Spinner and Bottlenose Dolphins

Spinner and Bottlenose Dolphins 

Hawaiian Name: Nai‘a

[Illustration: Spinner dolphin flipping in air]

 

Spinner Dolphins (Stenella longisrostris)

Endemic Hawaiian subspecies

They leap up from the sea in synchronous motion, their shimmering bodies arcing gracefully through the air and then disappearing again beneath the water’s surface.  These are Hawai‘i’s dolphins—playful, intelligent, perceptive, curious, and often quite friendly to humans. 

An ancient Hawaiian proverb states, “He nai‘a, he i‘a lele.” (“It is the nai‘a, a leaping fish”), and is “...said of one who jumps to conclusions.”[i]

[Illustration: Spinner dolphin]

 

Spinner dolphin adults average from 5 to 7 feet (1.5 to 2.2 m) long and weigh from 110 to 160 pounds (50 to 73 kg).  The dolphin’s back is dark, the sides are pearl gray, and the belly is pure white.  The slim beak of the spinner dolphin is blackish-colored and lined with 180 or more delicate interlocking conical teeth.

Spinner dolphins earned their name because of their fondness for jumping out of the water and doing acrobatic flips and spins in the air, a sight that is common in Hawaiian waters. 

Spinner dolphins travel in pods, which sometimes number in the hundreds.  It’s not uncommon to see several dolphins leap from the water perfectly together.  The dolphins often leap high up out of the water, doing two or even three flips in the air while at the same time spinning their bodies around.

Researchers believe that the energetic flips and spins done by spinner dolphins may serve the purpose of acting as geographical markers for the pod, as well as to let other dolphins know their location.  Dolphins often let out a loud sound below the surface right before they leap out and spin.  It is possible the leaps and spins may also be a form of play.

Pods of spinner dolphins often swim in large, slow circles nearshore in the early morning.  This is likely their resting phase.  Dolphins may alternate brain halves while “sleeping,” allowing one side of the brain to rest while the other side stays active (an important survival trait in an ocean environment).

Spinners dolphins live to about 20 years of age, on average.  Pregnancy in spinner dolphins lasts about 10½ months.  Baby dolphins are seen around the Hawaiian Islands mostly in the late spring and summer. 

About 1 time in 100, a spinner dolphin will have twins, and when that does occur, the other adult females in the pod may take care of one of the babies.

[Photograph: Spinner Dolphin]

 

 

Bottlenose Dolphins (Tursiops truncatus)

Indigenous

Bottlenose dolphins are also seen around the Hawaiian Islands quite regularly, though not in such great numbers as spinner dolphins.  An adult bottlenose dolphin may be 8 to 11 feet (2.4 to 3.4 m) long and weigh just less than 500 pounds (227 kg).

[Illustration: Bottlenose dolphin]

 

Bottlenose dolphins may eat more than 20 pounds (9 kg) of food per day, including fish, squid, shrimp and crabs.  Dolphins usually dive less than 200 feet (61 m) for food, but one bottlenose dolphin trained by humans demonstrated its deep-diving ability by diving to almost 1,800 feet (549 m).

The bottlenose dolphin is considered the smartest of all of dolphin species, and perhaps the smartest of all animals.  Bottlenose dolphins actually have a cerebral cortex more convoluted (folded) than the human cerebral cortex, though it is thinner than the human cerebral cortex. 

The cerebral cortex is the area of the brain that corresponds with social communication, abstract information processing, problem solving and higher level thinking.  The structural complexity of the dolphin’s cerebral cortex may explain the learning capabilities that are highly evolved in bottlenose dolphins.

Bottlenose dolphins are also known to sometimes create elaborate bubble rings, and then to play with these bubble rings in a variety of ways.  Some dolphins have shown the ability to produce stable, lasting bubble rings, including fabricating long helices (spirals) of air by swimming in a curved path. 

The dolphin releases air into the vortex it creates, and then uses its dorsal fin to bring the bubbles together and curl them into a helix up to 15 feet (4.6 m) long.

[Photograph: Bottlenose dolphin]

 

Echolocation

Dolphins purposefully bounce sounds off objects and then interpret the returning echo.  The dolphin uses this echolocation to get an acoustic picture of its surroundings by emitting sound waves from its forehead region, known as the melon.  All toothed cetaceans (Odontocetes) are believed to be capable of echolocation.

[Illustration: Echolocating bottlenose dolphin with sound waves reflecting out and reflecting back and/or dolphin creating bubble rings]

 

 The time lapse between the dolphin emitting the signal and receiving the echo indicates the distance to the object being echolocated, and also the target’s density.  By determining the target’s density, dolphins are able to recognize particular species, even in the total darkness of the deep ocean. 

Dolphins may also analyze their prey using this echolocation “sonar,” and may even stun their prey using echolocation pulses.  The advanced capabilities of dolphins using echolocation may also allow dolphins to be able to tell if a human is in distress.  There have been numerous reports of dolphins saving drowning people.

 

Dolphin Research

A National Marine Fisheries Service sponsored study took place from May 3 to June 13, 2003, and found that bottlenose and roughtooth dolphins are more abundant around Kaua‘i and Ni‘ihau that around the other Hawaiian Islands.  The precise reason for this remains unclear, but contributing causes may include the better availability of the dolphins’ food sources, which include fish and squid, and this in turn may be related to ocean currents around the islands.

Species surveyed in the study included not just roughtooth and bottlenose dolphins, but also pilot whales, dwarf sperm whales, false killer whales, and spotted dolphins.  Also seen were beaked whales, which are considered a deeper-water species along with rough-toothed dolphins.  Another deeper-water species encountered was the striped dolphin (Stenella coeruleoalba), rarely seen in Hawaiian waters.

During the study the scientists encountered at least 12 different whale and dolphin species.  Many photographs were taken, and skin samples of various species were collected for DNA analysis and comparison with other Hawaiian cetacean populations to determine if there are island-specific differences.[ii]

A 2003 study by researchers at the University of Hawai‘i’s Institute of Marine Biology Marine Mammal Research Program discovered that dolphins reduce the volume of the echolocation pulses they emit as they approach the object they are echolocating. 

The dolphins first use echolocation to check the distance to the object, then adjust their volume and use echolocation to determine the object’s size (the more waves that are reflected back indicates a bigger size object). 

This is different than the echolocation method used by bats, and human-built mechanical echolocation devices, which maintain the same volume of emission but adjust the receiving sensitivity when getting closer to an object being echolocated.

[Photograph: Dolphin pod]



[i]p. 90, Pukui, Mary Kawena.  ‘Ōlelo No‘eau: Hawaiian Proverbs & Poetical Sayings.  Honolulu: Bishop Museum Press, 1983.  Proverb 833.

[ii] See http://is.dal.ca/~whitelab/rwb/robin.htm.