Blue whale

The blue whale is the largest known animal to have ever existed.[3][13][14][15] The International Whaling Commission (IWC) whaling database reports 88 individuals longer than 30 meters (98 ft), including one of 33 meters (108 ft), but problems with how the measurements were taken suggest that any longer than 30.5 meters (100 ft) are suspect.[16] The Discovery Committee reported lengths up to 31 meters (102 ft);[17] however, the longest scientifically measured individual blue whale was 30 meters (98 ft) from rostrum tip to tail notch.[18] Female blue whales are larger than males.[5][19] Hydrodynamic models suggest a blue whale could not exceed 108 ft (33 m) because of metabolic and energy constraints.[20] The highest recorded weight for the species is 199 metric tons (196 long tons; 219 short tons).[16]

The average length of sexually mature female blue whales is 22.0 meters (72.1 ft) for Eastern North Pacific blue whales, 24 meters (79 ft) for central and western North Pacific blue whales, 21–24 meters (68–78 ft) for North Atlantic blue whales, 25.4–26.3 meters (83.4–86.3 ft) for Antarctic blue whales, 23.5 meters (77.1 ft) for Chilean blue whales, and 21.3 meters (69.9 ft) for pygmy blue whales.[16][21][22]

In the Northern Hemisphere, males weigh an average 100 metric tons (220,000 lb) and females 112 metric tons (247,000 lb). Eastern North Pacific blue whale males average 88.5 metric tons (195,000 lb) and females 100 metric tons (220,000 lb). Antarctic males average 112 metric tons (247,000 lb) and females 130 metric tons (290,000 lb). Pygmy blue whale males average 83.5 metric tons (184,000 lb) to 99 metric tons (218,000 lb).[23] The weight measured of the heart from a stranded North Atlantic blue whale was 0.18 metric tons (400 lb), the largest known in any animal.[24]

A blue whale skull measuring 5.8 meters (19 ft)

A blue whale's age is most reliably measured using ear plugs. Blue whales secrete earwax throughout their lives, forming long, multilayered plugs. Each deposited light and dark layer indicates a switch between fasting during migration and feeding. As one set is laid down per year, the number of layers is an indicator of age.[25][26][27] The maximum age of a pygmy blue whale determined this way is 73 years.[28] Before the ear plug aging method, layers in baleen plates were used; however, these wear down and are not as reliable. The blue whale's ovaries form a permanent record of the number of ovulations (or perhaps pregnancies), in the form of corpora albicantia—fibrous masses that are permanent scars and were once used as an indication of age.[29] In a female pygmy blue whale, one corpus albicans is formed on average every 2.6 years.[28]

The genus name, Balaenoptera, means winged whale[10] while the species name, musculus, could mean "muscle" or a diminutive form of "mouse", possibly a pun by Carl Linnaeus[4][10] when he named the species in Systema Naturae.[30] One of the first published descriptions of a blue whale comes from Robert Sibbald's Phalainologia Nova,[31] after Sibbald found a stranded whale in the estuary of the Firth of Forth, Scotland, in 1692. The name "blue whale" was derived from the Norwegian "blåhval", coined by Svend Foyn shortly after he had perfected the harpoon gun. The Norwegian scientist G. O. Sars adopted it as the common name in 1874.[32]

Blue whales were referred to as 'Sibbald's rorqual', after Robert Sibbald, who first described the species.[31] Herman Melville called the blue whale "sulphur bottom" in his novel Moby Dick[11] because of the accumulation of diatoms creating a yellowish appearance on their pale underside.[4][5]

Blue whales are rorquals, in the family Balaenopteridae[33] whose extant members include the fin whale (Balaenoptera physalus), sei whale (Balaenoptera borealis), Bryde's whale (Balaenoptera brydei), Eden's whale (Balaenoptera edeni), common minke whale (Balaenoptera acutorostrata), Antarctic minke whale (Balaenoptera bonaerensis), Omura's whale (Balaenoptera omurai), and humpback whale (Megaptera novaeangliae).[34] A 2018 analysis estimates that the Balaenopteridae family diverged from other families in the late Miocene, between 10.48 and 4.98 million years ago.[35] The earliest discovered anatomically modern blue whale is a partial skull fossil found in southern Italy, dating to the Early Pleistocene, roughly 1.5–1.25 million years ago.[36] The Australian pygmy blue whale diverged during the Last Glacial Maximum. Their more recent divergence has resulted in the subspecies having a relatively low genetic diversity,[37] and New Zealand blue whales have an even lower genetic diversity.[38]

