Exp^Sta. } LIFE HISTORY OF THE KANGAROO EAT. 17 



centages of breeding and nonbreeding individuals, and this, coupled 

 Avith the fact that the importance of recording carefully all non- 

 breeders was not at first recognized, makes it impossible to tabulate 

 such information reliably. The total of females taken in April, for 

 example, is only 3, of which 1 was breeding; while in June, during 

 the course of poisoning operations, 45 females were examined, of 

 which 21 were breeding. 



Five breeding females were taken in January, all during the last 

 three days of the month. One of these was a suckling female, the 

 young of which were secured alive and were probably at least a week 

 old when taken. This must have been exceptionally early for young, 

 since of a number of adult kangaroo rats taken during the first weeJv 

 of January none have been found to be breeding. Two records from 

 Vernon Bailey are as follows: May 19-June 8, 1903. young specimen 

 in nest (Santa Rosa, N. Mex.) ; June 12, 1889, one female, two em- 

 bryos (Oracle, Ariz.). 



The considerable proportion (which we believe to be more than 50 

 per cent) of nonbreeding females taken during all those months in 

 which breeding has been found to occur may also indicate an ex- 

 tended period of breeding, with a small percentage breeding at any 

 one time. This period also furnishes ample time for the rearing of 

 two litters a year by some females, but we have no evidence as to the 

 occurrence of two litters. Young of the year, practically grown, are 

 taken during and after the month of April. 



The mammae are arranged in three pairs, pectoral, y; inguinal, f. 



Kangaroo rats are among those rodents in which the vagina be- 

 comes plugged with a rather solid material, translucent, and of the 

 consistency of a stiff gelatine, after copulation. This must occur 

 very soon after coitus, since in those individuals taken in this con- 

 dition no definite evidence of the beginning of development of em- 

 bryos could be detected by examination. 



The length of the gestation period of spectdbilis is unknown. The 

 young are born naked, a fact inferred by failure to find an}' fetus 

 showing noticeable hair development, and from the conditions ob- 

 served in such young as have been seen. A suckling female was 

 taken by Vorhies, January 31, 1920, and her den immediately exca- 

 vated in the hope of securing the young. Two juveniles were found 

 in a special nest chamber (see p. 30). These were estimated to be 

 perhaps two weeks old. A serious effort was made to raise the little 

 animals by feeding milk with a pipette and keeping them warm with 

 a hot water bottle, but they survived only 10 days, without the eyes 

 having opened. The uneven temperature as well as the character of 

 the food was probably responsible for their deaths. On February 3 

 they Avere measured and weighed, with the following results : 

 107600—22 4 



