may be received because of the progressive diminution in volume. The sound is of a quality to carry well, yet even at very close range it rarely sounds loud. The direction of the performer is usually hard to fix. This shifting, ventriloquistic quality goes well with the shimmering landscape and elusive behavior of the animal, with which it is usually associated in our experience.
The breeding season begins about the first of March and, in its various phases, lasts ordinarily until the end of May. At the highest altitudes the program is evidently retarded some because of the later advent of warm weather. There is nothing to indicate that more than one litter is produced each year by one female. The instances of late appearance of young (for example, in August), where not accounted for by altitude, would seem likely to be due to individual variation in time of development of the reproductive instincts or else to abortion or early death of the first litter. The following is the more or less exact breeding data given in the field notes on file in the Museum of Vertebrate Zoology.
The earliest date for embryos is February 20 (1910) near Needles on the Colorado River; the number of embryos was eight. On March 11 (of the same year), in Chemehuevis Valley, south of Needles, a female was taken containing five embryos. The weather was yet cold, ice on standing water at night. It seems to be a rule with the squirrel family in the desert that the breeding season is so timed that the young of the year are well grown long before the period of intensest summer heat.
March 11 to 16 (1918) thirty Antelope Squirrels were trapped or shot in the vicinity of Mohave. Of these, seventeen were males and thirteen were females, all adult and in breeding condition. The testes of the males were huge, measuring up to three-fourths of an inch in length. The uteri of the females were heavy-walled, but in only one case were there yet any embryos; one taken March 11 contained five well-developed embryos. The males were lean; the females all more or less fat. On March 18 (1914) at Victorville two females were taken containing thirteen and fourteen embryos, respectively. On March 27 (1907), at the same place, a female was found to contain eight embryos. On April 6 (1918), at Olancha, two females contained nine and ten embryos, respectively; and on April 12 one was found to contain seven embryos. On April 24 (1912) at Keeler a female was taken which contained six embryos.
The average number of young per litter as figured from the above records of embryos is close to nine, with five and fourteen as extremes. Stephens (1906, p. 75) considers five to eight as the usual number. Nelson (1918, p. 443) gives four to twelve. Mearns (1907, p. 301) records that near Mountain Spring, in May, 1894, ten small young of uniform size were caught from one hole.
The mammæ are generally in five pairs, occasionally in six, rarely five on one side and six on the other.
The earliest date we have for the appearance of young aboveground is April 23 (1917) at Furnace Creek Ranch, Death Valley; one youngster scarcely one-fifth grown (its weight was but 17.7 grams) was found wandering about weakly under a mesquite (see fig. 27). The next date is May 13 (1908) for third-grown young at Cabezon, and records for
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