starve a horse, cutting feed costs sharply.	It is notoriously stubborn and will refuse tasks it deems unsafe or excessive.
It carries up to 30% of its body weight over steep, rocky paths daily.	It cannot match a horse's speed, galloping only briefly at roughly 50 km/h.
It resists many equine diseases and requires fewer veterinary interventions overall.	It is prone to laminitis if fed rich grain or lush pasture, requiring strict diet control.
It lives 30 to 40 years, giving decades of service from a single animal.	Its long lifespan means owners face decades of commitment and end-of-life care.
It stays calm in chaotic environments where horses panic and bolt.	Its strong self-preservation makes it hard to force into a trailer or unfamiliar space.
It needs less water than a horse, working a full day on a single drink.	Its braying is loud and frequent, disturbing neighbours in suburban settings.
It has hard, durable hooves that rarely need metal shoes in dry terrain.	Its hooves grow fast in wet climates, requiring regular trimming to prevent cracks.
It bonds strongly with handlers and can be trained for guarding livestock.	It shows aggression toward unfamiliar dogs and can kick with deadly accuracy.
It breeds reliably and produces hybrids like mules with superior strength.	It has a slower maturation rate than horses, taking up to 4 years to reach full work capacity.
It is cheap to buy and maintain, making it accessible to low-income farmers.	It has poor cold tolerance and suffers frostbite in harsh winter conditions.

What Is Mule?

Mule is a hybrid animal produced by breeding a male donkey with a female horse. It exists to combine the donkey’s endurance and sure-footedness with the horse’s size and strength, creating a working animal prized for hauling and riding in challenging terrain.

Definition of Mule

A mule is the sterile offspring of a jack (male donkey) and a mare (female horse). It possesses 63 chromosomes, an odd number that prevents successful reproduction. Mules exhibit hybrid vigour, showing greater physical toughness, disease resistance, and longevity than either parent species.

Key Characteristics of Mule

Characteristic	What It Means in Practice
Hybrid vigour	Mules show superior strength, stamina, and disease resistance compared to both parent species.
Sure-footedness	Mules navigate rocky, steep, or slippery trails with a careful, deliberate gait that prevents falls.
High intelligence	Mules assess situations before acting, refusing dangerous tasks that a horse might blindly attempt.
Sterility	Mules cannot produce offspring, so every mule must be bred anew from a donkey and horse.
Long lifespan	Mules commonly live 35 to 40 years, outliving horses and donkeys by a significant margin.
Heat tolerance	Mules sweat efficiently and endure extreme desert heat better than most horse breeds.
Sure memory	Mules remember routes, handlers, and past experiences for years, making them reliable on repeat journeys.
Hardy feet	Mule hooves are naturally tough and require less frequent shoeing than horse hooves.
Low maintenance	Mules eat less than horses of equal size and tolerate poorer forage without losing condition.
Strong back	Mules carry heavy loads, up to 20% of their body weight, across long distances without fatigue.

Common Examples of Mule

Missouri Fox Trotter mule – a gaited riding mule bred for smooth, comfortable travel over long distances.
Army pack mule – used by the U.S. military for decades to transport artillery and supplies in mountain warfare.
Grand Canyon mule – carries tourists and supplies down steep canyon trails on daily scheduled trips.
Mammoth jack mule – a very large draft mule standing over 16 hands, used for heavy farm and logging work.
Spanish Jennet mule – a smooth-gaited trail mule popular for endurance riding competitions in North America.
Mule deer hunter's pack mule – carries camp gear and harvested game out of remote, roadless wilderness areas.
Draft mule team – pairs of heavy mules pull wagons, ploughs, and sleds on small farms and Amish operations.
U.S. Border Patrol mule – historically used to patrol rugged desert and mountain sections of the border.
Zambian mining mule – transports ore and equipment in underground tunnels where machinery cannot operate.
French Poitou mule – a massive breed from the Poitou region, historically used for heavy agricultural haulage.

