Porpoises belonging to the family Phocoenidae. Usually porpoise family consist of three genera, Phocoena, Phocoenoides, and Neophocaena. Porpoises definition aquatic marine mammals. Porpoise specifically belongs to the toothed whales’ classification and they look similar to the dolphin. However, the physical characteristics of porpoise are closely related to narwhals and belugas than true dolphins. Porpoise species have seven extant species, and the smallest species among them was toothed whales. Porpoises have flattened, spade-shaped teeth and they lack a pronounced beak, on the other hand, dolphins have conical teeth. Porpoises and other cetaceans are belonging to the clade. Here, Cetartiodactyla with even-toed ungulates characteristics has the closest living relationship between hippopotamuses, which have diverged from them about 40 million years ago.
Vaquita is a species of porpoises range with 1.4 meters in length and 54 kilograms of weight. The Dall’s porpoises are at 2.3-meter length and 220kg in weight. Many porpoise animals exhibit sexual dimorphism, by which the females’ porpoise looks larger than males. Porpoises streamlined bodies and two limbs are slightly modified into flippers. Porpoises use echo sense to find the location as echo sensing is part of their primary sensory system. Porpoises produce ultrasonic clicks, which helps them in both social communication and navigation (echolocation). Some species of porpoises are well adapted for diving into the great depths. Here, all the porpoise animals have a layer of fat, or blubber, under the skin to maintain their warmth in cold water.
Porpoises are abundantly found in a multitude of environments. This includes rivers, coastal and shelf waters, and open oceans. They are spread over in various water temperatures from polar to tropical regions. The finless porpoise can spot in rivers, harbour porpoise, vaquita can spot at coastal and shelf waters. Dall's porpoise and spectacled porpoise can spot in open oceans. The sea of Cortez, vaquita are widely spread over in tropical regions. Greenland, harbour porpoise can found in polar regions.
Porpoises will take fishes and squids as their feed like the rest of the odontocetes. While coming to known about reproductive behaviour, female porpoise will give birth to one calf every year under its favourable conditions. Most of the calves are born in the summer and spring months, but they remain dependent on the female until the following spring.
Like dolphins, porpoises will not occur in a group. But, still many countries are hunting porpoises on large scale. The large challenges facing porpoises are extensive bycatch in gill nets, competition for food from fisheries, and marine pollution. They contain heavy metals and organochlorides, which may cause danger to the health of human beings. The vaquita, a species of porpoise nearly became extinct in the twentieth century, as they are widely bycatch in gill nets, and its predicted population is fewer than 100 individuals. Since the extinction of the baiji, the vaquita species are large, they are considered as the most endangered cetacean.
Still some species of porpoises are kept under captivity and trained for research, education, and public display.
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Evolution of Porpoises
Porpoises, whales, and dolphins all have similar characteristics and are descendants of land-living ungulates (hoofed animals). They entered the ocean around 50 million years ago, this time period is denoted as mya. During the Miocene (23 to 5 Mya), mammals were modern, Which means they can change physiologically according to time. According to the study of cetaceans diversified, and fossil evidence, it is found that porpoises and dolphins have diverged from their last common ancestor around 15 Mya. The oldest fossils are identified from the shallow seas around the North Pacific region, and those animal fossils are spread over the European coasts and Southern Hemisphere and only much later, during the Pliocene.
Characteristics of Porpoises
Porpoises have a bulbous head, a non-flexible neck, no external ear flaps, limbs modified into flippers, a torpedo-shaped body, and a tail fin. The skull of porpoises contains small eye orbits, small, blunt snouts, and its eyes placed on the sides of the head. The smallest porpoises Vaquita are ranged at 1.4m long and 54 kg in weight. Likewise, the largest porpoise Dall averagely ranges at 2.3m long and 220 kg. They are considered a dwarfed species while comparing to other cetaceans. Most of the species are showing sexual dimorphism, based on which females look larger than the males. But this may vary from species to species. The teeth of Odontocetes are made up of cementum cells with overlying dentine cells, and they have cementum outside the gum.
The stomach of porpoises has three chambers, which include a fore-stomach and fundic and pyloric chambers. Porpoises have only one blowhole harbour like other odontocetes, which helps for breathing, forming an upward, steamy spout and entering fresh air into the lungs. Porpoise animals have thick blubber, which can help with insulation from the harsh underwater climate, protecting them from some extent as predators. The blubber gets thicker when they are getting fats. Young calves are born with only a thin layer of blubber, but they gain the fat and became a thick layer from the milk.
