| Spinner Dolphin[1] | |
|---|---|
| Size compared to an average human | |
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Subclass: | Eutheria |
| Order: | Cetacea |
| Suborder: | Odontoceti |
| Family: | Delphinidae |
| Genus: | Stenella |
| Species: | S. longirostris |
| Binomial name | |
| Stenella longirostris (Gray, 1828) |
|
| Spinner Dolphin range | |
The Spinner Dolphin (Stenella longirostris) is a small dolphin found in off-shore tropical waters around the world. It is famous for its acrobatic displays in which they spin longitudinally along their axis as they leap through the air.
Contents |
The Spinner Dolphin is sometimes referred to as the Long-snouted Dolphin, particularly in older texts, to distinguish it from the similar Clymene Dolphin which is often called the Short-snouted Spinner Dolphin. The species was described by John Gray in 1828. There are four named subspecies:
However the species display greater variety than these subspecies might indicate. A hybrid form characterized by its white belly inhabits the eastern Pacific. Other less distinct groupings inhabit other oceans.
The specific name comes from the Latin term for long-beaked.
The spinner dolphin is a long and slender species. Adults range from 129–235 cm and reach a body mass of 23–79 kg.[3] The rostrum of this species is relatively long and narrow.[4] It also have a triangular or sub-triangular dorsal fin.[4] Spinner dolphins generally have a tripartite color pattern consisting of a dark gray dorsal field or cape, lighter lateral field and white or very light-grey ventral field.[5] There is also a dark band that runs from the eye to the flipper, bordered above by a thin light line. However, variation in body form and color pattern is more pronounced in spinner dolphins than in any other cetacean. Spinner dolphins from the pelagic eastern Pacific tend to have smaller skulls and have shorter rostra from those from the central and south Pacific to the west.[4] Large males of some subspecies may have relativity large dorsal fins that are erect and canted forward.[6] There is also a dwarf form of spinner dolphin that occurs around southeast Asia .[7] In these some subspecies, the tripartite nature of the color pattern may be obscured by a dark dorsal overlay.[6] More offshore subspecies have a less dense and less extensive dorsal overlay, lending a faint cape and a more extensive field.[8]
The spinner dolphin has a pantropical distribution. It occur in all tropical and subtropical waters between 40°N and 40°S.[9] The species is usually associated with inshore waters, islands or banks. However, in the eastern tropical Pacific, dolphins live hundreds of miles from the nearest land in waters of mixed shallow, shoal and sharp thermocline and relatively low variation in surface temperature.[10] There appear to be seasonal shifts in the preferred habitat of spinner dolphins along with as year-to-year variation in habitat distribution.[11] The dwarf spinner dolphin of southeast Asia apparently inhabits shallow coral reefs.
The spinner dolphin feed mainly on small mesopelagic fish, squids and sergestid shrimps and will dive 200-300m to feed on them.[12] Spinner dolphins of Hawaii feed on many organisms in deep scattering layers when they move to the surface. The dwarf spinner dolphin may eat mostly on benthic fish in reefs and shallow water.[7] Off Oahu, Hawaii, spinner dolphins foraging at night and cooperatively herd their prey in highly dense patches.[13] They swim around the edge of a circle that is around 28–40 m in diameter at least 5 times and herd prey in this area. After this, pairs of dolphins from opposite sides of the circle will change positions and start swimming through the "donut" of high density prey which they have formed.[13] Spinner dolphins are themselves preyed on by sharks. Other possible predators include then killer whale, the false killer whale, the pygmy killer whale and the short finned pilot whale.[14] They are also susceptible to parasites, both external ones like barnacles and remoras, and internal ones like nematodes, trematodes, cestodes and acanthocephalans.[4]
In certain regions, such as Hawaii and northern Brazil, dolphins spend the daytime resting in shallow bays near deep water.[15] At dusk, they travel offshore to feed. They may move some distance along the shore when feeding, and the same animals may not be present in the same bay on two successive days.[15] Some dolphins move slowly along the shore between successive nights, and not all of them go into the rest coves every day. However in Hawaii, site-fidelity in dolphins is strong, and at least one and up to three animals were re-sighted northwest of Oahu 20 years after the first reported sighting.[16]
Spinner dolphins live in a fluid social organization.[17] The spinner dolphins of Hawaii live in family groups but also have associations with others beyond their groups.[4] The bond between mothers and calves are persistent. Spinner dolphins seem to have a promiscuous mating system, with individuals moving freely among several sets of companions over a period of minutes, hours, or weeks. Adult males will form coalitions of a few to a dozen individuals.[17] The function of these coalitions is not understood, although they may form the several associated groups that make up a school.[4] Acoustic signals of spinner dolphins include whistles, which may be used to regulate the organization and function of the school; burst-pulse signals, thought to have a evocative and vocative nature and echolocation click trains.[18] The spinner dolphin has a gestation period of about 10 months, and nursing lasts 1–2 years. Females reach sexual maturity at 4–7 years, and may calve every 3 years, while males become sexually mature at 7–10 years.[4] Breeding is seasonal, more so in certain regions than others.[4]
Spinner dolphins are well known for their acrobatics and aerial behaviors. They leap high out of the water and rotate as many as 14 longitudinally.[19] A spinner dolphin begins by emerging, rostrum first, from the surface and then rotates it body as it moves upward.[20] While in the air, the dolphin reaches its maximum height and fells back into the water while making a parabolic trajectory. In addition, during its re-entry, the spinner dolphin will rotate to present the side on the water’s surface.[20] The number of spins a dolphin can perform in a leap ranges from 2-5.5. The number of spins depends on the relationship between the swim speed and angular speed during while spinning underwater.[20] While it is not well understood, it is possible that this spinning behavior is related to breathing patterns, swimming energetics, play, hunting, removal of ectoparasites, territoriality or acoustic communication.[20] Other observed aerial behaviors include nose-outs, tail slaps, flips, head slap, "salmon leaps" and side and back slaps.[19]
Over tens of thousands of spinner dolphins, mostly eastern and white-bellied varieties, were killed in the thirty years after purse seine fishing for tuna was introduced in the 1950s.[4] The process killed probably half of all Eastern Spinner Dolphins. See Pantropical spotted dolphin for a discussion. They have also be contaminated by pollutants such as DDT and PCBs.[4] Spinner dolphins, as with other species impacted by the ETP tuna purse-seine fishery are managed both nationally by the coastal countries and internationally by the IATTC. The IATTC has imposed annual stock mortality limits on each purse seine and promulgated regulations regarding the safe release of dolphins.[2] Spinner dolphins in Hawaii have multiple daily visits to their nearshore resting grounds. The Big Island, Hawaii on the Kona Coast is a popular area for Spinner Dolphins. Boats take people out daily to snorkel and interact with the local dolphin population.
The eastern tropical Pacific and Southeast Asian populations of the Spinner dolphin are listed on Appendix II[21] of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), since they have an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.
In addition, the Spinner dolphin is covered by Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU) and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).
*Hawaii Spinner Dolphin Encounters