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: “polar compass” or “inclination compass”. Among vertebrates the compass type has been identified for the 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 the field experiments in a large circular arena we identified the direction of stereotypic migration axis on a total of 581 frogs caught during migration from river to ponds or in a breeding pond. We also found that motivation of the frogs varied throughout the day, likely 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 inclination magnetic compass, as do newts, birds, and sea turtles, and potentially use it during the spring migration. The vertical magnetic field confuses the frogs, apparently due to 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.