Page 46 - Summer2017

Peter L. Tyack
Postal:
Sea Mammal Research Unit School of Biology University of St Andrews St Andrews, Fife KY16 8LB Scotland United Kingdom
Email:
plt@st-andrews.ac.uk
Marine Mammal Acoustic Behavior
Marine mammals exploited acoustic properties of the ocean for tens of millions of years before human acousticians.
Introduction
The species that are called marine mammals actually represent many different lin- eages of terrestrial mammals that independently adapted to marine life. The terres- trial ancestor most closely related to the polar bear is the brown bear; the sea otter is related to the river otter, and manatees and dugongs are related to elephants. In this article, I discuss pinnipeds (seals, sea lions, and the walrus), whose ancestor diverged from a terrestrial carnivore about 35 × 106 yr ago (Higdon et al., 2007), and cetaceans (whales and dolphins), which separated about 54 × 106 yr ago from a lineage including ungulates such as the hippopotamus (Arnason et al., 2004).
The terminology we use in English for whales and dolphins does not match how biologists distinguish the main division among the cetaceans. For biologists, the two major divisions of the whales are toothed whales and baleen whales, which do not have teeth. Baleen whales are named for the baleen plates that grow down from the upper jaw, forming a sieve that allows baleen whales to engulf whole patches of small prey at one time. The family of toothed whales includes large whales such as pilot, killer, and sperm whales but also includes the smaller dolphins and por- poises. In fact, to a biologist, the killer whale is just a large dolphin.
When mammals evolved a return from terrestrial back to marine life, they were forced to compete with life forms that had been adapting to the ocean since life originated there. Most marine animals take the oxygen they need directly from the water, and most maintain their bodies at the same temperature as the water around them. Mammals would seem at a disadvantage competing against competitors that do not need to ascend to the surface to breathe and that do not need to use meta- bolic energy to maintain a body temperature tens of degrees above ambient. Was there anything about life on land that could give the returning mammals an advan- tage as a marine predator?
There does seem to be an advantage for a predator to maintain a body temperature above that of its prey because its responses may be faster and the higher metabo- lism may give it more power. The costs of needing to generate heat are reduced in larger animals, which have a lower ratio of surface area to volume, and many of the large marine dinosaurs actually maintained body temperatures well above ambi- ent (Bernard et al., 2010). The baleen whales include the largest animals to have evolved on Earth, the blue and fin whales. Their large size makes locomotion so efficient that large baleen whales can spend the winter breeding season in temper- ate or tropical waters, then migrate thousands of kilometers to more polar waters to take advantage of the pulse of productivity that produces great patches of their prey. The larger an animal is, the more expensive it can be to chase individual prey; baleen whales have solved this problem by being able to engulf thousands of small prey in one gulp. A key to their ecological success lies in their ability to roam the
44 | Acoustics Today | Summer 2017 | volume 13, issue 2 ©2017 Acoustical Society of America. All rights reserved.