Guide to beastly behaviours

David Burnie and colleagues' Animal: The Definitive Visual Guide to the World's Wildlife is a remarkable collection of photographs and descriptions of the earth's fauna. From the intimidating stare of the male mandrill monkey on the cover, right through to the striking colour micro-photographs of tiny tardigrades, Animal 's set of 5,000 or so vivid colour images is unsurpassed among wildlife books.

Animal makes it look easy. The photographic collection is organised around the major divisions of animals; it is augmented by technically up-to-date sections on classification, anatomy and behaviour and important aspects of species' ecology and conservation; and it provides basic descriptive facts about each species. But producing a visual guide to the world's animals is a courageous undertaking, and not just because there are so many from which to choose.

What is an animal? We all know the easy ones - our neighbour's dog, goldfish, parrots, snakes, beetles and slugs. But is a sponge an animal? Are jellyfish animals? Most will guess that jellyfish are animals because we call them "fish", but they have no brain. Shouldn't proper animals have a brain? Sponges are animals, even if not, perhaps, wildlife. That soft thing you wash with is the wretched sponge's skeleton. What about sea anemones and corals? They too are animals.

These quite un-animal-like animals raise the question of whether there is a natural way to recognise one. Although still contentious, a DNA-based definition of animals is emerging. Everything we think of as an animal contains a common set of Hox genes that guide their development from a single-celled embryo right on up through adulthood. This tells us that all animals are descended from a single common ancestor - the ur-animal - that evolved once, perhaps a billion years ago. Quite possibly the Creator was lazy when it came to animals, maybe becoming an animal is not easy, or perhaps once the first ones were in place all other attempts were excluded.

Modern biological science has little truck with the first of these explanations but, equally, little of any value to say on the other two.

The modern genetic perspective gives Burnie and colleagues a way to include and exclude species. But how to winnow down the list from the perhaps 20 million to 30 million species with Hox genes to the 2,000 or so that fill the pages of Animal ? Were one to sample the animals by drawing names from a hat, Animal would have ended up being a book about the invertebrates, as they alone comprise more than 97 per cent of animal species. On the other hand, to include more than one of the only 5,000 or so mammals would overrepresent them statistically, risking charges of mammal-ocentrism.

Not surprisingly, the editors chose that strategy so widely adopted in European politics - the middle way. They carefully identified all of the groups of animals and then selected the best known and most interesting species in each. Mammals, birds, fish, amphibians and reptiles are overrepresented - but why not since these are the species on our scale and with which we are most familiar.

Only boffins will quibble at the statistical underrepresentation of the millions upon millions of invertebrate species. The selective coverage of these groups still means that about 15 per cent of the book's pages and a greater percentage of the photographs are devoted to them.

What is missing? Animal is a collection of the world's extant species, so readers will not, for example, find a section on dinosaurs, or for that matter on the perhaps hundreds of millions of other species that once inhabited the earth but which are now extinct. This represents the overwhelming majority of animal species that have ever existed and so it may be reasonable to ask whether a collection of only the living species gives a biased view of evolution and diversity. The simple answer is that it almost certainly does. Had Burnie sat down with his editorial team only a little more than 545 million years ago, there would have been very few animals from which to choose - fewer yet that anyone would recognise today.

A mere 20 million years later, this picture would have changed dramatically owing to an event that palaeontologists call the Cambrian "explosion", a period of exceptionally rapid diversification of animal forms not seen before or since. But about 250 million years ago, and again about 65 million years ago there were catastrophic extinctions; the second one killing, among many others, the dinosaurs.

The earth's fauna are in anything but a state of equilibrium. And yet, collections of photographs make it all too easy to think that the earth was always as they depict - that lions have always lazily walked the savannah and have done so under the gaze of baboons munching nuts and fruits overhead in the trees. But it would be unfair to hold Burnie and his editorial team responsible for this perception. Anything beyond even a cursory attempt at describing the past would have spoiled the focus of this project.

And what is that focus? In the age of dazzling wildlife film photography a collection of mere still photographs might seem dusty and anachronistic. Anyone with a television will have experienced what it is like to fly on a goose's wing, to run like a cheetah, or, and one must tread gingerly here, to mate an elephant. Breathtaking and awe-inspiring as such images are, they can often fail to engage the viewer. By leaving nothing to the imagination, watching modern wildlife films can all too easily become a passive act of being told how everything works and fits together, with little or no room for one's own experience, imagination and emotions. Still photographs retain their appeal and force imaginative leaps upon the viewer, thereby engaging in ways that film, ironically, does not. Animal , then, represents a collection of what the editorial team must hope are iconic photographs, the value of which extends beyond the mere representation of form, to what can be a far more personal collection of image-inspired thoughts and emotions.

The cumulative impression that arises from flicking through Animal is that there are an astonishing number of different ways to be an animal. Have all these different ways evolved by natural selection gradually sculpting animals to fit into pre-existing environmental niches (ways of making a living)? Or, do animals evolve just about any shape or form and then slither off to find a corner of the environment in which they are successful? There are no banana-eating snakes in this book, nor are there any grass-eating bats, suggesting that not all possible ways of making a living are equally likely to be successful. This in turn suggests that the forms that abound on the pages of Animal are not accidental tourists but have been actively crafted by the effects of natural selection. Armed with this point of view, Animal may unwittingly make a statement about the number of different dimensions there are to the current state of the natural environment on earth. This is not the obvious point it may seem. Why, for example, are there no extant land animals as big as the dinosaurs once were?

Animal is therefore distinctive perhaps not just for its marvellous collection of photographs, but also for what it does not explicitly show and say. This volume reveals that there is still a place for the grand picture book, and this one will make a fine addition to anyone's coffee table.

Mark Pagel is professor of evolutionary biology, University of Reading.