Dusky dolphin

Appearance

The dusky dolphin is small to medium in length compared with other species in the family. There is significant variation in size among the different population areas. The largest dusky dolphins have been encountered off the coast of Peru, where they are up to 210 cm in length and 100 kg in mass. The size for dusky dolphins in New Zealand have been recorded to be a length range of 167–178 cm and a weight range of 69–78 kg for females and a length range of 165–175 cm and a weigh range of 70–85 kg for males.

Almost no sexual dimorphism occurs in this species, although males have more curved dorsal fins with broader bases and greater surface areas. The back of the dolphin is dark grey or black, and the dorsal fin is distinctively two-toned; the leading edge matches the back in colour, but the trailing edge is a much lighter greyish white. The dusky dolphins has a long, light-grey patch on its fore side leading to a short, dark-grey beak. The throat and belly are white, and the beak and lower jaw are dark grey. Two blazes of white colour run back on the body from the dorsal fin to the tail. Right between the white areas remains a characteristic thorn-shaped patch of dark colour, by which the species can easily be recognised. Aside from that, dusky dolphins may be confused with other members of their genus when observed at sea.
It can be distinguished from the common dolphin, which has a more prominent and longer beak and yellow flank markings. The skull of a dusky dolphin has a longer and narrower rostrum than that of an hourglass dolphin or Peale's dolphin of similar age and size.

Distribution

The dusky dolphin has a discontinuous semi-circumpolar range. The dolphins can be found off the coasts of South America, southwestern Africa, southern Australia and Tasmania, New Zealand, and some oceanic islands. Off South America, they range from southern Peru to Cape Horn in the west and from southern Patagonia to around 36°S in the east. Its range also includes the Falkland Islands. They are particularly common from Peninsula Valdes to Mar de Plata. In comparison, they are uncommon in the Beagle Channel and the inshore waters of the Tierra del Fuego region.

Dusky dolphins are found throughout New Zealand waters. The dolphin population in this area centres around East Cape and Cape Palliser on the North Island to Timaru and Oamaru on the South Island. They are especially common in the cold waters of the Southland and Canterbury currents. In Africa, the dusky dolphin ranges from Lobito Bay, Angola in the north to False Bay, South Africa in the south. Within Australian waters, dusky dolphins have been recorded in colder waters off Kangaroo Island, eastern Tasmania, and Bass Strait, although they are uncommon and those that are sighted there are possibly transients from New Zealand. They are also found around Campbell, Auckland, and Chatham in the western South Pacific, Tristan da Cunha in the South Atlantic and Île Amsterdam, and Île Saint-Paul in the southern Indian Ocean.

Status

The dusky dolphin, protected in much of its range, is listed as Data Deficient by the IUCN because; "... assessment of global population status is not possible with the currently available estimates of abundance and removals. The subpopulation off Peru has probably been overexploited, but present data do not allow estimation of present decline". Dusky dolphins may fall victim to the small cetacean fisheries of Peru and Chile. The expansion of these fisheries could have started in Peru when the anchoveta fishery collapsed in 1972. Dolphins have also been caught in gill nets in New Zealand, but catches appear to have dropped since the 1970s and 1980s. In Peru, dusky dolphins are killed in large numbers and used as shark bait or for human consumption. The dolphins are also thought to have been harpooned off South Africa, but the numbers are not considered large.

The dusky dolphin is listed on Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals as it has an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.

Behavior

Dusky dolphins prefer cool, upwelling waters, as well as cold currents. They largely live in inshore waters and can be found up to the outer continental shelf and in similar zones in offshore islands. They can move over great distances , but have no well-defined seasonal migrations. However, dolphins off Argentina and New Zealand make inshore and offshore seasonal and diurnal movements. In Argentina, dusky dolphins associate closely with southern right whales and South American sea lions. They have been found around bottlenose dolphins, but apparently do not interact with them, and may share feeding areas with Risso's dolphins. They also associate with various seabirds, such as kelp gulls, cormorants, terns, shearwaters, petrels, and albatrosses. In New Zealand, dusky dolphins mingle with common dolphins. Dusky dolphins have also been observed with southern right whale dolphins and pilot whales off southwestern Africa.Dusky dolphins live in a fission-fusion society, with most group size increases occurring during foraging and decreases in group sizes occurring during resting and traveling. In the Golfo San José off the Valdes Peninsula, dolphins commonly switch between small traveling groups and large sociosexual groups, and encounter a variety of associates. Studies of dolphins off Kaikoura, New Zealand, showed the dolphins normally live in large groups that split into smaller subgroups. These subgroups are composed of mating adults , mothers with calves and nonbreeding adults. Dusky dolphins have a promiscuous mating system in which both males and females mate with multiple partners. Mating groups are generally made of around 10 males and a single female. These mating groups can be found in both shallow and deep water but more often gather near shore.:162

In the mating groups, the males pursue a female in high-speed chases. Females seem to prefer males with great speed and agility rather than size, strength, or aggression.:164:175 Females may extend the chase as long as possible so only the best male remains. Females may try to evade males that are not vigorous or lack social skill.:170 Males also may form alliances to catch females.:167–69 Unlike male bottlenose dolphins, male dusky dolphins cannot monopolise females.:166 The time when female dusky dolphins first reproduce varies between regions. New Zealand dolphins first reproduce at about seven to years, with possibly six to seven years for Argentine dolphins. A study of dusky dolphins off the coast of Peru showed the reproductive cycle lasts around 28.6 months, with mother dolphins pregnant for 12.9 months, lactating for a further 12 months, and resting for 3.7 months before the cycle begins again. During copulation, females tend to be on the top.:170 As with all species where females mate with multiple partners, male dusky dolphins have large testicles for sperm competition.:166 Dusky dolphins sometimes engage in sexual behaviour for reasons other than reproduction, perhaps for greeting, communication, or strengthening social bonds. Homosexual behaviour between males has been observed. Dolphins having sex for social reasons tend to be more relaxed.:175

