The reasons why whales migrate are complex. They may move to a warmer place in the winter to reduce the risk of predators attacking vulnerable newborn calves, or they may travel to areas where food is more abundant. However, some researchers have suggested that the primary reason is to give birth and raise young.
This explanation is based on the observation that many baleen whale species have long, annual cycles of molting their outer layer of skin. Molting typically occurs in the tropics, and researchers believe that this is the primary reason why some whales migrate to the tropics in the winter, to molt their skin in a warm environment.
Other researchers have observed that some high-latitude whales, such as belugas in the Arctic, gather each summer in river estuaries, which are typically warmer and shallower than their main habitat. They initially thought that this was a unique behavior, but now scientists think that it could be the rule rather than the exception for high-latitude whales.
In addition to these common explanations, some researchers have also proposed that whales migrate to take advantage of rich feeding grounds in the colder areas during the summer and then return to their warmer tropical breeding and calving areas in the winter. This allows them to maximize their energy reserves during a season of intensive feeding and minimizes the risks of giving birth and raising young in the harsher, higher latitude conditions where they would be more susceptible to predators and environmental changes such as large ice caps.