Top 10 Animals with the Most Unusual Hearts in the World

Like other insects, cockroaches have an open circulatory system, meaning their blood does not flow through blood vessels. Don Moore III, a senior scientist at the Smithsonian National Zoo in Washington, D.C., explains that the cockroach's blood flows through a unique structure with 12 to 13 chambers.

Moore adds, 'Cockroaches and other insects breathe through openings on their body surface rather than lungs, so their blood doesn’t need to transport oxygen.' Instead of blood, the substance is called hemolymph, which contains nutrients and is either white or yellow. The cockroach’s heart doesn’t beat on its own. Muscles in its body expand and contract to help push hemolymph throughout its body.

The heart of a flightless cockroach is usually smaller than that of its flying counterparts, and its heartbeat is similar in pace to that of a human heart.

Among the 4600 species of cockroaches, around 30 live in close proximity to humans. About four species are known to be pests. The earliest cockroach fossils date back to the Carboniferous period, around 354–295 million years ago. However, these fossils differ from modern cockroaches in that they had long egg-laying organs and were the ancestors of both praying mantises and modern cockroaches. Fossils of modern cockroach species with egg-laying organs inside their bodies appeared in the early Cretaceous period.

Want to win the heart of an earthworm? Unfortunately, that’s impossible because they don’t have a heart. Instead, earthworms have five pseudo-hearts around their esophagus. These pseudo-hearts don’t pump blood; rather, they squeeze blood vessels to help circulate blood throughout the body. Earthworms don’t have lungs either; they absorb oxygen through their moist skin. Earthworm blood is red and contains hemoglobin, an oxygen-carrying protein, but unlike humans, they have an open circulatory system.

One fascinating trait of earthworms is their remarkable ability to regenerate parts of their body. If an earthworm is cut in half, the two halves can still survive. However, they will remain as separate parts of the same individual. Earthworms cannot regenerate into two new individuals from a split body.

Earthworms are capable of regenerating lost segments, but this ability varies by species. Moreover, if the injury is too severe, they cannot survive. Contrary to popular belief, if an earthworm is cut in half, it will not grow into two new worms. The front half, which contains the head, can continue to live and regenerate its tail. However, the back half, which contains the tail, cannot grow a new head and will die.

Earthworms thrive in moist environments, often found in fields, farmlands, or anywhere the soil is damp. They are particularly abundant in areas rich in organic matter, where the soil is fertile and humid.

The seahorse is another animal with a fascinating heart. In addition to having one atrium and one ventricle, it also has two unique structures not found in humans: a venous sinus (a sac located in front of the atrium) and a ductus arteriosus (a tube situated just behind the ventricle).

But why do seahorses have such an unusual heart structure? This is because their gills are delicate and could be damaged if the blood pressure were too high. The ductus arteriosus, much like the ventricle, is highly elastic, unlike the muscular nature of the human heart.

Moreover, the seahorse’s heart has the ability to regenerate. If the heart is injured, it can immediately generate a new one to replace the damaged organ. Studies show that seahorses can regenerate up to 20% of the damaged heart tissue in just two months. Their heart consists of a single atrium and ventricle, with two additional structures that are completely different from those found in humans.

The zebrafish, also known as the striped horsefish, is a freshwater fish from the carp family. Native to South Asia, it is a popular aquarium fish, often sold under the trade name zebra danio (and thus commonly referred to as “tropical fish,” although it can be both tropical and subtropical). The zebrafish is a vital model organism in vertebrate biology and is widely used in scientific research, particularly in drug development, especially preclinical studies. It is also known for its regenerative abilities and has been genetically modified to create various transgenic strains.

The zebra danio gets its name from the five horizontal blue stripes along its body, which resemble the stripes of a zebra, extending to the tail fin. Its body is fusiform and compressed laterally, with an upward-facing mouth. The males have a torpedo-like shape with golden stripes between the blue stripes, while females have a larger belly, white coloring, and silver stripes instead of gold. Adult females have a small genital papilla at the base of the anal fin. Zebra danios can grow to 4–5 cm in length, though they typically measure 1.8–3.7 cm in the wild, with variations depending on the habitat. Their lifespan in captivity is about two to three years, although under ideal conditions, it can extend to over five years. In the wild, they are typically annual species.

The myxini, also known as the hagfish, is an unusual marine animal resembling an eel and capable of secreting up to nearly 4 liters of slime. This slime forms a fibrous cocoon around the myxini for protection. The creature’s body is quite strange, with primitive vertebrae and an unusual skull structure.