Whole genome sequencing suggests that blue whales are most closely related to sei whales with gray whales as a sister group.[35] This study also found significant gene flow between minke whales and the ancestors of the blue and sei whale. Blue whales also displayed a high degree of genetic variability (i.e., heterozygosity).[35]

Hybridization between blue and fin whales has been documented across multiple ocean basins. The earliest description of a possible hybrid between a blue and fin whale was a 20-meter (65 ft) anomalous female whale with the features of both the blue and the fin whales taken in the North Pacific.[39] In 1983, a 20-meter (65 ft) sexually immature male specimen was taken. Based upon the number of layers in the earwax, the animal's age was determined to be seven years. In 1984, whalers caught a female hybrid between a fin and a blue whale off northwestern Spain. Molecular analyses revealed a blue whale mother and a fin whale father.[40]

In 1986, a 21-meter (70 ft) pregnant female whale was caught. Molecular analyses of the whale showed it was a hybrid between a female blue whale and a male fin whale, and that the fetus had a blue whale father. It was the first example of any cetacean hybridization giving rise to a fertile offspring.[41] Two live blue-fin whale hybrids have since been documented in the Gulf of St. Lawrence, (Canada), and in the Azores, (Portugal).[42] DNA tests done in Iceland on a blue whale killed in July 2018 by the Icelandic whaling company Hvalur hf, found that the whale was a hybrid of a fin whale father and a blue whale mother;[43] however, the results are pending independent testing and verification of the samples. Because the International Whaling Commission classified blue whales as a "Protection Stock", trading their meat is illegal, and the kill is an infraction that must be reported.[44] Blue-fin hybrids have been detected from genetic analysis of whale meat samples taken from Japanese markets.[45]

There is reference to a humpback-blue whale hybrid in the South Pacific, attributed to marine biologist Michael Poole.[10][46]

At least four subspecies of blue whale are recognized, some of which are divided into population stocks or "management units".[47][48] They are distributed in all major ocean basins, except the Bering Sea and the Arctic Ocean (although they have been sighted near the ice edge in the North Atlantic).[48][49][50]

Aerial view of adult blue whale

• Northern subspecies (B. m. musculus)
  • North Atlantic population - This population feeds mainly from eastern Canada to Greenland, particularly in the Gulf of St. Lawrence, during summer though some individuals may remain there year-round. There have also been increased sightings in the Norwegian Sea. Their wintering areas are less understood, but they are reported to migrate near the West Indies, the Azores and northwest Africa.[47]
  • Eastern North Pacific population - Whales in this region mostly feed off California during the summer and into fall and then migrate to secondary feeding sites off Oregon, Washington State, the Alaska Gyre and Aleutian Islands. During winter and spring, blue whales move south to feed and breed mainly in the Gulf of California and the Costa Rica Dome.[47]
  • Central/Western Pacific population - This stock is documented off the Kamchatka Peninsula and, some individuals may remain there year-round. They have been recorded wintering in Hawaiian waters, though some can be found in the Gulf of Alaska during fall and early winter.[47]
• Northern Indian Ocean subspecies (B. m. indica) - This subspecies can be found year-round in the northwestern Indian Ocean, though some individuals have recorded travelling as far south as the Crozet Islands late summer and early fall.[47]
• Pygmy blue whale (B. m. brevicauda)
  • Madagascar population - This population migrates between the Seychelles and Amirante Islands in the north and the Crozet Islands and Prince Edward Islands in the south where they feed, passing through the Mozambique Channel.[47]
  • Australia/Indonesia population - Whales in this region appear to winter off Indonesia and travel to their summer feeding grounds along Western Australia, with major concentrations at Perth Canyon and an area between the Great Australian Bight and Bass Strait.[47]
  • Eastern Australia/New Zealand population - This stock may winter in the Tasman Sea and the Lau Basin around Tonga and forage mostly at the South Taranaki Bight. Blue whale have been detected year-round in New Zealand waters.[47]
• Antarctic subspecies (B. m. intermedia) - This subspecies includes all populations found around the Antarctic. They have been recorded to travel as far north as eastern tropical Pacific, the central Indian Ocean, and off southwestern Australia and northern New Zealand.[47]

