Hearts, and the Heartless, in the Animal Kingdom

We all take our hearts for granted: the fascinating organ inside everyone that beats continuously to keep blood pumping through our bodies. Blood flow ensures that oxygen, nutrients from food, hormones, and waste products get to the correct cells. The heart is essential for keeping humans and most animals alive. Hearts are even more interesting when we examine what they do, how they look, how they work, and the similarities and differences in the hearts of species across the planet. Is a giraffe heart similar to a human heart? Which animal survives despite having no heart? Can a heart really beat over 1,500 times a minute? From dinosaurs to insects, humans to dogs, this paper looks at what is really happening on the inside, exploring the world of heart anatomy.


HOW MANY HEARTS DO WE HAVE?
You surely know that humans and gira es have just one heart, as most animals do-but not all. Octopuses and squids (animals called cephalopods) have three hearts. Two hearts pump blood to the gills to CEPHALOPODS Animals without backbones, including squids, octopi, and nautiluses. Cephalopods have symmetrical bodies, prominent heads, and tentacles. take up oxygen, and the other pumps blood around the body ( Figure  ). Worms are also unusual, with five structures called aortic arches acting as basic hearts. The hagfish, sometimes called the slime eel, has one true heart plus three accessory pumps helping the blood to move. Just when you thought you had heard it all, some animals are heartless. Jellyfish, starfish, and even corals manage very well without hearts. Starfish do not even have blood, so this explains why no heart is required. Instead, they use small hair-like structures called cilia to push seawater through their bodies and they extract oxygen from the water.
For Dr. Who fans, the fictional Time Lords have two hearts, but real humans very rarely do. In extremely unusual cases, people with the disease cardiomyopathy have a second heart attached onto their own heart by doctors. The healthy and damaged hearts work together to share the load. Also, twins that are born connected to each other (conjoined twins) can have two hearts naturally.

Figure
The basic structures of animal hearts. Bird and mammal hearts have four chambers (two atria and two ventricles). A frog, which is an amphibian, has a heart with three chambers (one ventricle and two atria), and fish hearts have two chambers (one atrium and one ventricle). An octopus heart system contains three hearts-one main heart (H ) pumping blood to the body and two other hearts (H and H ) pumping blood to the gills. A, atrium; V, ventricle.

Figure Figure
Hearts have adapted di erently to best suit every animal. (A) The gira e has a very large left ventricle so that the heart muscle can pump blood to the body and all the way up the long neck to the head. The gira e's right ventricle is smaller, as it only pumps blood to the lungs. Microscope images of the heart muscle of a chick (B) and a dog (C) are also shown. The chick is young and therefore only has one ventricle and one atrium. Later, the chick heart will become four-chambered, with two atria and two ventricles.

MAMMAL AND BIRD HEARTS
It is not just the number of hearts that can change between species. The basic structure of this vital organ can be extremely di erent from one species to the next. Hearts mostly consist of muscle that contracts and relaxes, causing blood to move through blood vessels to and from the lungs and around the body ( Figure ) . As mammals, we have four di erent (Figure ).

REPTILE AND DINOSAUR HEARTS
Reptile hearts have three chambers, two atria and one ventricle ( Figure  ). The exception is crocodilians, which have four-chambered hearts, just like mammals and birds. However, there is a hole in the crocodile chamber wall, so whether there are three or four heart chambers is up for debate. People often wonder whether dinosaurs evolved from birds or reptiles. Finding a dinosaur heart is very rare because, unlike bone, the heart is a soft tissue, so it is not often preserved. One potentially fossilized heart appeared to show that dinosaurs had four heart chambers, more like birds than reptiles. Sadly, as this specimen was investigated further using more advanced scientific technology, it was found not to be dinosaur tissue, so we still do not know enough about dinosaur hearts to predict which animals dinosaurs evolved from [ ].

AMPHIBIAN HEARTS
Amphibians are an interesting group, as their hearts vary greatly. Living on land and in water, many get oxygen using their lungs, but also take it up via their skin. Most amphibians, including frogs and toads, have three-chambered hearts, with two atria and one ventricle ( Figure  ). However, lungless salamanders do not have a structure called a septum to divide the atrium into two separate parts, so this animal has just one atrium and one ventricle. Some lesser known amphibians seem to have a septum in between their ventricles, so perhaps ancient amphibians had four-chambered hearts, like mammals and birds.

FISH AND INSECT HEARTS
Fish hearts have just two chambers, an atrium and a ventricle ( Figure  ). Insects often have just a tube that pumps hemolymph (the HEMOLYMPH Fluid, similar to blood, that some simple animals have to move nutrients and oxygen around their bodies. name for the insect equivalent of blood) freely around the entire body, with a vessel to help it move. Cockroaches, however, have heart chambers!

