Taxonomy of baleen whales
(whales, dolphins and porpoises)
Family Balaenidae (right whales)
Balaena mysticetus Bowhead whale
Eubalaena australis Southern right whale
E. glacialis North Atlantic right whale
E. japonica North Pacific right whale
Estrichtius robustus Gray whale
Family Balaenopteridae (rorquals)
Balaenoptera musculus Blue whale
B. physalus Fin whale
B. borealis Sei whale
B. edeni Bryde’s whale
B. acutorostrata Minke whale
B. bonaerensis Antarctic minke whale
Megaptera novaeangliaeHumpback whale
Caperea marginata Pygmy right whale
The humpback whale is one of the rorqual whales, a family that includes the blue whale, Balaenoptera musculus, fin whale, B. physalus, Bryde’s whale, B. edini, sei whale,
B. borealis, and minke whale, B. acutorostrata. The rorquals have two distinctive features in common, a dorsal fin and groves running along the ventral surface from the lower jaw to the umbilicus.
The humpback whale is the 5th largest of the baleen whales and is stockier than other rorquals. It displays reverse sexual dimorphism, with males measuring 12.2-14.6 m with the females slightly larger at 13.7-15.2 m. The overall size range is from 10.9-15.8 m although the largest recorded humpback was a female of 26.8 m caught off Bermuda. They weigh 25-30 tonnes. Calves range from 3-4.8 m and average 1-2 tonnes at birth.
The head of the humpback whale is large in proportion to the body and is up to a third of the total body length. Unlike other rorquals, the midline ridge on top of the head is indistinct and the rostrum is covered in tubercles each about the size of a golf ball and each with a stiff sensory hair or vibrissa around 1.2 to 2.6 cm long which has a rich blood supply and is connected to a nerve suggesting a function as a sensory organ - perhaps to detect current and temperature changes in the water to aid in navigation. The tubercles are found along the upper and lower jaws and along the lips. The mandible, or lower jaw, has a rounded protuberance near the tip that appears to increase in size with age. Its function, if any, is unknown.
The humpback whale has long pectoral fins up to a third of the body length (around 5 m) with 10 phalangic knobs on the leading edge corresponding to the phalanges and joints. Although the upper side is usually black in Pacific populations, it is mostly white with black markings in Atlantic populations. The underside is usually mottled and mainly white. It is thought that the function of these unusually long pectoral fins is to aid manoeuvrability.
The dorsal fin is situated nearly two thirds along the dorsal surface and is reduced to a fleshy hump.
Humpback whales display countershading with the dorsal surface generally black or dark grey, and the ventral surface mottled black and white to varying degrees. The northern hemisphere humpback whales tend to be dark whilst the southern hemisphere populations have more white pigmentation on the ventral and lateral sides of the body. Calves in both hemispheres are light grey at birth and darken during the first month of life.
The caudal peduncle is relatively thin with ‘knuckles’ visible on the dorsal surface. These are the caudal vertebrae, which become more visible when the animal looses weight. On the ventral side of the caudal peduncle is a protrusion or ventral keel known as the carina. The carina is found on all whales and whilst its function is obscure, it may aid stability in the water.
Anterior to the carina is the anal slit followed by the genital opening and mammary slits in females, which are located on either side of the genital opening. This is identified in females as a ‘hemispheric lobe’. In males, the distance between the genital opening and the anus is almost 2.5 times longer than is seen in females. The genital opening in males is more anterior than the female. Around the midline is the umbilicus.
Diagram of male (left) and female (right) humpback whale genitals. Below is a male humpback showing the genital slit. The carina can be seen between the genital slit and the fluke.
The breath, known as the ‘blow’ appears as a single bushy spout, extending to 3 m in height. The blow/roll sequence begins with head emerging from the water at around the same time as the blow. The head is kept low and followed by a broad, progressively-arched dorsal surface and fleshy dorsal fin. In adult humpback whales, dives usually last 10-15 minutes (sometimes up to 45 minutes) followed by 3-4 blows at the surface at 15-30 second intervals. In the breeding grounds the humpback whale blows 3-6 times between dives.
Before a deep dive, they usually raise their flukes at the surface, known as a ‘fluke up’ dive. They are thought to dive to around 120 m. Their swimming speed averages 8 km/hr on migrations but they can reach 32 km/hr in short bursts. Humpback whales can be highly active at the surface with breaching, pec slapping and tail slapping, particularly at the breeding grounds.
It is possible to identify individuals from the markings on the undersides of the fluke, also to some extent from the shape of the dorsal fin and pigmentation on the flank.
