6 Strangest Hearts in the AnimalKingdomby Laura Geggel, Staff Writer | February 13, 2015 08:47am ET

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

Three-chambered frogs

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

In general, the heart takes deoxygenated blood from the body, sends it tothe lungs to get oxygen, and pumps it through the body to oxygenate theorgans, he said. In humans, the four-chambered heart keeps oxygenated

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blood and deoxygenated blood in separate chambers. But in frogs,grooves called trabeculae keep the oxygenated blood separate from thedeoxygenated blood in its one ventricle.

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

T he 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 (Speabombifrons). "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 thedepartment of vertebrate zoology at the Smithsonian Institution.T he bluewhale's heart is the largest of all the animals living today. Like othermammals, it has four chambers.

T he organ is responsible for supplying blood to an animal the size of twoschool buses, said Nikki Vollmer, a National Oceanic and AtmosphericAdministration (NOAA) and National Research Council postdoctoral fellowat the National Systematics Lab at the Smithsonian.

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

Three hearts for cephalopods

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T here's nothing half-hearted about cephalopods. T hese tentacularmarine creatures, including the octopus, squid and cuttlefish, have threehearts apiece.

T wo brachial hearts on either side of the cephalopod's body oxygenateblood by pumping it through the blood vessels of the gills, and thesystemic heart in the center of the body pumps oxygenated blood fromthe gills through the rest of the organism, said Michael Vecchione,director of the NOAA National Systematics Laboratory at the Smithsonianand a curator of cephalopods at the National Museum of Natural History.

Cephalopods are also literally blue-blooded because they have copper intheir blood. Human blood is red because of the iron in hemoglobin. "Justlike rust is red, the iron in our hemoglobin is red when it's oxygenated,"Vecchione said. But in cephalopods, oxygenated blood turns blue.(T aonius 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 flowsthrough a single structure with 12 to 13 chambers, said Don Moore III, asenior scientist at the Smithsonian's National Zoo.

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T he dorsal sinus, located on the top of the cockroach, helps to sendoxygenated blood to each chamber of the heart. But the heart isn't thereto move around oxygenated blood, Moore said.

"Roaches and other insects breathe through spiracles [surface openings]in the bodies instead of lungs, so the blood doesnt need to carry oxygenfrom one place to another," Moore said.

Instead, the blood, called hemolymph, carries nutrients and is white oryellow, he said. T he heart doesn't beat by itself, either. Muscles in thecavity expand and contract to help the heart send hemolymph to the restof the body.

T he heart is often smaller in wingless cockroaches than in flying ones,Moore said. T he cockroach's heart also beats at about the same rate asa human heart, he added. (Photo Credit: skynetphoto |Shutterstock.com)

False hearts

T he earthworm can't take heart, because it doesn't have one. Instead,the worm has five pseudohearts that wrap around its esophagus. T hesepseudohearts don't pump blood, but rather squeeze vessels to helpcirculate 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 staymoist, dissolves in the skin mucous, and the oxygen is drawn into the cellsand blood system where it is pumped around the body," Moore said.

Earthworms have red blood that contains hemoglobin, the protein thatcarries 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

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

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

However, fish have unique hearts. T hey have one atrium and oneventricle, but they also have two structures that aren't seen in humans.T he "sinus venosus" is a sac that sits before the atrium and the "bulbusarteriosus" 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. T he atrium then pumps the blood into the ventricle.

T he ventricle has thicker, more muscular walls, and pumps the blood intothe bulbus arteriosus. T he bulbus arteriosus regulates the pressure of theblood as it flows through the capillaries surrounding the fishs gills. It is inthe gills where there is oxygen exchange across cell membranes and intothe blood, Moore said.

But why does the fish need the bulbus arteriosus to regulate bloodpressure?

"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."T he bulbous arteriosus itself is apparently a chamber with very elasticcomponents compared to the muscular nature of the ventricle." (Photocredit: Annette Shaff | Shutterstock.com)

Follow Laura Geggel on T witter @LauraGeggel. Follow Live Science@livescience, Facebook & Google+.

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Laura Geg g el on

Laura GeggelAs a staff writer for Live Science, Laura Geggel covers general science, includingthe environment and amazing animals. She has written for the SimonsFoundation, Scholastic, Popular Science and The New York Times. Laura grewup in Seattle and studied English literature and psychology at WashingtonUniversity in St. Louis before completing her graduate degree in science writingat NYU. When not writing, you'll find Laura playing Ultimate Frisbee, biking orbrowsing local green markets. Follow Laura on Google+.

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