Advantages and Limitations of Mule

Advantages	Limitations
Mules outperform horses on steep, rocky terrain due to superior balance and careful footing.	Mules are often stubborn and will refuse to move if they perceive danger or unfair treatment.
Mules require less feed and water than horses of comparable size, cutting upkeep costs.	Mules are almost always sterile, so they cannot be bred to produce more mules.
Mules resist many equine diseases and parasites better than horses or donkeys.	Mules are highly intelligent but can learn bad habits quickly, making training mistakes costly.
Mules have a longer working life, often staying productive well past age 20.	Mules are not suitable for racing or high-speed work, lacking the horse's top-end speed.
Mules show extreme patience and caution, making them safe for novice handlers.	Mules can be aggressive toward unfamiliar animals, especially other equines, requiring careful management.
Mules tolerate extreme heat and harsh climates better than most horse breeds.	Mules are slower than horses, covering ground at a steady walk rather than a fast canter.
Mules have naturally hard hooves that resist cracking and need minimal shoeing.	Mules can be difficult to breed, requiring a patient and experienced breeder for success.
Mules display remarkable self-preservation, avoiding injuries that would cripple a horse.	Mules are not ideal for jumping or dressage, lacking the horse's natural athletic flexibility.
Mules carry heavy loads efficiently, packing more weight per body size than horses.	Mules have a strong independent streak and may ignore commands they consider unreasonable.
Mules form strong bonds with handlers, showing loyalty and consistent work ethic.	Mules can be unpredictable around dogs or sudden noises, reacting with a defensive kick.

Similarities Between Donkey and Mule

Shared Aspect	How Donkey and Mule Are Alike
Animal Classification	Both donkey and mule are members of the Equidae family, alongside horses and zebras.
Working Purpose	Donkey and mule both serve as pack animals, carrying heavy loads across rough terrain.
Domestication History	Donkey and mule have both been domesticated for thousands of years for human labor.
Dietary Needs	Donkey and mule both eat grass, hay, and grains as their primary food sources.
Water Intake	Donkey and mule both require daily access to fresh, clean drinking water.
Digestive System	Donkey and mule both possess a hindgut fermentation system for breaking down fibrous plants.
Hoof Structure	Donkey and mule both have single, solid hooves that require regular trimming and care.
Social Behavior	Donkey and mule both form strong bonds with other equines and prefer living in groups.
Lifespan Range	Donkey and mule both commonly live between 25 and 30 years with proper care.
Reproductive Sterility	Donkey and mule both share ancestry where hybrids are typically sterile, though donkeys breed normally.
Vocal Communication	Donkey and mule both use braying sounds to communicate with handlers and other animals.
Temperament Stability	Donkey and mule both exhibit calm, steady temperaments when handled consistently and gently.
Intelligence Level	Donkey and mule both display high intelligence and strong problem-solving abilities in daily tasks.
Training Approach	Donkey and mule both respond best to patient, positive reinforcement rather than harsh punishment.
Vaccination Schedule	Donkey and mule both require annual vaccinations against tetanus, influenza, and encephalitis viruses.
Deworming Routine	Donkey and mule both need regular deworming treatments every six to eight weeks.
Dental Care	Donkey and mule both need routine dental exams and floating to prevent sharp tooth edges.
Shelter Requirements	Donkey and mule both need access to a dry, draft-free shelter for protection from weather.
Grooming Needs	Donkey and mule both benefit from regular brushing to maintain healthy coats and skin.
Terrain Adaptability	Donkey and mule both navigate rocky, mountainous, and desert landscapes with sure-footed ease.
Load Capacity	Donkey and mule both can carry roughly twenty percent of their own body weight comfortably.
Speed Range	Donkey and mule both travel at similar walking speeds of about four miles per hour.
Cold Tolerance	Donkey and mule both grow thick winter coats that protect them from cold temperatures.
Heat Tolerance	Donkey and mule both handle hot climates well, though both require shade and water breaks.
Veterinary Costs	Donkey and mule both incur similar annual veterinary expenses for routine checkups and care.
Feeding Costs	Donkey and mule both consume comparable amounts of hay, costing owners similar monthly feed bills.
Common Ailments	Donkey and mule both face risks from colic, laminitis, and parasitic infections throughout their lives.
Working Lifespan	Donkey and mule both remain productive working animals well into their late teens or twenties.
Companion Value	Donkey and mule both provide loyal companionship and form lasting bonds with their human owners.
Historical Utility	Donkey and mule both have powered agriculture and transport in civilizations across continents for centuries.

