Trapping in February and March for the purpose of securing greater numbers of female specimens, begun with the idea that these months were most likely to be the breeding months, has invariably yielded an unsatisfactory number of nonbreeding specimens and males.  Unfortunately, the numbers of females secured in some months were not sufficient to be significant if worked out in percentages of breeding and nonbreeding individuals, and this, coupled with the fact that the importance of recording carefully all nonbreeders 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, N. Mex.); June 12, 1889, one female, two embryos (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 extended 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, 1/1; inguinal, 2/2.

Kangaroo rats are among those rodents in which the vagina becomes 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 condition no definite evidence of the beginning of development of embryos could be detected by examination.

The length of the gestation period of spectabilis is unknown.  The young are born naked, a fact inferred by failure to find any fetus showing noticeable hair development, and from the conditions observed in such young as have been seen.  A suckling female was taken by Vorhies, January 31, 1920, and her den immediately excavated 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 were measured and weighed, with the following results: