THE ENVIRONMENT AND REPRODUCTION 4I 



10. PSYCHOLOGICAL FACTORS IN 

 REPRODUCTION 



Evidence has been reviewed that suggests the existence 

 of an internal physiological reproductive rhythm which 

 tends to stimulate the growth of the gonads at some 

 particular season of the year, and to this has been added 

 the information that an animal is commonly dependent 

 on some regularly varying environmental factor in 

 order to render the timing of this rhythm more precise. 

 It is now possible to go one step further, for, granted 

 that by these means the gonads have reached potential 

 maturity, yet another set of requirements, psychological 

 in nature, must be satisfied before successful breeding 

 can occur. In the lower vertebrates these are not obvious, 

 although undoubtedly they will be found to exist when 

 these groups are studied in more detail. Already an 

 example is provided by the bitterling. This is a fish with 

 a complex courtship ceremony which must be performed 

 before the female will lay its eggs, and also necessary at 

 this time is the presence of fresh- water mussel into which 

 the egg can be deposited.** 



Examples of factors of this kind are best understood in 

 birds, and since they have been reviewed in detail by 

 Armstrong^ it is only necessary to mention a few 

 examples here. When the gonads of a bird are approach- 

 ing maturity certain needs must usually be satisfied 

 before the gametes are shed and successful copulation 

 can occur. In the first place the immediate environment 

 must be satisfactory. It is unthinkable that a guillemot 

 might be found nesting in the middle of a field, or that a 

 hedge sparrow might lay its eggs on a seashore. The 

 incompatibility of the environment is probably one of 

 the most important of the factors that tend to inhibit 

 breeding among animals in zoological gardens. 



Even if the bird is in its normal nesting locality it is 

 common to find that as a necessary preliminary to 

 breeding it must lay claim to and occupy an area of 