Females with calves tend to gather in nursery groups in shallow water. Nursery groups likely provide mothers and calves more time to rest, which is important for both. While the behaviours of nursery groups vary by month, resting is the predominant behaviour during most months.:188 The formation of nursery groups in shallow waters also allows members to hunt prey species that inhabit in these waters. Both adults and calves have been observed to chase and catch fish, and the adults may be teaching the calves how to hunt.:188–89 In contrast to shallower waters, hunting in deep water at night may be too dangerous for calves.:189 Calves are particularly vulnerable to predators like killer whales and use of shallow water by nursery groups may be a way to avoid predation.:183 Nursery groups tend to avoid mating groups.:174 Adult males in these groups will aggressively herd and chase females. They can separate calves from their mothers and harass them, as well.:185 Calves may also become even more vulnerable to predators as they become exhausted and disoriented.:185 Mother dolphins may look after calves that are not their own.:192Dusky dolphins perform a number of aerial displays, including leaps, backslaps, headslaps, tailslaps, spins, and noseouts. They also perform head-over-tail leaps which have been called the most "acrobatic" of the displays. A headfirst re-entry is performed when a dolphin leaps entirely water and positions its back in a curve while it flips the tail to land back in the water head-first. "Humping" is similar, except the snout and tail remain in the water when the dolphin is the arch. Leaps, head-over-tail leaps, backslaps, headslaps, tailslaps, and spins are often done over and over again. Young dusky dolphins apparently are not born with the ability to perform the leaps and must learn to master each one.:190 Calves appear to learn the leaps in this order: noisy leaps, head first re-entries, coordinated leaps, and acrobatic leaps.:190–91 Adults may perform different leaps in different contexts, and calves may independently learn how to perform leaps, but learn when to perform these when interacting others.:191

Habitat

Dusky dolphins prefer cool, upwelling waters, as well as cold currents. They largely live in inshore waters and can be found up to the outer continental shelf and in similar zones in offshore islands. They can move over great distances , but have no well-defined seasonal migrations. However, dolphins off Argentina and New Zealand make inshore and offshore seasonal and diurnal movements. In Argentina, dusky dolphins associate closely with southern right whales and South American sea lions. They have been found around bottlenose dolphins, but apparently do not interact with them, and may share feeding areas with Risso's dolphins. They also associate with various seabirds, such as kelp gulls, cormorants, terns, shearwaters, petrels, and albatrosses. In New Zealand, dusky dolphins mingle with common dolphins. Dusky dolphins have also been observed with southern right whale dolphins and pilot whales off southwestern Africa.

Reproduction

Dusky dolphins live in a fission-fusion society, with most group size increases occurring during foraging and decreases in group sizes occurring during resting and traveling. In the Golfo San José off the Valdes Peninsula, dolphins commonly switch between small traveling groups and large sociosexual groups, and encounter a variety of associates. Studies of dolphins off Kaikoura, New Zealand, showed the dolphins normally live in large groups that split into smaller subgroups. These subgroups are composed of mating adults , mothers with calves and nonbreeding adults. Dusky dolphins have a promiscuous mating system in which both males and females mate with multiple partners. Mating groups are generally made of around 10 males and a single female. These mating groups can be found in both shallow and deep water but more often gather near shore.:162

In the mating groups, the males pursue a female in high-speed chases. Females seem to prefer males with great speed and agility rather than size, strength, or aggression.:164:175 Females may extend the chase as long as possible so only the best male remains. Females may try to evade males that are not vigorous or lack social skill.:170 Males also may form alliances to catch females.:167–69 Unlike male bottlenose dolphins, male dusky dolphins cannot monopolise females.:166 The time when female dusky dolphins first reproduce varies between regions. New Zealand dolphins first reproduce at about seven to years, with possibly six to seven years for Argentine dolphins. A study of dusky dolphins off the coast of Peru showed the reproductive cycle lasts around 28.6 months, with mother dolphins pregnant for 12.9 months, lactating for a further 12 months, and resting for 3.7 months before the cycle begins again. During copulation, females tend to be on the top.:170 As with all species where females mate with multiple partners, male dusky dolphins have large testicles for sperm competition.:166 Dusky dolphins sometimes engage in sexual behaviour for reasons other than reproduction, perhaps for greeting, communication, or strengthening social bonds. Homosexual behaviour between males has been observed. Dolphins having sex for social reasons tend to be more relaxed.:175

Females with calves tend to gather in nursery groups in shallow water. Nursery groups likely provide mothers and calves more time to rest, which is important for both. While the behaviours of nursery groups vary by month, resting is the predominant behaviour during most months.:188 The formation of nursery groups in shallow waters also allows members to hunt prey species that inhabit in these waters. Both adults and calves have been observed to chase and catch fish, and the adults may be teaching the calves how to hunt.:188–89 In contrast to shallower waters, hunting in deep water at night may be too dangerous for calves.:189 Calves are particularly vulnerable to predators like killer whales and use of shallow water by nursery groups may be a way to avoid predation.:183 Nursery groups tend to avoid mating groups.:174 Adult males in these groups will aggressively herd and chase females. They can separate calves from their mothers and harass them, as well.:185 Calves may also become even more vulnerable to predators as they become exhausted and disoriented.:185 Mother dolphins may look after calves that are not their own.:192