HOW BIG IS YOUR HEART?
It goes without saying that heart size varies in di erent animals. After all, a whale could not survive with a mouse-sized heart. An adult human heart weighs about . lbs. If you make a fist, that is roughly the size of your heart. The gira e heart is a hefty lbs, but the blue whale really tips the scales at lbs. The world's smallest heart belongs to the fairyfly. This tiny animal is just . mm long, and a microscope is needed to see its heart.
Hearts are normally about . % of an animal's body mass. Dogs and wolves have relatively large hearts in comparison with their weight, at . %. Meanwhile, the cat heart is just . % of a cat's body weight. Hopefully, relative heart size and the amount of love animals feel are not related. If they were, the world's smallest mammal, the Etruscan shrew, would have a lot of love to give! The mouse-like shrew weighs a tiny g and is around cm long, but its heart makes up . % of its body weight [ ]. Scientists also found that the shrew heart can shrink in cold weather. This shrinkage helps shrews survive harsh climates, by reducing the amount of food it needs. This little mammal eats twice its body weight in food every day, so hibernation is not an option. They rarely even sleep.

SWIFT AND SLUGGISH HEART RATES
The way animals' hearts work varies between species, too. The heart rate, measured in beats per minute (bpm), varies in di erent species.

HEART RATE
The number of times per minute that a heart beats.

kids.frontiersin.org
October | Volume | Article | Generally, larger animals have slower heart rates. A large slug has a VIDEO Early on, when a chick is growing in an egg, its heart can be seen on the outside of its body. Over time, the heart will move inside the body. This video shows a chick's heart beating when it is just a few days old. The heart is still a tube containing two chambers, a ventricle and an atrium, but as it grows it will eventually have four chambers. You can see the blood pumping through the heart chambers.
heart rate of around bpm, while smaller slug species are in the s. Many whales have heart rates of -bpm, gira es are -, and cats are around . In even smaller animals, the number increases: an adult chicken has a heart rate of bpm, a chick is (Video ), and the hamster heart beats away at bpm. The little Etruscan shrew clocks bpm, more than times higher than a person. The highest recorded shrew rate was , bpm, a world record for a warm-blooded animal [ ].
People have heart rates of around -bpm, but children often have slightly higher rates. At month old, -is fairly normal, -for --years-olds, and -by the time a child is years old. A growing baby inside its mother starts at bpm in week , -at weeks old, and around just before it is born. Interestingly, the human heart starts as a tube in the --days-old embryo, rather like a fish, but gradually rotates, develops, and separates into four chambers over the next weeks.

EXERCISE, TEMPERATURE AND HIBERNATION ALL CHANGE HEART RATE
Heart rate changes in most exercising animals. Running gira es can reach bpm and humans can hit , but ideally it should be a little less. A crocodile heart rate at • C is -bpm, at • C it hits -, and once it reaches over • C, the heart can become damaged. Young swifts (birds) lower their heart rates when in the nest, to avoid starvation. Hibernating animals can also lower their heart rates. Grizzly bears normally clock bpm, which lowers to bpm during hibernation; a human heart would usually stop working if it went that low. Emotions, such as fear, love, and excitement, as well as hormone levels, sickness, oxygen levels, and other factors both inside and outside of the body can change the heart rate.

MENDING A BROKEN HEART
In our previous Frontiers for Young Minds paper "Mending a Broken Heart," we looked at fixing failing hearts [ , ]. The zebrafish has a very important ability: it can regenerate (regrow), so if it gets injured or its heart has a problem, it can often repair itself. Human bodies are fantastic, and they continuously try to replace heart cells and repair heart tissue, but the zebrafish is a real professional at mending a broken heart.

CONCLUSIONS
In this article, we looked at some of the similarities and di erences between the hearts of various species. Mammals and birds have kids.frontiersin.org October | Volume | Article | very similar hearts due to evolution, while reptiles, fish, insects, and other animals evolved hearts that are slightly di erently from those of mammals. The environment both inside and outside of each animal helps control the heart's structure and function, but lifestyle choices, such as exercise and food, can have huge impacts on your heart's health. Your friends' hearts are more similar to yours than to a gira e's for example, and even more similar than to a frog or spider heart, but every person's heart is unique. Your heart is constantly adapting to the situations you are in, reacting to food, exercise, emotions, and illness. Unlike a squid, you only have one heart, and it will beat around . billion times in your life, so it is worth keeping it healthy.

ACKNOWLEDGMENTS
The authors would like to thank our young reviewers, Erin and Joshua Rutland. Part of this work was made possible due to funding from the Anatomical Society with a Public Engagement and Outreach grant to Catrin titled Anatomy for ALL-Making Anatomy Accessible. We would also like to thank the British Science Association and University of Nottingham for awarding Catrin with a BSA Media Fellowship .
. Heart and respiratory rates and their significance for convective oxygen transport rates in the smallest mammal, the Etruscan shrew Suncus etruscus. J. Exp. Biol.
. Genomic insights into cardiomyopathies: a comparative cross-species review.