Arizona \ 
Exp. Sta. J 
LIFE HISTORY OF THE KANGAROO RAT. 
17 
centages of breeding and nonbreeding individuals, and this, coupled 
with 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 week 
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, X. 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. T ; inguinal, \. 
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 spectaMUs is unknown. The 
young are born naked, a fact inferred by failure to find any fettis 
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 witli 
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 
