The causal factors and function of 'yawning' in Microspathodon chrysurus (Pisces: Pomacentridae).

INTRODUCTION The motor pattern 'yawning' has been described for all vertebrate groups. The most intensive study of this behaviour pattern has been made by SAUER & SAUER (1967) in the South African Ostrich. They suggest that yawning assists in the regulation of the physiological state of these birds. It is primarily released and activated by endogenous stimuli, but can also be facilitated and triggered by environmental stimuli when the organism is sufficiently motivated. It is correlated with the periodic appearance of fatigue in the Ostrich and occurs typically in birds whose motor activities have been reduced to a minimum. It was found to induce a relaxation of tension in a group and trigger sleepiness, assuring excited birds of the absence of danger. Apart from this one intensive study, yawning has been mentioned in many ethological investigations, but its causation and function have usually been merely suggested. Only the motor pattern 'yawning' described for fish will be considered here, as there is some doubt as to whether this behaviour is directly comparable to that seen in birds and mammals. MORRIS (1954) noticed that males of the River Bullhead (Cottus gobio), when first presented with a female, tended to yawn frequently, these yawns being interspersed with the courtship actions. In a later paper on the Tenspined Stickleback Pygosteus pungitus (1958), he suggested that yawning could not be termed a 'displacement activity', but was merely a jaw stretching that occurred most often when the fish was 'unoccupied'. He stated that species with strong jaws tend to yawn more intensely and frequently than other species and that the motor pattern 'yawning' in fish was similar to that observed in mammals. TUGENDHAT (1960) in her study of feeding behaviour in the Three-spined Stickleback, Gasterosteus aculeatus, found that yawning was associated with 'excess initiation' to feeding and with a slow build-up of feeding motivation. It was found to occur at moments of balance between feeding and interrupted feeding and also when balance was maintained for some time. This she attributed to a slow rate of build-up of feeding motivation while it was increasing from a point below threshold level to one just above it. and postulated that yawning should thus appear at the end rather than at the beginning of an interruption, which was found to be the case. A similar association between yawning and interruption of an ongoing behaviour has been noticed in free-living Chromis …