MARINE MAMMALS, EXTINCTIONS OF Glenn R. VanBlaricom,* Leah R. Gerber,† and Robert L. Brownell, Jr.‡ *U.S. Geological Survey and University of Washington, †University of California, Santa Barbara, and ‡National Marine Fisheries Service I. Introduction principal source for taxonomic nomenclature, includ- II. Patterns and Case Studies of Extinction in ing common names, is the recent review of Rice (1998). Marine Mammals The order Cetacea includes whales, dolphins, and III. Discussion porpoises (Table I). The ‘‘pinnipedia’’ is a group of species in three families in the mammalian order Carni- vora (Table I). The pinnipeds include the seals, fur seals, sea lions, and walrus. The term pinnipedia is no I. INTRODUCTION longer recognized formally by marine mammal taxono- mists, but it continues to appear in the systematic ver- A. Taxonomic Definition of nacular as a matter of tradition and convenience. The order Sirenia includes the extant manatees and dugong ‘‘Marine Mammals’’ and the extinct Steller’s sea cow (Table I). The order The marine mammals include one extinct order and Desmostylia is the only recognized order of marine three major extant taxa that were or are fully aquatic, mammals to become entirely extinct. in most cases occurring entirely in the marine habitats Two largely terrestrial families of the order Carnivora of the major ocean basins and associated coastal seas also include species recognized as marine mammals and estuaries. In addition, a few species of largely terres- (Table I). Sea otters and chungungos (family Mustel- trial taxa are currently regarded as marine mammals. idae) live entirely or primarily in marine habitats. Polar We consider 127 recent mammal species in total to bears (family Ursidae) also spend a significant propor- be marine mammals for purposes of this review. We tion of time at sea. acknowledge that species numbers within any taxon Many other species of mammal utilize aquatic or are subject to revision as new systematic methods and marine habitats, including monotremes, ursids, mustel- philosophies emerge. Our primary bases for defining ids, canids, primates, rodents, bats, and ungulates. Ulti- our list of marine mammal species are the protocols of mately, the distinction among aquatic, marine, and the U.S. federal government, determined largely by the terrestrial taxa is arbitrary. Thus, our reliance on defini- U.S. Marine Mammal Protection Act (MMPA) of 1972 tions and protocols of MMPA, NMFS, and FWS is sub- [16 U.S.C. §§1361-62, 1371-84, and 1401-07 (Supp. jective, although it is consistent with common practice IV 1974)] as amended (MMPA) and managed by two at least in the United States. U.S. federal agencies, the National Marine Fisheries We use the term ‘‘marine’’ to refer to large, contigu- Service (NMFS) and the Fish and Wildlife Service ous aqueous habitats with significant dissolved salt con- (FWS). Our choice of defining criteria is arbitrary. Our tent in ambient waters. Thus, we apply the term marine Encyclopedia of Biodiversity, Volume 4 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 37 38 MARINE MAMMALS, EXTINCTIONS OF TABLE I TABLE II Major Taxa and Species Numbers of Marine Mammalsa Distinguishing Characteristics of the Major Marine Mammal Taxa Taxon No. of species Characteristic Cetacea Sirenia Pinnipedia Cetacea: Whales 83 Mysticeti: Baleen whales 12 Body streamlined x x x Balaenidae: Right whales 2 Limbs modified x x x Neobalaenidae: Pygmy right whale 1 Rear limbs modified as x Eschrichtidae: Gray whale 1 flippers Balaenopteridae: Rorquals 8 Rear limbs and pelvic girdle x x Odontoceti: Toothed whales 71 absent Physeteridae: Sperm whales 1 Propulsion by caudal spine x x Kogiidae: Pygmy sperm whales 2 and flukes Ziphiidae: Beaked whales 20 Loss of pelage x x Platanistidae: Indian river dolphin 1 Iniidae: Amazon river dolphin 1 Subcutaneous blubber layer x x x Lipotidae: Chinese river dolphin 1 Simplification of dentition x x x Pontoporiidae: La Plata dolphin 1 Expansion of anterior skull x Monodontidae: Beluga and narwhal 2 Development of acoustic capa- xa xa Delphinidae: Dolphins 36 bility for communication Phocoenidae: Porpoises 6 and echolocation Carnivora, ‘‘Pinnipedia’’ 36 Amphibious capability x Otariidae: Sea lions and fur seals 16 Odobenidae: Walrus 1 a Echolocation capability is known only for the odontocete ceta- Phocidae: Seals 19 ceans. Carnivora, other marine taxa 3 Mustelidae: Marine otters 2 Ursidae: Polar bear 1 Sirenia: Manatees, dugongs, and sea cows 5 lated approximately with the duration of the evolution- Trichechidae: Manatees 3 ary history of the major marine mammal taxa. Dugongidae: Dugong and Steller’s sea cow 2 Although marine mammals are largely defined by marked departures from the terrestrial mammalian Total species 127 model, it is instructive to consider major features of a terrestrial mammals retained in marine mammals. In Following the conventions of Rice (1998). the context of extinction processes in general, and an- thropogenic extinctions in particular, two retained fea- to the world’s oceans, seas, and estuaries. We