Who Needs Necks?
Necks are good for a lot of things: One of them is getting your head into places your whole body can't go. Sauropods had longer necks than any other animal that ever lived; some stretched almost nine meters (just under 30 feet). Having such a long reach allowed the animal access to a lot of food without needing to move its body too much--a good thing, when you weigh between 18 and 30 tons!
Computer simulation has revealed that Apatosaurus couldn't reach as high as some have thought, but it could reach unexpectedly low--even below ground level. Some experts think the huge plant eaters may sometimes have stood on the edges of lakes and swamps, peaceably grazing on vegetation growing in shallow water. If they did, their long necks were one way to keep huge bodies on higher--and firmer--ground.
Form and Function
From AMNH video © 2005
University of Oregon at Eugene Computer Scientist Kent Stevens.
These vertebrae--which sat about midway down the neck of Apatosaurus--are now very heavy, but before being fossilized they were remarkably light for their size.
Like the bones of modern birds, these vertebrae were thin-walled and hollow. In another similarity to bird bones--and to modern airplane wings--a network of internal struts provided reinforcement.
An Engineering Masterpiece
In a sauropod's neck, each bone fits into the one in front like a boiled egg into an eggcup, or a fist in a catcher's mitt. This ball and socket joint, as it is called, allows the neck to move freely.
In addition, four winglikewinglike facets--two at the front of the bone and two in back--meant the vertebrae could glide smoothly over one another when the dinosaur flexed its neck. They also limited side-to-side motion. When the neck of a living animal is relaxed, each facet is centered over its counterpart on the bone ahead and behind. Researchers think the same was true for sauropods.
© Kent Stevens, University of Oregon
Scientists refer to the neck posture that is easiest and most natural as the neutral position. An animal's neutral position can be a window into its lifestyle.
The sauropods Apatosaurus and Diplodocus had similar neutral positions. Both held their necks angled slightly down from their shoulders, bringing their heads close to ground level. The story is different for some modern plant eaters, such as deer. Their necks are curved up in neutral position; it is the head-down pose that takes the effort. This makes sense, because deer are constantly on the lookout for predators.
Humans are two-legged creatures who stand upright; in the neutral position our necks are almost vertical, or straight up and down. Try touching your chin to your chest--you can probably do it, but you wouldn't want to do it for long! And our neutral position makes a lot of sense. What would happen if you tried to run down the street with your chin on your chest?
Heads Up/Heads Down
Giraffes don't use much energy keeping their heads up. Their neck bones stack naturally in a steeply sloping position, just right for predator surveillance--and for nibbling trees.
For bison, the head-down, grazing position is easiest to hold. Though tiny compared to sauropods, bison are big enough to have few natural enemies, so they don't need to scan their surroundings constantly.