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Thar she blows … and dives, and feeds, and talks, and hears, and thinks: The anatomical adaptations of aquatic mammals

2007; Wiley; Volume: 290; Issue: 6 Linguagem: Inglês

10.1002/ar.20542

1932-8494

Jeffrey T. Laitman,

Ecology and biodiversity studies

Two of the most interesting people that I know are Kurt Albertine, the Editor-in-Chief of this journal, and Joy Reidenberg, the Guest Editor for this Special Issue. Among the things that fascinate me about them is their enormous knowledge about animals, all sorts of animals. I know things about animals too—but mostly from looking at their inner parts. (Want to know about the epiglottis of a sea lion, patas monkey, or even a dear-departed Neanderthal? I'm your man.) But these folks really know animals—from the inside and outside. They're what we call in the business, “animal people.” They know stuff the rest of us just don't. After a recent meeting and symposium in Salt Lake City for editors of this journal, Dr. Albertine decided that we had enough inside air and needed to get outside to see the world. While crossing from the colorful mountains of northern Utah to the windswept high plains of Wyoming, he decided to wax prolific about an animal found in abundance in the region, the moose. I was all ears, as moose have long been one of my favorite animals, a love stemming from my adoration of the ever-friendly TV and cartoon moose “Bullwinkle” in my youth (at least some of you must remember “Rocky and Bullwinkle”?) “Now, Jeff” my learned leader began, “this isn't New York. Animals here aren't in museums. There are lot's of them out here, and you can come upon them at any time, and you have to be prepared.” (Like in the Boys Scouts, I remember, I'm listening.) “For example, take the moose. You can usually smell them, but there are times that you can be caught unawares if you are not cognizant of your surroundings,” he continued (by the way, what do moose smell like?) “Be very, very, very careful about ever surprising or allowing yourself to come within a few feet of one, particularly a bull.” (I was riveted, but did not want to share that in the hamlet of my birth, Brooklyn, we are genetically programmed never to go near anything with antlers the size of Volkswagens.) “You can sense the moose is perturbed by watching it's ears,” he continued, “and then the bad stuff can happen.” (Bad stuff?) “A moose's attack mode is to first knock down their target with their powerful hindlimbs and then, once down, stomp the intruder to death. Yep, got to be real careful around moose areas.” (Goodbye Bullwinkle. Hello, stuffed museum moose). I'm just about to amble into the inviting blueness of the Gulf of Mexico—stealing a little time away from a research meeting session in St. Petersburg Beach—when I hear my colleague, Dr. Reidenberg call. “Wait,” she shouts, “I want to tell you about some of the local wildlife I saw before sunrise.” (OK. Animal stories are neat, and I'll never be up before sunrise anyway, so I'm all ears). “Early this morning, I saw a number of sharks swimming near shore. Magnificent” (Sharks? Did she say Sharks?) “Now, it is very, very, very important that you know how to distinguish a shark from a dolphin when you are in the water,” she continued. “A key anatomical difference that you can see at close range rests in the dorsal fins. The shark's is usually triangular, whereas a dolphin has the characteristic sickle or curved shape. Of course, with many sharks you can also see the smaller tail fin piercing the water, whereas the dolphin's fluke is horizontal and submerged unless it is arching out of the water. So it's imperative that you look at these differences.” (Why is she telling me this? If anything large and dark approaches me in the water, I'll be gone from a heart attack in seconds.) “Oh, in addition, you can also tell the size of the shark or dolphin … hey, why you going into the pool?” (That was 1998, the last time I went into the Gulf of Mexico). Exuberantly sharing knowledge about animal form and function (even if it frightens a poor city boy) is what makes Albertine and Reidenberg so fascinating—and in the case of the latter, is what she has done with this special issue of The Anatomical Record, “The Anatomical Adaptations of Aquatic Mammals” (2007). In this issue, Reidenberg has put under her watery microscope an array of cutting-edge science and in-depth, scholarly reviews that together bring to the fore both the breadth and depth of current thoughts on seminal aspects of our aquatic kin. Besides conveying the biological bases of how bits and pieces of our flippered relations have morphed to acclimate to their environment, these articles collectively transmit some of the mystery and fascination that surrounds a world of relatives so close in many ways, yet so alien in many others. The allure of sea-beasts of all types has a history that has fascinated us landlubbers for millennia with myths and tales that prevail even until today. While the forests, deserts, and jungles of the surface of our planet were gradually demystified by the end of the 19th century, the murky depths of