8. The Age of Giants, Spying on Whales

8. THE AGE OF GIANTS

Take in a deep breath. And now relax. You just shared oxygen with the largest animal—ever, in the history of life on Earth. In a single blow, this animal expels a column of water vapor that would reach the top of a two-story house; the air that passed through its lungs is enough to fill half a cement-mixer truck. Its blood cells pass through a heart with vessels the diameter of dinner plates and run through a body with over twenty billion miles of arteries, capillaries, and veins. Blood must carry oxygen to every cell in this animal—all 1,000 trillion of them—including nerves whose fibers reach over one hundred feet from its brain stem to every extremity, including its tail fluke. With a flip of its fluke, the animal descends to a depth beyond the limits of light, where it makes the most acoustically powerful sound made by any organism, a low tone that spreads for over nine hundred miles, echoing off undersea canyons. Everything about this animal, in all the ways that we can measure, is superlative. This animal is, of course, a blue whale.

Extremes enthrall us all, and the idea of an ultimate size champion among living things is a captivating thought. Ask an elementary-school kid about the biggest animal of all time, and the short list of possible contenders very quickly becomes a battle between two titans: a blue whale versus a sauropod dinosaur. This comparison is enshrined in textbooks, usually with a stylized composition of a blue whale, midgulp, floating above a sauropod, sloping neck and tail extending in opposite directions. Sometimes you might see a string of African elephants or school buses for scale.

The winner of this runoff, of course, depends a bit on how you measure the contestants. The longest of the sauropod dinosaurs probably exceeded 110 feet in total length, based on relatively complete skeletons. That linear distance comes close to the longest blue whale ever measured—a 109-foot female from the Southern Ocean, killed in 1926 by Norwegian whalers. But whales are the true heavyweights. At most, the largest sauropods might have weighed 120 tons, but the best estimates place the largest ones closer to 70 tons. By comparison, the heaviest blue whale reliably measured (a female, also from the Southern Ocean) weighed 136.4 tons, or just over 300,000 pounds—more than the takeoff weight of a Boeing 757. And this particular whale was only an eighty-nine-foot-long female. A blue whale closer to one hundred feet, especially if pregnant, would have weighed much more. From the standpoint of biomass, it’s not much of a contest: blue whales are the most massive animals ever in the history of life, and we just happen to live alongside them.

For us, a whale’s size may be the most striking thing about them, but the fascination is not just about sheer bulk. How can a creature of that magnitude keep itself alive? Size determines a lot about how much air a whale needs, how deep it can dive, how much food it needs to eat, and how far it can swim. But step back for a moment from these physiological questions: how did whales become giants in the first place? We know the beginning and end points: within the fifty million years that took whales from land ancestry to dominance in the oceans, the weight difference between Pakicetus and a blue whale increased about ten thousand times. How exactly did this change unfold? And how can we know?

Those questions loomed over me for many years. As a graduate student, I began to chip away at them, starting with the question of whether the roles extinct whales played in past ecosystems were similar to the niches they filled today, or if those ancient roles had since become obsolete. As a student, I had already read enough about walrus whales, for instance, to know that there were some cast members of past whale worlds that lacked any descendants.

I also spent a lot of time measuring whale skulls, keeping a log of the same set of measurements for every specimen I encountered. I knew the more data points I had, the more useful the information would be in helping me (and others) predict the total length of any fossil whale when only the skull was available. (As mentioned previously, whole skeletons, nose to tail, are essentially never recovered, Cerro Ballena notwithstanding.) I came to realize something odd about the fossil record of both baleen whales and toothed whales as well: none of them were as big as even their middling descendants today.

I hadn’t deeply probed this surmise with analytical tools. At the time, I was more focused on the challenge of reconstructing a single whale’s body size given scanty inputs. But I knew there was something to my observation. At Wadi Al-Hitan in Egypt, perhaps the best snapshot into the life of whales during the late Eocene, Basilosaurus was the whale titan; yet it was only as long as today’s humpback whales (a midsize baleen whale at about fifty feet long) and weighed probably five to six times less with its comparatively much smaller head, short chest cavity, and long tail. At Sharktooth Hill in California, the largest whales were even shorter, no more than thirty or so feet long. While those Miocene whales had modern proportions, none were nearly as large as a modern humpback. Even for all of the millions of bone shards in the Sharktooth Hill bonebed, not a single one represented a fossil whale species that remotely approached the gigantic sizes of today’s blue whales.