apply the tures are of particular importance. First, although most term ‘‘aquatic’’ to aqueous habitats without significant marine mammals spend most of their lives immersed measurable dissolved salt concentrations in ambient at sea, they retain largely terrestrial respiratory architec- waters, such as lakes and rivers above the elevation of ture and must surface and breathe in order to exchange significant mixing with marine waters, and to inland respiratory gasses. Second, marine mammals are ho- saline lakes that lack outlet streams connecting to ma- meothermic, with core body temperatures typically near rine habitats. ‘‘Terrestrial’’ habitats are those lacking 38ЊC, like their terrestrial relatives. The need to breathe standing water in normal circumstances. As indicated at the surface and the need for major anatomical adjust- in Table I, our list of ‘‘marine mammals’’ includes ma- ment to minimize rates of heat loss are constraints that rine and aquatic species. foster vulnerability to unsustainable rates of exploita- tion and to certain types of pollution. The significance 1. General Features and Habitat Boundaries of these constraints is developed in the case studies we Compared to terrestrial mammals, marine mammals are present later. characterized by many striking modifications in anat- The diving capabilities of marine mammals define omy, physiology, and ecology (Table II). In some cases, the three-dimensional nature of their habitats at sea. the modifications are sufficiently extreme that phyloge- Nearly all extant marine mammals dive to forage, al- netic linkages to terrestrial ancestry are obscured and though the ranges of diving capability and pattern difficult to resolve. The degree of modification is corre- are broad. Most marine mammals also spend signifi- MARINE MAMMALS, EXTINCTIONS OF 39 cant time submerged while traveling, socializing, or with small breathing holes, beluga whales and narwhals breeding. are also taken as food by polar bears. Thus, the extent Among cetaceans, sperm whales and beaked whales to which the at-sea habitat of marine mammals is truly likely dive the deepest and longest compared to other three-dimensional varies widely among the major taxa species. Sperm whales can dive to 1500 m, remaining and the individual species. Within species, there is also submerged for 20 min or more. The diving behavior of marked ontogenetic variation in diving capability and beaked whales is poorly known, but there is emerging pattern. evidence that beaked whales may also routinely make The marine mammals are geographically ubiquitous repetitive dives of long duration to great depth. Baleen in the world’s oceans, seas, and estuaries. Cetaceans whales may make long deep dives during breeding sea- occur in marine environments at all latitudes. For exam- son. Foraging dives of baleen whales normally are rela- ple, killer whales and minke whales may have the largest tively shallow and brief. Many of the smaller cetaceans natural geographic ranges of the earth’s mammals. Most commonly dive for less than 10 min at a time to depths of the mysticetes and some of the larger odontocetes no greater than a few hundred meters. have global ranges or are distributed antitropically. Among pinnipeds, elephant seals (Phocidae) have Smaller cetaceans are widely dispersed as well, although maximum diving capabilities comparable to the sperm individual populations typically concentrate in regions whales, and they are known to make remarkably long of predictably high local biological productivity. Several sequences of repetitive deep (to 1500 m), long (20 min species of small cetacean, including two delphinids, a or more) dives with surface intervals of only 2 or 3 phocoenid, and the four monotypic families of river min. These sequences may be maintained day and night dolphins, are found in major river systems in South for tens of days at a time. Many other phocid seals are America and Asia. Beluga whales also spend significant thought to have similar capabilities. The sea lions and time in river habitats. Pinnipeds occur in all the world’s fur seals (Otariidae), in contrast, usually dive for only major marine habitats, but most species are concen- a few minutes at a time, and usually to maximum depths trated in middle or high latitudes, in close association of a few hundred meters, although many otariids are with regions of high productivity. In addition, there known to be capable of continuous sequences of repeti- are several pinniped species or populations confined to tive shallow dives of 10–12 hr or more. Walruses are isolated large lakes in Europe, Asia, and North America. known to dive as deep as 80 m, with maximum dura- Most sirenians are limited to tropical or subtropical tions of 10 min. latitudes, in shallow seas that provide adequate macro- In contrast to cetaceans and pinnipeds, sirenians are phytic food and refuge from predation, and are ther- weak divers, normally remaining in shallow water (Ͻ20 mally tolerable. The sea otter is confined to the coastal m) and diving for only 2 or 3 min when active.