Donkey or Mule: Which Should You Choose?

The single deciding variable is your terrain and workload. Choose Donkey for rocky, steep, or narrow trails where sure-footedness and low maintenance matter most. Choose Mule when you need greater strength, speed, and endurance for heavier loads on longer, more moderate routes.

When to Use Donkey

Choose Donkey when you need maximum sure-footedness on treacherous, narrow, or rocky terrain where a mule's larger frame cannot safely pass. Donkeys also win when your budget is tight, as they cost less to buy, feed, and house. They excel at guarding livestock, carrying light packs (up to 20% of their weight), and thriving on sparse forage where richer feed is unavailable.

When to Use Mule

Choose Mule when you need 25-30% more carrying capacity and greater speed than a donkey offers on the same route. Mules excel when your work involves long distances, heavy equipment, or agricultural hauling on established trails. They also suit riders who want a horse-like gait with donkey-level hardiness, and they tolerate heat and flies better than horses while demanding less feed than a horse of equal size.

Common Misconceptions About Donkey and Mule

Common Myth	The Reality
A mule is just another name for a donkey.	A mule is a hybrid offspring of a male donkey and a female horse, while a donkey is its own distinct species.
All donkeys and mules are the same size.	Donkeys range from miniature 36-inch standards to 15-hand Mammoth Jacks, while mules inherit varying horse and donkey size traits.
Mules are sterile, so they are useless for breeding.	Most male mules are sterile, but female mules occasionally produce foals when bred to a horse or donkey stallion.
Donkeys are simply stupid horses with long ears.	Donkeys possess exceptional memory, problem-solving skills, and a strong self-preservation instinct that differs from horse behavior.
A mule cannot reproduce under any circumstances.	Rare fertile female mules, called molly mules, have successfully birthed foals, though male mules remain almost always infertile.
Donkeys and mules bray identically to one another.	Donkeys produce a classic "hee-haw" bray, while mules combine donkey brays with horse whinnies into a unique hybrid vocalization.
Mules are more stubborn than donkeys in every situation.	Mules show careful caution and refuse unsafe demands, while donkeys display firm independence; both respond well to patient handling.
You can train a donkey exactly like you train a horse.	Donkeys require slower, reward-based training methods because they think before acting and do not respond to forceful horse techniques.
Mules have 64 chromosomes just like their horse mothers.	Mules carry 63 chromosomes, an odd number from combining a donkey's 62 and a horse's 64, which causes most sterility.
A hinny is the exact same animal as a mule.	A hinny comes from a male horse and female donkey, while a mule comes from a male donkey and female horse, with size differences.
Donkeys cannot be kept as companion animals indoors.	Donkeys adapt well to small farms and backyards, though they need shelter, hoof care, and another donkey companion for mental health.
Mules are always larger than every donkey breed.	Miniature mules from small donkey dams can stand under 40 inches, while Mammoth Jack donkeys exceed 14 hands, proving size varies widely.
Donkeys are wild animals that cannot be domesticated.	Donkeys were domesticated over 5,000 years ago in Africa and now serve as working animals, companions, and livestock guardians worldwide.
Mules lack the endurance that horses naturally possess.	Mules generally surpass horses in endurance, heat tolerance, and sure-footedness, making them preferred pack animals on rugged mountain trails.
Donkeys cannot interbreed with horses at all.	Donkeys and horses interbreed successfully to produce mules and hinnies, though the offspring usually inherit an odd chromosome count.
Mules are always sterile, so breeding them is pointless.	Rare fertile molly mules have produced offspring, and breeders still value mules for work, so sterility does not make them pointless.
Donkeys have no natural defense against predators.	Donkeys kick powerfully, bite aggressively, and bray loudly to alert herds, making them effective guardians for sheep and cattle.
Mules eat far more food than donkeys of equal size.	Mules typically require less feed than horses of similar weight, though donkeys need even less and thrive on lower-quality forage.
Donkeys cannot learn tricks or complex commands.	Donkeys learn tasks quickly when motivated by food or praise, and they remember trained behaviors for years without constant repetition.
A mule's ears are always shorter than a donkey's ears.	Mules inherit long ears from donkey fathers, though their ears are usually slightly shorter and more horse-like than pure donkey ears.
Donkeys are too small to carry adult human riders.	Standard donkeys carry riders up to 200 pounds, while Mammoth Jacks and large mules easily support heavier adults on trails.
Mules cannot survive in hot desert climates.	Mules inherit the donkey's heat tolerance and thrive in arid conditions where many horses struggle, making them ideal desert pack animals.
Donkeys are aggressive and bite humans without provocation.	Donkeys bite mainly when frightened, protecting themselves, or defending territory; well-socialized donkeys are gentle and affectionate with handlers.
Mules have no natural hoof care requirements.	Mules need regular trimming every 6-8 weeks, just like horses and donkeys, to prevent cracks, lameness, and painful overgrowth issues.
Donkeys cannot live with horses in the same pasture.	Donkeys and horses coexist peacefully when introduced gradually, though owners should separate them at feeding time to prevent resource guarding.
Mules are always calm and never spook at sudden noises.	Mules spook less than horses but still startle at loud sounds, requiring consistent desensitization training to build reliable trail confidence.
Donkeys are solitary animals that prefer living alone.	Donkeys are highly social herd animals that bond deeply with companions, and solitary donkeys often develop depression and behavioral problems.
Mules cannot be used for dairy or meat production.	Mules are rarely used for dairy, but some cultures raise them for meat, and their primary value remains labor, transport, and companionship.
Donkeys live shorter lives than horses in captivity.	Donkeys often live 30-50 years with proper care, frequently outliving horses, whose typical lifespan ranges from 25 to 30 years.
Mules are just stubborn horses with donkey ears.	Mules are distinct hybrids with unique intelligence, hybrid vigor, and survival instincts that make them safer and hardier than either parent species.