However, the most remarkable feature of the myxini is that it has four hearts. One heart pumps blood through the body and is known as the brachial heart, while the other three serve supportive functions. The hearts of the myxini are distributed in different parts of its body.

The hagfish is one of the few ancient creatures still alive today. They have existed for over 300 million years. Also known as the “ninja fish,” the scientific name of this parasitic species is *Petromyzon marinus*. Hagfish are most commonly found in the Atlantic Ocean and the Mediterranean Sea.

Hagfish have long, slimy bodies that can grow up to 90 cm. Their skin is smooth and fat, without bones, only white cartilage extending along their body. As for their coloration, they are typically brownish-white or grayish-black on the back and belly. In addition, hagfish are equipped with numerous mucus-secreting glands along their bodies. When they encounter seawater, they can produce liters of mucus in a short time.

The human heart beats about 72 times per minute, but during the same time, a hummingbird's heart beats 1,260 times while in flight. The hummingbird's heart rate can reach up to 1,200 beats per minute, and its breathing rate is 250 breaths per minute.

Moreover, the hummingbird holds many records in the bird world, such as being the fastest flier, the only bird capable of flying backward, and the smallest bird. Here are 19 interesting facts about the hummingbird that you may find fascinating. Hummingbirds are the only birds capable of flying backward because their wings can move freely in the direction of their shoulders. This unique wing structure also allows them to hover in place while keeping their head steady. Hummingbirds are known for their incredibly fast wingbeats, ranging from 70 to 80 times per second. With a body length of around 8 cm and weighing only 2-20 grams, hummingbirds are the smallest birds on Earth.

According to hummingbird experts, these tiny birds have excellent vision, allowing them to spot food from up to 1.3 kilometers away. Hummingbird nests are very small, about the size of a walnut. There are around 400 species of hummingbirds, and they primarily inhabit tropical regions near the equator. Despite having a brain no larger than a grain of rice, hummingbirds have exceptional memory when it comes to locating food. They can remember which flowers they've fed from and how long it takes for a flower to regenerate its nectar.

The male ruby-throated hummingbird measures about 7-9 cm in length, weighs approximately 2.83 grams, and can migrate more than 600 miles (approximately 965 km). Only male ruby-throated hummingbirds have the red throat. When fully grown, males are smaller than females. Female hummingbirds do all the work, from building nests and laying eggs to caring for the young, without help from the male.

The human heart weighs about 0.3 kg, but a giraffe's heart can weigh up to 12 kg. This is because the giraffe needs a strong heart to pump blood through its long neck. The giraffe's heart is massive, weighing around 12 kg. When a giraffe lifts its head, the blood vessels in its head direct almost all of the blood to the brain, bypassing other parts of the head, such as the cheeks, tongue, or skin. At the same time, its thick skin and a peculiar set of muscles in the veins – which are usually muscle-free – help increase the pressure in the veins, enabling them to carry blood back to the heart.

When the giraffe lowers its head to the ground, blood rushes to the head, causing its blood pressure to double. When it raises its head to nibble on tree leaves, the blood is quickly drained back.

This is similar to what happens to us. You might feel lightheaded if you hang upside down, and your face will turn red. If you flip back, you'll feel better. If your blood pressure is too low and not enough blood reaches your brain, you could faint.

With its long neck, a giraffe spends most of its time lifting its head from low to high positions. Therefore, it needs a system that helps blood circulation to the brain so that it doesn't get dizzy. Scientists once believed that the blood vessels in the giraffe's neck helped transport blood from the heart to the brain. However, research by Mitchell has shown that giraffes use a powerful pump to send blood to the brain, and their blood pressure is twice as high as ours.

During winter in the Arctic, temperatures drop drastically, making food scarce. Polar bears tend to lower their heart rate to conserve energy during hibernation. Their sleep is typically shallow. Their heart rate can drop from 70 beats per minute to as low as 8, and their body temperature remains stable. They can wake up instantly when needed. While in their dens, they do not eat and live off their body fat; during this period, they neither eat nor urinate.

As the temperature drops and food becomes harder to find, polar bears begin their hibernation. They have a thick layer of fat, up to 10 cm, that helps keep them warm, even when the temperature reaches -40°C. Polar bears are excellent insulators, and if observed using infrared cameras, only their paws are visible. In unfavorable winter conditions or when pregnant, female polar bears simply retreat into their dens, curl up, and sleep to conserve energy and avoid the cold.