oceans and rivers lie beyond our collective reach and have afforded fertile ground for assumptions, speculations, and stories that endure. (Want to know the biggest business around Loch Ness? Hint: it's not selling souvenir kilts.) What is under those foreboding ocean waves or within those dark riverine waters? What is the anatomy really like of the many creatures below, and how do their respective derived morphologies function compared with those of more accessible (and understandable) land mammals? And how did those creatures get to be that way any way…and why? Most of the world is covered with water of some sort, yet our knowledge about the mammals that live in it palls in comparison to that which we know about our terrestrial relatives. Indeed, more myths and stories arguably revolve around sea creatures than all others. I can remember my own first confusions: “And the Lord appointed a great fish to swallow up Jonah; and Jonah was in the belly of the fish three days and three nights” (Jonah 1:17). Fish? My picture-book as a child said it was a “whale,” but the “real” Book says fish, and even as a child I knew that a fish was not a whale (the American Museum of Natural History here in New York had both stuffed fish and stuffed whales so I knew the difference; I was also very familiar with a local fish known as the “gefilte”). This apparent confusion really bothered me as a child as I always thought that “You-Know-Who” would not make a mistake; if He wanted Jonah eaten by a whale he would have said whale and that's that. If the Almighty can be ambiguous about the identity of sea animals then it is not surprising that well-meaning scholars could also on occasion miss-the-mark. While the great Aristotle (Peck, 1965) may have been the first to describe aspects of the anatomy and behavior of dolphins and other marine mammals in his 4th century BC Historia animalium subsequent chroniclers, such as Conrad Gesner in his pictorial masterpiece, Historia animalium (1551–1587; illustrated by woodcuts, many from Albrecht Durer), can be forgiven for a melding of the real and the mythical in descriptions of sea and land creatures, such as his depiction of the unicorn as a mix of a horse and narwhal. While Gesner may have, on occasion, lapsed into fancy (or, more likely, just re-iterated long-standing legends) regarding the pelagic unknown, some serious comparative biology was also getting under way at that time. Most notable in this regard was the comparative observations by the French anatomists Pierre Belon (1551, 1553) and Guillaume Rondelet (1554) who, in their respective treatises on fish and other sea animals, often provided extensive and insightful observations on aspects of the anatomy of assorted sea mammals, including cetaceans. (For you noncognoscenti, living cetaceans consist of the mysticetes, or baleen whales, and the usually smaller odontocetes, the toothed whales that also include dolphins and porpoises). Whaling—in one sort of another—likely played an important role in the lives of peoples from coastal northern Europe and Scandinavia from at least the 14th century, and it was thus likely that many had some familiarity with cetaceans. While thus having some glancing acquaintance with them, it was—according to one of the leading whale biologists of this century, E.J. Slijper (1979)—apparently not until 1654 that the first systemic dissection of a cetacean was performed and recorded. This was of a porpoise by noted anatomist Thomas Bartholin (Bartholinus) who apparently did this dissection in the presence of his august majesty, King Frederick III of Denmark (talk about performance anxiety). Following this royal entertainment, bits and pieces of cetacean anatomy—largely on the smaller, toothed whales, usually the more commonplace porpoises of northern European waters—began to come under investigation. Notable among these were works by Ray (1671), Major (1672), and Tyson (1680), all focusing on the porpoise as well. Information on the larger whales was apparently not easy to obtain, and although whaling expeditions clearly must have provided tantalizing anatomic morsels over time, it was not until the work on whales by the great English anatomist John Hunter that reliable anatomical descriptions were advanced (1787). Following Hunter, the end of the 18th to 19th centuries saw the era of the great naturalists/anatomists emerge with their respective treatises delving into the magnificence of the animal world, including that of our aquatic friends. Paramount in this group was the “Big Three” of Georges Cuvier, Georges Buffon, and Richard Owen. Each in their own way brought extensive knowledge of the anatomy and world of various cetacean and other aquatic mammals to the attention of both the scientific world as well as the lay populace. Cuvier's Leçons d'Anatomie Comparée (1800–1805) and Owen's On the Anatomy and Physiology of Vertebrates (1866–1868) still rank among the greatest comparative works of all time, and Buffon's Histoire Naturelle (1828–1833) remains to me the unsurpassed tour-de-force of attempts to convey the entirety of the natural world. Indeed, the nature and anatomy of cetaceans were first