Skulls engineered to take hard knocks

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The braincase of a skull may well be, as advertised, a strongly built and cleverly engineered structure, but listening to all that incessant banging coming from the direction of the crab apple tree in the garden, one has to wonder: Is it really strong enough to keep a woodpecker from having the most terrible headache?

And what about those rams and deer you see butting heads with such determined ferocity during springtime? The echoing, crashing sounds, the visions of extreme and repeated violence, the frequent tangling of horns -- how do animals with an instinctive need for such brutish behavior prevent their brains from turning into rice pudding?

Some answers are to be found in the upstairs bedroom of a decidedly unremarkable suburban house on the outskirts of the city of Coventry, in the English Midlands. This is where a collector named Alan Dudley has spent the last four decades bent to a curious obsession. By day he works selecting veneers for gluing to the dashboards of expensive automobiles. After work, he collects skulls.

He now has thousands, from the great hulk of a hippopotamus skull to the tiniest and most delicate tissue-like skull of a wren, and his collection makes up one of the finest and most comprehensive known. Included in it are many skulls of creatures that do strangely violent things to their heads -- the ram and the woodpecker among them.

Both creatures happen to have very dense skulls, especially in that rounded rear area known as the braincase, where they are built like armored cars. Crucially, their braincases are also unusually smooth inside.

The brains of most animals that are prone to head banging -- these include deer and other antlered mammals, as well as various birds -- are relatively small and (unlike a human's) smooth-surfaced; and they're bathed in only small amounts of cerebrospinal fluid, leaving little room for the brain to move and be shocked by the sudden decelerations and accelerations of their weaponized heads.

Moreover, both rams and woodpeckers are scrupulous in the precise, single-direction fashion in which they smash their heads into things, whether trees or one another: The aim is such that there's very little side-to-side torsion exerted on the brain, none of the movement that induces whiplash injury and other kinds of damage.

Gannets have solved a similar problem. These magnificent black-and-white seabirds, with wingspans of as much as 6 feet, catch fish by spectacular dives into the ocean. Starting from heights of 100 feet or more, they enter the water at 60 mph and hurtle downward far beneath the surface, pursuing their chosen fish underwater, like penguins, using their wings to swim.

It's an awesome performance -- not least because they are so successful as hunters: They are eagle-eyed (if the avian metaphor may be allowed) and they have, unusually for birds, true binocular vision, which helps them lock on target. If lucky, they will consume the fish while still underwater, only eventually bobbing back to the surface to take off, something they do very clumsily, and resume high-altitude patrol.

However, their fishing success is one thing. Their survival is quite another. To dive into water from 100 feet may not be lethal for a gannet, but it would, or should, get a fearful migraine. Yet that doesn't seem to happen. Gannets manage to bob to the surface with all their mental faculties intact, their brains entirely unhurt.

And how? Skull modifications, just as with the ram and the woodpecker. In this case, to mitigate the brain-shattering trauma of a 60-mph collision with a wall of water, air sacs built into the gannet's face act as cushion; its extremely long and narrow beak helps the bird enter the water with only a very stealthy kind of impact; and it has no nostrils that would allow water to gush inward and do serious damage to the delicate tissues inside. A gannet's skull is built like the nose of a Concorde: strong, delicate, unpierced and able to tilt downward on landing but hold straight ahead when passing at great speed through the water.

Modifications of the skull are many, all produced by evolution to give each animal maximum advantage in adapting to its environment and its lifestyle niche. Some skulls are narrow and delicate -- the gazelle's, say -- while others, like that of the lion, are squat and fat and powerful: The owner of one grazes and has the grace to prance away; the owner of the other stays put and waits for chance to bring him the opportunity to crush and to kill.

Invariably the melding of form and function is displayed perfectly in an animal's skull -- so that one can quite easily deduce the manner in which the animal behaves, or the environment in which it lives, by examining, or even by casually glancing at, the skull it leaves behind.

Compare the massive jaws of a wolf, for instance, with the more modest arrangements of the mouth of a hare; or look at the great ridges along the uppermost part of the skulls of some species -- a mountain lion, for example, which has a sail-like arrangement known as a sagittal crest, to which the jaw muscles are anchored to give it even greater crushing power.

If you come across an otherwise modest animal with a large sagittal crest -- a coati-mundi, for instance -- you'd do well to avoid it, or at least to keep it happy: When a coati fights, it bites, and when it bites, the muscles attached to its sagittal crest allow it to come down hard with its canine teeth, the ones that really hurt and do damage. In less threatening territory, the eye orbits on certain skulls can be spectacular -- the immense orbits of a tarsier, for example, are often as large as the entire rest of the skull; they provide a classic example of a skull detail that suggests how well or ill an animal can use a certain sense, in this case, its vision.

Similarly, structures called the auditory bullae can show, in bony form, just how well an animal can hear. Springhares and rabbits have very large auditory bullae: It's said that the chamber enclosed by the bullae resonates perfectly to the whooshing sound of a downward-swooping owl, alerting the rabbit to dive for its burrow and live to enjoy another day.

Until its skull was examined, no one had any idea of how a rabbit was able to do such a thing: The owl flew down, the rabbit vanished -- such magic long seemed as mysterious as the butting of rams and the vacant stares that follow.

science


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