I didn’t know what to make of the broad observation and repeatedly pushed the issue back under other more immediate layers of inquiry. But the question always resurfaced. Years later, at the Smithsonian, I came across a postdoctoral research fellow who could finally help me get to the answer. Now a professor, Graham Slater specializes in analyzing evolutionary trends in the fossil record. Using my working file of skull measurements, we charted the sizes of all living and fossil species across their family tree; we focused particularly on baleen whales (excluding toothed whales, which feed at a variety of different water depths, by echolocating on prey). We finally had an evolutionary map for how size evolved in whales over the past thirty or so million years.

Our analysis revealed that very large body size in baleen whales had evolved several times across different lineages. Extreme gigantism in whales—body lengths of more than sixty feet or weights over 200,000 pounds—appeared on different parts of the tree in species unrelated to one another. Blue whales and fin whales, the longest and second-longest whales today, for example, are actually not very closely related to each other. And neither of these whales is particularly closely related to right and bowhead whales, which can, at their maximum sizes, also tip the scales at one hundred tons. When a notable feature arises several times on distantly related branches of an evolutionary tree, it’s a sure sign that something interesting is going on.

Our evolutionary tree mapping also confirmed another fact I knew casually: the world today is devoid of very small baleen whales, compared with other times in the geologic past. Many of the fossil baleen whales that Remington Kellogg described in detail, including those from Sharktooth Hill, were very small. In some cases, their skulls were small enough to cradle in your arms. The total length of these filter-feeding whales would have been about the size of a car—similar in size or smaller than today’s enigmatic pygmy right whale, the smallest baleen whale. In the past fifteen million years or so, whales at this size seemed relatively common in the fossil record; our analysis showed that they went extinct very recently, some species only a few million years ago.

Past whale worlds were very different, based on the story of size alone. At this evolutionary moment, we live in an age of giants, sharing our planet with the largest whales ever, some of which are the largest animals to ever exist, period, in the history of life. There’s nothing the size of today’s blue whales, fin whales, or any of the skim-feeding whales in the fossil record. The immediate questions then become these ones: What drove the history of these changes in the fossil record of baleen whales? Does it have more to do with the whales themselves or the worlds they lived in? And what’s preventing them from getting even bigger?

You might expect baleen to hold some answers. Baleen whales are, after all, the largest of all whales, and they have a unique anatomical apparatus that makes them stand apart from all other mammals, alive or extinct. Look inside the mouth of a whale, and you’ll know immediately which of the two major living groups it belongs to: if it has rows and rows of plasticlike plates hanging from the roof of its mouth, it’s a baleen whale. If it doesn’t, it’s a toothed whale. Toothed whales all echolocate, and despite their name, a few species don’t actually have teeth, as they hardly need to chew or break up their food after seizing or sucking it into their mouths.

Baleen is a soft, pliable structure made up of keratin, just like fingernails, hooves, and hair. It grows from the roof of the mouth, soon after a whale’s birth, emerging in a series of plates, numbering in the hundreds, which form a racklike structure. These triangular plates are ensconced in a bed of flesh, are fed by blood vessels and nerves, and grow in layers the way our own nails and hair do. Baleen develops frayed edges as its tubules are worn down from the friction of feeding; these tubules spring outward, like uncooked spaghetti, interdigitating with tubules from adjacent baleen plates to form a mesh facing the inside of the mouth. When baleen whales close their mouths around a gulp of water laden with krill, fish, or other zooplankton, the racks of baleen create a filter that traps prey inside.

So how long has baleen been around, and does its appearance relate to whale gigantism? It turns out that the first baleen whales didn’t have baleen, meaning, yes, they once had teeth. There are three major pieces of evidence that underlie this point. First, modern baleen whale fetuses have tooth buds, primordial bits of tooth enamel and dentin resorbed by the body prior to birth. Baleen itself grows from ridges on the roof of the mouth during the first year of life, but in the womb the genetic machinery for creating teeth is still in place, a vestige of baleen whales’ deep past as much as tiny hind limbs in Basilosaurus.