Blue whales off the Chilean coast may be a separate subspecies based on geographic separation, genetics, and unique song types.[51][52][53] Chilean blue whales may overlap in the Eastern Tropical Pacific with Antarctica blue whales and Eastern North Pacific blue whales. Chilean blue whales are genetically differentiated from Antarctica blue whales and are unlikely to be interbreeding. However, the genetic distinction is less with the Eastern North Pacific blue whale and there may be gene flow between hemispheres.[54]

The blow of a blue whale

Blue whale populations may go on long migrations, traveling to their summer feeding grounds towards the poles and then head to their winter breeding grounds in more equatorial waters.[50] The animals appear to use memory to track the shifting hotspots of predictable feeding areas.[55] There is evidence of alternative strategies, such as year-round residency, and partial (where only some individuals migrate) or age/sex-based migration. Some whales have been recorded feeding in breeding grounds.[56] The traveling speed for blue whales range 5–30 kilometers per hour (3.1–18.6 mph).[5]

The greatest dive depth reported from tagged blue whales was 315 meters (1,033 ft).[57] Their theoretical aerobic dive limit was estimated at 31.2 minutes,[58] however, the longest dive duration measured was 15.2 minutes.[57] The deepest confirmed dive from a pygmy blue whale was 510 meters (1,660 ft).[59] A blue whales' heart rate can drop to 2 beats per minute (bpm) at deep depths, but upon surfacing, can rise up to 37 bpm, which is close to its maximum possible heart beat.[60]

The small dorsal fin of this blue whale is just visible on the far left.

The blue whale's diet consists almost exclusively of krill.[5] They have also been observed near Magdalena Bay (along the western coast of Baja California, Mexico) feeding on pelagic red crabs.[61][62] They may accidentally eat small fish that are in the swarms of krill (although this is rare).[63][64] Blue whales capture krill through lunge feeding, they swim towards them at high speeds with the mouth wide open.[5][57] They may engulf 220 metric tons (220 long tons; 240 short tons) of water at one time.[65] They squeeze the water out through their baleen plates with pressure from the throat pouch and tongue, and swallow the remaining krill.[5][57] Blue whales have been recorded making 180° rolls during lunge-feeding, allowing them to engulf krill patches while inverted. They rolled while searching for prey between lunges, which has been hypothesized as allowing them to survey the prey field and find the densest patches.[66]

Blue whales maximize the intake of energy by increasing the number of lunges they make during a dive while targeting dense krill patches. This allows them to acquire the energy necessary for everyday activities while storing additional energy necessary for migration and reproduction. Because of the high cost of lunge feeding, it has been estimated that blue whales must target densities greater than 100 krill/m3.[57][67] They can consume 34,776–1,912,680 kilojoules (8,312–457,141 kcal) from one mouthful of krill, which can provide up to 240 times more energy than used in a single lunge.[57] The daily food requirement for an average-sized blue whale is estimated to be 1,120 ± 359 kilograms (2,469 ± 791 lb) of krill.[68][69]

There is little to no direct evidence of interspecific competition between blue whales and other baleen whale species.[70] Surveys using tagging, line-transect surveys, hydroacoustic surveys, and net sampling have found that despite the overlap with blue whales and other baleen whales, there appears to be niche separation[71][72][73] in space and/or time and selection of prey species.[74][75][65] In the Southern Ocean, baleen whales have been found to preferentially feed on Antarctic krill of specific sizes, which would result in reduced interspecific competition.[76]

A blue whale calf with its mother

Blue whales generally reach sexual maturity at 8–10 years. In the Northern Hemisphere, the length of which they reach maturity is 21–23 meters (69–75 ft) for females and 20–21 meters (66–69 ft) for males. In the Southern Hemisphere, the length of maturity is 23–24 meters (75–79 ft) and 22 meters (72 ft) for females and males respectively.[77] Male pygmy blue whales average 18.7 meters (61.4 ft) at sexual maturity.[78][79] Female pygmy blue whales are 21.0–21.7 meters (68.9–71.2 ft) in length[21] and roughly 10 years old at the age of sexual maturity.[21][22][80]

Blue whales exhibit no well-defined social structure[81] other than mother-calf bonds from birth until weaning.[82] They are generally solitary or found in small groups.[83] Little is known about mating behavior, or breeding and birthing areas.[15][77] They appear to be polygynous, with males competing for females.[77][84] A male blue whale typically trails a female and will fight off potential rivals.[85] Mating occurs in the fall and winter.[15][77]

