596 



SCIENCE 



[N. S. Vol. XLVII. No. 1224 



we have had excellent success with this food. 

 If table scraps are not available, cooked beans, 

 nuts and meat two or three times per week 

 should be provided. The amount of food 

 given should always be greater than will be 

 consumed if one is desirous of quick returns. 

 When rats are fed on a sufficient quantity of a 

 well-balanced ration they are very prolific and 

 grow rapidly. 



A pair should be placed in each compart- 

 ment. The female comes in heat about every 

 five days and the period of gestation is ap- 

 proximately 21 days. The period of gestation 

 is prolonged a few days if the female is nurs- 

 ing a litter of young during this time. Nu- 

 merous instances are on record where mating 

 has occurred the same day that young were 

 born. 



The number of young in a litter varies from 

 one to fifteen, the average number being about 

 six or eight. They grow rapidly and can be 

 weaned at 30 or 35 days of age. If one de- 

 sires to maintain a pure breed the young 

 should never be allowed to remain with the 

 parents after they have reached the age of 

 fifty days, as breeding is likely to occur. A 

 litter should always be weaned regardless of 

 age as soon as (preferably just before) a new 

 litter is born to prevent starvation of the new- 

 born. In case the weaned litter is very young 

 (25 to 35 days) milk should be added to their 

 diet. To prevent inbreeding the sexes should 

 be separated at weaning and confined in sepa- 

 rate cages. With proper food, however, in- 

 breeding can go on without apparent detriment 

 for a number of generations. 



The sexes in the young may be distinguished 

 by the following characteristics: 



The males may be recognized by a greater 

 distance between the anus and the genital 

 papilla. 



In the m'ale the genital papilla is larger 

 than in the female. 



At about 15 days of age the nipples are 

 visible in the female. 



After the hair covers the body a strip ex- 

 tending from the anus to the genital papilla 

 remains almost bare in the female, while in the 

 male this region is covered with hair except a 



small area immediately below the anus which 

 later becomes the scrotum. 



After the descent of the testes into the 

 scrotum the males can readily be distin- 

 guished. 



The age at which the young females become 

 sexually mature varies between rather wide 

 limits, but usually between 70 and 90 days. 

 The earliest age at which we have found them 

 sexually mature is 69 days. Lantz' records 

 a case of sexual maturity at the early age 

 of 35 days and Jackson^ one at 49 days. Sexual 

 activity in the females may continue until 

 they have reached the age of 600 days. We 

 have not determined the ages at which sexual 

 activity begins and ceases in the male. 



A great difference is noticed in the ability 

 of females to produce young. Some appear to 

 be sterile, while others have given birth to as 

 many as nine litters. If one is desirous of 

 securing numbers, the offspring from prolific 

 breeders should be selected for breeding. 



The rats do best in a well-ventilated room 

 of fairly uniform temperature. Extreme tem- 

 perature should be avoided. 



Since these animals need daily attention 

 they can not be shipped long distances unless 

 provision is made for watering and feeding 

 them en route. Our available supply is quite 

 limited but we can generally furnish a few 

 pairs to any one within shipping distance who 

 is willing and able to breed rats for purposes 

 of supplying the government, or for general 

 scientific research. 



The three main items for success are clean- 

 liness, a sufficient quantity of a balanced 

 ration, and avoidance of great changes in 

 temperature. With these carefully looked 

 after success is assured. 



J. E. Slonaker 



Physiological Laboratories of 

 Stanford XJniversitt 



1 David E. Lautz, ' ' Natural History of the Eat, ' ' 

 In ' ' The Eat and its Eelation to the Public 

 Health, ' ' by various authors. P. H. and M. H. 

 Service, Washington, 1910. 



- C. M. Jackson, ' ' On the Eecognition of Sex 

 through External Characters in the Young Rat," 

 Biological Bulletin, Vol. XXIII., No. 3, August, 

 1912. 