Second, baleen whales share common ancestry with toothed whales; these two branches that comprise the surviving houses of the whale family tree diverged around 35 million years ago, during the last days of Basilosaurus. The first baleen whales hardly looked anything like a blue whale; they looked much more like an extremely shortened version of Basilosaurus, with tiny versions of jagged, saw-blade teeth lining their mouth. They weren’t eel-like in body proportions but built more along the lines of a bottlenose dolphin. But we know that these first, toothed members of the lineage that eventually led to filter-feeding whales are more closely related to blue whales than they are to any dolphin, because they share key traits located in their unique, pebblelike ear bones.

Third, although baleen is a soft-tissue structure, it does occasionally show up in the fossil record. Its preservation may have to do with a unique chemical environment on the seafloor. Fossil baleen has been reported from rocks in California and Peru, although no more than about fifteen million years in age—about half as old as the oldest baleen whales.

One important caveat is how we define whale groups when talking about baleen whales: because the earliest baleen whales didn’t have baleen, describing these extinct forerunners using common words is challenging. Scientific nomenclature helps—“mysticetes” instead of “baleen whales,” “odontocetes” instead of “toothed whales.” At some point on the separate branches leading to today’s baleen whales and toothed whales, filter feeding and echolocation evolved, respectively. But we don’t know exactly when, how, or how many distinct times. Consider birds and dinosaurs: when scientists discovered dinosaurs with feathers, it became clear that feathers do not necessarily make a bird. Instead, we need to define organisms as lineages separate from traits that diagnose their identity.

Some scientists have proposed that early baleen whales had a combination of teeth and primordial baleen—teeth outboard, baleen inboard—but fossilized primordial baleen hasn’t been found in any of the species where such a scenario might be expected. Other scientists have proposed that the first baleen whales evolved a half-and-half solution, but oriented not outside-in as in the other proposal, but front to back: front teeth still in place but lacking them in the back half of their mouth, instead bearing a primitive ridge of baleen. Still another possibility is an intermediate state with neither teeth nor baleen—simply a toothless, gulping early baleen whale. Most of these scenarios involve interpreting fragmentary fossils that don’t preserve the critical soft-tissue structures; no one has yet, for instance, demonstrated how a whale with both outboard teeth and inboard baleen would actually be able to feed. Absent more fossils with clear evidence for what the first baleen plates looked like, all that we can do with these ideas is to test them using biomechanical or computer models. Until scientists figure out a way to record video inside the mouths of today’s filter-feeding whales—a logistical hurdle that may be impossible to surmount—we need to shore up our knowledge about how filter feeding works in living baleen whales before jumping to any conclusions about how it may have worked in their ancient fossil relatives.

At the Smithsonian I can place all of the skulls and jawbones from the first baleen whales from one geologic epoch on a single long table. The skulls of many of these first baleen whales—from a time called the Oligocene, around thirty million to twenty million years ago—have prominent large eye sockets and flat, beaklike snouts, with sockets for teeth. Some of these fossils preserve teeth in place, which makes me wonder about all the ways we could test their feeding style. The chocolate and coffee tones of the bones contrast against the gray mud-and-siltstone matrix that encases them. The rock matrix for bones this ancient can be so hard as to require years of careful exhumation by pneumatic chisel, acid baths, and the sharp picks of dental tools.

Many of these Oligocene whales hail from the storm-washed beaches of the Pacific Northwest and adjoining sea cliffs. Amateur scientists are responsible for nearly all of the Smithsonian’s collection of fossils from this part of the world. For these fossil finders, the brutal weather can make the finds more satisfying: winter storms wash the shores with heavy waves that pull away sand to reveal boulder after boulder, many with bits of bone inside if you know how to read the rocks. And, as always, you need to be lucky.

The ear bones among these Oligocene fossils reveal their identity as close forerunners of today’s baleen whales. Otherwise their needlelike and slightly cusped teeth recall some kind of strange, monstrous seal, or might make you think they belong in the Cretaceous, along with marine reptiles in the time of dinosaurs. But they are indeed whales, albeit smaller versions of today’s giants transposed tens of million years into the geologic past.

Their descendants, still alive in today’s oceans, have skeletons that vastly eclipse these early whales in size. Baleen came first, then, much later, enormous sizes. Even at Cerro Ballena, a site that samples the diversity of whales from nine to seven million years ago, no single baleen whale skeleton reaches more than about thirty feet in length. The origin of baleen doesn’t neatly explain why blue whales and other filter-feeding whales became modern-day giants. We have to keep digging.