Friday, July 10, 2009

Humpback Whales

Found in oceans and seas around the world, humpback whales typically migrate up to 25,000 kilometres each year. Humpbacks feed only in summer, in polar waters, and migrate to tropical or sub-tropical waters to breed and give birth in the winter. During the winter, humpbacks fast and live off their fat reserves. The species' diet consists mostly of krill and small fish. Humpbacks have a diverse repertoire of feeding methods, including the bubble net feeding technique.
Like other large whales, the humpback was and is a target for the whaling industry. Due to over-hunting, its population fell by an estimated 90% before a whaling moratorium was introduced in 1966. Stocks of the species have since partially recovered; however, entanglement in fishing gear, collisions with ships, and noise pollution also remain concerns. There are at least 80,000 humpback whales worldwide. Once hunted to the brink of extinction, humpbacks are now sought out by whale-watchers, particularly off parts of Australia and the United States.
Humpback whales are rorquals (family Balaenopteridae), a family that includes the blue whale, the fin whale, the Bryde's whale, the Sei whale and the Minke whale. The rorquals are believed to have diverged from the other families of the suborder Mysticeti as long ago as the middle Miocene. However, it is not known when the members of these families diverged from each other.
Humpback whales can easily be identified by their stocky bodies with obvious humps and black dorsal colouring. The head and lower jaw are covered with knobs called tubercles, which are actually hair follicles and are characteristic of the species. The tail flukes, which are lifted high in some dive sequences, have wavy trailing edges.There are four global populations, all being studied. North Pacific, Atlantic, and southern ocean humpbacks have distinct populations which make an annual migration. One population in the Indian Ocean does not migrate. The Indian Ocean has a northern coastline, while the Atlantic and Pacific oceans do not, thereby preventing the humpbacks from migrating to the pole.
The long black and white tail fin, which can be up to a third of body length, and the pectoral fins have unique patterns, which enable individual whales to be recognised. Several suggestions have been made to explain the evolution of the humpback's pectoral fins, which are proportionally the longest fins of any cetacean. The two most enduring hypotheses are the higher maneuverability afforded by long fins, or that the increased surface area is useful for temperature control when migrating between warm and cold climates.
A humpback whale tail has wavy rear edges. The tail of each humpback whale is visibly unique. Humpbacks have 270 to 400 darkly coloured baleen plates on each side of the mouth. The plates measure from a mere 18 inches (460 mm) in the front to approximately 3 feet (0.91 m) long in the back, behind the hinge. Ventral grooves run from the lower jaw to the umbilicus about halfway along the bottom of the whale. These grooves are less numerous (usually 16–20) and consequently more prominent than in other rorquals. The stubby dorsal fin is visible soon after the blow when the whale surfaces, but has disappeared by the time the flukes emerge. Humpbacks have a distinctive 3 m (10 ft) heart shaped to bushy blow, or exhalation of water through the blowholes. Early whalers also noted blows from humpback adults to be 10 - 20 feet (6.1 m) high. Whaling records show they understood each species has its own distinct shape and height of blows.
Newborn calves are roughly the length of their mother's head. A 50-foot (15 m) mother would have a 20-foot (6.1 m) newborn weighing in at 2 short tons (1.8 t). They are nursed by their mothers for approximately six months, then are sustained through a mixture of nursing and independent feeding for possibly six months more. Humpback milk is 50% fat and pink in color. Some calves have been observed alone after arrival in Alaskan waters. Females reach sexual maturity at the age of five with full adult size being achieved a little later. According to new research, males reach sexual maturity at approximately 7 years of age. Fully grown the males average 15–16 m (49–52 ft), the females being slightly larger at 16–17 m (52–56 ft), with a weight of 40,000 kg (or 44 tons); the largest recorded specimen was 19 m (62 ft) long and had pectoral fins measuring 6 m (20 ft) each. The largest humpback on record, according to whaling records, was killed in the Caribbean. She was 88 feet (27 m) long, weighing nearly 90 tons.Females have a hemispherical lobe about 15 centimetres (6 in) in diameter in their genital region. This allows males and females to be distinguished if the underside of the whale can be seen, even though the male's penis usually remains unseen in the genital slit. Male whales have distinctive scars on heads and bodies, some resulting from battles over females.
