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HOW COME? Downside, and upside, of giraffe's neck

How does a giraffe's long neck affect its body? asks reader Johnnie Borders.The tallest basketball players can thank their long legs and torsos for their towering height. But giraffes win the mammal height contest -- by a neck. The tallest male giraffes, from the bottom of their hooves to the top of their heads, measure 20 feet high. And about 6 feet of a giraffe's height is all neck.

A very long neck allows a giraffe to nip at tasty leaves high up in trees, out of the reach of many other plant eaters. The long neck is also startlingly heavy: At up to 600 pounds, a male giraffe's neck and head can weigh as much as three heavyweight boxers. Between meals, a male giraffe wields his weighty neck as a weapon in fights with other males.

How? Swinging their necks in battle, male giraffes -- who can weigh up to 3,500 pounds each -- slam heads, with sometimes bone-breaking results. Researchers say that males with the longest, heaviest necks tend to win the violent mating contests, ensuring their genes are passed down.

But long necks have their downside, too. Our brains control our bodies, and a giraffe's brain is located up to 6 feet away from his beating heart. So a giraffe's heart must pump extra-hard to push blood up to his treehouse brain.

Bend over or hang upside down, and blood rushes to our heads. Stand up too quickly, and as blood leaves the head, we may faint. But when a giraffe dips his head to the ground, then raises his neck 15 feet in the air, he doesn't faint -- even though his heart is much farther from his head.

How come? Scientists note that giraffes' hearts are much heavier than ours, weighing in at up to 26 pounds. Still, for a body tipping the scales at a ton or more, giraffe hearts are relatively small.

But a 2009 study found that the walls of a giraffe's heart are incredibly thick. This allows the heart muscle to contract strongly enough for blood to reach a giraffe's faraway brain. Normal human blood pressure with each heartbeat is about 120 mm/Hg or less. In a giraffe's upper body, the pressure with each beat can reach 260 mm/Hg -- a force that might cause blood vessels to burst in humans.

In addition, when a giraffe's head lifts from ground to treetop, blood vessels channel blood away from the face and into the brain. Special muscles in a giraffe's neck veins also keep blood from draining too quickly to the distant heart. Which is why you're unlikely to encounter a woozy giraffe: Giraffes have evolved to be virtually faint-proof.

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