Hearts have become iconic symbols of Valentine's Day, but when it comes to hearts in the real world, one size doesn't fit all — particularly in the animal kingdom. The human heart beats about 72 times a minute, but in that same time, a hibernating groundhog's heart beats just five times and a hummingbird's heart reaches 1,260 beats per minute during flight. The human heart weighs about 0.6 pounds (0.3 kilograms), but a giraffe's weighs about 26 pounds (12 kg), since the organ needs to be powerful enough to pump blood up the animal's long neck. Here are some other creatures with strange hearts. 

Three-chambered frogs

Mammals and birds have four-chambered hearts, but frogs have just three, with two atria and one ventricle, said Daniel Mulcahy, a research collaborator of vertebrate zoology who specializes in amphibians and reptiles at the Smithsonian Institution in Washington, D.C.

In general, the heart takes deoxygenated blood from the body, sends it to the lungs to get oxygen, and pumps it through the body to oxygenate the organs, he said. In humans, the four-chambered heart keeps oxygenated blood and deoxygenated blood in separate chambers. But in frogs, grooves called trabeculae keep the oxygenated blood separate from the deoxygenated blood in its one ventricle.

Frogs can get oxygen not only from their lungs, but also from their skin, Mulcahy said. The frog's heart takes advantage of this evolutionary quirk. As deoxygenated blood comes into the right atrium, it goes into the ventricle and out to the lungs and skin to get oxygen.

The oxygenated blood comes back to the heart through the left atrium, then into the ventricle and out to the major organs, Mulcahy said.

Mulcahy snapped this photo of a plains spadefoot toad (Spea bombifrons).  "We have a saying," he said, that "not all frogs are toads, but all toads are frogs." (Photo credit: Daniel Mulcahy)


A whale of a heart

"It is the size of a small car and has been weighed at about 950 pounds [430 kg]," said James Mead, a curator emeritus of marine mammals in the department of vertebrate zoology at the Smithsonian Institution.The blue whale's heart is the largest of all the animals living today. Like other mammals, it has four chambers.

The organ is responsible for supplying blood to an animal the size of two school buses, said Nikki Vollmer, a National Oceanic and Atmospheric Administration (NOAA) and National Research Council postdoctoral fellow at the National Systematics Lab at the Smithsonian.

"The walls of the aorta, the main artery, can be as thick as an iPhone 6 Plus is long," Vollmer told Live Science. "That is a thick-walled blood vessel!" (Photo credit: © AMNH | D. Finnin) 


Three hearts for cephalopods

There's nothing half-hearted about cephalopods. These tentacular marine creatures, including the octopus, squid and cuttlefish, have three hearts apiece.

Two brachial hearts on either side of the cephalopod's body oxygenate blood by pumping it through the blood vessels of the gills, and the systemic heart in the center of the body pumps oxygenated blood from the gills through the rest of the organism, said Michael Vecchione, director of the NOAA National Systematics Laboratory at the Smithsonian and a curator of cephalopods at the National Museum of Natural History.

Cephalopods are also literally blue-blooded because they have copper in their blood. Human blood is red because of the iron in hemoglobin. "Just like rust is red, the iron in our hemoglobin is red when it's oxygenated," Vecchione said. But in cephalopods, oxygenated blood turns blue. (Taonius borealis squid, Photo credit: Michael Vecchione)


La cucaracha

Like other insects, the cockroach has an open circulatory system, meaning its blood doesn't fill blood vessels. Instead, the blood flows through a single structure with 12 to 13 chambers, said Don Moore III, a senior scientist at the Smithsonian's National Zoo.

The dorsal sinus, located on the top of the cockroach, helps to send oxygenated blood to each chamber of the heart. But the heart isn't there to move around oxygenated blood, Moore said.

"Roaches and other insects breathe through spiracles [surface openings] in the bodies instead of lungs, so the blood doesn’t need to carry oxygen from one place to another," Moore said.

Instead, the blood, called hemolymph, carries nutrients and is white or yellow, he said. The heart doesn't beat by itself, either. Muscles in the cavity expand and contract to help the heart send hemolymph to the rest of the body.

The heart is often smaller in wingless cockroaches than in flying ones, Moore said. The cockroach's heart also beats at about the same rate as a human heart, he added. (Photo Credit: skynetphoto | Shutterstock.com) 


False hearts 

The earthworm can't take heart, because it doesn't have one. Instead, the worm has five pseudohearts that wrap around its esophagus. These pseudohearts don't pump blood, but rather squeeze vessels to help circulate blood throughout the worm's body, Moore said.

It also doesn't have lungs, but absorbs oxygen through its moist skin.

"Air trapped in the soil, or aboveground after a rain when worms can stay moist, dissolves in the skin mucous, and the oxygen is drawn into the cells and blood system where it is pumped around the body," Moore said.

Earthworms have red blood that contains hemoglobin, the protein that carries oxygen, but unlike people they have an open circulatory system. "So the hemoglobin just kind of floats among the rest of the fluids," Moore said. (Photo credit: alexsvirid | Shutterstock.com)


Underwater hearts

If a zebrafish has a broken heart, it can simply regrow one.  A study published in 2002 in the journal Science found that zebrafish can fully regenerate heart muscle just two months after 20 percent of their heart muscle is damaged.

Humans can regenerate their liver, and amphibians and some lizards can regenerate their tails, but the zebrafish's regenerative abilities make it a prime model to study heart growth, Moore said. 

However, fish have unique hearts. They have one atrium and one ventricle, but they also have two structures that aren't seen in humans. The "sinus venosus" is a sac that sits before the atrium and the "bulbus arteriosus" is a tube located just after the ventricle.

As in other animals, the heart drives blood throughout the body. Deoxygenated blood enters the sinus venosus and flows into the atrium, Moore said. The atrium then pumps the blood into the ventricle.

The ventricle has thicker, more muscular walls, and pumps the blood into the bulbus arteriosus. The bulbus arteriosus regulates the pressure of the blood as it flows through the capillaries surrounding the fish’s gills. It is in the gills where there is oxygen exchange across cell membranes and into the blood, Moore said.

But why does the fish need the bulbus arteriosus to regulate blood pressure?

"Because the gills are delicate and thin-walled — any fisherman knows this — and can be damaged if the blood pressure is too high," Moore said. "The bulbous arteriosus itself is apparently a chamber with very elastic components compared to the muscular nature of the ventricle." (Photo credit: Annette Shaff | Shutterstock.com)

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