Females typically breed every two or three years. The gestation period is 11.5 months, yet some individuals can breed in two consecutive years. Humpback whales were thought to live 50–60 years, but new studies using the changes in amino acids behind eye lenses proved another baleen whale, the Bowhead, to be 211 years old. This was an animal taken by the Inuit off Alaska. More studies on ages are currently being done.
Humpbacks frequently breach, throwing two thirds or more of their bodies out of the water and splashing down on their backs. The humpback social structure is loose-knit. Usually, individuals live alone or in small transient groups that assemble and break up over the course of a few hours. Groups may stay together a little longer in summer in order to forage and feed cooperatively. Longer-term relationships between pairs or small groups, lasting months or even years, have been observed, but are rare. Recent studies extrapolate feeding bonds observed with many females in Alaskan waters over the last 10 years. It is possible some females may have these bonds for a lifetime. More studies need to be done on this. The range of the humpback overlaps considerably with many other whale and dolphin species, while it may be seen near other species (for instance, the Minke Whale), it rarely interacts socially with them. Humpback calves have been observed in Hawaiian waters playing with bottlenose dolphin calves.
Courtship rituals take place during the winter months, when the whales migrate toward the equator from their summer feeding grounds closer to the poles. Groups of two to twenty males typically gather around a single female and exhibit a variety of behaviours in order to establish dominance in what is known as a competitive group. The displays may last several hours, the group size may ebb and flow as unsuccessful males retreat and others arrive to try their luck. Techniques used include breaching, spy-hopping, lob-tailing, tail-slapping, flipper-slapping, charging and parrying. "Super pods" have been observed numbering more than 40 males, all vying for the same female.
Whale song is assumed to have an important role in mate selection, however, scientists remain unsure whether the song is used between males in order to establish identity and dominance, between a male and a female as a mating call, or a mixture of the two. All these vocal and physical techniques have also been observed while not in the presence of potential mates. This indicates that they are probably important as a more general communication tool. Recent studies showed singing males attract other males. Scientists are extrapolating possibilities the singing may be a way to keep the migrating populations connected. It has also been noted that the singing begins when the competition ends. Studies on this are ongoing.
The humpback has the most diverse repertoire of feeding methods of all baleen whales. Its most inventive technique is known as bubble net feeding: a group of whales blows bubbles while swimming in circles to create a ring of bubbles. The ring encircles the fish, which are confined in an ever-tighter area as the whales swim in a smaller and smaller circles. The whales then suddenly swim upward through the bubble net, mouths agape, swallowing thousands of fish in one gulp. This technique can involve a ring of bubbles up to 30 m (100 ft) in diameter and the cooperation of a dozen animals. Some of the whales take the task of blowing the bubbles through their blowholes, some dive deeper to drive fish toward the surface, and others herd fish into the net by vocalizing. Humpbacks have been observed bubblenet feeding alone as well.Humpback whales are preyed upon by Orcas. The result of these attacks is generally nothing more serious than some scarring of the skin, but it is likely that young calves are sometimes killed.
Both male and female humpback whales can produce sounds, however only the males produce the long, loud, complex "songs" for which the species is famous. Each song consists of several sounds in a low register that vary in amplitude and frequency, and typically lasts from 10 to 20 minutes.Songs may be repeated continuously for several hours, humpback whales have been observed to sing continuously for more than 24 hours at a time. As cetaceans have no vocal cords, whales generate their song by forcing air through their massive nasal cavities.Whales within an area sing the same song, for example all of the humpback whales of the North Atlantic sing the same song, and those of the North Pacific sing a different song. Each population's song changes slowly over a period of years, never returning to the same sequence of notes.
Scientists are still unsure of the purpose of whale song. Only male humpbacks sing, so it was initially assumed that the purpose of the songs was to attract females. However, many of the whales observed to approach singing whales have been other males, with the meeting resulting in a conflict. Thus, one interpretation is that the whale songs serve as a threat to other males. Some scientists have hypothesized that the song may serve an echolocative function. During the feeding season, humpback whales make altogether different vocalizations, which they use to herd fish into their bubble nets.