The Spinner dolphin (Stenella longirostris), a marine mammal in the family of oceanic dolphins, engages in formation of small schools. This cetacean species evinces 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 (Norris, 1991). 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 colouration and size, but can be identified by its relatively long, slender beak and triangular dorsal fin. The most common colour 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.
Mating in spinner dolphins appears to be promiscuous, and like many small dolphins, true courtship behaviour can be observed, such as mutual caressing between the male and female. The breeding system may vary geographically, with some populations showing a greater degree of polygyny than others. Calves are born every three years, after a gestation period of about ten months. The mother nurses the calf for up to two years, and they form a bond that lasts a lifetime. Females reach sexual maturity between four and seven years, whereas males do not reach maturity until between seven and ten years.
Spinner dolphins are polygynandrous. The male senses when the female is ready to mate and pursues her. Mating happens within the school with no real mate selection.
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 (Norris, 1991). Males reach sexual maturity at about 10-12 years old, while the females' age at sexual maturity ranges from 5.5-10.0 years old. Adult females give live birth to one calf every two or three years. Gestation period averages 10.6 months (Klinowska, 1991). Females nurse their calves for a minimum of seven months.
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. Pelagic spinner dolphins feed primarily on small mesopelagic fish, squids and shrimps, and dive down to depths of 300 meters to catch their prey. The dwarf spinner dolphin feeds on reef fishes and other benthic organisms.
The purpose of the energetic spinning behaviour of the spinner dolphin is not known. It has been suggested that the large cloud of bubbles created by the powerful spin and splash landing 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 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, yellowfin, and skipjack tuna (Klinowska, 1991). 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 (Norris, 1991).
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. Stenella longirostris performs many kinds of jumps. The spinning jump is the trademark jump for this species; they are named for their amazing aerial spinning maneuvers. They do this most frequently at night.
"Whales may use ultrasonic noise like a stun gun against fish…Working with captive Hawaiian spinner dolphins (Stenella longirostris), California University cetologist Kenneth Norris found that when they direct ultrasonic beams at a shoal of fish, they can stun or even kill some of them. The beams may cause the fishes' air-filled swimbladders to resonate so intensely that their body tissues also vibrate, disorienting them." (Shuker, 2001)
The acrobatic spinner dolphin is the most common small cetacean in many tropical open seas, where it can be seen spinning high in the air, (hence its common name), or riding the bow waves of boats.
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 subtropical and warm temperate waters.
Predation and Feeding Habits
The pelagic spinner dolphin feeds primarily on small mesopelagic fish, squids and shrimps, and dive down to depths of 300 meters to catch their prey. The dwarf spinner dolphin feeds on reef fishes and other benthic organisms.
Spinner dolphins are carnivorous. They consume mesopelagic fish and epipelagic and mesopelagic squid as well as shrimp (Klinowska, 1991). Most of the prey they take are vertically migrating species.
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 bycatch 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 the tuna industry (Klinowska, 1991).
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