Eye Wide Shut
Bottlenose dolphins sleep with one eye open!
Bottlenose dolphins are found in warm and temperate seas the world over. Sometimes, they are also found in the Arctic and the Antarctic oceans, but they rarely venture too far South or North. Dolphins are one of the most popular animals in the world, and are very popular as aquarium entertainers. Their high intelligence has also resulted in them being used by armed forces for marine threat identification.
Bottlenose dolphins are part of the Delphinidae family. They constitute the genus Tursiops. Three species of bottlenose dolphins, previously thought to be identical, can be found in this genre:
- Common bottlenose dolphin, T. truncatus: has a cosmopolitan distribution
- Indo-Pacific bottlenose dolphin, T. aduncus: is found in the Indian Ocean and around Australasia
- Burrunan Dolphin, T. australis: has a cosmopolitan distribution
Bottlenose dolphins are one of several aquatic mammals, like porpoises and whales. Though they resemble fish, they are evolutionarily more closely related to us humans. Dolphins, porpoises and whales are thought to have evolved from one of several ancient terrestrial mammals which gradually increased their dependence on water to the extent that they couldn’t come back onto dry land. Similarly, whales are thought to have evolved from hippo-like creatures.
Bottlenose dolphins grow to about 2-4 meters long and generally weigh about 200-300 kg, though they can weigh as much as 600 kg. Their length has been found to be dependent on their geographical location. Dolphins in colder waters were found to be more than a meter longer than their tropical counterparts.
They live for up to 40+ years, although 20-30 years is the norm.
Bottlenose dolphins can swim at a constant speed upwards of 15 miles per hour, and are able to reach 18 mph.
Since they are mammals, they breathe through lungs, not gills. This means that they need to surface to breath. If a dolphin is unable to rise to the surface when it needs to breathe, it will drown!
Bottlenoses can extract more than 80% of the available oxygen from their breaths, whereas healthy humans can extract less than 20%.
Bottlenose dolphins are generally dark gray in color on the dorsal side, but get steadily lighter towards the underside. The undersides of the bottlenose dolphins are of a very light gray shade — almost white. This design of coloring, known as countershading, makes them harder to spot from above and below.
Pod of Bottlenose Dolphins
Bottlenose dolphins live in moderately large groups, containing up to 12 individuals. Males don’t stay in a pod permanently, and live in groups of up to three. Small groups can sometimes come together to form large groups consisting of hundreds, sometimes even more than a thousand individuals.
Bottlenose dolphins have a unique method of sleeping. Unlike most terrestrial mammals, their breathing is a voluntary process, i.e. dolphins have to consciously surface and expel the stale air in their blowholes. So, they can’t just ‘go to sleep’ like other mammals, and instead rest one hemisphere of their brain at a time, alternating between the two for 8-10 hours. The active hemisphere controls the essential bodily processes, while the other hemisphere sleeps.
While sleeping, dolphins swim close to the surface, since not being able to rise from deep waters due to their half-asleep state would be fatal. Since eyesight is not their primary mode of navigation, bottlenose dolphins may even close one eye while sleeping!
Dolphins are curious about humans, and are more tolerant of humans than pretty much any other marine creature. Several dolphins, in fact, developed close bonds with neighboring human communities by playing with the residents, performing acrobatics of their own free will, and following boats. Famous examples of such dolphins are Moko, a dolphin who led a pair of stranded pygmy killer whales to safety, and would play with residents regularly, and Opo, who gained fame as a friendly, fun-loving dolphin.
Dolphins have a weird knack for being of help to other marine animals, and even humans, altruistically. This has been seen in many instances of dolphins leading stranded humans towards the shore and vice versa with beached marine animals. This mysterious desire to altruistically help others is not natural, and researchers are not sure about its origin.
Bottlenose dolphins use the method of echolocation to track their prey. This method, which is also used slightly differently by bats, is very similar to sonar. Dolphins create high-pitched sounds (inaudible to human ears) that bounce off the various objects in the dolphin’s path, and enable the dolphins to create a virtual ‘sound-map’ of their surroundings.
Even though it is not their primary mode of navigation, bottlenose dolphins are known for their sharp eyesight. They have a good vision in air and underwater.
Like all members of the odontoceti family, bottlenose dolphins use a variety of sounds to communicate. This primarily includes a variety of clicks and squeaks. They also communicate via gestures such as leaping out of the water, and slapping the surface with their tails.
Feeding A Captive Bottlenose Dolphin
They eat small fish such as mullet, tuna, and mackerel. Sometimes, they also eat crabs, squids, shrimp, and other such small animals.
Their high intelligence makes them formidable predators. Pods of dolphins are extremely successful hunters, and even a solitary dolphin rarely has any problems.
During the mating season, males compete for mating rights. This often involves acts such as head butting. Groups of males have even been observed following and ‘herding off’ females, waiting for them to come into estrus.
Dolphins are one of the very few animals thought to have sexual intercourse for mere pleasure, and are probably the only non-primate species to do that. Sperm production in dolphins is quite taxing and tiring, and slows down after the mating season to minimize its inconsequential loss.
The bottlenose dolphin’s gestation period is around one year. Nearly all pregnancies result in a single calf; twins are the only other possibility, and is extremely rare.
The young ones of the Bottlenose dolphins live with their mother till they reach 6 years of age, which is among the longest periods in the animal world. The male Bottlenose dolphins do not help in raising their young ones.
Adult bottlenose dolphins have few predators. They are impossible to hunt for any lone predator; even solitary great white sharks and killer whales usually stay away from adult dolphins. Though juveniles are targeted and are often killed, the communal structure of dolphin pods increases their chances of survival against much larger predators, such as killer whales (orca). They can out-swim most predators and even use complicated evasive techniques thanks to their higher intelligence, but dolphin pods can also become extremely aggressive in self-defense. Whole pods often ‘mob’ predators, a fierce tactic that can — rarely — result in the death of the predator. Groups of sharks and orcas can prey on solitary or smaller groups of bottlenoses, but orcas are also known to swim with bottlenose dolphin pods.
These interesting facts about Bottlenose dolphins give you but a glimpse into the lives of these beautiful, intelligent creatures. Here’s hoping that bottlenoses will continue to thrive and populate the vast oceans of the blue planet!