The Causal Factors and Function of Yawning' in Microspathodon Chrysurus (Pisces: Pomacentridae)

The behaviour pattern 'yawning' in Microspathodon chrysurus has been shown to be initiated by high levels of excitement and low levels of kinetic activity. High excitement can be either endogenously produced by frustration and thwarting or by exogenous visual stimuli of a non-specific type. Addition of the hormone ACTH to the aquarium water also increases yawning frequency. When kinetic activity is artificially raised, even though excitement is high, yawning frequency is much decreased and there is no increase in yawning when visual stimuli are presented. This reciprocal relationship between kinetic activity level and yawning indicates that this behaviour probably functions as a regulator for excitement/kinetic activity level discrepancies. It does not appear to have a 'dearousing' function in fish, but functions rather in raising kinetic activity, activity in general having been shown to increase after the performance of a yawn. Yawning in fish is not thought to be analogous to the motor pattern of the same name in birds and mammals from three points of view: I) It is not associated with sleepiness and relaxation of tension, but more with increased activity, 2) It does not appear to be associated with breathing and oxygen/carbon dioxide discrepancies in the brain 3) It does not have the 'infectious' quality in fish that it has in the higher vertebrates.