Some 55 million years ago, in the Early Eocene epoch, a number of mammals forsook their terrestrial homes to return to life in the sea, beginning one of the strangest and most interesting chapters in evolutionary history. They were the ancestors of today's marine mammals, which (unlike fishes and most other aquatic creatures) arose from land-living, air-breathing animals. On the shallow shores of the ancient Tethys Sea, which stretched across what is now southern Asia and the Middle East, slender, cat-sized creatures gave rise to the cetaceans (whales, dolphins, and porpoises), and squat, elongate elephant relatives evolved into sirenians (manatees and dugongs). These and other fascinating creatures are described in Annalisa Berta's wideranging and fact-packed new book, Return to the Sea: The Life and Evolutionary Times of Marine Mammals.
Berta is a professor of biology at San Diego State University, where she specializes in vertebrate morphology and evolution—fields of expertise that figure prominently in this engaging tour through the highlights of marine mammal history. How did these ancestors survive the tremendous challenges of their new environment? In turns serious and playful, Berta outlines many of the extraordinary adaptations that enabled these animals—and their living descendants—to survive and thrive. For example, walruses have countercurrent heat exchangers in their flippers, and the males make gong-like sounds to attract females. Blue whales are born with rudimentary teeth, which are later reabsorbed, before they grow 300 plates of filtering baleen, which are made of the same keratin as are in our hair and fingernails. Sperm whales can dive close to two miles deep and can stay below the surface for over two hours, all on a single breath of air. Abundant facts are woven into a compelling story of the history and biology of marine mammals that will delight while it informs readers.
Berta writes in an engaging, jargonfree style, which makes the book readily accessible to students and teachers at both the graduate and the undergraduate levels, but it is especially recommended for undergraduate students just learning, for example, the difference between crown and stem groups. Even specialists who may lack a firm grasp of the evolutionary history of marine mammals can find space on their bookshelf for this volume. Given that marine mammals are one of the most widely used examples of major evolutionary transitions—with numerous intermediary fossils bearing now-lost limbs and other former features—most biologists and educators will view this evidence-based book to be as useful as it is stimulating.
Return to the Sea covers a lot of ground. Lessons are presented on the biology of seals, whales, sea otters, polar bears, and other marine mammals living and extinct, with excellent coverage given to basic anatomy, physiology, ecology, and behavior. Lavish illustrations of long-gone species, especially Carl Buell's softly shaded yet detailed paintings of fossil reconstructions and James Sumich's crisp line drawings, are compelling. The highlight and theme of the book, however, is the discussion of the many factors involved in the origin and radiation of various marine mammal lineages. Berta's expertise in paleontology substantiates her accounts of the different species through time, which ties the expansive material in the book together.
It is Berta's presentation of these lineages that directs the book toward the fundamental concepts of evolution within speciation, exaptation, and extinction. She uses marine mammals as the cast of characters with which to demonstrate such ideas as paedomorphosis (in porpoises), mating leks (in walruses), and regulatory Hox and PITX1 genes (presumably relating to the loss of limbs). There is also a heavy dose of oceanography, including upwellings, the deep scattering layer, and marine sanctuaries, but the marine mammals take center stage. Berta moves briskly through the anatomical descriptions of these creatures, from lobed kidneys to compartmentalized stomachs, then onto their vocal communications (e.g., pops, clicks, and whistles).
Although a bibliography is absent, the book ends with a slim list of recommendations for further reading, which contains excellent online sources. The book is not without errors, including some mistaken figure captions. One switches the families of whale and megamouth sharks; another misidentifies an ancient giant dugong. Readers unfamiliar with all of the living marine mammals described in the text would have been well served by a table (or an appendix) listing extant species or by additional evolutionary tree diagrams. Most of all, I found myself wanting richer descriptions, and especially more illustrations, of lesser-known fossil species, including desmostylians and aquatic sloths. These are minor quibbles, however, particularly for such a wide-ranging, readable survey.
In some ways, Berta's book is reminiscent of Carl Zimmer's equally recommended At the Water's Edge: Fish with Fingers, Whales with Legs, and How Life Came Ashore but Then Went Back to Sea (Free Press, 1999), although Zimmer's book delves into many asides, has few figures, and is now a bit outdated. The information in Return to the Sea is admirably up to date, in terms of both paleontological findings and current research methods, which range from satellite telemetry to DNA barcoding and fingerprinting. An entire chapter is devoted to fossil dating techniques and other key elements of paleontology; another is focused on conservation issues, especially those involving human impacts (e.g., anthropogenic sound, fisheries bycatch, strandings, military sonar). This last of six sections reveals many recent discoveries, some of which are optimistic (e.g., whales may play a key role in marine nitrogen recycling) and some alarming. Sea lions are now threatened (from over-fishing) by a junk-food diet that lacks fatty fish. Climate change threatens ice-bound algae that feed krill, which, in turn, feed many whales and other marine mammals.
Berta explains that the first marine mammals likely turned to the sea in search of food. It would be a shame if a lack of food now doomed these remarkable creatures. Then again, perhaps resources might now lure some species back onto land, continuing—with a new twist—the long and enthralling evolutionary story of marine mammals.
