_Enemies._--Bailey (_loc. cit._) lists hawks, owls, and weasels as natural predators on _Z. princeps_. Stanford (1931:362) records the garter snake (_Thamnophis_) as a predator of jumping mice. A large snake of this genus obtained by him regurgitated two jumping mice a few hours after its capture. Grinnell, Dixon, and Linsdale (1937:232) report that on Parker Creek, in California, H. C. Bryant frightened a weasel that dropped a freshly killed jumping mouse. Crowe (1943:407) reported _Cuterebra_ fly larvae in the inguinal region of a _Z. princeps_ obtained at Invermere, British Columbia. Several mice of this species taken at Moccasin Lake, 19 mi. W and 4 mi. N of Lander, 10,000 ft., Fremont County, Wyoming, were heavily infested with mites of the family Laelaptidae.
_Food._--In early September in central Utah, Moore (1928:154) found only a white, starchy, glutinous paste in stomachs of six _Z. princeps_ and only traces of a brown seed coat in a seventh. The main seeds eaten seemed to be from an introduced brome-grass which was abundant in the vicinity of capture. Bailey (1932:226) wrote of _Z. princeps_ in New Mexico, that "In feeding they cut down the tall grass, beginning at the bottom and cutting the stem at intervals as high as they can reach until the seed part of the grass is brought down." He (_op. cit._:227) remarked that the food was almost entirely seeds of grass and grasslike plants and that the stomach contents almost always were perfectly clean white dough from the shelled kernels of small seeds.
_Reproduction._--Females with embryos have been collected from late May to mid-July and lactating individuals until late August. Possibly there is only one litter per season as Davis (1939:336) suggests is the case in Idaho.
Embryos in 25 pregnant females averaged 5 (2-7). The mammae of the female are arranged in four pairs (two abdominal, one pectoral, and one inguinal).
_Z. princeps_ builds a grass nest on the ground which is placed under cover of vegetation or surface litter. Bailey (1932:227) writes that in New Mexico jumping mice of this species use fibers of grass to construct a ball-shaped nest. The nest usually has one opening but sometimes there are two. In the Ruby Mountains, of Nevada, Borell and Ellis (1934:37) found the globular nests of this mouse on the ground in tall grass.