Aseasonal Migration of a Northern Bottlenose Whale Provides Support for the Skin Molt Migration Hypothesis.

Why animals migrate is a fundamental question in biology. While the adaptive significance of some animal migrations is well understood (e.g., to find food, to pursue more-favorable habitats, to spawn, or to give birth), others remain unknown. The adaptive significance of whale migration, for example, is unresolved and multiple hypotheses have been proposed to explain it. One recently proposed hypothesis that challenges the long-standing "feeding-breeding" whale migration model is a "feeding-molting" model, where whales undertake latitudinal migrations to warmer waters to molt skin. In July 2019, we attached satellite-tracking tags to northern bottlenose whales () in the Canadian Arctic. One of these tagged whales completed a round-trip movement between the Arctic and the temperate western North Atlantic, traveling 7281 km in 67 days (and spanning 27° of latitude). The whale was tagged in sea-surface temperatures of ~4°C, but migrated south, reaching ~23°C surface waters, where it remained for 7 days before returning to the Arctic. The whale's occupancy of warm water was accompanied by a distinct shift in dive behavior, remaining near the ocean's surface. Four other tagged whales initiated similar long-distance movements. We conclude that feeding or breeding were unlikely reasons for this movement and that northern bottlenose whales migrate to warmer latitudes to molt skin.