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Put your hand on your heart. Can you feel your heartbeat? Like most vertebrates, humans have only one heart, which is usually located closer to the front and middle of the chest, slightly to the left of the sternum. The heart’s main function is to pump blood and distribute oxygen throughout the body. The heart is an important organ and part of the circulatory system, which includes blood and lymph vessels and glands. Cells need oxygen to function, so without a heart, our body’s cells would lose access to oxygen and slowly die. Unfortunately for humans, we only have one heart, and without it our life would quickly come to an end. However, did you know that there are animals that have multiple hearts?
Animals that have multiple hearts have usually developed separate hearts for different functions. Rather than developing one heart to perform all the basic functions of the circulatory system, these animals have developed specialized hearts for individual tasks. In this article, we’ll explore five of these incredible creatures with multiple hearts. As an added bonus, we will discuss some of the animals that almost qualify as having multiple hearts but not for one reason or another. With that said, here are five awesome animals with multiple hearts.
Bonus: animals with “almost” more than one heart
Some animals have developed unique physiologies that almost qualify them as having multiple hearts. For example, horses have additional pumps in their circulatory system known as frogs. The frog is part of the horse’s hoof and looks like a triangle. In addition to acting as a shock absorber and relieving stress on the bones and joints of the horse, it also pumps blood down the horse’s leg upon contact with the ground. Essentially, frogs act like blood-filled pillows that work with the horse’s weight and kinetic energy, pumping blood up his legs and back to his heart.
In most vertebrates, the heart has two, three, or four chambers. The roles of the heart chambers include receiving or pumping blood throughout the body. However, some animals have more complex chambers that can give the impression of having multiple hearts. A case in point is the cockroach, which has an impressive 13 heart chambers. Oxygenated blood flows into each of the tubular chambers through tiny holes called spines. While this doesn’t technically count among animals with multiple hearts, it’s still a marvelous evolutionary adaptation.
Another animal that can technically have multiple hearts is humans. Although most people have only one heart at birth, today there are people with multiple hearts. This usually happens when a person is living with a serious heart condition such as cardiomyopathy. To treat this life-threatening condition, surgeons may perform heterotopic or combination heart transplants. During this procedure, a second heart is transplanted onto the patient’s existing heart to help compensate for the decreased performance of the person’s original heart. While this is not an original heart, it cannot be said that people cannot have two hearts.
Without looking at these bonus animals, let’s now dive into our list of five animals that have multiple hearts.
# 5: octopus
Instead of one or two hearts, octopuses have three hearts. Two hearts perform the same function, and the third arose for a different reason. The systemic heart pumps blood through the body of the octopus, and its two gill hearts pump blood through two gills. Since octopus blood is very rich in copper, it is extremely viscous. As a result, he needs significant pressure to pump blood through his body. To compensate for this, the octopus has developed three separate hearts to relieve stress from its systematic heart and to ensure that it receives sufficient oxygen to its gills. In addition, research shows that hemocyanin in octopus blood is more efficient at transporting oxygen when exposed to low temperatures. This may help explain why larger octopus species tend to live in deeper and colder waters, where they can get more oxygen, given that their hearts have a much harder time pumping warm water.
The systemic heart contains one ventricle and two atria, one on each side of its body. Blood vessels include arteries, capillaries, and veins that carry blood throughout the body. First, blood moves through the aorta and capillary system, then into the vena cava, and then, finally, it is pumped through the gills and returns to the systemic heart. Due to the amount of energy required, the systemic heart shuts down when the octopus swims, leaving only the gill hearts that move oxygen through the gills. This causes octopuses to quickly drain themselves while swimming and helps explain why they spend most of their time crawling or swimming.
# 4: squid
Similar to octopuses, squid have also developed three hearts. His systemic heart pumps blood throughout his body, and his two gill hearts pump oxygen through the gills. All three hearts rest in the cavity of the squid mantle, a bag filled with seawater, which contains the organs that carry out respiration and excretion. Like the octopus, the squid’s heart consists of three chambers, including the lower ventricle and two atria. Squid blood also contains copper-rich hemocyanin, which appears dark blue when the host is in a colder, low oxygen environment. Once blood from the systemic heart returns to the gill hearts, the squid excretes urine, carbon dioxide, and other waste products through holes called nephridial appendages. These holes allow squid to exchange gas and expel waste through the mantle cavity. Basically, the mantle works like a closed circuit that takes care of every part of the basic functions of the squid organs, from breathing to waste management.
# 3: Hagfish
Due to its incredible quirkiness, myxina is one of the first among animals with many hearts. A slimy eel-shaped fish, myxina is the only known surviving animal that has a skull but lacks a spine. Its strange, alien appearance probably contributed to its less flattering name. Plus, it also contains four hearts, one more than an octopus or squid. One heart serves as the systemic heart, responsible for pumping blood throughout the body. The other three work as auxiliary hearts that help the circulatory system pump blood. According to a 2019 report, myxin hearts can beat for 36 hours in the absence of oxygen. This fantastic quality probably explains why myxins have the lowest blood pressure of any known vertebrate. In addition, they also record the highest blood volume-to-body weight in all chordate species, which means that myxins are practically bursting with blood.
# 2: cuttlefish
Unlike most molluscs, cuttlefish have a closed circulatory system. Like squid and octopus, cuttlefish have three hearts. In addition, their hearts work in the same way as their cephalopod cousins. As you already know, the systemic heart pumps blood throughout the body, while the only role of the two gill hearts is to move blood through the gills of the cuttlefish. Due to the high levels of hemocyanin, their blood appears more green-blue than red. In addition, their blood must flow faster in order for them to remain properly oxygenated. This is due to the fact that hemocyanin carries much less oxygen than hemoglobin, so cuttlefish compensate for this deprivation by moving their blood more quickly than is required for most other animals.
# 1: earthworm
The last among animals with multiple hearts are earthworms without actually a heart. Instead, they have a heart-shaped system called the aortic arch. This closed circulatory system carries food, waste, and respiratory gases throughout the earthworm’s body. In total, the aortic arch is made up of five blood vessels. The dorsal vessel is located above the digestive tract and primarily propels blood forward. Meanwhile, the other four vessels, including the ventral, subneural, and two lateroneural vessels, work to carry blood back.
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