All three species are migratory, moving seasonally to feed or give birth. The warm equatorial waters form a barrier that isolates the northern and southern species from one another. In the Northern Hemisphere, Right whales tend to avoid open waters and stay close to peninsulas and bays and on continental shelves, as these areas offer greater shelter and an abundance of their preferred foods. In the Southern Hemisphere, right whales feed far offshore in summer, but a large portion of the population occur in near shore waters in winter. Right whales feed mainly on copepods but also consume krill and pteropods. They may forage the surface, underwater or even on the ocean bottom. During courtship, males gather into large groups to compete for a single female, suggesting that sperm competition appears to be an important factor in mating behavior. Although the blue whale is the largest animal on the planet, the testes of the right whale are actually ten times larger than those of the blue whale[9] â€" with each weighing up to 525 kilograms (1,157 lb), they are by far the largest of any animal on Earth.[10] Gestation tends to last a year, and calves are born at 1 short ton (0.91 t; 0.89 long tons) in weight and 4â€"6 m (13â€"20 ft) in length. Weaning occurs after eight months. Right whales were a preferred target for whalers because of their docile nature, their slow surface-skimming feeding behaviors, their tendencies to stay close to the coast, and their high blubber content (which makes them float when they are killed, and which produced high yields of whale oil). Today, the North Atlantic and North Pacific right whales are among the most endangered whales in the world,[11] and both species are protected in the United States by the Endangered Species Act. The western populations of both are currently endangered, with their total populations numbering in the hundreds. The eastern North Pacific population, on the other hand, with less than 50 individuals remaining, is critically endangered[12] â€" further still, the eastern North Atlantic population, which numbers in the low teens at best, may already be functionally extinct.[11] Although no longer facing a threat from whaling, mankind remains by far the greatest threat to these species: the two leading causes of death are from being struck by ships and from entanglement in fishing gear. Regarding the North Atlantic right whale, for example, these two anthropogenic factors alone account for 48% of all known right whale deaths since 1970.[13]
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