With a barrel-shaped body and pectoral fins protruding like oversized wings, the humpback whale is not designed for speed. Yet the marine mammal, which can reach lengths of 50 feet, is known for its impressively long travels between warm-water breeding grounds in the winter and cold-water feeding grounds during the summer.

“Humpbacks arguably make the longest documented migration of any mammal,” says marine mammal research biologist John Calambokidis, who has studied the whales since 1986.

While some humpback whales have been known to migrate from the Antarctic Peninsula all the way to the tropical waters off Costa Rica, the marine mammals are not exactly known for their ability to swim fast.

“The speeds that a lot of these whales that make long migrations travel at is often not that impressive,” Calambokidis says. “In other words, they can be just as slow as three to five miles an hour. But the impressive part is they are doing that 24 hours a day. That means they can be covering 100 miles in a day. They can cover these rather long distances . . . in a month or two.”

Although humpbacks were once hunted to near extinction, since achieving federal endangered species status their population has rebounded. They are found in oceans all over the world. The Northern Hemisphere populations reside in the North Atlantic Ocean and the North Pacific Ocean. The southern group spends its time feeding off the coast of Antarctica. During the winter, both groups head to warmer waters for breeding and raising their young.

According to Calambokidis, there is some scientific debate about why humpback whales migrate so far. It might be because the giant creatures, and their vulnerable newborn calves, need warmer waters to reduce energy loss during the winter.

There’s also the possibility that humpback whales migrate as a strategy to escape predation by killer whales. Calambokidis says that humpback whales are able to defend themselves against killer whale attacks easier if they are in shallow, warm waters rather than deeper cold-water ocean regions.

During their three-week to two-month migrations, humpback whales seldom eat. They live off body fat accumulated before embarking on their journey. Calambokidis says that it’s hard to tell if humpbacks fatten up as a way of preparing for their travels or because they somehow know that prey will be less abundant in the near future. Humpback whales mostly eat tiny shrimp called krill, which are found in the icy waters of the Arctic and Antarctic.

“You could say they are fattening up for the long migration,” he says. “If they do the long migration because of prey that aren’t as abundant in the winter, it’s kind of a chicken and egg thing. It’s preparing to migrate by feeding heavily, but the reverse is also true: the long migration is driven by the fact that [the whale] can only feed heavily during part of the season.”

How the humpback whales know where to travel during their migrations still has scientists stumped. “I am not sure anyone has a clear answer to this other than speculation,” Calambokidis says.