Summer Movements of Adult Leopard Frogs, Rana Pipiens Schreber, in Northern Michigan

The purpose of this study was to determine the nature of the movements of adult Rana pipiens in summer and the factors controlling them. Movements in a population in Cheboygan County, Michigan, were studied by means of a trailing device for following individual frogs, by weekly censuses of the study areas, and by direct observation. Trailing was accomplished by means of a spool of thread, mounted in a holding device on the backs of adult frogs; this unwound as the animals moved, marking their routes. One hundred thirty—six frogs were trailed for a total of 681 frog—days (one frog/one day); individuals were trailed continuously for periods up to 35 days. In fair weather in summer, leopard frogs on their home ranges in the fields typically spent more than 95% of a day's time sitting quietly in "forms," made by clearing the wet soil of dead vegetation. Several remained in the same form for more than 24 hr, one for more than five days. Travel on the home range occurred only infrequently during each day, and usually consisted of a shift from one resting spot (form) to another, seldom more than 5 to 10 m apart and usually much less. Such movement took place at all times of the diel period, but almost two—thirds of the total distance moved by frogs on their home range was covered between 8 pm and 8 am, presumably during the night. Movement within the home range formed complicated patterns of turns, doublings, and criss—crossed paths. Many frogs returned repeatedly to particular parts of their range, often to certain forms which seemed to have some significance as homes. Occasional brief trips into adjoining regions uninhabitable by frogs may have been exploratory, or attempts to reoccupy former parts of the home range which were no longer suitable for occupancy. Leopard frogs which had home ranges in wet woods seldom construct forms but instead, frequently used crevices and cavities as retreats. It is suggested that Rana pipiens may be a more common resident in wet forests than is generally realized, but is seldom seen in such places owing to its secretive habits. In nocturnal rains leopard frogs occasionally made extended excursions off their ranges. Such movement differed from home range movement in being direct, more or less continuous through the night, and often covering distances of 100 m or more; one trailed frog moved 159 m in a single night. These migratory movements stopped at daybreak, the frogs commonly remaining in the region they had reached for several days, unless forced by unfavorable moisture conditions to move to more moist regions. Occasionally the migration was continued on the night following the initial movement; one trailed frog traveled 240 m in two consecutive nights. Heavy rains of long duration in which almost the total population emigrated produced a marked, though temporary, effect on dispersion. Although leopard frogs were almost entirely confined to regions of moist soil in fair weather, during and immediately after heavy rains they scattered widely. Migratory frogs, even those which reached areas suitable for permanent occupancy, showed a strong tendency to return to their ranges following nocturnal excursions. Of 30 subadult and adult frogs captured off their range during or after rains, 25 were later taken on their home range or had mowed toward it. The function of the nocturnal migratory excursions is now known; they probably enable the animals to become acquainted with a large area around their home ranges. The most rapid rate of movement determined for a trailed frog during a migration was 46.6 m/hr. The distance traveled, hence rate of movement, showed a significant positive correlation with temperature. Rain appeared to be the environmental factor initiating migratory movement.