CHAPTER 15 

 THE BREEDING BEHAVIOR OF ANIMALS 



Reproduction in which both sexes are involved is dependent upon the 

 uniting of the germ cells, proper conditions for the development of 

 the fertilized egg, and conditions suitable for the development of the 

 immature animal. The parents often do more or less to ensure these 

 events and conditions, to guard against accident to the immature off- 

 spring, and to help it over the period of its own helplessness. These 

 services of the parents are habitual and are known collectively as breed- 

 ing behavior. 



Breeding behavior in the animal kingdom is exceedingly varied. 

 There are two apparent reasons for this variety. First, different forms 

 have different modes of life, and the breeding habits must be suited to the 

 manner of living if they are to accomplish their purpose. Second, the 

 increasing complexity attained in the higher forms of life apparently 

 necessitates in them a longer period of prenatal development. At least, 

 the development before birth or hatching is longer in the complex forms 

 than in the simpler ones. The differences in behavior are not characters 

 that distinguish large groups of related animals from one another, for 

 within these groups there is considerable dissimilarity in breeding habits. 

 Even closely related tree frogs, for example, may employ very different 

 means of assisting the processes of reproduction and development. 

 Because of this diversity no attempt will be made to describe in detail 

 the various breeding habits of animals, but rather to classify and sum- 

 marize and to introduce just enough detail to illustrate in concrete 

 manner the several types of breeding behavior. 



Urinogenital Systems. — Since some features of the breeding habits 

 of animals are dependent upon the structure of their reproductive organs, 

 these must first be examined. In vertebrate animals the reproductive 

 and excretory systems are intimately connected and together they com- 

 prise the urinogenital system. The excretory system of the frog has 

 already been described (page 13G). In both sexes of the frog the gonads 

 (meaning testes or ovaries) develop ventrally to the kidneys and here they 

 hang suspended in sacs of peritoneum. This relation is most plainly seen 

 in the male and in young females whose ovaries have not yet become 

 voluminous. 



The oviducts are coiled tubes passing by the ovaries (Fig. 147, left). 

 Each oviduct takes its origin in a ciliated funnel which lies near the 



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