Spinner dolphins (Stenella longirostris) derive their name from a habit of leaping from the water and warping their bodies into graceful curves, or spinning lengthwise before splashing back. The motives for this behavior are not known but such actions are often in themselves enough to distinguish this species.
Common names for the geographic varieties of Stenella longirostris include: the Costa Rican, Eastern, Whitebelly, and Hawaiian or Gray’s dolphins. A dwarfed form of this species is found in the Gulf of Thailand.
General Description
Like other dolphins of the genus Stenella, spinner dolphins are relatively small, reaching lengths of up to 2 metres and weighing approximately 59-77 kilograms at maturity. They have long, slender snouts or beaks that are black above and white below.
Coloration is dark gray dorsally fading to lighter gray on the sides and the belly is white. A dark stripe extends from the flipper to the eye.
Spinner dolphins’ pectoral flippers are used to steer them through the water. They also use them to stroke one another, increasing and affirming social bonds. Dolphin “friends” may swim along, touching flippers and stroking each other. Dolphins that appear to be closely bonded may swim along in synchrony, twisting, turning and swimming in perfect harmony together.
The spinner dolphin can leap into the air and make as many as seven complete spins before diving back into the ocean again, they usually perform a series of spins, each spin tending to be made with less energy, finally finishing up with an emphatic side slap. The power of the spin comes from the tremendous acceleration under the water and the torque of the tail just as the dolphin breaks the surface. The aftermath of the spin — the sound of the slap, the splash on the surface, and the dense bubble cloud underwater, which even distant dolphins can pick up through their echolocation may be the real purpose of the spin.
Spinner dolphins maximize the effect of this splash by twisting around to land in a belly-flop, or back-flop. Spins are most frequently performed while the school is spread out across the water.
The most common cause of spinning is assumed to be in order to rid the spinner dolphin of parasites, such as remora fish.
Spinning may also serve as a courtship display or to eject water from the upper respiratory tract, reset the organs of balance, help mix fluid in the gut and venous reservoirs, or simply be for fun. It might also be important to maintain the spinner’s thermal budget, since the dolphins’ core and subcutaneous temperature are higher when the spinner dolphins are spinning. In order to live most of their lives underwater, spinner dolphins are conscious breathers: they think about when to breathe (probably as much as we think about walking). Therefore, in order to breathe, they have to be conscious. Dolphins sleep by isolating one side of the brain first to sleep then switching to the other side after a few hours. In this way, the animal is never completely unconscious, but still gets the rest it needs. This practice also prevents the dolphins from losing body heat and getting too cold.
Spinner dolphins can be very noisy under and above the water. Echolocation enables dolphins to track objects in dark water or water with poor visibility in order to “see” much further than their eyes will allow. Their complex array of whistles are a means for the dolphins to communicate with one another. The spinners also identify themselves with sound they make whilst trailing bubbles from their blowholes. These sounds are known as signature whistles.
Eating
Spinner dolphins feed at night on mesopelagic fish, shrimp and squid that are found about 650-1,000 feet below the surface of the water. The mesopelagic boundary layer stays in deep waters in the ocean during the day. At night they move up in the water column (vertical migration), and inshore (horizontal migration). Dolphins follow these diel migrations of their prey in order to maximize foraging efficiency.
Reproduction
Adult females give birth to a single calf at two year intervals. Parturition usually occurs in early summer but can occur in any season.
Spinner dolphins carry their young inside their womb and gestation lasts as long as 10 months. The baby emerges tail first, and will suckle from its mother for up to 2 years.
Calves are usually about 75 cm long at birth and can be seen swimming near their mothers for about a week before they swim nearer to her dorsal fin. This is called echelon swimming. Other features of a newborn spinner dolphin are fetal folds; wrinkles along their sides where they were curled up in the womb, and floppy dorsal fins. The dorsal fin, made of cartilage, takes a few days following birth to become hard and rigid. The softness allows it to fold over the calf’s back during birth.
Multiple males may mate with one female in short, consecutive intervals. Calving intervals for males average three years. Maturity occurs at around seven and maximum longevity is twenty years.
Where To Find Spinner Dolphins
Spinner dolphins often associate with spotted dolphins, common dolphins and small to medium sized whales (e.g. pilot whales)
They usually occur in groups of 30 to several hundred but may number into the thousands
In most places, spinner dolphins are found in the deep ocean where they likely track prey. They tend to rest in bays and protected areas during the day and then fuse into larger groups to feed in deeper water on fish and squid at night.
Spinner Dolphins can be found in all tropical and subtropical oceans around inshore waters, islands, banks or shallow coral reefs.
Wicked Diving sees Spinners on many of our expeditions in more remote places and open waters. We have seen them a few times a year in Thailand and once a month or so on our Komodo Liveaboard trips.
Conservation
Spinner dolphins are classified as Lower Risk, however, due to the as yet unexplained association between large yellowfin tuna and schooling dolphins, fisheries have been known to use spinner dolphins in order to track tuna. As a result spinners often become trapped in the nets and drown. Stress from being pursued by the fisheries has also been documented as a very serious threat to dolphins. Currently, fishing methods for tuna imported under the Dolphin Safe Programme do not allow fishing practices, such as setting on dolphins.
Interactions with tourists is also a growing threat to the spinner dolphin population; because the species is active at night, daytime interactions with tourists inhibit necessary rest and sleep time.
As a result Environmentalists and tour operators are at odds as to wether you should dive with the spinner dolphin.
The 1972 Marine Mammal Protection Act prohibits any “taking” of marine mammals, i.e., to “harass, hunt, capture, or kill.” As a “Level B harassment” which was later defined in the act as “any act of pursuit, torment, or annoyance which . . . has the potential to disturb a marine mammal or marine mammal stock in the wild by causing disruption of behavioral patterns.”
While the research is ongoing, some experts believe that when humans come to the spinners’ resting grounds, they could change the dolphins’ behavior and perhaps endanger them. The spinners go to those areas “for a purpose, and that’s because they need to rest,”
If the dolphins don’t get adequate downtime, that could make it more difficult for them to fish and to avoid predators at night.
There are some guidelines, in place to protect the spinner dolphin however, many tour operators simply aren’t adhering to them, and the majority of tourists are completely unaware of them.
On a spinner dolphin encounter, the usual procedure is to have snorkelers, swimmers, divers assume a passive float position in a safe, non-intrusive manner, other guidelines request that if swimming, snorkeling or diving with spinners that you remain at least 45 metres from the dolphin and that you limit your time observing to a maximum of 30 minutes. Always remember that spinner dolphins should not be encircled or trapped between boats or the shore.
If approached by a spinner dolphin while on a boat, put the engine in neutral and allow the animal to pass. Boat movement should be from the rear of the animal.
It is up to your discretion if you want to have an encounter with these playful creatures. It is not yet illegal to swim, snorkel or dive with spinner dolphins however various environmental groups are currently working on legislation to make it so. Therefore, before you jump into the water examine both your conscience and the local laws to ensure that you are making the right decision.
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