The evolution of whales

Whales, dolphins and porpoises, collectively known as cetaceans (for their classification in the mammalian order Cetacea) have long captured the attention of humans. From early drawings etched on the walls of Paleolithic caves, to 21st century satellite tags tracking the underwater movements of these denizens of the deep, the lives of humans and whales have been inexorably entangled. This connection has been a compelling story of the discovery and scientific study of large, charismatic sea animals tempered by darker times of intense human hunting and whaling that left many species hovering on the brink of extinction.
The approximately 90 living species of cetaceans inhabit nearly every ocean basin in addition to a few river systems, occupying diverse habitats that include polar, temperate and tropical waters. Whales evolved more than 50 million years ago in present-day India and Pakistan. Evidence from anatomy as well as genetics supports a close relationship between whales and even-toed ungulates (e.g. deer, giraffes, hippos, pigs, cows) with hippos positioned as their closest kin. The land to sea transition made by whales involved anatomical and physiological adaptations (e.g. feeding, locomotion and respiration) that are well represented in the fossil record. For example, recent discoveries indicate that the earliest whales like their terrestrial ancestors had well developed fore and hind legs and lived on land as well as the water.
One of the best-documented examples of evolutionary change in the fossil record is the loss of hind limbs in whales and recent study of whale embryos has revealed its genetic basis.
Whales are divided into two major groups: odontocetes (toothed whales) and mysticetes (baleen whales). Odontocetes are more diverse with 76 living species compared to 14 species of mysticetes. As the name suggests odontocetes possess teeth and most eat fish although some have very reduced dentitions and are specialized for suction feeding squid.
A key adaptation that enables odontocetes to pursue prey involves using high frequency sounds produced in the nasal region.
Mysticetes do not echolocate and recent research suggests that they may find prey using sensitive vibrissae (whiskers) on the rostrum. Although mysticetes possess teeth at birth (also present in early fossil mysticetes) they are resorbed and adults feed using baleen, keratin based structures. Plates of baleen are suspended in racks from their enormous mouths that act as a comb to bulk filter feed large aggregations of fish and zooplankton.
The biology of whales has been enriched by remarkable recent advances in integrated research in paleontology, ecology, behavior, and genetics. Modern techniques such as attaching digital acoustic tags (DTAG) to whales have elucidated extraordinary feeding behaviors and foraging strategies. Not only do these tags provide information on body orientation (i.e. acceleration, pitch, roll and heading) they also record sounds made by and heard by the tagged whale as well as recording environmental parameters such as water temperature and depth. Isotope studies reveal ocean temperature changes through time providing evidence that the diversification of modern whales was associated with increased food production.
Exciting breakthroughs in understanding the anatomical structures and pathways involved in sound production and reception in whales have been facilitated by CT scans and 3D imagery. Genetic and genome studies have explored evolutionary relationships among cetaceans as well as providing valuable life history and population data critical to the development of thorough management and conservation plans.
Knowledge of the biology of whales provides a framework that is essential for helping us understand how best to protect and conserve them. Further, as top predators, whales are indicator species of the health of the ecosystem and serve a vital role as sentinels of climate change.
Beautifully illustrated throughout, “Whales Dolphins and Porpoises” edited by Annalisa Berta, combines highlights from the latest scholarly studies of the nature and behaviour of the world’s whales, dolphins and porpoises, with a fully comprehensive species directory, that offers detailed profiles of each species alongside all the information needed to identify them in the wild.
Annalisa Berta has been Professor of Biology at San Diego State University, California, for more than 30 years, specialising in the anatomy and evolutionary biology of marine mammals. Past President of the Society of Vertebrate Palaeontology and co-Senior Editor of the Journal of Vertebrate Palaeontology, Berta has authored and co-authored numerous scientific articles and several books for the specialist and non-scientist.
Edited by Annalisa Berta
Available 21 September 2015
ISBN: 978-1-78240-152-0
£29.99

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