Hearts are extraordinary. A powerful muscular pump, they keep our blood oxygenated and circulating, help to feed our cells with nutrients, and help to remove waste products (like carbon dioxide) from our bodies.
Across the animal kingdom, there are some hearts that beat more than 1000 times a minute and some that beat just twice a minute. Some species have hearts as big as a piano, while others have hearts so small that they are invisible to the naked eye.
How big are human hearts?
Human hearts are about the size of a fist, and weigh around 300 grams. That’s the weight of a grapefruit or half a pint of milk. As a percentage of total body weight, our hearts are usually around 0.4 – 0.6%, and this percentage is pretty standard across mammals.
Biggest hearts in the animal kingdom
While gigantic animals tend to have gigantic hearts, there are some smaller species that have much bigger hearts than we would expect based on their body size.
Below are 10 animals with the biggest hearts in the world, starting off with a heart the size of a piano…
Blue Whale (Balaenoptera musculus)
The biggest animal to have ever lived, the blue whale is a true giant. It can grow up to 33 metres long. That’s longer than two buses parked end-to-end.
Considering that a blue whale’s heart needs to pump blood around its gigantic body, it is probably no surprise that it has a giant heart – but how big is it, exactly?
The biggest blue whale heart ever weighed (indeed, the biggest heart ever weighed) was 440 pounds. That’s the weight of three adult humans. This heart, which belonged to a whale that washed up in Newfoundland in 2014, was about 5 feet long, 4 feet wide, and 5 feet tall – about the size of a piano.
Even though the blue whale has the biggest absolute heart size, its heart is actually pretty small compared to its body size, at around 0.1 – 0.2% of its total body weight.
Astonishingly, when a Blue Whale dives, its heartrate slows right down to as little as two beats per minute. This is so that the whale doesn’t use its oxygen up too quickly, meaning it can stay underwater for longer.
African bush elephant (Loxodonta africana)
While the blue whale has the biggest heart in the world, the African Bush Elephant comes out on top when we look at land animals.
The African bush elephant’s heart is about the size of a basketball, and weighs around the same as a six-year-old child. It is roughly 0.5% of an elephant’s total body weight – which means it has roughly the same relative heart size as humans.
African bush edlephant hearts beat around 30 times per minute. Strangely, their hearts speed up when they lie down. This is the opposite to what happens in most other animals, and it’s because when an elephant lies down, its lungs become compressed. This limits the amount of oxygen the elephant can breathe in, and so the heart must speed up to compensate.
Spotted hyena (Crocuta crocuta)
Spotted Hyenas are the most numerous large predator in Africa, and they often hunt their prey – such as gazelles, buffalo and even young rhinos – in packs.
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These hunts often involve pursuing prey over long distances and sometimes running as fast as 60 kilometres per hour. For all this high-energy hunting, hyenas need hard-working hearts.
This is thought to be why hyena hearts are double the size we would expect. Instead of the usual 0.4 – 0.6% of body weight, their hearts are 1% of their body weight. They need bigger, more powerful hearts in order to supply their muscles with the oxygen and energy that they need to catch their next meal.
Grey Wolf (Canis lupus)
Grey wolf hearts are also double the size we would expect given their body size. Like spotted hyenas, they are endurance athletes. They often spend 30% of each day travelling, often walk more than 30 miles per day, and can run in bursts of up to 40mph. A big heart is needed to power their bodies through these physical challenges.
Interestingly, captive wolves – who are much less active than their wild counterparts – have smaller hearts. This means that the size of wild wolf hearts is not only a result of genetics, but also the result of regular high-endurance exercise.
We see this effect in human athletes, too. Marathon runners typically have bigger hearts than sedentary people.
Domestic dog
Our pet dogs are descended from wolves, so it is no surprise that they, too, have big hearts. Like wild wolves, their hearts are often around twice the size we would expect given their body size.
For most of our sofa-loving, floor-lounging furry friends, their big hearts probably can’t be explained by regular high-endurance exercise. The reason why their hearts are so big is not well understood, but is likely to be highly influenced by genetics.
Golden-collared Manakin (Manacus vitellinus)
A small bird native to Colombia and Panama, the golden-collared manakin’s courtship has to be seen to be believed. The males do elaborate dances to impress females, darting quickly between trees and making peculiar sounds as they fly. It is one of the most acrobatic courtship displays in the animal kingdom, and it requires a huge amount of energy.
To keep up with the energetic demands of the dance, a male’s heart can beat up to 1300 times per minute, and can weigh up to 1.1% of their body weight.
In general, birds have bigger hearts than mammals of the same size. This is because flight is energetically costly, and therefore bigger hearts are needed to meet the oxygen demands of the body. But even so, a heart that weighs 1.1% of total body weight is pretty impressive, and means the golden-collared manakin has safely secured a position on this list.
Etruscan shrew (Suncus etruscus)
These tiny rodents are the smallest mammals in the world. Found between Europe and Malaysia, they are 4cm long and weigh the same as half a teaspoon of water, or one paperclip. And yet they have one of the biggest hearts, relative to body size, of all animals – their hearts are 1.2% of their body weight.
The reason for these big hearts is that they burn through energy very quickly. This is because they are so small, meaning they have a high surface-area-to-volume ratio, and therefore lose heat very quickly. If they don’t constantly burn energy to keep warm, they risk dying from hypothermia.
To keep up with the high energy demands of their bodies, their hearts must beat quickly and powerfully. Their hearts can beat at an unbelievable 1500 beats per minute.
Rock Ptarmigan (Lagopus muta)
Native to arctic and subarctic regions, rock ptarmigans live in bitterly cold conditions, often at high altitudes. Their large heart – at a whopping 2% of their body weight – is thought to be an adaptation to a few different lifestyle factors.
One factor is temperature. Rock ptarmigans must contend with temperatures as low as -40°C. It is energetically expensive to keep warm at this temperature, and therefore the heart needs to work hard to pump oxygen around the body.
Another factor is altitude. Rock ptarmigans often live at high altitude, where oxygen levels are lower. This means their hearts need to work harder than low-altitude animals to pump the same amount of oxygen around their bodies.
Small Big-eared Brown Bat (Histiotus montanus)
A little bat with a big heart, the Small Big-eared Brown bat – native to South America – has an exceptionally large heart, at 2.2% of its body weight. This means it has one of the highest relative heart sizes of any animal. But why?
Researchers think it is because of the high energetic demands of flight – but there is a twist.
While all bats must deal with the energy costs of flying, small big-eared brown bats have an extra cost to contend with. They have huge ears, which evolved to help them hunt insects without using sonar (a noise some insects have learned to avoid), but these huge ears create a lot of drag during flight. This means flying is extra energetically demanding for this species.
So, surprisingly, Small Big-eared Brown bats might have evolved big hearts as a result of their big ears!
Ruby-throated Hummingbird (Archilochus colubris)
The Ruby-throated Hummingbird, native to Central and North America, is a tiny bird that weighs the same as a single grape. Yet it has the biggest relative heart size of any animal, at 2.5% of its body weight.
Ruby-throated hummingbirds beat their wings around 60 times per second during normal flight, but their wings can beat up to 200 times per second. Beating their wings so quickly requires a huge amount of energy (about ten times more than marathon-runners use while exercising at their limit), and therefore their hearts have evolved to be gigantic – relative to body size, of course. These huge hearts can beat astonishingly fast, at over 1200 times per minute.
If humans had hearts the same size – in proportion to body size – as Ruby-throated Hummingbird hearts, our hearts would be the size of a toaster instead of the size of a fist!
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