Adaptations for Life in the Ocean
Whales, dolphins and porpoises are mammals that have adapted to live their entire lives in the ocean. Like all mammals, they give live birth, have hair (very little), breathe air and nurse their young. Therefore, they are much more closely related to humans, bats and koala bears than they are to fish. In fact, whales are most closely related to hoofed mammals like pigs and hippopotamuses. Click HERE to learn more about the classification and evolutionary history of humpback whales.
Humpback whales are well-adapted to move through the ocean. Their bodies are streamlined with only a few bristles for hair on top of the head, no external ear flaps, and internal genitalia. Each whale has a nasal opening on the top of its head that allows it to exhale efficiently and inhale without lifting its head out of the water. Their powerful flukes, a modified 15-foot wide tail, propel them through the water. Humpbacks have the longest flippers (pectoral fins) of any whale, and the leading edge of the fin has ridges that increase their hydrodynamic efficiency.
Humpback whales need to maintain body temperature despite the fluctuations in water temperature between their feeding and breeding grounds. To maintain body temperature, all whales have blubber. Blubber is a highly specialized skin adaptation where fat cells are trapped in a mesh of collagen. Their large body size (40-45 feet) and torpedo shape also helps reduce heat transfer because they have little surface area compared to their body volume. With lots of mass to produce heat and little surface area to transfer it, humpbacks lose heat more slowly. In fact, in warm tropical water whales must keep from overheating by sending blood to their flukes and pectoral fins to allow heat to transfer to the water.
As mammals, humpbacks must come to the surface to breathe. Typically, humpbacks dive for 5-10 minutes although they can hold their breath for much longer if necessary. Unlike land mammals who store the majority of air in their lungs, whales store less than 25% in the lungs with almost half of the oxygen contained in hemoglobin molecules. They also store oxygen in myoglobin molecules in the muscles. When a whale dives, their metabolism and heart rate decrease so that they use oxygen stores more slowly. At the same time blood is shunted away from the extremities. These adaptations allow them to breathe much less frequently than land mammals.
Scraping Together a Meal
Humpback whales target very small prey compared to their body size. They filter-feed primarily on small shrimp-like invertebrate (zooplankton, often krill or euphausiids) and small schooling fish such as capelin, herring and sand lance. They consume about half a ton per day to get their daily requirement of calories. They have special adaptations that help them to forage efficiently. A humpback whale’s jaw bones can flex to maximize the volume of seawater engulfed and when coupled with 14-22 expandable pleats along the lower jaw, a whale can engulf up to 15,000 gallons of water. Once they have a large mouthful, they expel the water by straining it through a tightly packed row of about 400 baleen plates hanging from each side of the upper jaw, and trapping their prey. Each baleen plate is about two feet long, and is fringy on the interior of the mouth to catch the prey, and smooth on the side that faces the outside of the mouth, so the whale can expel the water efficiently.
Whales use a variety of behaviors to capture their food. Humpback whales concentrate prey by taking advantage barriers such as shorelines, kelp beds, the sea floor and the sea surface, as well as oceanographic features such as tide rips and currents and even bubbles. Humpback whales expel bubbles to herd or trap prey, feeding alone, or in a coordinated group using a circle of bubbles, called bubble-net feeding. Click HERE to find out more about this specialized feeding style.
The most common feeding behavior seen is a whale showing its flukes and disappearing below the surface for 4-8 minutes, resurfacing and breathing for a few minutes to replenish oxygen and remove built up carbon dioxide and then repeating the dive. It is not well understood how many mouthfuls of food a humpback whale gets from a single feeding dive, but results from studies of tagged whales indicate that a whale completes several subsurface lunges before coming up to breathe.
Humpback whale foraging is particularly impressive because whales must store enough energy in their blubber to sustain them while they fast during a long annual migration to wintering areas for mating and calving. Sighting histories and genetic studies show that a humpback whale will often return every year to the same feeding grounds where their mother brought them as a calf. In southeastern Alaska about three-quarters of the whales have been documented returning to feed in these waters each year. In some cases, whales join the same feeding groups and even return to the same exact locations within southeastern Alaska to feed at the same time each year.
Migration, Mating and Calving
Most North Pacific humpbacks that spend their time feeding in southeastern Alaska breed and calve in Hawaii with a small proportion migrating to Mexico. The fastest documented humpback migration from Alaska to Hawaii (2,800 miles) was 36 days. Whales start to arrive in Hawaii in October, numbers peak in mid February and March and most have left by April.
Calves usually arrive with their mothers, presumably born along the way, or are born in Hawaii. Occasionally a calf from the previous year will travel with the mother to Hawaii, now as a yearling. Female humpback whales in southeastern Alaska have their first calf at eight years old, at the earliest. Most have their first calf at age 11, and some are not sighted with their first calf until age 14. Conception occurs during the winter with an 11 month gestation. Newborn calves are about 15 feet in length. After their first calf, females generally have one calf every 2-3 years and nurse their calves for about 10 months.
Whale behavior during the winter is much different than summer. While on the breeding grounds males often display aggressive behavior toward other males in competition for access to females. Competitive behaviors include blowing various kinds of bubbles underwater, slapping their heads or tails on the surface of the water and even physical contact with other males. Females, including those with calves, are usually accompanied by at least one male. Females are very rarely seen with another female. During the summer, humpbacks are found alone or in pairs, with little aggressive behavior observed.
Another behavior likely related to mating because it is done only by males and occurs primarily during the breeding season is “singing.” Song is distinct from other vocalizations because it is long and highly structured. A song may last between 10-15 minutes, but is usually repeated, sometimes for hours with the singer usually surfacing to breathe between repetitions. At any given time, all the males at a breeding ground sing the same song although changes to the song occur over the course of the breeding season. Scientists are still unsure why whales sing and how they change their song, apparently simultaneously.
An estimated 28,000 North Pacific humpbacks were killed as a result of commercial whaling, reducing the population to about 10% of pre-whaling numbers, although these estimates are controversial. The International Whaling Commission banned non-subsistence hunts of North Pacific humpback whales in 1965. Humpback whales were further protected under the U.S. Endangered Species Act of 1973.
Recent results of a large study called SPLASH estimated of 18,000- 20,000 individuals in the North Pacific with a growth rate at 4-7 % per year. The humpback whales observed in southeastern Alaska and much of British Columbia are considered one feeding aggregation. This was determined from tracking movements of individuals and genetic studies. SPLASH estimated 3,000 to 5,000 humpbacks return to feed in these waters each summer.
Although most humpback whale populations in the North Pacific are doing well there are areas of concern. The numbers of humpbacks in Asia, which were subjected to prolonged and intensive hunting, are still below historical levels. Also, as humpback whales continue to increase there will be a higher risk of interactions with human activities in terms of vessel strikes and entanglement with fishing gear and marine debris.
Click HERE for a list of references used in this summary.