As you work through this course you will come across boxes, like the one below, 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 'A Winning Design' on the DVD and read LoM Chapter 1.
To gain maximum benefit from the question that follows the next paragraph, make an attempt at answering the question before you reveal the answer. You will probably find it helpful to write down an answer to the question, in note form at least, and then compare it with the one given.
Mammals come in a bewildering variety of shapes and sizes and yet all of the 4700 or so species have some characteristics in common. Indeed, it's the existence of these common features that justifies the inclusion of all such diverse types within the single taxonomic group (or class) called the Mammalia.
On the basis of your study of LoM Chapter 1 and the associated TV programme, list up to three biological features that define mammals.
Mammals are distinguished by:
the production of milk;
the possession of hair (or similar structures, e.g. bristles);
and (along with birds) the ability to regulate their body temperature, usually being warm-blooded.
For much of this course, you will be concerned with exploring these diagnostic features of mammals in more detail. Sections 2 and 3 explore topics that emerge from the reading and viewing you've done already on the less 'conventional' mammals - the monotremes and the marsupials. Many of the facts and ideas in these early sections should therefore be familiar, though you'll also encounter new technical terms, especially in Section 3. One new issue that will come up a good deal is how we should best use the terms 'primitive' and 'successful'.
The title of this course emphasises the notion of mammalian design. This concept is a useful 'hook' on which to hang some introductory ideas; for example, how the structure of a particular part of an animal - a bone or a limb, perhaps - relates to its function, but biologists worry that using the term might imply that something complex has to be designed. (If you work through the series of units, you'll appreciate why any such implication is so strongly resisted.) Another idea emphasised is the success of mammals. You'll recall images from the TV programme of mammals flourishing in seemingly hostile environments. The fact that mammals are very widely distributed - such that David Attenborough (from here on referred to as DA) has to travel from the Arctic to the scrubland of Australia to sample mammalian diversity - is an expression of their success as an animal group. Another useful measure might be numbers of species - though the 4700 or so species of mammal is dwarfed in comparison with the known number of insect species in the world, which some estimate to be as many as 10 million. Other expressions of the success of mammals that are worth considering include the degree of physiological sophistication of their systems (which can be difficult to assess) and the number of individuals of a particular species or broader group that exist in total.
Mammals have also been around for a long time; LoM [p. 14] mentions the shrew-like 200-million-year-old fossil named Megazostrodon. Rather than shuffling along, with splayed-out limbs in the manner of many reptiles, this animal had limbs that were more erect and aligned under the body. Fossil evidence shows that the skulls of very early mammals have a distinctive lower jaw structure and sites on the skull for the attachment of chewing muscles. We can be confident that between 225 and 195 million years ago, mammal-like reptiles evolved into true mammals, though for the next 100 million years or so these unobtrusive animals, none larger than a pet cat, continued (in DA's words) to 'scuttle around the feet of the dinosaurs'. Their diversity did not appear until more than 100 million years later, during a period of geological time that witnessed the demise of the dinosaurs and their close relatives.