Observations on breeding habits have consisted mainly in taking records
from the females trapped at all seasons of the year throughout the
course of the investigation, and from examinations made during poisoning
operations, and yet from this source the number of pregnant females
taken or of young discovered is disappointingly small. The records
indicate a breeding period of considerable length, extending from
January to August, inclusive. It is possible that the length of the
period may be increased by a second litter from the earliest breeding
females in summer, but the large percentage of nonpregnant or
nonbreeding animals which occurs throughout the season would indicate a
wide variation in the time of breeding of different individuals.
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:
---------------------------------------------------
| | Measurements (in millimetres).
| Weight |-------------------------------
| (in | Total | Tail | Hind
| grams). | length. | vertebrae. | foot.
---------|---------|---------|------------|--------
No. 1 | 13.3 | 90 | 38 | 24
No. 2 | 12.6 | 93 | 38 | 24
---------------------------------------------------
At this stage the young were partially clothed with a coat of fine
velvety fur, more especially on the bodies, the tails being still nearly
naked. The body color was dark plumbeous, just the color of the dark
underfur of the adult, or a shade darker, while the characteristic white
markings of the adult stood out sharply as pinkish-white areas against
the dark background (see Pl. IX, Fig. 2, at p. 32). The proportions were
much as in the adult, except that the tails were relatively much shorter
and the feet relatively longer.
Only one other record of young is at hand, that by Bailey, who secured
the young after capture of a suckling female at Santa Rosa, N. Mex. In
this case the litter contained only one. This was squeaking when found,
but was not large enough to crawl away. Its eyes and ears were closed,
and its soft, naked skin was distinctly marked with the pattern of the
adult, the colors being as given for the other two. This juvenile lived
only a week. Young less than half grown were not trapped or noted in our
poisoning operations outside the dens.
Kangaroo rats, if _spectabilis_ be representative, reproduce at a slow
rate as compared with many other small rodents. We have records of 67
females with embryos or scars showing the number produced, and of the
two litters of young described above. Of the 69 females thus recorded,
15, or 21.7 per cent, had but one offspring each; 52, or 75.3 per cent,
but two each; while only 2 individuals, or 2.9 per cent, had three.
Three young is the maximum litter recorded. This, taken in connection
with the protracted breeding season and lack of sure evidence of the
production of two broods a year, gives a surprisingly low rate of
reproduction, indicating relative freedom from inimical factors.
Our breeding records for _merriami_ are fewer than for _spectabilis_,
but are very similar in every way so far as they go, both as to the time
of year and number of young.