Female blue whales give birth every two to three years.[77] Pygmy blue whales are estimated to give birth every 2.6 years.[28] Pregnant females gain roughly four percent of their body weight daily,[86] amounting to 60% of their overall body weight throughout summer foraging periods.[77][87] Gestation may last 10–12 months with calves being 6–7 meters (20–23 ft) long and weighing 2–3 metric tons (2.0–3.0 long tons; 2.2–3.3 short tons) at birth.[77] Estimates suggest that because calves require 2–4 kilograms (4.4–8.8 lb) milk per kg of mass gain, blue whales likely produce 220 kilograms (490 lb) of milk per day (ranging from 110 to 320 kilograms (240 to 710 lb) of milk per day).[88] The first video of a calf thought to be nursing was filmed in New Zealand in 2016.[89] Calves may be weaned at 6–8 months old at a length of 16 meters (53 ft).[77][90] They gain roughly 37,500 pounds (17,000 kg) during the weaning period.[14]

Blue whale vocalizations are among the loudest and lowest frequency sounds made by any animal,[49][91] and their inner ears appear well adapted for detecting low-frequency sounds.[92] The fundamental frequency for blue whale vocalizations ranges from 8 to 25 Hz.[93] Blue whale songs vary between populations.[94]

Vocalizations produced by the Eastern North Pacific population have been well studied. This population produces pulsed calls ("A") and tonal calls ("B"), upswept tones that precede type B calls ("C") and separate downswept tones ("D").[95][96] A and B calls are often produced in repeated co-occurring sequences and sung only by males, suggesting a reproductive function.[96][97] D calls are produced by both sexes during social interactions while foraging and may be considered multi-purpose contact calls.[97][98][99] Because the calls have also been recorded from blue whale trios from in a what was considered to be a reproductive context, it has been suggested recently that this call has different functions.[85]

Blue whale calls recorded off Sri Lanka have a three‐unit phrase. The first unit is a pulsive call ranging from 19.8 to 43.5 Hz, lasting 17.9 ± 5.2 seconds. The second unit is an FM upsweep from 55.9 to 72.4 Hz lasting 13.8 ± 1.1 seconds. The final unit is a long (28.5 ± 1.6 s) tone that sweeps from 108 to 104.7 Hz.[100] A blue whale call recorded off Madagascar, a two‐unit phrase,[101] starts with 5–7 pulses with a center frequency of 35.1 ± 0.7 Hz and duration of 4.4 ± 0.5 seconds followed by a 35 ± 0 Hz tone lasting 10.9 ± 1.1 seconds.[100] In the Southern Ocean, blue whales' calls last roughly 18 seconds and consist of a 9-second-long, 27 Hz tone, followed by a 1-second downsweep to 19 Hz, and another downsweep to 18 Hz.[102][103] They also produce short, 1–4 second-duration, frequency-modulated calls ranging in frequency between 80 and 38 Hz.[103][104]

At least seven blue whale song types appear to have decreased in tonal frequency over time, though at different rates.[105][106][107] The Eastern North Pacific blue whale tonal frequency is 31% lower than it was in the early 1960s.[105][106] The frequency of pygmy blue whales in the Antarctic has decreased at a rate of a few tenths of a hertz per year starting in 2002.[107] One hypothesis is that as blue whale populations recover from whaling, this is increasing sexual selection pressure (i.e., a lower frequency indicates a larger body size).[106]

The only known natural threat to blue whales is the orca, although the rate of fatal attacks by orcas is unknown. Photograph-identification studies of blue whales have estimated that a high proportion of the individuals in the Gulf of California have rake-like scars, indicative of encounters with orcas.[108] Off southeastern Australia, 3.7% of blue whales photographed had rake marks and 42.1% of photographed pygmy blue whales off western Australia had rake marks.[109] Documented predation by orcas has been rare. A blue whale mother and calf were first observed being chased at high speeds by orcas off southeastern Australia.[110] The first documented attack occurred in 1977 off southwestern Baja California, Mexico, but the injured whale escaped after five hours.[111] Four more blue whales were documented as being chased by a group of orcas between 1982 and 2003.[112] The first documented predation event by orcas occurred September 2003, when a group of orcas in the Eastern Tropical Pacific was encountered feeding on a recently killed blue whale calf.[113] In March 2014, a commercial whale watch boat operator recorded an incident involving a group of transient orcas harassing a blue whale in Monterey Bay.[114] A similar incident was recorded by a drone in Monterey Bay in May 2017.[115] In 2019, orcas were recorded to have killed a blue whales on three separate occasions off the south coast of Western Australia, including an estimated 18–22 meters (59–72 ft) individual.[116]

