Tuesday, 20 September 2011

The Spinner Dolphin (Stenella longirostris)

Photo by/ Khaled M.Saied
 
Spinner dolphins are aptly named for their above-water actions. They can leap into the air and make as many as seven complete spins before diving back into the sea again.
The spinner dolphins 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. 

Photo by/ Khaled M.Saied

The spinner dolphin has a pantropical distribution. It occur in all tropical and subtropical waters between 40°N and 40°S. The species is usually associated with inshore waters, islands or banks. However, dolphins live hundreds of miles from the nearest land in waters of mixed shallow, shoal and sharp thermocline and relatively low variation in surface temperature.There appear to be seasonal shifts in the preferred habitat of spinner dolphins as well as year-to-year variation in habitat distribution.

Photo by/ Ahmed F.Gad

Dolphins breathe through their blowhole located at the top of their head. Oceanic dolphins have evolved a method of breathing without surfacing from the water. They blow a bubble when near the water surface and then quickly draw breath in when the bubble forms a bridge between the blowhole and the air, through the water. A dolphin may empty and refill its lungs in less than a fifth of second. As the dolphin breathes the air leaves the blowhole at speeds of over 100 miles per hour. . 

Photo by/ Khaled M.Saied
 
In order to live most of their lives underwater, 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. So what if they don't wake up in time? Dolphins cope with this by putting one half of the brain to sleep at a time (literally sleeping with one eye open). In this way, the animal is never completely unconscious, but it still gets the rest it needs. The two hemispheres "take turns" cycling up and down for a few hours. This also keeps the dolphins from losing body heat and getting too cold (taking up energy to keep warm). Think how you get cold when you sleep, even when the temperature hasn't changed. Social Behavior
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 in synchrony, twisting, turning and swimming in perfect harmony together.

Photo by/ Khaled M.Saied
Spinner Dolphins can be very noisy under and above water. Echolocation enables dolphins to track objects in dim or dark water, in effect to "see" much further than their eyes alone will allow. Their complex array of whistle sounds are the way that dolphins talk to one another. The spinners also identify themselves with sounds they make while trailing bubbles from their blowholes — sounds called signature whistles.

Photo by/ Khaled M.Saied
The ability to determine which direction a sound is coming from underwater (directional hearing) is vital for dolphins and other toothed whales. They pinpoint their prey through echolocation, in which some of the click train sent out bounces off an object and returns to them in hologram form. Dolphins then interpret this returning echo to determine the object's distance, shape and other characteristics. 

Photo by/ Ahmed F.Gad
 
Newborn calves can be seen swimming near their mothers head for about a week before they swim nearer to her dorsal fin. This is termed as echelon swimming. Dolphins carry their young inside their womb and gestation is about 10 months for a spinner. The baby emerges tail first, and will suckle from its mother for up to 2 years. Other details seen in a newborn 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 after birth to become hard and rigid. The softness allows it to fold over against the calf's back during birth. Much more pleasant for Mom!

Researchers have long speculated as to how dolphin calves are able to keep up with their fast-swimming mothers. One favored theory has been that they employ "drafting," defined as "the transfer of forces between individuals without actual physical contact between them." Kinda like when you sucked into a passing truck on the freeway, a free ride for awhile.

Photo by/ Khaled M.Saied
Photo by/ Ahmed F.Gad

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