The Spinner dolphin (Stenella longirostris), a marine mammal in the family of oceanic dolphins, gets its name from the spinning behavior it shows when it leaps out of the water. This cetacean species lives in schools showing a dominance hierarchy and displays complex interactions among individuals. S. longirostris also engages in intricate echolocation underwater acoustics. Spinner dolphins attract tourists for dolphin watching. The species is of special interest for scientific investigation because of its remarkable capacity to learn.
The spinner dolphin is six to seven feet long and exhibits a three part colour pattern on its body. The pattern consists of a dark gray back, a pearl-gray side panel and a white belly. Males possess a postanal hump and are generally larger than the females. Spinner dolphins that live farther away from land are morphologically different from those that live close to land.
The small and slender spinner dolphin varies geographically in coloration and size, but can be identified by its relatively long, slender beak and triangular dorsal fin. The most common color pattern is three-part: dark grey on the back, lighter grey along the sides, and white or very light grey underneath. A darker grey stripe runs from the eye to the flipper, bordered above by a narrow, light line.
Spinner dolphins are polygynandrous. Mating in spinner dolphins appears to be promiscuous, and like many small dolphins, true courtship behavior can be observed, such as mutual caressing between the male and female. The male senses when the female is ready to mate and pursues her. Mating happens within the school with no real mate selection. The breeding system may vary geographically, with some populations showing a greater degree of polygyny than others.
Spinner dolphins mate when their hormone levels are high, which is one or two times a year. The male swims upside down underneath the female and inserts his penis into the female's reproductive tract. Females reach sexual maturity between four and ten years, while males reach sexual maturity between seven and twelve years. Adult females give live birth to one calf every two or three years. The gestation period averages 10.6 months. Females nurse their calves for a minimum of seven months up to two years, and they form a bond that lasts a lifetime.
Spinner dolphins move about the oceans in schools; groups that vary in size from just a few dolphins to over a thousand. They commonly school with other species such as pantropical spotted dolphins, or small toothed whales. In such schools, spinner dolphins are known to undertake migrations, following prey or warm water currents. In Hawaii, spinner dolphins usually spend their days resting in shallow bays near deep water, and then move offshore at dusk and feed as they move substantial distances along the shore.
Spinner dolphins may occur in schools with as many as 1000 individuals, but it is common to have 200 or fewer to a school. They are very social with each other and with other species of ocean dwellers. Spinner dolphins have been known to associate with spotted dolphins as well as yellowfin and skipjack tuna. Spinner dolphins rest in shallow waters, usually inlets. They tend to go back to the same area each day. When they are done resting, they quickly swim out to the deep area to feed on the vertically migrating fauna. When they are in the deep, darker waters, they are more susceptible to predation.
There is a dominance hierarchy within the schools of spinner dolphins. It is sustained by a descending order of threats and by behaviors that involve caresses. Threats are usually a simple nudge or abrupt gesture. These hierarchies are active when the school is in an enclosed area, and not in the open sea.
Spinner dolphins communicate with each other by echolocation, caressing each other, and using aerial patterns. They are most active when they have just finished resting.
The spinning jump is the trademark jump for this species. Spinner dolphins may rotate up to seven times while they are in the air! They do this most frequently at night. The purpose of the energetic spinning behavior of the spinner dolphin is not known. The back-flop or belly-flop that occurs when they fall back into the water produces a large cloud of bubbles that may act as an echolocation target, to allow a widely dispersed school of dolphins to communicate. Another theory is that the spinning may dislodge hitch-hiking remoras, or the spinning may, at times, simply be play.
Spinner dolphins are found in the tropical and subtropical waters in the Atlantic, Indian and Pacific oceans. They can also be found in some warm temperate areas. Spinner dolphins often occur near islands.
At present, four subspecies are recognised: S. l. longirostris (Gray's spinner dolphin), which occurs in all tropical seas; S. l. orientalis (Eastern spinner dolphin), found in open waters of the eastern tropical Pacific; S. l. centroamericana (Costa Rican or Central American spinner dolphin), which inhabits continental shelf waters off western Central America and southern Mexico; and S.l. roseiventris, (dwarf spinner dolphin) which inhabits shallow waters of Southeast Asia. Oceanic tropical and subtropical zones in both hemispheres. Limits are near 40ºN and 40ºS.
Stenella longirostris is mostly pelagic, and the species spends time in both shallow waters and deeper water farther from land. The spinner dolphin is typically thought of as a tropical high seas species, but it also inhabits shallow reef areas, coastal areas, and warm subtropical temperate waters.
Predation and Feeding Habits
The pelagic spinner dolphin is a carnivore that feeds primarily on small fish, squids and shrimps, and dives down to depths of 300 meters to catch its prey. The dwarf spinner dolphin feeds on reef fishes and other benthic organisms. Spinner dolphins use their excellent communication skills to allow them to hunt in groups. They can produce ultrasonic noise that stuns, or kills, fish making them easier to capture.
Conservation Status and Threats
The IUCN classifies this species as Data Deficient. Spinner dolphins in the eastern tropical Pacific have been killed incidentally since the early 1960s by tuna purse seine fisheries. They were caught in such large numbers that the population of S. l. orientalis was reduced to less than one third of its original size. Following raised awareness of the number of dolphins killed in tuna purse seine fisheries, measures were implemented to reduce dolphin by-catch. Today spinner dolphins continue to be killed in this way, although in greatly reduced numbers. However, continued chase, capture and release of large numbers in the fishery may be preventing the population from recovering.
In Sri Lanka and the Philippines, large numbers of spinner dolphins have also been captured in gillnets and killed by harpoons for the past 20 years, and local harpoon fisheries exist in several more locations throughout the world. Incidentally captured dolphins are consumed by local people, or used as shark bait, and this has led to the development of markets and fisheries directed at dolphins. The takes in these fisheries may be unsustainable.
The major threat to spinner dolphins is by-catch entrainment in tuna nets. There is also habitat destruction in some areas due to tourism infrastructure development. Spinner dolphins are protected in some countries. In the United States, special efforts have been made to monitor and reduce deaths due to the tuna industry.
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