The World’s Weirdest Hearts

From multiple hearts to none at all, animals run the gamut when it comes to the large, powerful organ.

Size Matters

An adult human resting heart rate is normally 60 to 100 beats per minute, while shrews clock in at “over 1,000 beats per minute—that’s over 16 times a second,” Mark Oyama, cardiology professor at the University of Pennsylvania School of Veterinary Medicine, says via email.

The pygmy shrew, which weighs in at less than an ounce, has the fastest heartbeat of any mammal at 1,200 beats per second, according to the National Wildlife Federation.

Animal heart rates tend to correlate to their size, with smaller animals having quicker rates, Oyama explains.

This tiny pygmy shrew has the fastest heartbeat of any mammal—1,200 beats per second. [Photograph by Scott Tilley, Alamy]

Several whale species have heart rates as slow as 10 to 30 beats a minute.

The 100-foot (30-meter) long blue whale, the biggest animal on Earth, not surprisingly has the largest heart of any animal, weighing in at 400 pounds (180 kilograms).

Well, that’s physically. Many people would say the biggest heart in the world belongs to their dog. Dogs do indeed have “slightly higher heart-to-body weight ratios than cats and other animals,” Oyama says. The average weight of a dog heart is about 0.7 to 0.8 percent of their body weight, whereas in cats it's about 0.35 percent.

Lopsided Heart

Giraffe hearts aren’t unusually large for their size, Rachel Brand, an independent behavioral ecologist in Namibia, says via email—but you could say they’re a little one-sided.

A giraffe heart’s left wall ventricle can be up to 3.24 inches (about 8 centimeters) thick, compared with its right ventricle wall, which is about 0.6 inch (1.5 centimeters) thick. Thickness indicates muscle power.

That disparity is because the right side only has to pump blood to the lungs, while the left side has the tough job of pumping blood over 6 feet (about 2 meters) high to reach the giraffe’s head.

Have a Heart … Or Three

When they were handing out hearts, some animals forgot to say when. Cephalopods like squid and octopus usually have three hearts: One systemic heart that pumps blood through the rest of the body after the hearts have pumped it to the gills, where oxygen is taken up.

And then some animals have no heart at all.

A sea star, alas, does not have a heart. [Photography by Paul Nicklen, National Geographic]

Sea stars, aka starfish, and other echinoderms “do not have a ‘heart’ or anything analogous to it,” Chris Mah, a marine invertebrate zoologist at the Smithsonian National Museum of Natural History in Washington, D.C., says via email.

Nor do they have blood to pump with one.

Instead, starfish have millions of tiny, hair like structures called cilia that beat constantly, pumping seawater via “a system of internal pipes and bags,” Mah says.
Their internal cavity also has “all the various cells needed for transporting nutrients, immune cells, and so forth.”

It’s okay if they’re heartless. We heart them anyway.

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