CHAPTER 1 

 BIOLOGY 



1. Life history. 2. Span of life. 3. Puberty. 4. Period of gestation. 

 5. Superfecundation — Superfetation. 6. Fertility. 7. Sex ratio. 8. Recogni- 

 tion of sex. 9. Body weight according to sex. 10. Behavior. 11. Comparison 

 with man. 



1. Life history. The rat breeds at all seasons of the ^'•ear, 

 but most readily in the spring, and judging by other animals, 

 it is probable that those born at this season tend to be more 

 vigorous. The albino rat is born blind, hairless, with a short 

 tail, closed ears and undeveloped limbs. It responds to con- 

 tacts and oKactory and taste stimuli, utters a squeaking sound 

 and is capable of some locomotory movements which are a 

 combination of wriggUng and paddhng. The head is always 

 searching. The young can find their way back to the mother 

 at about ten days of age (Watson, '03). 



The ears open between 2| to 3| days of age but there is a 

 cellular plug in the meatus which may persist for a short time 

 longer. 



According to Wada ('23) rats can hear at from 9-12 days of 

 age. The earher date is exceptional. Lane ('17) reports 12 

 days as the earliest age at which he obtained a response. 



The rat is a swimming animal. The newborn young sink. At 

 two days they make paddling movements mostly with the fore- 

 legs, but remain below the surface. At eight days they swim 

 with the nose above water, while previous to this they have 

 been swimming with the nose more or less submerged. 



The eyes open at from the 14th to the 17th days, most often 

 on the 15th or 16th. King has also observed that in a given 

 Utter the eyes of the females usually open some hours before 

 those of the males. For some seven days more, i.e., up to the 

 time when the young rats are 21-22 days of age, they are 



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