Although they hibernate for long periods, polar bears manage to maintain their strength and muscle mass. After about three months of hibernation, they wake up, still retaining more than three-quarters of their muscle strength without eating or drinking a single drop. In comparison, humans would lose around 90% of their strength if inactive for such a duration.

Many other animals also have to hibernate in winter because they cannot escape, find food, or don’t have thick enough fur to stay warm. These animals, such as marmots or brown bears in the Pyrenees, hibernate for up to six months.

Daniel Mulcahy, a collaborator specializing in amphibians and reptiles at the Smithsonian Institution's National Museum of Natural History in Washington, explains that mammals and birds have a four-chambered heart, but frogs have only three chambers. In most species, the heart's job is to carry deoxygenated blood from the body to the lungs for oxygenation, which is then distributed to the organs. In humans, oxygenated and deoxygenated blood are kept in separate chambers. However, in frogs, structures known as trabeculae help separate oxygenated blood from deoxygenated blood in the same chamber.

Mulcahy also points out that frogs can absorb oxygen not only through their lungs but also through their skin. When deoxygenated blood enters the right atrium, it flows into the ventricle and is then sent to the lungs and skin for oxygen. Even more surprisingly, the hearts of some frog species can freeze. The heart of the wood frog completely stops beating when the frog freezes during hibernation.

The blue whale's heart holds the record for being the largest among living animals today. James Mead, a researcher in the vertebrate zoology department at the Smithsonian National Museum of Natural History, states that the heart is about the size of a small car and weighs around 430 pounds (430 kg). Like other mammals, the whale's heart has four chambers. When a blue whale dives deep into the ocean, its heart rate slows to as low as 4 beats per minute, which helps extend its breath-holding time during dives and can even reduce the risk of decompression sickness.

The blue whale is the largest animal that exists and the heaviest ever known to have lived. Its mouth is large enough to swallow a football team of 11 players, and its heart is the size of a four-seat car. Generally, blue whales in the North Atlantic and Pacific are smaller than those in the waters near Antarctica. However, an average weight of 150-170 tons has been recorded for individuals measuring 27 meters in length. A 30-meter long whale can reach up to 180 tons, according to the National Marine Mammal Laboratory. The largest blue whale identified here was a female weighing 177 tons.

A giant animal means that its body parts are also of colossal proportions, such as the blue whale's tongue, which is estimated to weigh about 3 tons. For comparison, the average weight of an elephant is around 2.7 tons, meaning that the blue whale's tongue weighs more than the entire elephant.

In terms of size, the blue whale even surpasses extinct creatures like dinosaurs. One of the largest dinosaur species from the Mesozoic era, the Argentinosaurus, weighed just 90 tons, comparable to the average blue whale. Meanwhile, the long-necked dinosaur often seen in films and documentaries, Amphicoelias fragillimus, despite being 58 meters long and estimated to weigh 122.4 tons, is still lighter than the blue whale.

Known for possessing three hearts within a single body, soft-bodied creatures like squid and octopuses breathe by pumping oxygen through their bloodstream using two hearts on either side of their body, while the central heart circulates oxygen to the remaining organs. Cephalopods, as these animals are called, also have blue blood, literally, because their blood contains copper-based molecules.

Octopuses are famous for their eight tentacles, a feature reflected in their scientific name, derived from the Greek word ὀκτώπους, meaning 'eight feet.' These tentacles function as a form of hydrostatic muscle, providing them with remarkable flexibility. Unlike most other soft-bodied animals, many octopuses in the suborder Incirrina have fully soft bodies with no internal skeleton. They lack the external shell seen in creatures like snails or the internal shells or bones found in squid and cuttlefish. The only hard part of their body is their beak, which they use to squeeze through narrow crevices when escaping predators. Octopuses in the suborder Cirrina have two fins and an internal shell, which limits their ability to squeeze into small spaces.

Octopuses have relatively short lifespans, with some species living only six months. The giant Pacific octopus can live up to five years under ideal conditions. However, reproduction contributes to the short lifespan: male octopuses often die shortly after mating, and females die shortly after their eggs hatch.

With three hearts, octopuses are unique in their circulatory system. Two hearts pump blood through the gills, while the third heart pumps blood throughout the rest of the body. Their blood contains hemocyanin, a copper-based protein that carries oxygen. While less efficient than the iron-based hemoglobin of vertebrates, hemocyanin is dissolved in plasma rather than in red blood cells, giving octopus blood its characteristic blue color. Water is drawn into their cavities and passes through the gills, which are lined with blood vessels for oxygen exchange.