Paul Walton

Avian diving, respiratory physiology and the marginal value theorem

Behavioural studies of diving birds have reported that the ratio of dive duration to the duration of the subsequent period on the surface displays a characteristic relation to dive duration. For short dives, the dive to surface ratio increases with dive duration, whereafter the relation peaks, and for longer dives decreases with increasing dive duration. Such a relationship is not a general prediction of existing marginal value models which have been used to predict optimal diving behaviour. This may be because the smooth curve used to describe the oxygen gain rate of individuals after surfacing is not a good reflection of the respiratory physiology of birds. Here we argue that on physiological grounds, the oxygen gain curve for avian divers will not be smooth, but will have two distinct regions (representing oxygen recovery in the respiratory tract, and in haemoglobin and myoglobin, respectively). Modifying two of the classical diving models by incorporating such a kinked curve causes them to predict the humped relationship between dive to surface ratio and dive duration under many circumstances. We also present data on the duration of dives and surface periods from three species of diving seabirds: the shag, Phalacrocorax aristotelis, the black guillemot, Cepphus grylle and the common guillemot, Uria aalge. All three species showed a humped relationship for dive to surface ratio as a function of dive duration. In line with the predictions of our model, when oxygen stores on surfacing were greatly depleted, the dive to surface ratio peaked at short dive durations. Copyright 1998 The Association for the Study of Animal Behaviour.

Bad News for Trade Unionists

Ever since the invention and development of the printing press by Gutenberg in 1457 mass media technology has played an increasingly important role in the development, form and struggle over ideas. The first mass use of any media technology was by Luther and his supporters in 1517. Luther’s demands nailed up on the door of the Cathedral at Wittenberg were reproduced on posters and within weeks had appeared throughout Germany and France. By 1815 the power of the Press had become widely acknowledged among ruling elites; indeed Napoleon shrewdly suggested in his maxims that, ‘Four hostile Newspapers are more to be feared than a thousand bayonets.’