Porpoises have two flippers one at its front, and one near to its tail fin. Porpoises do not possess fully developed hind limbs. But, they possess discrete rudimentary appendages, which may contain feet and digits. Porpoises can swim faster than seals, typically they can cover 9–28 km/h. Its fusing neck increases stability and improves speed during swimming. But its neck is less flexible, so they are impossible to turn its head. While swimming, the porpoises will move their tail fin and their lower body up and down and aiding for their vertical movement. Likewise, flippers are mainly used for steering. Flipper helps to move faster even out of the water, while they are jumping. Its well-defined triangular dorsal fin, allowing them to steer in the water. Unlike their dolphin counterparts, they are adapted for coastal shores, bays, and estuaries.
The porpoise’s ear has special characteristics in the marine environment. The human ear has a middle ear, which works as an impedance equalizer between the outside air's low impedance and the cochlear fluid's high impedance present in the outer and inner ear. But porpoise and other mammals have no great difference between the outer and inner environments. Porpoise will use the throat to receive sound instead of the ear. From which they pass through a low-impedance fat-filled cavity to the inner ear. The porpoise ear is acoustically separated from the skull by its air-filled sinus pockets, these allow the porpoise for greater directional hearing underwater. Odontocetes release high-frequency clicks from their body is known as a melon, which is made up of fat. The large bulge located on the porpoise’s head is due to the presence of melon.
The eyes of the porpoise present on the sides of its head so, they can have a view of two fields. Even, the eyes of porpoise are relatively small in size, by they have a good degree of eyesight. Its eyes have a lens and cornea, which can correct themself according to the refraction of light. They also contain rods and cone cells to view in both dim and bright light. But they are lack short-wavelength sensitivity. So, cone cells can recognize limited colours. Many porpoises have enlarged pupils, slightly flattened eyeballs, slightly flattened corneas, and a tapetum lucidum; So, they can adapt and allow large amounts of light to pass through their eyes. So that, they make a very clear image of its surrounding area. Since olfactory lobes are absent in porpoises, they cannot sense the smell. Porpoises taste buds do not contain atrophied, so they can sense the taste.
Porpoises have conscious breathers but they cannot afford to become unconscious for long as like other mammals because they may drown. The knowledge of sleep is limited in wild cetaceans. One side of its brain always records its atmospheric activities during its sleep time. So they can breathe consciously, swim, and avoid both predators and other social contacts during their period of rest
The hemispheres of porpoise take turning the waves between slow-wave sleep and being awake. If one hemisphere shows the slow waves and the other will display wake patterns in its electroencephalogram. Its brainstem controls its activity. Porpoises will employ a minimal amount of suppressed REM sleep while swimming.
Behaviour of Porpoises
Porpoise meaning fully aquatic mammals. The female porpoise will deliver its young one after a gestation period, which will last for a year. The entire calving process takes place under the water, with the fetus positioned for tail-first delivery, this helps to prevent drowning. The female porpoise has mammary glands, but its shape and seal around the nipple do not support the shape of a newborn calf’s mouth. So, the mother squirts milk into the calf's mouth instead sucking the milk. The milk of porpoise contains high-fat content, which helps for the development of blubber. The fat consistency in milk looks like toothpaste. The claves are weaned up to 11 months of age. Male porpoises do not play any part in rearing calves. The calf will live as dependent for one to two years. Further, it will mature after seven to ten years, which will vary from species to species. Usually, porpoise reproduces countable offspring, but its probability of survival is high.
Porpoises generally take a wide variety of creatures as their feed. Harbour porpoises stomach supports the benthic fish and pelagic fish. They also intake benthic invertebrates. In some rare cases, they take Ulva Lactuca, a kind of algae. Atlantic porpoises will intake baitfish, like herring, and varies their diet pattern depends on season. The stomach of Dall's porpoises mainly supports baitfish, like capelin and sardines as its feed.
The finless porpoise is also known to follow seasonal migrations. So, porpoises are widely found in the mouth of the Indus river, which migrates to the seas from April to October for feed and annual spawning.