Humpback whale

Appearance

The adult humpback whale is generally 14–15 m, though longer lengths of 16–17 m have been recorded. Females are usually 1–1.5 m longer than males. The species can reach body masses of 40 metric tons. Calves are born at around 4.3 m long with a weight of 680 kg.

The body is bulky with a thin rostrum and proportionally long flippers, each around one-third of its body length. It has a short dorsal fin that varies from nearly non-existent to somewhat long and curved. As a rorqual, the humpback has grooves between the tip of the lower jaw and the navel. They are relatively few in number in this species, ranging from 14–35. The mouth is lined with baleen plates, which number 270-400 for both sides.

Unique among large whales, humpbacks have bumps or tubercles on the head and front edge of the flippers; the tail fluke has a jagged trailing edge. The tubercles on the head are 5–10 cm thick at the base and poke up to 6.5 cm. They are mostly hollow in the center, often containing at least one fragile hair that erupts 1–3 cm from the skin and is 0.1 mm thick. The tubercles develop early in the womb and may have a sensory function as they are rich in nerves.

The dorsal or upper-side of the animal is generally black; the ventral or underside has various levels of black and white coloration. Whales in the southern hemisphere tend to have more white pigmentation. The flippers can vary from all-white to white only on the undersurface. The varying color patterns and scars on the tail flukes distinguish individual animals. The end of the genital slit of the female is marked by a round feature, known as the hemispherical lobe, which visually distinguishes males and females.

Status

As of 2018, the IUCN Red List lists the humpback whale as least-concern, with a worldwide population of around 135,000 whales, of which around 84,000 are mature individuals, and an increasing population trend.

Regional estimates are around 13,000 in the North Atlantic, 21,000 in the North Pacific, and 80,000 in the southern hemisphere. For the isolated population in the Arabian sea, only around 80 individuals remain, and this population is considered to be endangered. In most areas, humpback whale populations have recovered from historic whaling, particularly in the North Pacific. Such recoveries have led to the downlisting of the species' threatened status in the United States, Canada, and Australia. In Costa Rica, Ballena Marine National Park was established for humpback protection.

Humpbacks still face various other human-made threats, including entanglement by fishing gear, vessel collisions, human-caused noise and traffic disturbance, coastal habitat destruction, and climate change. Like other cetaceans, humpbacks can be injured by excessive noise. In the 19th century, two humpback whales were found dead near repeated oceanic sub-bottom blasting sites, with traumatic injuries and fractures in the ears. Saxitoxin, a paralytic shellfish poisoning from contaminated mackerel, has been implicated in humpback whale deaths. While oil ingestion is a risk for whales, a 2019 study found that oil did not foul baleen and instead was easily rinsed by flowing water.

Whale researchers along the Atlantic Coast report that there have been more stranded whales with signs of vessel strikes and fishing gear entanglement in recent years than ever before. The NOAA recorded 88 stranded humpback whales between January 2016 and February 2019. This is more than double the number of whales stranded between 2013 and 2016. Because of the increase in stranded whales, NOAA declared an unusual mortality event in April 2017. Virginia Beach Aquarium's stranding response coordinator, Alexander Costidis, stated the conclusion that the two causes of these unusual mortality events were vessel interactions and entanglements.

Habitat

Humpback whale groups, aside from mothers and calves, typically last for days or weeks at the most. They are normally sighted in small groups though large aggregations form during feeding and among males competing for females. Humpbacks may interact with other cetacean species, such as right whales, fin whales, and bottlenose dolphins. Humpbacks are highly active at the surface, performing aerial behaviors such as breaching and surface slapping with the tail and flippers. These may be forms of play and communication and/or for removing parasites.

Humpbacks rest at the surface with their bodies laying horizontally. The species is a slower swimmer than other rorquals, cruising at 7.9–15.1 km/h. When threatened, a humpback may speed up to 27 km/h. They appear to dive within 150 m and rarely below 120 m. Dives typically do not exceed five minutes during the summer but are normally 15–20 minutes during the winter. As it dives, a humpback typically raises its tail fluke, exposing the underside.

Reproduction

Mating and breeding take place during the winter months, which is when females reach estrus and males reach peak testosterone and sperm levels. Humpback whales are promiscuous, with both sexes having multiple partners. Males will frequently trail both lone females and cow–calf pairs. These are known as "escorts", and the male that is closest to the female is known as the "principal escort", who fights off the other suitors known as "challengers". Other males, called "secondary escorts", trail further behind and are not directly involved in the conflict. Agonistic behavior between males consists of tail slashing, ramming, and head-butting.

Gestation in the species lasts 11.5 months, and females reproduce every two years. Humpback whale births have been rarely observed. One birth witnessed off Madagascar occurred within four minutes. Mothers typically give birth in mid-winter, usually to a single calf. Calves suckle for up to a year but can eat adult food in six months. Humpbacks are sexually mature at 5–10 years, depending on the population. The length at maturity is around 12.5 m. Humpback whales possibly live over 50 years.

Food

Humpback whales feed from spring to fall. They are generalist feeders, their main food items being krill and small schooling fish. The most common krill species eaten in the southern hemisphere is the Antarctic krill. Further north, the northern krill and various species of "Euphausia" and "Thysanoessa" are consumed. Fish prey include herring, capelin, sand lances and Atlantic mackerel. Like other rorquals, humpbacks are "gulp feeders", swallowing prey in bulk, while right whales and bowhead whales are skimmers. The whale increases its mouth gape by expanding the grooves. Water is pushed out through the baleen. In the southern hemisphere, humpbacks have been recorded foraging in large compact gatherings numbering up to 200 individuals.

Humpbacks hunt their prey with bubble-nets. A group of whales swim in a shrinking circle while blowing air from their blowholes, capturing the prey above them in a cylinder of bubbles. They may dive up to 20 m performing this technique. Bubble-netting comes in two main forms; upward spirals and double loops. Upward spirals involve the whales blowing air from their blowhole continuously as they circle towards the surface, creating a spiral of bubbles. Double loops consist of a deep, long loop of bubbles that herds the prey, followed by slapping the surface and then a smaller loop that prepares the final capture. Combinations of spiraling and looping have been recorded. After the humpbacks create the "nets", the whales swim into them with their mouths gaping and ready to swallow.

Using network-based diffusion analysis, one study argued that whales learned lobtailing from other whales in the group over 27 years in response to a change in primary prey. The tubercles on the flippers stall the angle of attack, which both maximizes lift and minimizes drag. This, along with the shape of the flippers, allows the whales to make the abrupt turns necessary during bubble-feeding.