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Studying mammals: Return to the water
Studying mammals: Return to the water

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1 The pinnipeds, sirenians and cetaceans

As you work through this course you will come across boxes, like this one, which give you advice about the study skills that you will be developing as you progress through the course. To avoid breaking up the flow of the text, they will usually appear at the start or end of the sections.

As well as the course text, you will be using The Life of Mammals book (LoM) and related The Life of Mammals DVDs, as described in the introduction to this course. Before you go any further, watch 'Return to the Water' on the DVD and read LoM Chapter 7. Unless stated otherwise, all the page references you encounter in this course will be to LoM.

The versatility of mammals is a central theme of this series, but surely no environment has tested that versatility as much as the rivers and oceans of the world. Mammals are essentially a terrestrial group of animals: living mammals are descended from shrew-sized ancestors that evolved on land, and most mammals are still land-based animals.

For the first two-thirds of the evolutionary history of the mammals, from about 210 to 65 million years ago, dinosaurs dominated the plains and forests. But the dinosaurs conquered more than just the land: the seas and estuaries were home to other large aquatic reptiles, including ichthyosaurs and plesiosaurs. These animals disappeared at the same time as their terrestrial cousins - and their demise opened up new habitats and new sources of food that the mammals were quick to exploit.

As author David Attenborough (DA) notes in LoM Chapter 7, three major groups of mammals have independently adopted an aquatic way of life, the suborder Pinnipedia (part of the order Carnivora), the order Sirenia and the order Cetacea. These groups are the focus of our attention in this course. I'll have less to say about the smaller lineages that have become aquatic to some extent, such as the river and sea otters [pp. 185-188] that were one of the highlights of the TV programme 'Return to the Water'.

The suborder Pinnipedia

The name of this suborder comes from a Latin word meaning 'wing-footed', which refers to the modification of limbs into flippers. Most pinnipeds move onto the land from time to time to rest and bask in the sun, and in the breeding season they gather, often in large numbers, on beaches or flat areas of sea-ice to give birth. There are three main families: the eared seals, which include fur seals and sea-lions; the earless or true seals; and the walrus. Remember that these mammals are carnivores; in view of their continuing links to land, they are sometimes aptly called 'amphibious carnivores'.

The other two groups, the sirenians and cetaceans, are fully committed to a life in the water.

The order Sirenia

There are four living members of the order Sirenia: three species of manatees and the dugong. These animals are descended from ancestors that also gave rise to the elephants, and they are the only herbivorous, completely aquatic mammals. As DA points out [p. 200], the order is named after the Sirens of classical mythology, sea-nymphs who lured sailors to their death. Until relatively recently, there was a fifth species: Steller's seacow. This animal lived mainly on kelp and had no teeth, though it did have rough plates on its palate. Hunting and the decline of the kelp beds caused a drastic fall in its numbers, and it has the dubious record of the shortest period of time between discovery by western science (1741) and extinction (1768).

The order Cetacea (dolphins, porpoises and whales)

The cetaceans get their name from the Greek word for a whale (ketos). The order is divided into two suborders: the Odontoceti, or toothed whales, which include the sperm whale, killer whale, dolphins and porpoises; and the Mysticeti, or baleen whales, which include the blue whale and humpback whale, and are named for the brush-like plates that they use to filter krill from the water, as you saw in the TV programme at 35.40.

In moving to the water, aquatic mammals have had to survive, feed and reproduce using a set of biological characteristics that evolved in association with life on land. This course will explore how these characteristics have provided challenges, and opportunities, for mammals that spend some or all of their time in the water. I shall examine some of the important differences between a life in air and a life in water, and describe how the bodies and behaviour of aquatic mammals have changed in ways better suited to living and hunting in an aquatic environment. In particular, I shall explore the so-called 'diving response' (Section 3), and describe some of the problems involved in finding out how the animals behave on their dives. Finally, in Section 4, I shall look at what is now known about the evolution of the mammals that have made the most complete transition from the land to the water - the whales. So, in the early part of this course I'll be revisiting some of the topics introduced by DA in LoM and the TV programme; my aim here is to consolidate what you've learnt already. Later on, we move into uncharted waters to cover new topics that build on what's gone before. This course also gives you an opportunity to read two intriguing articles on diving mammals, one written by a science journalist and the other by a team of research scientists.

You have just met the scientific names of some mammalian groups (orders and suborders) with explanations of how the words have been derived. Many scientific names have Latin or Greek roots and with a good dictionary (and maybe a web search), you may be able to unravel some of these words for yourself. For example, the Weddell seal which you will meet later in this course has the scientific name Leptonychotes weddelli (remember that scientific names are always printed in italics; if you write them by hand, you should underline the name). My dictionary gives, at the end of the definition of the word 'lepton', the Greek word leptos meaning 'slender' or 'fine' and at the end of the definition of 'onyx', it gives onyx as a Greek word meaning 'nail' or 'claw'. So the name of the genus, Leptonychotes refers to the narrow claws on the flippers of the seal (actually, the hind flippers in this case). The species name weddelli is given in honour of Captain James Weddell, who described and illustrated the seal in his writings about an Antarctic voyage to catch seals in the 1820s. You might like to try to unravel an example for yourself - try the Pacific white-sided dolphin Lagenorhynchus obliquidens. (Hint: use a comprehensive dictionary to look up 'lagena', 'rhyncho-' and 'oblique'.)