Page:Natural History of the Ground Squirrels of California.djvu/34

620 Ground Squirrel has not to our knowledge been determined; yet the facts at hand, such as the general rate of development of the embryos, and of the young after birth, lead us to believe that it is close to thirty days.

The bulk of the young ground squirrels in any one locality make their appearance with remarkable uniformity as to size and regularity as to date. Our data is incomplete, as to exact time of birth; but we have plenty of records of embryos in various stages of development, and we can observe the time of appearance of the young squirrels above ground. In the lowlands the majority are probably born the last of March, and by the last of April the first born are beginning to appear aboveground, playing about the mouths of the burrows. In the higher altitudes the young are born later. Females in the Transition Zone and lower part of the Canadian Zone had not yet given birth to their young in June. "Spring" in the lowlands comes in April, while the spring of the higher altitudes does not occur until late June or July. Hence the young do appear at the same season, considering the differences in temperature conditions at the different elevations. The accumulation of a certain quantity of heat from without seems to be necessary each year to start the squirrels breeding.

Young California Ground Squirrels may be considered fairly precocious. They ordinarily begin to venture outside their nest burrows when yet very small, in ascertained cases only one-fourth or even one-fifth the weight of the adults. They are then probably not over four weeks old. At Snelling, Merced County, on May 28, 1915, C. L. Camp (MS) observed that young "evidently just emerging for the first time in their lives, seemed confused when they saw a horse and buggy and often ran almost directly under the wheels." Two months later, in the high mountains, the young squirrels behaved the same way. A probably abnormal occurrence was that of a very young squirrel found on April 29 wandering aimlessly about in the grass near a burrow entrance. This squirrel weighed only 61.5 grams, or less than one-tenth the weight of adults. It was practically helpless and would have fallen easy prey to any sort of predaceous animal.

The first litter of young ground squirrels seen aboveground in the season of 1918 by the junior author was noted on April 28 at 1,500 feet altitude on San Emigdio Creek, Kern County. In this litter there were six young at least one-third grown. Judging from the "sign" about the burrow, these youngsters had been foraging above ground for a week or ten days. The season at this altitude was at least ten days later than it was down on the lower parts of the San Joaquin Valley. Several burrows of small diameter and amateurish construction were found at the edge of a thick patch of alfilaria that grew near the nest burrow. These young squirrels in spite of their small size were busily harvesting the heads of the ripening alfilaria and when alarmed ran down the small burrows which each had dug for himself. While the observer was standing over one of these burrows a youngster came up halfway out of a hole six feet away, but catching sight of him gave a hasty alarm note and scurried back down the hole. Twenty-five minutes elapsed after this before any of the young squirrels reappeared above ground.