The Comparative Ethology of Vertebrate Breathing

Abstract

The ascent through water to the surface in order to breathe, in animals which spend much time under water, is regarded as an instinct whose consummatory act is the filling of the lungs. The behaviour of Triturus cristatus is different according as it is on land, or floating because its lungs are distended, or supported on a solid object below the water because it is heavier than water. The frequency of ascent to breathe is diminished when oxygen is breathed or dissolved in the water, and increased when nitrogen is substituted for air. It is also sharply increased when the lungs are partially collapsed by increasing the water pressure. Intention movements, and the interaction of breathing and courting movements, are described. A phylogeny of breathing behaviour is suggested: accidental swallowing of air; evolution of an instinct involving rising to seek it; evolution of organs to exploit it for respiration or buoyancy. The appetitive behaviour of this instinct led to land-living forms; development of a skin impermeable to carbon dioxide necessitated regulation by chemical stimuli. These, unlike the physical stimuli asssociated with the older appetitive phase, have an associated psychic component in man only in exceptional environments. In land animals appetitive behaviour is not seen, but may reappear when evoked by the environment, either exceptionally as in men or mice, or integrated with other instinctive activities such as sleep or reproduction, as in the aquatic mammals. It is associated with regulation of buoyancy. In man the consummatory act has lost a fixed action pattern and acquired the ontogenetic plasticity which seems common among mammalian movements. Vocalization is considered as a ritualization of breathing. The phylogenetically complementary drive to obtain water is discussed. If this is defined as an attempt to keep the internal salinity relatively constant, even among mammals it can have another appetitive behaviour, the seeking of air with a preferred humidity. This, including immersion of the whole body in water, seems the phylogenetically older, and drinking does not occur in amphibia. If this drive is called thirst, it is pointed out that it is one of the drives responsible of fish migration. Thirst and air hunger belong to a class of drives amenable to experiment, and easy to discuss from the standpoint of evolution, but whose corresponding sign stimuli are provided by the inanimate environment, and are therefore difficult to present to an animal artificially. A propos of the other class of drives whose sign stimuli are provided by the living environment and can be presented by dummies, we discuss the relation between chemical stimuli evoking the drives in the central nervous system and peripheral chemical sensation.