Marine Mammals: Evolutionary Biology
- Annalisa Berta &
- James L. Sumich
The study of marine mammals is a highly fragmented and multidisciplinary endeavour, often conducted in an intensive, but seldom eclectic, manner. The discipline is united solely by its objects of study, the 100 or so species of mammal that are considered to belong to the marine mammal club. Species from three mammalian orders make up the list: those from the Carnivora include the pinnipeds (seals, sea lions and walruses), sea otters and polar bears; those from the Cetacea include whales, dolphins and porpoises; and those from the Sirenia the dugongs and manatees.
These mammals spend most, if not all, of their lives in the sea. Interestingly, during evolution, the marine ancestors of dolphins in South America, India and China repeatedly and independently recolonized freshwater habitats. Sadly, and ironically, the study of cetaceans began in earnest only in the first part of last century, just as it was beginning to be recognized that they were vanishing from our oceans and sustainable catch quotas had to be established.
Various marine adaptations permitted warm-blooded, air-breathing mammals to leave the safe confines of land and return to the watery habitats of their distant ancestors. These include insulation (blubber) and circulatory adaptations such as countercurrent heat-exchange systems to deal with the extra heat loss in cold water. The animals' eyes, nose, ears and limbs also became highly modified, sporting sensory adaptations and key evolutionary innovations such as echolocation, baleen to allow filter-feeding, collapsible lungs that allow deep and prolonged diving, a tail and flippers. In the cetacean lineage, the evolution of many of these adaptations for marine life was apparently extremely rapid — much faster than in the closely related artiodactyls (which include cows, pigs and giraffes) and perissodactyls (donkeys, rhinos and horses).
The study of marine mammals, and particularly that of dolphins and whales, shares with primatology the cachet (or stigma, depending on your point of view) of intense public interest. Romantic notions of ‘talking with dolphins’, saving whales and emulating the French oceanographer Jacques Cousteau have eager students flocking to this field. But reliable data collection and controlled experiments on marine mammals can be extremely difficult, if not impossible. The great rarity of some species, frequently as a result of whaling, and the animals' often enormous body size prevent many experimental approaches. As a result, a lot of problems, particularly in behavioural ecology, are still unanswered. This remains the case in spite of advances in microelectronics such as satellite telemetry, and molecular approaches using the polymerase chain reaction and DNA fingerprinting, which have revealed many new and often unexpected insights into areas such as social systems and migration.
Comprehensive texts on marine mammal biology are few and far between. This is particularly true of books that use a comparative phylogenetic approach, which is based on studying the evolutionary relationships among species and interprets data within an explicitly evolutionary framework. For example, based on the knowledge of phylogenetic relationships among cetaceans and their extinct closest land-bound relatives — the four-legged artiodactyl group, the mesonychids — it is clear that whale locomotion evolved through various stages. In now extinct forms, locomotion advanced from being quadrupedal, to pelvic paddling, and through caudal undulation of feet and tail to the modern condition of vertical movements of the tail only, with the concomitant loss of the hind limbs.
There is great need for a text that takes a strongly evolutionary approach to the study of marine mammals. This book fills that niche. It will be useful for both upper-level undergraduates and graduates and for researchers in marine mammal science. It is well researched, lucidly written and bang up-to-date. I was also impressed by the informed and balanced treatment of current debates, such as that on the phylogenetic relationships within and among cetaceans.
The systematics of marine mammals is relatively well known, largely through molecular phylogenetic investigations over the past ten years. However, questions such as the relationship of the walrus to the other two groups of pinnipeds, the seals and sea lions, or the position of beaked whales to other whales, remain unresolved. Another topic still being debated by molecular phylogeneticists is whether cetaceans should have their own order, or be nested within the artiodactyls, with hippos as their closest living relatives. What is well established is that whales are more closely related to cows and their relatives than they are to donkeys and horses and their perissodactyl relatives. Rates of morphological evolution have apparently been very fast in the whale lineage, rendering their appearance almost totally different from that of their artiodactyl relatives. Yet morphological traits and molecular evidence strongly indicate that donkeys don't belong with cows and whales.