The Biology of Marine Mammals
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Romero, A. 2009. The Biology of Marine Mammals. The Biology of Marine Mammals Aldemaro Romero, Ph.D. Arkansas State University Jonesboro, AR 2009 2 INTRODUCTION Dear students, 3 Chapter 1 Introduction to Marine Mammals 1.1. Overture Humans have always been fascinated with marine mammals. These creatures have been the basis of mythical tales since Antiquity. For centuries naturalists classified them as fish. Today they are symbols of the environmental movement as well as the source of heated controversies: whether we are dealing with the clubbing pub seals in the Arctic or whaling by industrialized nations, marine mammals continue to be a hot issue in science, politics, economics, and ethics. But if we want to better understand these issues, we need to learn more about marine mammal biology. The problem is that, despite increased research efforts, only in the last two decades we have made significant progress in learning about these creatures. And yet, that knowledge is largely limited to a handful of species because they are either relatively easy to observe in nature or because they can be studied in captivity. Still, because of television documentaries, ‘coffee-table’ books, displays in many aquaria around the world, and a growing whale and dolphin watching industry, people believe that they have a certain familiarity with many species of marine mammals (for more on the relationship between humans and marine mammals such as whales, see Ellis 1991, Forestell 2002). As late as 2002, a new species of beaked whale was being reported (Delbout et al. 2002), in 2003 a new species of baleen whale was described (Wada et al. 2003) and in 2005 a new species of dolphin was also described (Beasley et al. 2005). Some scientifically recognized species have never been seen alive by humans and are only known because very occasional remains found on beaches. Because of the paucity of obtaining specimens for certain species and variability in morphological characters, we continue to change the scientific names of some of them (e.g., van Helden 2002). With the increased use of molecular techniques and the discovery of new fossils, the systematics of these mammals is also changing almost every year. Given that there are only about 131 living species of marine mammals, such continued modification of our views of their basic classification underscores how little we still know about them. For example, we still have little understanding of the meaning of the sounds produced by these creatures despite the fact that bioacoustics was one of the first subfields to attract a lot of research. For instance, only relatively recently it was 4 discovered that among fin whales, only males sing loud songs and that they use such songs in order to attract females (Croll et al. 2002). In the evolutionary arena, key discoveries of fossils began to take place in the early 1980’s. These discoveries have helped us explain some of the key anatomical transformations leading to the adaption the aquatic environment. Also, some species and/or populations such as the Steller’s sea cow and the Caribbean monk seal as well as the North Atlantic population of gray whales have become extinct in the last few centuries. That has left us with many questions about their very basic biology. These are just a few examples of our ignorance. Yet, the little we know about these animals shows that these creatures are absolutely amazing. Even at the risk of falling into the realm of trivia, the facts presented in Box 1.1, are just some examples of pieces of knowledge that are astonishing. Box 1.1. Some Interesting Facts About Marine Mammals The Smallest: Among dolphins is the vaquita (1.2 m in length); among the pinnipeds is the Guadalupe fur seal with (1.35 m). The Longest: A 33.5 m female blue whale landed in the Southern Ocean, in 1909. The Heaviest: A 209-ton female blue whale caught in the Southern Ocean in 1947. The Loudest: The low-frequency pulses made by the blue whales and fin whales when communicating with members of their own species across the ocean have been measured at up to 188 decibels: the loudest sounds emitted by any living source (See Table 1.1.). The Longest and Most Complex Songs in the Animal Kingdom: Sung by male humpback whales; each song can last for half an hour or more and consists of several main components. The Longest Migration: Gray whales migrate between 15,000 and 20,000 km each year. The Fastest: The pantropical spotted dolphin can reach 40 km per hour in bursts. Deepest Dive: A sperm whale can dive as deep as 3,000 m or more. The Longest Dive: 2 hours and 18 minutes by five sperm whales. Their sound