HOW COME? Little hearts beat fast

How come the hearts of small animals beat faster than those of bigger animals? Shouldn't it be just the opposite? asks a reader.Your heart may beat faster when a certain card or box of chocolates is delivered on Valentine's Day. However, if you were a squirrel, your heart would beat faster still when a box of assorted acorns arrived.

The pace of our own human hearts -- about 60 to 80 beats at rest -- seems normal to us, but other creatures skip to different beats. A resting rabbit may have a heart rate of 130, which speeds up to 300 when the bunny darts across the lawn. But the silence between a blue whale's heartbeats may last 20 long seconds.

Hearts beat, of course, to send blood pulsing through the body, from heads to distant toes and tails. With each squeeze of your heart, up to half a cup of freshly oxygenated blood is pushed along through your arteries, nourishing cells as it goes.

Among mammals, from humans to squirrels to whales, heart rate varies according to size: The bigger the mammal, the slower the heart. So an elephant's 26- to 40-pound heart thumps about 30 times a minute. But a resting bat's small heart races along at more than 300 beats a minute.

It seems like a mystery. A larger body requires more blood, so we might expect a big animal's heart to beat faster. And isn't it easier to circulate blood throughout a tiny body?

But it turns out that the mass of a large animal is packed into a proportionately smaller body suit than the mass of a small animal. Which is why, compared to its mass, a mouse's body actually has more surface area than an elephant's. And since animals lose heat through their skin, the body of a smaller animal must work even harder to maintain its internal temperature. The result? Small animals burn calories very quickly; big animals have slower, thriftier metabolisms.

The average elephant tips the scale at more than 12,000 pounds. A typical field mouse weighs an ounce, about as much as a Valentine requiring just one stamp. But thanks to his slower metabolism, an elephant weighing about 200,000 times as much as a mouse will need only 10,000 times the calories.

The result: The faster metabolism of smaller animals requires that their hearts supply oxygen at a higher rate, beating extra-fast to keep up with the demand. Which is why, at about 400 beats a minute, a mouse's tiny heart beats more than 10 times faster than an elephant's. But compared to the heart of a hovering hummingbird, racing up to 1,200 beats a minute, a mouse's heart is a slowpoke.