Stenella longirostris
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.
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
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