Enemies.—V. Bailey (1927:119) reports that A. K. Fisher found 50 skulls of Zapus in barn owl pellets taken from the towers of the Smithsonian Institution, Washington, D. C. Dearborn (1932:32) reported mink as having fed on jumping mice. Surface (1906:197) records taking a Zapus from the stomach of a rattlesnake. Pearson and Pearson (1947:138) found remains of Z. hudsonius in pellets of barn owls. Quimby (1951:74) reports two cases of predation on Z. hudsonius; one was by a northern pike, Esox lucius Linnaeus and the other was by a weasel, Mustela sp. Vergeer (1948:91) collected a green frog, Rana clamitans Latreille, which had eaten a jumping mouse.
Quimby (1951:74) frequently found the fleas, Megabothris quirini Rothschild, and Megabothris wagneri (Baker), and occasionally a larval tick, Dermocenter variabilis (Say), on Z. hudsonius. Sheldon (1934:296) remarks that captive animals are burdened with numerous fleas. Hamilton (1935:191) removed a louse from a jumping mouse. One mouse had a hole in the throat and three others had holes in the inguinal region; presumably bot-flies had emerged from these holes. Test (1943:507) found a single Cuterebra larva in the inguinal region of a Z. hudsonius, and Sheldon (1938:328) found Z. hudsonius infested by larvae of Cuterebra fontinella Clark. Here, as in other cases, these larvae were found immediately below the skin. Erickson (1938:252) examined 18 Z. hudsonius obtained in Minnesota, and found that three were parasitized. He found a bot-fly larva, Cuterebra sp., nematodes of the genera Subulura and Spirocerca, and a fluke of the genus Notocotylus.
Food.—Quimby (1951:85-86) studied the food preferences, by presenting to caged Z. hudsonius the plants and invertebrate animals normally available to these mice in nature, and indicates that in general, the starchy fruits of the Gramineae and the less fleshy fruits of various groups of plants are more heavily utilized than other plant materials. His observations indicate that these rodents are highly insectivorous and that they consume many insects under natural conditions. Goodwin (1935:148) reports that the stomach contents of several individuals obtained at South Woodstock, Connecticut, consisted exclusively of blackberries, and that others had subsisted principally on cranberries. Hamilton (1935:197) remarks that seeds are the favored food but that berries, nuts, fruits of various kinds, roots, and insects are also utilized. Stoner (1918:123) writes that the food in cultivated areas of Iowa is various grains as well as grass and weed seeds; in wooded places the mice feed on seeds and nuts of trees. Vernon Bailey (1927:118) states that the examination of a great many stomachs of these jumping mice [in North Dakota] revealed nothing “but the fine white pulp of carefully shelled, well-masticated seeds. Generally these are from grasses, although grain and a variety of other plant seeds are eaten.” Schmidt (1931:116) examined the stomach contents of several Z. hudsonius taken in Clark County, Wisconsin, and in most stomachs found the remains of finely chewed roots; however, two from Hewett had eaten several geometrid caterpillars.
Lyon (1938:279), Stoner (1918:123), and J. W. Bailey (1946:263) present information which indicates that Z. hudsonius stores food in its nests or burrows. Possibly these mice awaken at intervals from hibernation and eat.
“These rodents characteristically seize the material to be eaten with the front feet and devour it while reclining on their haunches. The following observation of a caged animal is typical of their feeding habits. The mouse selected a head of yellow foxtail, Setaria glauca (Weig.) Stuntz, from several in the cage, separated it by gnawing through the supporting stem, seized it with the front feet, held it up to the mouth and began to gnaw at one end, stripping all parts from the rachis. The grass head was slowly rotated and shifted sideways until nothing remained but the rachis which was discarded. Actually the seeds were the only parts eaten …” (Quimby, 1951:73). Sheldon (1934:294) remarks that Z. hudsonius eats from a squatting position and holds the piece of food in the forepaws. The mouse seems to bite off a seed, and then, holding it in the forepaws, transfers it to the mouth.
According to Sheldon (op. cit.:295) and Quimby (loc. cit.), caged jumping mice drink water. When drinking, the mouths of the mice are in contact with the water, but neither observer determined whether the mice lapped or sucked the water. Sheldon (loc. cit.) observed these mice passing stems of long grass through their mouths as though to squeeze out moisture, and thought that the mice obtain most of their required moisture from green plants.
Reproduction.—The breeding season begins shortly after the jumping mice emerge from hibernation in the spring, and reproduction continues until a few weeks before they hibernate in the autumn. The extent of the breeding period probably varies geographically and possibly seasonally. For example, Quimby’s (op. cit.:70) information suggests that the 1947 period of parturition occurred between June 15 and August 30 in the area of Centerville, Minnesota. In Michigan, Blair (1940:246) found a peak of breeding activity in spring and another in late summer with little activity in the intervening midsummer. Brimley (1923:263) records a female in North Carolina, with eight embryos on June 13, 1895, and another with seven embryos on September 17, 1891, indicating a strong possibility of two litters per year there. Vernon Bailey (1927:118) records young born in May or June in North Dakota and thinks that there is time for only one litter per year. Petrides (1948:76) captured a female on September 22, 1944, at Athens, Georgia, that gave birth to six young on September 29. This late parturition date indicates a longer breeding season in the southeastern part of the range of Z. hudsonius.
The gestation period of nonlactating, caged Z. hudsonius, Quimby (1951:63) thinks, “is approximately 18 days … [but] gestation is prolonged in lactating females.”
Data from museum labels indicate that embryos in 62 pregnant females averaged 5.4 (2-8) per female. Quimby (1951:67) found the average number of embryos per female for 14 females taken in Minnesota, to be 5.3 and that litters of young found in nests averaged 5.8. Sheldon (1938:330) reports two litters of seven young each and one of four young for Z. hudsonius in Vermont. Petrides (1948:76) records a litter of six young for Z. hudsonius in Georgia. Brimley (1923:263) records one lot of seven and one lot of eight embryos for Z. hudsonius in North Carolina. Vernon Bailey (1923:120) reports six embryos for a female of Z. hudsonius taken in Washington, D. C. Ivor (1934:8) obtained a litter of five young Z. hudsonius from Erindale, Peel County, Ontario. Hamilton (1935:195) records litters of two, four, and five young and embryo counts of four, two, four, and four for Z. hudsonius in New York.
There seems to be two litters per year. According to Quimby (1951:69), “most adult females breed soon after emergence from hibernation and produce the first litters within a month. The remaining females do not breed immediately but produce the first litter,” he says, “in the second month after emergence.” Both early-breeding females and late-breeding females produce at least 2 litters per year. Those that breed early may have 3 litters.