Book excerpt: We teach our students of behavioural science that one first defines a research problem, and then the most appropriate animal is selected to investigate hypotheses. The reverse order of events is improper: a particular class of animals should not be studied for its own sake. In the case of the Pinnipeds (seals, sea lions, fur seals and walruses) the organism and the problem are essentially the same. The research questions presented in this volume in one way or another relate to survival in two worlds, the ocean for foraging, and the terrain at its edge or frozen above it for breeding. The evolution of Pinniped behaviour and the mechanisms which underlie it are a consequence of having to cope with two seemingly incompatible sets of environmental constraints. The physiological adaptations for concomitant functioning in two media with very different physical characteristics have produced correlated behav ioural modifications. The energetic demands of reproduction and foraging are idiosyncratic because each activity occurs on opposite sides of the air/water interface. As a result, the mating system must reconcile aquatic design for such functions as locomotion and thermoregulation, with the terrestrial requirements for successful pupping. Similarly, the ecology of this dual habitat prescribes the rules governing the behaviour of the neonate and its interactions with its mother.