brought to the attention of the public in large part through these works, particularly the profusely illustrated tomes of Cuvier, Buffon, and other continental naturalists of the age such as Lacépède (1804) and Geoffroy Saint-Hillaire (Geoffroy Saint-Hillaire and Cuvier, 1824–1842). To this day, as for over a century, one cannot stroll amid the booksellers along the Seine without seeing an array of color plates of mysticetes and dolphins liberated from the aging volumes of Cuvier and Buffon. Indeed, so much of our knowledge of our aquatic mammalian relatives flows from the thoughts, ideas, and images of these Gallic luminaires. More so than with any other broad group of mammals, those who inhabit aquatic habitats, be they pelagic cetaceans or riverine hippos, have undergone enormous modifications to interact with their world. While general pictures of their morphologies have often been advanced—obviously, with considerable enhancements since the 19th century (see, e.g., reviews in Reynolds and Rommel, 1999; Perrin et al., 2002)—many aspects remain as unknown today as they were in Aristotle's day, and often as mysterious. This Special Issue brings many of these to the fore, and Reidenberg has put her own anatomical and evolutionary antennae to good use by choosing areas that represent both a diversity of topics and yet those that collectively transmit the current controversies, confusions, and arguments in the field today. A central theme that runs through many of the studies and reviews surrounds the evolution of aquatic mammals, centering on marine cetaceans, but also broadening the scope by highlighting changes in other groups such as sirenians (manatees and dugongs), pinnipeds (seals, sea lions, and walruses), sea otters, and hippos as well. True to the issue's focus (Reidenberg, 2007), most articles place their anatomical observations within the context of adaptational interactions with the watery environment, perspectives that logically lead to discussions of evolutionary mechanisms and scenarios. Entwined through many of the papers are also discussions of fossil remains, particularly the extinct lineage of cetaceans known as the archeocetes (Archeoceti), which further place discussions in a broader evolutionary picture. The range of anatomical regions covered spans the body. While some of the issues and topics will be well-recognized by like-minded specialists in their respective inch of the world, most nonaquatic, comparative biologists and functional morphologists will find the hypotheses tested, data presented, and discussions offered to open up areas of thought they may have not ventured into before. Topics covered literally range from those exploring the outside (fascinating, anatomical study of the integument of whales and comparisons to that of terrestrial animals) to the inside (several studies and reviews assessing the nature of feeding mechanisms—including hyolaryngeal, cranial, and gut adaptations—in groups from large whales to manatees). The general subject of skeletal biology is a focus of several contributions, with insightful studies examining the following: adaptations that have evolved in bone density as it relates to buoyancy; modifications that produce marked asymmetry in the skulls of certain cetaceans; evolutionary changes that have occurred in the cetacean manus; the nature of fluke functional anatomy as revealed by new assessments via means of three-dimensional (3D) computed tomographic analysis of their geometry; the use and evolution of tusks in dugongs; adaptations and evolutionary changes in the vertebral column and skeletal anatomy of sirenians; and detailed assessments of forelimb myology and structure of the pygmy hippopotamus, and what this may tell us about its evolutionary history. Adaptations that affect how cetaceans, pinnipeds, and others behaviorally interact with their environment through the anatomy underlying their special senses are also an important theme. In this regard can be found insightful studies exploring the following: the nature and size of cetacean brains, and what this may mean for intelligence and communication; aquatic and aerial influences on the eyes of sea mammals, and how they have evolved in different species; both the detailed anatomy of hearing by use of computerized tomography to recreate 2D and 3D images of the whale cochlea, and assessments of fossil and extant species to assess changes that may have occurred when ancestors of whales returned to the sea; previously unreported, anatomical mechanisms for the source of sound production in mysticetes; and the anatomy underlying an unusual behavior of certain whales that may place their respiratory tract—and them—at great risk. Anyone reading the variety of articles in this issue will swim away with a renewed—or new—appreciation at once of how much we still do not know about our aquatic brethren and how much exciting information our colleagues are gathering with new eyes, approaches, and tools. Too bad that the issue did not cover the most ubiquitous, semiaquatic mammal of my region: the New York sewer rat. Now there's an animal that I can teach Albertine and Reidenberg a tad about.