vii. 21 BEHAVIOUR OF FISHES 225 



bladder is prevented by atropine (Fig. 142) but not by ergotoxine 

 (Fig. 141), though the latter is the drug that in mammals often inhibits 

 sympathetic motor-fibres. In these fishes, therefore, it is not possible 

 to divide up the autonomic nervous system into sympathetic and 

 parasympathetic divisions by either anatomical, physiological, or 

 pharmacological criteria. Presumably the two 'antagonistic' systems 

 found in mammals are a late development, allowing for a delicate 

 balancing of activities for the maintenance of homeostasis. 



21. Behaviour patterns of fishes 



The well-developed receptors and brain of the teleostean fishes 

 constitute perhaps the most important of all factors in giving them 



Fig. 143. The red belly of the stickleback releases attacking behaviour in other males 



and following by females. Of the above models only the two on the left acted as 



releasers. (From Tinbergen, Wilson Bulletin, 1948.) 



their great success. Varied habits and quick actions enable the fish to 

 make full use of the possibilities provided by the special features of 

 their structure — the air bladder, mouth, and so on. The receptors and 

 brain make it possible for the fish to learn to react appropriately to 

 many features of its surroundings. Thus the eyes besides orientating 

 the fish to movements in the visual field allow the discrimination of 

 wavelengths and distinct reactions to differing shapes (see Bull, 1957). 



The social behaviour of many species includes the development of 

 special 'releasers', shapes, colours, or postures that are displayed by 

 one individual and elicit specific reactions in another (Fig. 143). 



There is no doubt that fishes possess great powers of learning. 

 They can form conditioned reflexes involving discrimination of tones, 

 also second-order conditioned reflexes, in which after the animal has 

 learnt to give a certain behaviour in response to a visual stimulus it is 

 then taught to associate the latter with an olfactory stimulus. There 

 are many other examples of such powers, but unfortunately we have 

 as yet little information as to the way in which they are brought about 

 by the brain. Nor have the naturalists provided us with very clear 

 examples of the use of these powers by fishes in nature. There are 



