What Is a Marine Mammal?
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© Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION 1 2 CHAPTER 3 4 5 What Is a 6 7 8 Marine Mammal? 1 9 10 11 12 13 CHAPTER OUTLINE 14 15 Characteristics of Mammals Exploring the Depths: International Union for Conservation of Nature 16 Mammalian Evolution Exploring the Depths: Sea Wolves and Sea Sheep? 17 Defining a Marine Mammal Exploring the Depths: Eurasian Otters: A Marine Mammal? 18 19 Quasi-Marine Mammals Exploring the Depths: Steller’s Sea Monkey: Were There More Species of Marine Mammals Historically? 20 Greater Bulldog Bat (Noctilio leporinus) Why Are Marine Mammals Special? 21 Fishing (or Fish-Eating) Bat (Myotis vivesi ) 22 Arctic Fox (Vulpes lagopus) Selected References and Further Reading 23 24 Before we embark on this book about the biology and In addition to hair, sweat, and mammary glands, mam- 25 conservation of marine mammals, we should first define mals also possess specialized teeth and, like birds, are en- 26 exactly what they are. Marine mammals are a diverse col- dothermic (or homeothermic, commonly referred to as 27 lection of species grouped together not because of a com- warm-blooded), metabolically generating their own heat 28 mon evolutionary history, but because they inhabit marine and maintaining a constant internal body temperature. 29 environments. Nevertheless, they are all mammals and as They also possess a four-chambered heart and their brain 30 such have many characteristics in common. has a neocortex (the outer part of the cerebral cortex that 31 is responsible for sensory perception, spatial awareness, 32 Characteristics of Mammals and “higher functions,” such as reasoning and language 33 skills). 34 Mammals (members of Class Mammalia) have several fea- 35 tures. They all possess hair composed of keratin, a protein; 36 three bones in their inner ear; and sweat glands. In females 37 some of these sweat glands have been modified to become 38 milk-producing mammary glands, or mammae, which give 39 this group of animals their name. Most mammals have teats 40 or nipples except for the monotremes (Subclass Proto- 41 theria, Order Monotremata), which exude milk directly 42 from their pores to be lapped up by their young. The mono- 43 tremes, which include the echidna, or spiny anteater, and 44 the aquatic platypus (Figure 1.1) are also unusual in that they 45 lay eggs, unlike the rest of the mammals (Subclass Theria), 46 which produce live young. The monotremes also have a 47 common cloaca, or orifice, through which they lay eggs, 48 urinate, and defecate. These remaining (Theria) mammals 49 are separated into the Metatheria, which include the marsu- 50 pials, and the Eutheria, or placental mammals. All marine FIGURE 1.1 A member of the mammalian order Monotremata, 51 mammals are members of the Eutheria. the platypus. 52 L 83440_CH01_Parsons_v3.indd 1 Achorn International 03/09/2012 01:33PM © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION 2 CHAPTER 1: What Is a Marine Mammal? 1 2 Eocene Miocene Pliocene Recent Oligocene Cretaceous Paleocene 3 Pleistocene – 4 Sirenia 5 Hyracoidea Proboscidea 6 7 Artiodactyla 8 9 Cetacea 10 Perissodactylala 11 Tubulidentataa 12 Pinnepedia 13 Carnivora 14 15 16 Chiroptera 17 Insectivora 18 Dermoptera 19 Primates 20 21 Rodentia 22 23 Lagomorpha 24 Pholidota 25 Edentata 26 Marsupialia 27 Multituberculata 28 Monotremata 29 30 A 31 32 Pakicetidae 33 Pakicetus 34 35 Ambulocetidae 36 Ambulocetus 37 38 Remingtonocetidae 39 Dahlanistes 40 41 Protocetidae 42 Rhodocetus 43 44 Basilosauridae 45 Basilosaurus 46 47 48 49 Mysticeti 50 51 52 B Odontoceti L FIGURE 1.2 (A) A timeline showing mammalian evolution and (B) the sequence of evolution of various cetacean groups. 83440_CH01_Parsons_v3.indd 2 Achorn International 03/09/2012 01:33PM 83440_CH01_Parsons_v3.indd 3 Achorn International 03/09/2012 01:33PM © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION CHAPTER 1: What Is a Marine Mammal? 3 Mammalian Evolution 1 2 The evolution of the mammals really begins in the middle 3 of the Permian period (299–251 million years ago) (Fig- 4 ure 1.2) when reptile-like therapsids were one of the main 5 land-based predators. These creatures had several features 6 in common, including similar skull structures and incisor 7 teeth that were equal in size. A therapsid group that arose 8 in the late Permian period was the cynodonts (Figure 1.3), FIGURE 1.3 An artist’s impression of how a cynodont could have 9 animals with even more specialized teeth, including rear appeared. 