The Spinner Dolphin
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Species: Stenella longirostris
Common name: (Long snouted) Spinner dolphin
The Latin name derives from longus for long and rostrum for snout or beak. The common name for this dolphin derives from its behaviour - these dolphins typically spin around on their longitudinal axis as they breach, although they are often seen breaching in the normal way. The name Long-snouted spinner dolphin is now obsolete (Prof. W. Perrin, Scripps Institution of Oceanography, pers. com.) it derives from the time when scientists thought that the Clymene dolphin ( then called the short-snouted spinner dolphin) was of the same species. It is now known that they are two different species and so both names have been simplified. The long-snouted spinner dolphin is now known only as the Spinner dolphin and the short-snouted dolphin is known as the Clymene dolphin.
Spinner dolphins are found in the Atlantic, Indian and Pacific Oceans, where they are restricted to tropical, subtropical and less often warm, temperate regions. There are several varieties of spinner dolphin, these are geographically defined and differ in body shape, size and colour. At least four varieties have been recognised in the Eastern Pacific; the Grey Spinner Dolphin (which includes those found around Hawaii and the Islands of French Polynesia) the Whitebelly, the Eastern Pacific, and the Costa Rican spinner.
Spinner dolphins are characterised by a long slender beak with a black tip and black lips, and an erect dorsal fin. The body is mainly grey, with a three toned colouration; dark grey along the back, a predominantly grey body and a pale grey/creamy white or pinkish under belly. Adults of this species are about 1.8-2.1m / 5.9ft -6.6ft in size and weigh 75-95kg / 165-209lbs (Dr. Michael Poole, Director, Marine Mammal Research Program, French Polynesia [CRIOBE}, pers com.), new-borns are 75-85cm in length. Spinner dolphins used to occur in herds of pods of >1000 animals, although nowadays occurrences of herds around 300 hundred are high, and in fact pods of 20 to 100 animals are more common. These dolphins are oceanic and are usually found far out to sea or off the coasts of oceanic islands. The first time we saw spinner dolphins we were about 150km/100miles off the coast of Suriname. A pod of 13 animals came to our bow; and stayed for about 20 minutes. We were traveling at ca.10knots, but spinner dolphins can easily keep up with a speed of 20 knots although this tires them more quickly. We went on observed Spinner Dolphins off the Marquesas Islands and mast recently off the coast of Tahiti and Moorea in the French Society Islands, in each case they came to bow ride, but stayed only five to ten minutes.
Spinner dolphins often associate with spotted dolphins, common dolphins and small to medium sized whales (e.g. pilot whales). In the Pacific they frequently associate with Yellowfin Tuna - much to their cost, since the Tuna Fishery between Hawaii, Mexico and Peru exploits this association. The tuna swim underneath the spinners in these waters, so the fishermen purposefully catch the dolphins in order to catch the tuna underneath. The spinner dolphins and the tuna accompanying them are herded together by high speed skiffs and then encircled with huge purse-seine nets. Pursing of the nets creates a bag in which both the dolphins and tuna are trapped and the mortality rate for the spinners is very high. During the last 30 years, as a result of this exploitation over 6-8 million dolphins have died, and in some cases populations have been reduced by over 85% from their pre-exploitation levels (Dr Michael Poole, pers. com.)
As far as we know there is nobody on the South American mainland coast
carrying out research on pelagic dolphins and we do not know if the species has
been logged as part of the fauna of Suriname. Some research work is being
carried out on the island of Curacao off the coast of Venezuela by a Dutch
biologist named DeBrot, but he has mainly been reporting new species for the
island ( Prof. W. Perrin pers. com.) Extensive and ongoing work has been carried out on
Spinner Dolphins off the coast of Moorea in the French Society Islands. Dr
Michael Poole, Director of the Marine Mammal Research Laboratory on Moorea has
worked on spinner dolphins for over 10 years and has identified more than 200
spinner dolphins, 150 of which live around Moorea. He identifies them by the
pattern of nicks and scars on their dorsal fin - which he photographs and then
uses for comparison.
Spinners with calf
Spinner dolphins typically congregate together in groups or pods. These groups however are not really comparable to those formed by other mammals. The society is leaderless and very fluid. Associations change daily and there is no strict hierarchy of dominance. In each group however there is usually a core of individuals who associate on a regular basis (much as a group of teenagers will hang out together). The type of associations that spinner dolphins form are most comparable with those of Gorillas, Dr Jane Goodall (who worked on Gorillas for 37 years) called this "fission-fusion" -loose associations between varying individuals -the evidence that Dr Michael Poole is collecting suggests that the spinner dolphin groups are even more fluid than those formed by gorillas.
Spinner dolphins occur in deep tropical waters, where they feed on mid level small fish, squid and shrimp. They are however quite flexible in their choice of food and in shallow seas they may eat bottom-dwelling and reef organisms. They have 45-65 pairs of sharp teeth in each jaw. The dolphins mature by about 1.5-1.7m in size and adult females give birth to a single calf every second or third year after an average 10-11 months gestation period. Calves are suckled for 4-14 months on fat rich milk (40% fat compared to domestic cow milk 5% fat) but after weaning maternal association can last much longer.
Sense of smell and taste is reduced in dolphins, and although tests on captive dolphins have show that they have good visual activity they tend to rely more on sound than on vision. Dolphins have two voices: the more sonic (audible to humans) is a vocabulary of clicks and whistles uttered above and below water, these are used together with mechanical noises such as jaw-snapping, slapping of flippers or crash-dives after leaping at the surface. Secondly dolphins use directional sonar (echolocation) to navigate. By seeking the echoes it receives back from its own clicks and other emitted sounds the dolphin can not only locate an object but also determine size, shape, whether it is inorganic or living etc.
Many thanks to Prof. W. Perrin at the Scripps Institution of Oceanography and Dr Michael Poole, Director of the Marine Mammal Research Program in French Polynesia for their comments
This page will be updated if I receive further information
Dr. Janet Sumner Fromeyer