46 HELEN DEAN KING AND HENRY H. DONALDSON 



FEETILITY 



It has long been known from the writings of Buff on (1760), 

 Geoffroy Saint-Hilaire (1862), and Darwin (1875) that do- 

 mesticated animals, as a rule, are more prolific than their wild 

 representatives. To what extent fertility has been increased 

 has not, however, been determined with certainty for any 

 species, since data for wild forms are meager and, from a 

 statistical standpoint, cannot be considered as adequate for 

 purposes of comparison. 



In the rat fertility may be measured both by the number 

 of litters cast and by the number of offspring produced by 

 breeding females during their reproductive life. There are 

 but few statements in literature regarding litter production 

 in wild gray Norways. Zuschlag ('03) assumes eight litters 

 a year to be the average produced by a single pair of rats. 

 Lantz ('10) states that Norway rats probably breed from 

 three to five times a year, while Miller ('11) estimates that 

 five or six litters may be reared yearly. Eaton and Stirrett 

 ('28), from an investigation of the rat population in New 

 Bedford, Massachusetts, are of the opinion that females pro- 

 duce from three to four litters a year. The limitation of 

 numbers in nature is accomplished by a restriction of the 

 reproductive rate, probably through the influence of nutrition, 

 as well as through the action of natural enemies. That inade- 

 quate nutrition has a marked effect on reproduction in the rat 

 has been shown by the investigations of Osborne and Mendel 

 ('17), Evans and Bishop ('22 a), and Slonaker and Card 

 ('23). It is to the copious food supply, without the labor of 

 seeking it, that Darwin (1875) attributes the greater fertility 

 of animals under domestication. 



The average size of the litters in wild rats has been vari- 

 ously estimated by different investigators. Zuschlag 's theo- 

 retical table gives eight as the litter average. According to 

 Crampe (1884), Lantz ('10), and Miller ('11), litters contain 

 an average of ten young. Lloyd 's ( '09 ) observations on preg- 

 nant rats in India showed that litter size ranged from one 

 to twelve, with the average about six. Instances of twenty- 