Distribution and migration
Humpback whales are seen singly, in pairs or in groups of up to 15 or 20. They are found in all oceans of the world, from the tropics to polar waters, with apparent worldwide geographical segregation into at least 10-11 distinct populations with six separate populations in the Southern Hemisphere. Although some mixing does occur between populations, contact is unlikely between the Northern and Southern Hemisphere whales, as their migration is seasonal(although there are some exceptions - see below). In high latitudes, they are found in coastal zones within the continental shelf, where they feed.In low latitudes, they cluster around coast or offshore tropical reef systems such as Silver Bank in the West Indies and the Tongan and Hawaiian Islands. Humpback whales travel each year between summer feeding grounds and winter breeding grounds. The following map shows the known migration routes of humpback whales.
In the Northern Atlantic, humpback whales tend to form discrete clusters of subpopulations on the feeding grounds: the Gulf of Maine, Newfoundland, Labrador, Gulf of St. Lawrence, Baffin Island, western Greenland, Iceland and Norway. These sub-populations remain discrete during the summer feeding season, with high site fidelity, which is matrilineal (directed by the female). Recent molecular studies, using DNA sequencing, have found that site fidelity persists over evolutionary time scales of thousands of years and shows a high degree of heterogeneity between five different feeding aggregations in the North Atlantic. During the winter breeding season, whales from all subpopulations migrate to the West Indies where mixing and interbreeding take place. Some whales from the eastern North Atlantic migrate to the tropical coast of western Africa and the Cape Verde Islands, whilst one population in the Arabian Sea do not migrate at all.
In the eastern North Pacific, whales feed in the waters around the northern Pacific Rim: SE and South Central Alaska, as well as the Aleutian chain and these whales form clusters, which merge on the winter breeding grounds around the Hawaiian Islands. Separate stocks are found around the Californian coast, and migrate to warm Mexican waters to breed. There is thought to be some mixing of North Pacific stocks, since an individual humpback has been identified moving between Hawaii and Japan.
Data from whaling catches show that north and south bound migrations consist of a staggering of maturational classes with lactating females first to leave the feeding grounds followed by immature whales, mature males and females and finally pregnant females. The order is reversed on the return to the feeding grounds in spring. The behaviour of migrating humpback whales may be related more to breeding than feeding at least on this part of the migration. The most common group of animals seem to be male-female dyads (pairs) or groups of three or more males.
The humpback whale migration is one of the longest of any mammal and in the early days of whale research, discovery tags were used to record movements. These were shotinto the blubber and muscle of a whale and recovered when they were caught and killed during the whaling days. Now, other methods are used including satellite tags which can track whale movements in near real time. A fairly recent study using individual identification photographs of the tail flukes reported a migration distance of 8334 km undertaken between Antarctic feeding grounds and breeding grounds in Columbia, across the equator. Individuals also appear to make long-distance east-west movements, such as the one reported between Hawaii and Japan. Another study demonstrated that a male humpback whale also undertook an inter-ocean migration between the Indian Ocean and the South Atlantic Ocean. It is not known why humpback whales migrate although suggestions include predator avoidance and the energetic benefits of warm water during the winter when food production is low in the feeding grounds. However, models of the energetic costs and benefits of migration produce conflicting results and more work is needed in this area. This topic has generated some heated debate!
On the summer feeding grounds in the Gulf of Maine, most researchers broadly agree that humpback whales form small unstable groups which change frequently and move in response to prey patches. Female-male and female-female pairs are common and there is recent evidence that social interactions are more complex than previously thought.
On the winter breeding grounds whales also form small unstable groups showing transient affiliations. Larger groups, referred to as ‘competitive groups’, display aggressive behaviour and consist of several males competing for access to a female. This theory has been given credence by molecular DNA analysis of competitive groups in the West Indies. Individual roles were identified within competitive groups with a central ‘nuclear’ animal, generally confirmed to be a female, followed by a ‘principal escort’, ‘secondary escorts’ and ‘challengers’. No competitive groups have been found to contain more than one female.
Whaling intensified with the advent of modern whaling methods in the 1920s and decimated around 90 percent of the original humpback whale population, particularly around the rich feeding grounds of the Southern Ocean. Despite their status, they appear to be increasing in number, although population numbers were very low until the mid 1970s, possibly due to illegal catches up to 1970. They were given protection in 1963 by the IWC, and the worldwide humpback whale population has been estimated by several studies:
2804 females although, with a substantial proportion of females unavailable for sampling in the West Indies, it is likely to be an underestimate of the overall North Atlantic population. Smaller populations in the eastern North Atlantic and around the Cape Verde Islands increase the overall number.
In the eastern North Pacific, population size was estimated at ~600. The population which migrates between the Hawaiian breeding grounds and the Alaskan feeding grounds has been estimated by mark-recapture methods to be 1407, although more recent estimates gave a population size of between 2000 and 5000 whales. In 1997 it was estimated that the whole North Pacific breeding population was between 6000 and 8000 animals using mark-recapture methods. The South Pacific population, around the Tongan islands is <700, estimated from photo identification records of 337 individuals since 1991 and estimates for the southeastern Pacific stock were 2881-2917 whales in 2003-2004.
