Minkes tend to be solitary animals, though sometimes they are seen traveling in pairs or in small groups of 4 to 6. In the polar regions, where food is concentrated, it is common to find larger aggregations of feeding animals in an area. They appear to segregate by age and sex more than do the other baleen whales. Females remain close to shore, while males are farther out to sea. Some minkes migrate long distances, but others may move only within a restricted area. In some regions, minkes may be found year-round. Their life span is believed to be about 50 years. Killer whales are known to prey on minkes, especially in parts of the southern hemisphere. The taxonomy of minke whales is currently in question, and soon there may be three species of minke whales: the Antarctic minke whale (relatively large and lacking a flipper stripe), the dwarf minke (smaller than Antarctic, has a flipper stripe, lives in tropical southern hemisphere waters), and the true minke whale (flipper stripe present, lives in the northern hemisphere).