10 teeth that had crowns roughly resembling those of molar 11 teeth. They also had a bony palate in the roof of the mouth 12 that separated the mouth from their airways, so they could 13 the wolves and cats (Order Carnivora), even-toed hoofed eat and breathe at the same time. Bones in their jaw also 14 mammals such as deer, cows, and hippos (Order Artiodac- transformed, with two of the smaller jaw bones migrating 15 tyla), odd-toed horses (Order Perissodactyla), the elephants to become the inner ear bones, presumably giving better 16 (Order Proboscidea), the flying bats (Order Chiroptera), to hearing capabilities. It is hard to say to what extent or when 17 the fully marine manatees and dugongs (Order Sirenia) and these animals developed hair. 18 whales and dolphins (Order Cetacea), and to monkeys, apes, A mass extinction approximately 251 million years ago 19 and humans (Order Primates) (Figure 1.4). at the end of the Permian devastated terrestrial life, with 20 approximately 70% of land vertebrate species going extinct. Domain Eukarya 21 During the succeeding Triassic (251–199 million years ago) Kingdom Animalia 22 and Jurassic (199–145.5 million years ago) periods, the ar- Phylum Chordata 23 chosaurs (i.e., dinosaurs and crocodilians) overtook the Subphylum Vertebrata 24 therapsids as the dominant land predators and the proto- Superclass Tetrapoda 25 mammals appear to have shifted into a different ecological Class Mammalia 26 niche, becoming smaller, primarily nocturnal, and prob- 27 ably insectivorous, rodent-like creatures. This new niche 28 may have led to further adaptations, such as hair to keep 29 warm, improved hearing, and a higher metabolic rate. The Defining a Marine Mammal 30 first true mammals appeared in the early Jurassic, with the What is the definition of a marine mammal? In Marine 31 first marsupial, and possibly eutherians, appearing later in Mammals of the World, Systematics and Distribution (1998), 32 the fossil record at about 125 million years ago. Dale Rice lists marine mammals as sirenians (manatees 33 At the end of the Cretaceous period (145.5–65.5 mil- and dugongs), pinnipeds (seals, sea lions, and walruses), 34 lion years ago) another extinction event wiped out many of and cetaceans (whales, dolphins, and porpoises). He also 35 the land and sea reptiles, including most of the (nonbird) notes several species of bats and carnivores that inhabit 36 dinosaurs. Mammals are then believed to have expanded marine waters and are sometimes considered to be marine 37 and diversified, particularly in the Eocene (55.8–33.9 mil- mammals, although he does not specifically define what a 38 lion years ago). Recent molecular studies confirm a peak marine mammal is. These groups include the bulldog (Noc- 39 in diversification at about 50 million years ago. However, tilio leporinus) and fishing bats (Myotis vivesi), the polar 40 they also suggest a (larger) peak in mammalian diversifica- bear (Ursus maritimus), the Arctic fox (Vulpes lagopus), and 41 tion between 100 and 85 million years ago, producing most several otters, including the sea otter (Enhydra lutris), ma- 42 current mammalian orders by around 75 million years ago, rine otter (Lutra felina), and Eurasian otter (Lutra lutra). 43 thus before the late Cretaceous extinction. Reynolds et al. (1999, p.1) describe marine mammals as 44 There are approximately 5,400 species of mammals follows: 45 ranging in size from the tiny Kitti’s hog-nosed (or bumble- 46 bee) bat (Craseonycteris thonglongyai ) and Etruscan shrew (a) “occupy or rely on aquatic, if not strictly marine, 47 (Suncus etruscus), which weigh little more than a couple of habitats”; 48 grams, to the blue whale (Balaenoptera musculus), which (b) “have evolved similar anatomical features, 49 is believed to be the largest animal that has ever lived (see including large body size, streamlined shape 50 Chapter 11). Mammals range from mouse-like rodents (compared to terrestrial relatives), insulation in 51 (Order Rodentia) and shrews (Order Soricomorpha) to the form of blubber and dense fur, and in most 52 L 83440_CH01_Parsons_v3.indd 2 Achorn International 03/09/2012 01:33PM 83440_CH01_Parsons_v3.indd 3 Achorn International 03/09/2012 01:33PM © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION 4 CHAPTER 1: What Is a Marine Mammal? 1 2 3 4 5 6 7 8 9 10 11 12 13 A B 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 C D E 29 FIGURE 1.4 Examples of the diverse range of animals contained in Class Mammalia. (A) Etruscan shrew; (B) hippopotamus; (C) giraffe; (D) elephant; (E) orangutan. 30 31 32 33 34 35 cases, a modified appendicular skeleton resulting 36 in reduction in the size of appendages”; and 37 (c) “possess some similar physiological adaptations 38 (e.g., for diving, thermoregulation, 39 osmoregulation, communication, and 40 orientation).” 41 42 They consider sirenians, pinnipeds, cetaceans, polar bears, 43 and also marine otters and sea otters to be marine mam- 44 mals but do not include species such as the North Ameri- 45 can river otter (Lontra canadensis) or the bulldog or fishing 46 bats. They do note, however, that these animals can be de- 47 pendent, at least in part, on the marine ecosystem.