Conclusion

Difference Between Donkey and Mule comes down to parentage: donkeys are a species, mules are hybrids from a male donkey and female horse. Choose a donkey for predictable temperament and breeding. Choose a mule for superior strength, endurance, and sure-footedness in demanding work.

FAQs on Difference Between Donkey and Mule

What is the main difference between a donkey and a mule?: The main difference is that a donkey is a distinct species, while a mule is a hybrid offspring of a male donkey and a female horse, making it sterile.
Which is stronger, a donkey or a mule?: A mule is generally stronger and more enduring than a donkey, inheriting the horse's size and athleticism combined with the donkey's toughness and sure-footedness.
Is a mule more expensive to buy than a donkey?: Yes, a mule typically costs more than a donkey because mules are rarer, require a breeding program, and are often prized for their superior working capabilities.
Is it safer for a beginner to handle a donkey or a mule?: A donkey is often safer for a beginner because mules are highly intelligent and can be more stubborn and cunning, requiring an experienced handler to manage them effectively.
Can a donkey and a mule live together in the same pasture?: Yes, a donkey and a mule can live together peacefully, as they share similar social behaviors and dietary needs, though you should always introduce them gradually to prevent conflict.
What is a common beginner mistake when caring for a donkey or a mule?: A common beginner mistake is overfeeding either animal, as both are prone to obesity and laminitis, so they require a strict diet of mostly forage, not rich grains.
Can a donkey and a mule be used interchangeably for riding?: No, they are not interchangeable for riding because mules are typically larger and bred for riding, while donkeys are often too small and better suited for packing or driving.
What is the primary real-world use case for a mule over a donkey?: The primary real-world use case for a mule is for heavy trail riding and packing in rugged terrain, where its strength, endurance, and sure-footedness outperform a donkey.
Can I switch from owning a donkey to owning a mule without changing my setup?: No, you cannot switch without changes because mules generally require larger stalls, heavier-duty fencing, and more robust feed rations than a standard donkey setup provides.
What is a donkey's most defining characteristic compared to a mule?: A donkey's most defining characteristic is its species purity, with long ears, a braying call, and a straight back, whereas a mule has shorter ears, a whinny, and a horse-like body.