The Spinner Dolphin is found in off-shore tropical waters around the world. There are two species of the Spinner dolphin: the Short Snouted Spinner dolphin and the Long Snouted Spinner Dolphin.
Short Snouted Dolphin
Little is known about the Short Snouted Spinner, only that it ranges from the Gulf of Mexico to the Caribbean and from the mid-Atlantic to the northwest coast of Africa. Spinner Dolphins grow to a length of 7 feet (2 metres) and weigh 200 pounds (90 kilograms). Their flippers are small and pointed at the tips and its dorsal fin is curved and nearly pointed and is located in the middle of the back. Their upper body is colored dark grey, with a light grey area from the eyes along the sides to the tail. Its belly is white.
Long Snouted Dolphin
There are different forms of Spinner Dolphin that inhabit different locations in the Pacific. The Costa Rican, the eastern, the whitebelly, the Hawaiian (Greys) and a dwarf form found in the Gulf of Thailand. They are differentiated by their size which ranges from 5.5 to 6 feet (1.65 to 1.8 metres) to 7 feet (2 metres) and weights of 135 pounds (61 kilograms) and 200 pounds (91 kilograms). Spinner Dolphins can also be identified by their differences in shape and markings.
Spinner Dolphin Characteristics
The body size, shape, and color patterns of the various forms of Spinner Dolphin differ according to geographical location. These Dolphins in general all share common characteristics.
Spinner Dolphins have slender bodies and long, thin beaks (except for the Atlantic short-snouted spinner). Their flippers are small and pointed at the tips. Their coloring is in shades of dark grey, light grey and white. Most Spinner Dolphins have white bellies.
Spinner Dolphin Behaviour
Spinner Dolphins congregate in groups that vary from just a few dolphins to great schools numbering in the thousands. These Dolphins are famous for their acrobatic displays in which they will spin longitudinally along their axis as they leap through the air. They are also keen bow-riders. The reason for the animals spinning is not known.
One suggestion is that the great cauldron of bubbles created on exit and re-entry may act as a target for echolocation by other individuals in the school. It may also be simply play-acting. Individuals have been spotted completing at least 14 spinning jumps in quick succession.
Spinner Dolphins tend to do most of their hunting at night as the ‘scattering layer’ of marine life, which has spent the day at depths of 3000 feet, rises toward the surface to feed on microscopic plant material. Their diet is composed of fish, jellyfish, euphausiids (or krill), squid, shell-less snails, as well as copepods (a group of small crustaceans found in the sea and nearly every freshwater habitat).
Before diving into the layer, the pod of dolphins gather together in a kind of rally as if they are about to embark on a dangerous journey. These dolphins are taking a great risk because other predators have gathered as well, such as sharks, which are natural predators of dolphins.
Spinner Dolphins form small subgroups and spread out across the sea. Many times, the dolphins will dive down into the dark oceans at 800 feet, or more. Even though Spinner Dolphins have more teeth than any other dolphin (between 45 to 65 sharp, pointed teeth in each side of both the upper and lower jaws), they do not use them to chew their prey but rather to grasp and immobilize their prey.
Spinner Dolphin Communication
Spinner Dolphin sounds appear to be in the form of click-whistles and pulse sounds which are a mix of echolocation and communication. Echolocation sounds enable dolphins to track objects in dim or dark water and to see much further than their eyes will allow. Their complex array of whistle sounds are the way that dolphins talk to one another. The spinners can even identify themselves with sounds they make while trailing bubbles from their blowholes.
They also communicate by slapping the water with various body parts. For instance, ‘nose-outs’ occur when beak is thrust from the surface. This action is commonly used when the pod is emerging from a rest period. ‘Tail slaps’ are often used to indicate impending danger or to signal a dive.
Head slaps, side slaps and back slaps are most frequently seen as the school begins to pick up speed. Last, and most spectacular, are the spins themselves. Many animals spin repeatedly, with each spin tending to get smaller and smaller, finally finishing up with an emphatic side slap.
Females reach sexual maturity at about 4 to 7 years, males at about 7 to 10 years. A newborn calf averages 32 inches (80 centimetres) in length. The gestation period is 10 and a half months and calving is between 2 to 3 years. Calves nurse from 1 to 2 years.
Spinner Dolphin Predators
Known predators of the Spinner Dolphins are sharks, killer whales and possibly false killer whales, pygmy killer whales and pilot whales.
Spinner Dolphin Conservation
Spinner Dolphins are classed as an endangered species.
Whitebelly and eastern spinner dolphins in the eastern tropical Pacific have suffered enormous population losses as a result of entanglement in the nets of tuna fishermen. It is not yet known why they swim in herds above schools of yellowfin tuna. Fishermen are aware of this behaviour and instead of looking for the tuna, they seek out the spinner dolphins instead, knowing the yellowfin tuna will be beneath them. When they find them, they encircle the herd with large nets called ‘purse seine’ nets, capturing the dolphins along with the tuna.
It is believed that the stock has declined by as much as 80% since the 1960s, when purse seining operations began. They do well in captivity and have been displayed in Hawaii and elsewhere in the U.S., Indonesia, the Philippines and Hong Kong.