Over 100 000 humpback whales were taken in just 40 years in the Southern Hemisphere from a pre-whaling population of around 250 000. The current population in the Southern Ocean is unclear.
The most recent population estimates of humpback whale numbers is reported by the International Whaling Commission. Western North Atlantic (1992/93) 10 100-13 200; Southern Hemisphere (1997/98) 34 000-52 000; North Pacific (2007) at least 10 000. The healthiest population occurs in the western North Atlantic but current threats to humpback whales include:
-Illegal whaling by pro-whaling nations such as Norway, Iceland and Japan
-Habitat degradation, coastal seismic operations, defence operations, petroleum and mineral exploration
-Collisions with boats
-Entanglement in fishing gear/shark nets
-Pollution, including increasing amounts of plastic debris and polystyrene beads, oil spills and industrial waste dumping both at sea and in estuaries which flow into the sea resulting in the ingestion of contaminated prey, irritation of skin and eyes and the inhalation of toxic fumes
-Development of krill fisheries in the Southern Ocean (since krill oil is advocated as an alternative to fish oils) depleting food sources and causing a shift in the ecological balance with unknown consequences
The International Union for Conservation of Nature and Natural Resources (IUCN) describes the status of the humpback whale as VULNERABLE (VU) – ‘A taxon is Vulnerable when it is not Critically Endangered or Endangered but is facing a high risk of extinction in the wild in the medium-term future’.
Although there have been several studies on social structure and competitive behaviour at the Hawaiian breeding grounds behavioural studies of humpback whales have focused on the activities of competitive groups or social and reproductive aspects. There have been occasional studies on novel behaviour and the ‘sensory biophysics’ of humpback whales. A long-term study of surface behaviour was undertaken by Ruth Searle and is still the focus of much of the research at the Humpback Whale Research Foundation (see Research).
The pattern of humpback whale song shows a repeating series of sounds called phrases, which together make up a theme. The song consists of a series of themes sung in a specific order and can last from a few minutes to around 30 minutes. The song may be repeated continuously for many hours by lone male singers. Singing is heard on the winter breeding grounds and on migration and sometimes at other times such as late and early in the feeding season. Singers vocalise for longer periods as the season progressed. Song is different in each population and it changes slightly each year. A recent study reported that humpback whales may have an upper frequency limit of hearing as high as 24 kHz.
Studies suggest that singers are all male and various functions have been suggested, such as a sexual advertisement, a means of dominance sorting in males, a spacing mechanism in males and a mechanism of ‘organising’ males during the breeding season. It has also been suggestd that the humpback whale mating system could be characterised as a lek (as seen in birds) because of the presence of many singing males.
In Hawaii, singers stop when they either join other pods or are joined by other whales and they subsequently stay together for varying lengths of time from a few minutes to several hours. The same study also found that whales avoid song during playback experiments when song recorded from conspecifics was broadcast through underwater speakers.
There has been little study on vocalisations other than song, generally referred to as ‘social sounds’. One study in Hawaii found that social sounds were heard only in pods containing more than three whales and were associated with competitive pods of males. A recent study has documented vocalisations by calves in Hawaii and another has discovered (using DTAGs) acoustic clicks produced by night-time foraging humpback whales.
On feeding grounds, individual humpback whales tend to stay in a specific area for long periods, sometimes for several weeks. However, they do not seem to show a pattern of territoriality but move around in response to the patchy distribution of prey.
Females with calves tend to use shallow or nearshore waters in both breeding and feeding grounds. This may be to reduce the risk of predation or avoid harassment by males.
Sexing of humpback whales was made possible by the discovery of anatomical differences in the genital area of males and females, making inferences about behaviour and social structure more reliable. This method has proved valuable for sexing humpback whales during subsurface observations for our research work.
Females mate with different males in different years, as seen from molecular studies of calves in the Gulf of Maine who were shown to have the same mother but different fathers. The mating system of humpback whales is thought to be polygynous (one female mating with many different males). Males from different feeding grounds intermix on the breeding grounds.
Although the birth of a humpback whale has never been witnessed, the presence of young calves and on one occasion a fresh placenta in the water at Silver Bank suggests that calves are born in tropical areas. Early researchers determined from fetal length data during the whaling years that the peak time for births in the Southern Hemisphere was early August. Since then, surveys of live whales in the Northern Hemisphere have determined a peak for births in the Northern Hemisphere to be February.
Early studies from whaling catches found females without a calf but with milk in their mammary glands. From this data, they estimated that weaning and separation takes place at around ten to eleven months after birth. However, later research suggests that, although some calves separate from their mothers around this time, others have been sighted with their mothers during their second winter. It is possible that calves stay with their mothers longer than we currently believe.
© Ruth Searle, Humpback Whale Research Foundation
For more about humpbacks and the work of the Humpback Whale Research Foundation, go to RESEARCH