Nature's fabric stretches to all sizes

As any parent knows, the simple and often innocent question "why?" can have devastating effects on one's fortitude. Its use in the title of this book serves to draw the unwary browser into a topic that is intrinsically fascinating: how animals work. The book takes a rather Attenboroughesque approach to animals and their lives, choosing a combination of well-known and appealing creatures (elephants, hippos, rabbits and such) as well as less well-known but equally fascinating ones (elephant birds, mole rats, various reptiles and insects). Chris Lavers examines these in a scientific framework, exploring the interrelationship of factors such as shape and size, elementary mechanics, metabolic physiology and the environment.

Physiology (how organisms work) has always been a word to conjure with. As an undergraduate, I loved to learn how organisms were adapted to particular environments, but when wrestling with the details of reproductive physiology, muscle physiology and so on, the literature seemed vast, the books too heavy and my practical experiments doomed to failure. Lavers, however, is clearly intent on bringing animal physiology to the attention of the wider public, and he has done so with an accessible, fascinating and well-written book.

The book starts with a romp through the question in the title, but in doing so leaves an intriguing trail of questions: why are there no long, thin, snake-like mammals? Why are there no fly-sized mammals? Lavers poses questions about the things we take for granted in animals such as elephants: they are very big and many of the features that make them characteristically elephantine (pillar-like legs, long trunk, huge ears) are there because they are elephant-sized. These features are contrasted with other animals of different shape and size so that a better appreciation of animal design emerges rapidly and the reader becomes empowered with information and can deduce why, for example, King Kong and Godzilla are physical impossibilities.

The central theme is metabolic physiology - creating an understanding of the chemical "engines" that run the bodies of animals and why these vary from group to group, and the way that such engines are tuned to particular environmental conditions. Elephants are not only massive, they are powered by energetically expensive (endothermic) biological engines that generate chemical heat. Living on hot savannah with little tree cover, they are particularly thermally challenged: adults may be invulnerable to predators, but their body volume compared with the surface area of their skin would lead to a fatal build-up of internal heat were it not for big, flappy ears that act as thermal radiators.

Having established an extreme example of a large animal, Lavers demonstrates how unusual elephants are in the scale of animal life by surveying the total range of sizes from the 6-tonne elephant to the 0.000001g fairy fly. Using this much larger canvas, and bringing in evidence from the fossil record, Lavers takes the reader on a fascinating tour of the diversity and history of animal life from the perspective of environments and the evolution of metabolic regimes to suit changing conditions on earth.

The book ends on a rather sober note. The rise of carbon dioxide emissions in the past decades and concomitant rises in global temperatures have a similar signature (though on a far more rapid timescale) to rises in world temperatures in the distant past, which appear to have been linked to periods of large extinctions on a global scale. Such worries have been expressed at an international political level, but little response of any practical value has been forthcoming. Less widely argued has been the global mixing of organisms as a result of mankind's extraordinary mobility.

The Permian period - which witnessed "the mother of all mass extinctions", when perhaps 95 per cent of all species worldwide perished - occurred at a time when all the flora and fauna of the world could mix together simply because of a freakish accident of plate tectonics: the continents of the world slid together to create one enormous supercontinent (Pangea). The gradual collision would have taken place over a few million years. The question posed is: are we heading for a catastrophe on the scale of the Permian through the double whammy of global temperature rise and homogenisation of our biotas? We do not know. But readers of this book will be greatly sensitised to very important matters of the moment and will have learnt a great deal about animals, life and the earth they live in and on.

David Norman is professor of biology, Smithsonian Institution, Washington DC, United States.