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whose pelvis was less strongly coupled to its spinal column, permitting the first kind of flexibility for tail-driven propulsion in whales. Fossils belonging to relatives of Maiacetus extend over a far greater geographic range than those of previous ur-whales, suggesting that this still-quadrupedal animal was seafaring, though it still returned to the shore to give birth, like sea lions today. In this view, Maiacetus represented the last of the earliest whales; all subsequent whales, in the second phase of the evolutionary chronicle, had no weight-bearing limbs and were totally separated from land.

      The problem with this linear narrative is that we know the final result, which lets us pick and choose the likely path of least resistance toward the whales we recognize today. But evolution doesn’t work like that: it makes no concession for the future; it’s about what’s good enough in the moment. Selection operates on what’s available, sorting biological variation based on the demands of the immediate world. If you were somehow able to return to a late Eocene shoreline in the Tethys sea and happen upon the entire assemblage of early whales in one lineup—all of the early whales, four-legged and odd, scattered on the shoreline—you wouldn’t be able guess the eventual winner of the evolutionary sweepstakes. In its own time and habitat, each early whale was as well adapted as any crocodile, sea lion, or otter living today. It’s just that when we work with the fossil record, we’re afforded a view of the very long run, and the relative successes and failures in any particular group over millions of years. The eventual winners of the evolutionary sweepstakes were early whales that completely severed their ties to land, becoming fully aquatic, eventually yielding descendants that filter feed and echolocate.

      These first whales were merely semiaquatic mammals with specializations for life near the water to one degree or another. There was nothing predetermined about some of their descendants becoming fish-shaped leviathans many millions of years later. Retrospection, however, does cue us into specific features that show incremental transformations: shell-shaped ear bones being repurposed for underwater hearing; or the pelvis becoming unlinked from the backbone, allowing the whole back end to serve as a propulsion device. If you focus on tallying which species go extinct and which ones survive, you might lose sight of the important lessons about major evolutionary change told through bones over geologic time.

      Of all the two-hundred-odd bones in a whale’s body, skulls are probably the most important part to examine if you’re interested in the big picture of whale evolution. Like the skull of any vertebrate animal, whale skulls past and present conveniently house the primary organs for taste, smell, sight, sound, and thought all in one unit. Skulls are thus rich sources of functional information about the lives of whales and their transformation over time—after all, these senses are tweaked, enhanced, or diminished when lineages undergo major ecological transitions, such as the one from land to sea, over the course of evolution. Despite their durability, skulls are challenging objects for study. Their individual bones interlock with one another in complex and hidden ways, with blind corners, overlapping parts, and delicate connections. Soft tissues such as the eyes and brain all rest across several bones, like fruit sitting in a bowl made of interconnected puzzle pieces. To make things even more interesting, whale skulls are not only intricate but big. I’ve stared at whale skulls long enough that they feel familiar to me, but I always have to remember that whale skulls are, in very clear ways, unlike those belonging to any other mammal.

      Take the skull of a bottlenose dolphin, which rests comfortably on a desk but would require two hands to move carefully. It consists of two basic parts: a paddle-shaped beak, formed of elongate bones with rows of teeth like pencil tips; and a cranium of layered bones that cover a bowling ball–shaped braincase. About those teeth: You won’t find the traditional lineup of incisors, canines, premolars, and molars that mammals usually possess. At some point in their evolutionary history, toothed whales gave up chewing for merely seizing their prey with a snap of their jaws and then swallowing it whole. A bottlenose dolphin may flash what looks like a welcoming toothy grin, but I wouldn’t put my hands anywhere near it.

      Moving from the beak to the rest of the skull, the next-most-obvious feature is the orbit, the bone roofing where the eye would be, like a heavy eyebrow, still very much like that of other mammals. But behind the orbits, differences begin to accumulate. First, there’s the aperture that leads to nostrils, or the blowhole. You can peer down the curved passageway formed by these nostril bones to the underside of the skull, where you’ll see an origami construction of delicate, folded bones with paper-thin edges. The bones leading to the blowhole are actually behind where the eyes would be located, the complete opposite of any other mammal, where nostrils are located at the tip of the snout. If your nostrils were positioned like those of a dolphin, you’d blow your nose from the top of your forehead.

      Pakicetus, Maiacetus, and Remingtonocetus had nostrils toward the tip of their snout, and these structures slowly migrated backward in other stages of fossil whales, up to the bottlenose dolphins that we see today, with nostrils displaced well behind the eyes. Interestingly, whales aren’t alone in nostril migration: sea cows and manatees, also full-time aquatic mammals, have nostrils positioned high on the skull (though not behind the eyes), whereas their early fossil relatives have nostrils positioned more forward. This parallel migration of the nostrils lets fully aquatic mammals, such as whales and sea cows, orient their body in a more energy-saving horizontal position in the water, as opposed to doggy-paddling with their nose out of the water. But swimming horizontally is just part of the reason for the strangeness of the bottlenose dolphin skull before us.

      When viewed from the side, the top of a bottlenose dolphin’s skull is shaped like a scoop—if you were a dolphin, imagine having a skull with a dishlike forehead, right above your eyebrows. In life, for a dolphin this cavity contains a cone of fat called the melon, which gives toothed whales a domelike forehead. Tucked behind the melon, and underneath the blowhole, are empty sacks: air sinuses underlain by muscles and sealed with an organ that looks like a pair of lips. When these lips buzz like a trumpeter’s, they generate sound, which bounces around the inside of the head and then gets focused by the melon into a discrete path out of the head, as a high-frequency sound beam, like an acoustic beam emanating from a searchlight strapped to the dolphin’s forehead.

      By coordinating this process across a specialized set of anatomical parts to generate sound, toothed whales create a form of biological sonar—or echolocate—to see their underwater world in sound. All toothed whale species alive today echolocate, whether they are sperm whales, beaked whales, river dolphins, porpoises, or true dolphins. It’s how they wayfind, or hunt, in murky rivers or at ocean depth, sometimes a mile deep, with no light. Echolocation itself has evolved only a handful of times in other vertebrates; toothed whales are the only animals that do it underwater.

      Making high-frequency sound, however, is just one part of echolocation; after sound bounces off an object, it produces an echo that the animal needs to hear. (The classic dolphin chirps and squeaks are how they vocalize to communicate with one another at lower frequencies.) We can’t hear directionally underwater, but whales can because their ear bones float inside hollow pockets. In life, the ear bones of a dolphin hang in a sinus cavity full of spongy tissue, which acoustically isolates each ear, letting the brain detect small differences in the arrival time of sound between the right and left ear, helping to pinpoint the source in three dimensions. Whales have had the ability to hear like this since bthe time of Ambulocetus.

      But how does sound get to the ear bones, especially when whales don’t have external ears? Fats are conductive of sound, and along with the melon, toothed whales have large, fat bodies that fit hand in glove into hollows in the backs of their jaws and then branch out, backward, into lobes that directly connect to the ear bones. Akin to the way our ear canals funnel sound, these fat bodies provide a pathway for sound to reach the ears, although researchers still debate whether there aren’t other acoustic pathways in the head—testing the anatomical

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