European common frogs determine migratory direction by inclination magnetic compass and show diurnal variation in orientation.

Animals can use two variants of the magnetic compass: the 'polar compass' or the 'inclination compass'. Among vertebrates, the compass type has been identified for salmon, mole rats, birds, turtles and urodeles. However, no experiments have been conducted to determine the compass variant in anurans. To elucidate this, we performed a series of field and laboratory experiments on males of the European common frog during the spawning season. In field experiments in a large circular arena, we identified the direction of the stereotypic migration axis for a total of 581 frogs caught during migration from river to pond or in a breeding pond. We also found that motivation of the frogs varied throughout the day, probably to avoid deadly night freezes, which are common in spring. The laboratory experiments were conducted on a total of 450 frogs in a T-maze placed in a three-axis Merritt coil system. The maze arms were positioned parallel to the natural migration axis inferred on the basis of magnetic field. Both vertical and horizontal components of the magnetic field were altered, and frogs were additionally tested in a vertical magnetic field. We conclude that European common frogs possess an inclination magnetic compass, as for newts, birds and sea turtles, and potentially use it during the spring migration. The vertical magnetic field confuses the frogs, apparently as a result of the inability to choose a direction. Notably, diurnal variation in motivation of the frogs was identical to that in nature, indicating the presence of internal rhythms controlling this process.

Orientation in the European common frog Rana temporaria during the first wintering migration.

The orientation of naive animals during their first migration is extensively studied in birds and sea turtles, whereas the data for other groups such as amphibians are still scarce. To date, it is unknown whether young-of-the-year anurans perform a random or directional search for the hibernation sites, and what cues (global or local) they use. We conducted a series of field experiments to study the orientation behavior of juvenile common frogs during their first wintering migration. We captured 1614 froglets from two subpopulations with different directions of migration and assessed their orientation in large circular outdoor arenas (20 m in diameter) on the opposite sides of the river. Before the migration, froglets used local cues and moved back towards the forest (summer habitat). At the start of migration, the froglets did not move randomly: they navigated towards the river using local cues. Later, however, before approaching the hibernation site, they memorized the compass direction of migration and followed it using global cues. Orientation along a memorized compass heading begins to dominate in the hierarchy of orientation mechanisms, and this predominance is maintained even after reaching the hibernation site. Unlike in birds, no innate direction of migration was found.

Marsh frogs, Pelophylax ridibundus, determine migratory direction by magnetic field.

Orientation by magnetic cues appears to be adaptive during animal migrations. Whereas the magnetic orientation in birds, mammals, and urodele amphibians is being investigated intensively, the data about anurans are still scarce. This study tests whether marsh frogs could determine migratory direction between the breeding pond and the wintering site by magnetic cues in the laboratory. Adult frogs (N = 32) were individually tested in the T-maze 127 cm long inside the three-axis Helmholtz coil system (diameter 3 m). The arms of the maze were positioned parallel to the natural migratory route of this population when measured in accordance with magnetic field. The frogs were tested under two-motivational conditions mediated by temperature/light regime: the breeding migratory state and the wintering state. The frogs' choice in a T-maze was evident only when analyzed in accordance with the direction of the magnetic field: they moved along the migratory route to the breeding pond and followed the reversion of the horizontal component of the magnetic field. This preference has been detected in both sexes only in the breeding migratory state. This suggests that adult ranid frogs can obtain directional information from the Earth's magnetic field as was shown earlier in urodeles and anuran larvae.