Except diatoms (Cocconeis ceticola), which can create a yellowish sheen on individuals and remoras (which feed on sloughed skin), external parasites and epibiotics are rare on blue whales. They include the stalked barnacle Conchoderma auritum and acorn barnacle Coronula reginae; the former often attaches to the latter or to the baleen plates; the pseudo-stalked barnacle Xenobalanus globicipitis; the whale louse Cyamus balaenopterae; and the ciliate Haematophagus, which is very common in the baleen plates.[17]

Dead blue whale on flensing platform

Blue whales were initially difficult to hunt because of their size and speed.[49] Large-scale takes did not begin until 1864, when the Norwegian Svend Foyn invented the exploding harpoon gun which could be used on steam and diesel-powered ships.[127][128] Blue whale takes peaked from 1930–1931 when almost 30,000 animals were killed. Harvesting of the species was particularly high in the Antarctic, with 350,000–360,000 whales taken in the first half of the 20th century. In the same period, 11,000 whales were killed in the North Atlantic, mainly around Iceland, and 9,500 in the North Pacific.[77] The International Whaling Commission banned all hunting of blue whales in 1966 and gave them worldwide protection.[129] However, the Soviet Union continued to illegally hunt blue whales and other species through to the 1970s.[130]

Researchers examine a dead blue whale killed from a collision by a ship

Ship strikes are a significant mortality factor for blue whales, especially off the U.S. West Coast,[131] which has some of the greatest densities of commercial ship traffic in the world.[132] Nine blue whales were known to have been killed and one seriously injured by ship strikes between 2007 and 2010 off California.[121] The five deaths in 2007 were considered an unusual mortality event, as defined under the Marine Mammal Protection Act.[131][133] Lethal ship strikes are also a problem in Sri Lankan waters, where their habitat overlaps with one of the world's busiest shipping lanes.[134][135] Ship strikes killed eleven blue whales between 2010 and 2012 around Sri Lanka,[136] and at least two in 2014.[137] Ship strikes killed two Chilean blue whales in the 2010s off southern Chile.[138][139] Possible measures for reducing future ship strikes include better predictive models of whale distribution, changes in shipping lanes, vessel speed reductions, and seasonal and dynamic management of shipping lanes.[132][140][141] Few cases of blue whale entanglement in commercial fishing gear have been documented. The first report in the U.S. occurred off California in 2015, reportedly some type of deep-water trap/pot fishery.[142] Three more entanglement cases were reported in 2016.[143] In Sri Lanka, a blue whale was documented with a net wrapped through its mouth, along the sides of its body, and wound around its tail.[144]

Increasing man-made underwater noise impacts blue whales.[145][146] They may be exposed to noise from commercial shipping[147][148] and seismic surveys as a part of oil and gas exploration.[149][150] Blue whales in the Southern California Bight decreased calling in the presence of mid-frequency active (MFA) sonar.[151] Exposure to simulated MFA sonar was found to interrupt blue whale deep-dive feeding but no changes in behavior were observed in individuals feeding at shallower depths. The responses also depended on the animal's behavioral state, its (horizontal) distance from the sound source and the availability of prey.[152]

The potential impacts of pollutants on blue whales is unknown. However, because blue whales feed low on the food chain, there is a lesser chance for bioaccumulation of organic chemical contaminants.[153] Analysis of the earwax of a male blue whale killed by a collision with a ship off the coast of California showed contaminants like pesticides, flame retardants, and mercury. Reconstructed persistent organic pollutant (POP) profiles suggested that a substantial maternal transfer occurred during gestation and/or lactation.[154] Male blue whales in the Gulf of St. Lawrence, Canada were found to have higher concentrations of PCBs, dichlorodiphenyltrichloroethane (DDT), metabolites, and several other organochlorine compounds relative to females, reflecting maternal transfer of these persistent contaminants from females into young.[155]