Читать книгу The Homing Instinct: Meaning and Mystery in Animal Migration - Bernd Heinrich - Страница 7
ОглавлениеA skull I have lying on my desk is as big around as a somewhat flattened coffee mug, but it comes to a point at the front and has two large eye sockets. The bone on the top of the head is sculpted in ridges and furrows and looks like weathered stone. Seen from the back, the skull has a backward-opening hole on each side. The holes anchored the animal’s powerful jaw muscles. Between them is a much smaller hole. I am looking through this hole (foramen magnum) for the brain cavity, but this large head contains scarcely any brain at all, just an extension of the dorsal nerve cord that runs up from the neck.
It’s the skull of a snapping turtle, a female who met her end as she was crossing the road to dig a hole in the gravel to lay her clutch of eggs. As in previous years, she had come from her bog a mere hundred meters away.
The local snapping turtle is no great wanderer. But two sea turtles — the green turtle, Chelonia mydas, and the leatherback, Dermochelys coriacea — are renowned for migrations spanning entire oceans. The leatherback as an adult feeds in northern cold waters but nests on tropical beaches. It can weigh up to a ton, and although the skull of one that I had the opportunity to examine was as big as a basketball, the cavity holding its brain was, like the snapping turtle’s, only a slight expansion at the end of the vertebral column. It could barely have held a walnut. The green’s could have held two hazelnuts. Their skulls hardly differ from that of any turtle, whether of a species alive now or one that lived 215 million years ago, at a time at least three times more distant than that of the last dinosaurs.
The minute dimensions of some animals’ brains are as astounding as the homing capacity of some of their owners. Like albatrosses, sea turtles of various species lay their eggs in colonies with others of their kind on specific ocean beaches. After the young hatch from the eggs buried in the sand, they head for the water and spend years at sea. They may travel thousands of kilometers, and then, a decade or two later when they are ready to lay their eggs, they return to their birthplace. They mate in the water nearby, and the females then come ashore to dig their nest holes in the sand and to drop in their eggs. How are they able to find their old home after years of wandering in the vastness of the oceans, when we, if taken blindfolded to and then released in unfamiliar woods, would, despite our highly sophisticated massive brains, be as likely to head off in a wrong as a right direction? To get around in unknown territory most of us need a map with which to find at least one known fixed feature that we can both see on the ground and locate on that map, and a compass.
What knowledge and what kind of urges does it take for some birds to fly nonstop for nearly ten thousand kilometers, spending all day and night on the wing, until their body weight halves as they not only burn up all of their body’s food stores but even sacrifice muscle, digestive tract, and other entrails — almost everything except their brains?
