Fig. 21. Plot of a used burrow belonging to a female Belding Ground Squirrel, as excavated by J. Grinnell and T. I. Storer in a meadow near the Tioga Road, on Snow Flat, 8,700 feet altitude, Yosemite National Park, June 28, 1915.
Main opening of burrow at a (down this the squirrel went when shot, and was subsequently found at c); two other openings at a' and a"; at b and b' blind branches directed downward (the squirrel may have essayed to dig deeper at these points but was discouraged by water); food materials, including lily tubers, in chamber at f; in the enlargement at the point marked nest was an accumulation of dry grass, the remains of an old nest or the beginning of a new one (the young would not have been born for about two weeks); boulder at g bulged out over the portion of the burrow adjacent to it, affording protection to the supposed nest-chamber.
Diameter of burrow, 45 to 65 millimeters; average of several measurements, 52. Average depth beneath surface of ground, about 135 mm. Greatest depth, at b and b', 330 and 270, respectively. Total length of system, about 54 feet.
The burrow system above described belonged to just one squirrel, an adult female, which was found to contain five embryos, the only exact evidence we have as to the number of young to a litter. This was at 8,700 feet altitude, on Snow Flat, Yosemite Park, on June 28. Half-grown young were common on Tuolumne Meadows, 8,600 feet, July 16. Young two-thirds grown have been taken at Independence Lake, Nevada County, 7,000 feet altitude, as early as July 13. Three third-grown young were seen, and one of them photographed (fig. 22), at 6,800 feet altitude near Williams Butte, Mono County, June 28. There is thus probably some variation in time of appearance of the young, with altitude, the animals at the highest levels being born latest in the season. The young are out quite generally by the 15th of July. Only one litter
71
