Oldfield mouse

P. polionotus is omnivores and the principal diet is seasonal seeds of wild grasses and forbs, but blackberries, acorns, and wild peas may be consumed. Insects consumed include beetles, leaf hoppers, true bugs, and ants. Vertebrates are consumed. Beach populations consume the fruits and wind-deposited seeds of sea oats and sea rocket, and feed on invertebrates when seeds are scarce.[1][2][3]

The beach mouse burrows and leaves mounds of earth around the burrow entrance. The burrow slopes down from the entrance for a space then levels off with a nest at its end. A branch of the burrow may extend above the nest to just a few centimeters below the surface as an emergency exit. Should the burrow be disturbed, the mouse "explodes" through the sand via the exit and dash off. The mouse closes any burrows in heavy rains should flood threaten. Spiders, snakes, and other species may move into a burrow.[4]

The mice are monogamous. Gestation is 23–24 days, possibly 25–31 if the mother is still lactating from a previous litter. Mean litter size is three or four. When birth is imminent, the female assumes a crouching position and may rise to a more erect position as each young is born. She may aid the birth by gently pulling on the young with her fore feet, as one mouse has been observed doing. The placenta may be removed in the same manner. She then eats it. The umbilical cord is broken by pulling or chewing or when the placenta is consumed. She may stretch and groom between deliveries. Births usually occur in the daytime and usually over several minutes or even an hour.[4]

The pinkish newborn may be washed by the mother following the last delivery and weigh between 1.1 and 2.2 g (0.039 and 0.078 oz). The pinnae elevate in 3–5 days, the lower incisors erupt in 6–7 days, and the eyes open in 10–16 days. Weaning is gradual and occurs over 20–25 days. First estrus may occur as early as one month and first litter at 53 days of age. The juvenile pelage is gray. At one week, the young weigh 4 g (0.14 oz), at three weeks 6–7 g (0.21–0.25 oz), and at five weeks 8–10 g (0.28–0.35 oz). The young in one study dispersed 430 m (1,410 ft) before establishing their homes.[5]

Presumed predators are those that prey upon small mammals including house cats. Parasites found in mice from Florida were six species of nematodes, one each of trematodes and acanthocephalans, and two of fleas.[5] One mouse survived in captivity 5.5 years.[6]