clicking was recorded on 11 November 1983, in the Caribbean. The Most Teeth: Spinner dolphins between 172 and 252 in total. The Fewer Teeth: Adult baleen whales lack teeth. Many species of toothed whales, such as beaked whales, have only two teeth and in some cases their females do not erupt them. The Longest Tooth: Narwhals have the longest tooth, up to 267 cm long. Largest Heart: The blue whale has a heart the size of a small car. (Based on data in Carwardine et al., 1998, and information in this book) 5 Yet, that is only the beginning. Once we start to carefully examine the panoply of adaptations that these creatures underwent in 50 million years of evolution in the aquatic environment, is when we really appreciate them for their value to science. Only after we understand their reproductive behavior and ecology, we can really become fully aware of the impact that humans have had, and continue to have, upon them throughout history. Table 1.1. A sample of levels of noise from human-related sources Levels of Noise Sound Decibels (dBA) Quiet home 20 Normal talking 40 Ringing telephone 60 Air conditioner 75 Sustained exposure to noise over 85 dBAs can cause permanent damage Shouting Subway train 100 Honking horns Jack hammers Loud music 120 Jet plane 150 Rocket of the Space Shuttle 180 1.2. What is and what is not a Marine Mammal For the purpose of this book we will include all species of three different orders of mammals: cetaceans (whales, dolphins, and porpoises), pinnipeds (seals, sea-lions, walrus), and sirenians (manatees, dugongs, sea cows). We also include two species of otters (the sea otter and the marine otter) and the polar bear, all of which belong to the order Carnivora. Despite the fact that three species of cetaceans are exclusively freshwater (i.e., river dolphins) and one largely estuarine (the tucuxi), we include them in this book because they evolved from marine species and that helps us better understand a number of adaptations that we observe among cetaceans. The same thing can be said of the two species of pinnipeds (the Lake Baikal seal and the Caspian Sea seal) that are found today in enclosed bodies of waters due to the past distribution of their marine ancestors. One species of sirenian (the Amazonian manatee) is also fundamentally a freshwater one, but is derived from a marine ancestor. However, we are taking a different view when it comes to otters. There is a total of 18 species of otters and with the exception of two they all live in freshwaters. We will be studying only the two sea species. We also consider the polar bear a marine mammal because of its adaptations and impact on the marine ecosystem. We do not, however, include here some species of bats that capture marine fish because they do not really present any special adaptation to the 6 marine environment (they can also fish in freshwaters). All in all, there are 131 species of extant (living) or recently extinct mammals that will be examined in this book. The number of species of extinct marine mammals that have been found in the fossil record is in the hundreds and continues to increase rapidly. As for any other major animal taxon, there is a larger number of species in the fossil record than those that are alive today. There are also examples in the fossil record of species of marine mammals unrelated to the above-mentioned groups such as hippopotamus-like desmostylians, bizarre bear-like carnivores like Kolponomos, and the aquatic sloth Thalassocnus. 1.3. A Question of Names In the English-speaking world common names for marine mammals are confusing to say the least. That is particularly true when dealing with whales and dolphins. For example, in general, ‘whales’ are large in size, ‘dolphins’ are medium size, and ‘porpoises’ are the small ones. But this is confusing because some whale species are smaller than the largest species of dolphins and some species of porpoises are larger than the smallest dolphin species. Further, the term ‘great whales’ is applied to large baleen whales as well as to the largest species of toothed whales: the sperm whale. Also, there are six species of ‘whales’ (the killer whale or orca, the false killer whale, the pygmy killer whale, the short-finned pilot whale, the long-finned pilot whale, and the melon-headed whale) that should be called dolphins because they all belong to the family Delphinidae (the family of dolphins). The short-finned pilot whale is also called ‘blackfish’ in the Caribbean while there is a fish called ‘dolphin’ (Coryphaena hippurus). There are two right whale dolphins named after the northern and southern right whales since all four species have finless backs. Also, not all ‘white beak dolphins’ have white beaks.