Temporal range: 15.970–0 Ma Miocene to Recent
|The harbor porpoise (Phocoena phocoena)|
Porpoises are a group of fully aquatic marine mammals, that are sometimes referred to as mereswine, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). There are 8 extant species of porpoise. They are small toothed whales that are very closely related to oceanic dolphins. The most obvious visible difference between the two groups is that porpoises have shorter beaks and flattened, spade-shaped teeth distinct from the conical teeth of dolphins. Porpoises, and other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates, and their closest living relatives are the hippopotamuses, having diverged about 40 million years ago.
Porpoises range in size from the 1.4 metres (4.6 ft) and 54 kilograms (119 lb) Vaquita, the smallest cetacean to be discovered, to the 2.3 metres (7.5 ft) and 220 kilograms (490 lb) Dall's porpoise. Several species exhibit sexual dimorphism, in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Dall's porpoise is one of the fastest cetaceans discovered, with the ability to travel 40 knots. Porpoises have the ability to produce biosonar and it is their primary sensory system. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water.
Porpoises are not very widespread, with all specializing near the polar regions, usually near the coast. Porpoises feed largely on fish and squid, much like the rest of the odontocetes. Males typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Some porpoises produce a variety of clicks and whistles, which are thought to be primarily for social purposes. A few species, like the harbor porpoise, are highly sociable, but pods, generally, don't exceed ten individuals for most species.
Porpoises were, and still are, hunted by some countries by means of drive hunting. Some species are attributed with high levels of intelligence. At the 2012 meeting of the American Association for the Advancement of Science, support was reiterated for a cetacean bill of rights, listing cetaceans as non-human persons. The Vacquita nearly became extinct in the twentieth century, with a predicted population less than 100 individuals, and, with the extinction of the Baiji, is considered the most endangered cetacean. Besides drive hunting, they also face threats from bycatch, competition (from humans), and marine pollution. Porpoises are sometimes kept in captivity and trained to perform tricks, but breeding success has been poor.
Taxonomy and evolution
Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago (Mya). During the Miocene (23 to 5 Mya), mammals were fairly modern. The cetaceans diversified, and fossil evidence suggests porpoises and dolphins diverged from their last common ancestor around 15 Mya. The oldest fossils are known from the shallow seas around the North Pacific, with animals spreading to the European coasts and Southern Hemisphere only much later, during the Pliocene.
- ORDER CETACEA
- Suborder Odontoceti toothed whales
- Infraorder Delphinida
- Superfamily Delphinoidea
- Family Phocoenidae – porpoises
- Genus †Haborophocoena
- H. toyoshimai
- Genus Neophocaena
- N. phocaeniodes – Finless porpoise
- Genus †Numataphocoena
- N. yamashitai
- Genus Phocoena
- Genus Phocoenoides
- P. dalli – Dall's porpoise
- Genus †Septemriocetus
- S. bosselaersii
- Genus †Piscolithax
- P. aenigmaticus
- P. longirostris
- P. boreios
- P. tedfordi
- Genus †Haborophocoena
- Family Phocoenidae – porpoises
- Superfamily Delphinoidea
- Infraorder Delphinida
- Suborder Odontoceti toothed whales
Porpoises have torpedo shaped bodies with non-flexible necks, limbs modified into flippers, non-existent external ear flaps, a tail fin, and bulbous heads. Their skulls have small eye orbits, small, blunt snouts, and eyes placed on the sides of the head. Porpoises range in size from the 1.4 metres (4.6 ft) and 54 kilograms (119 lb) Vaquita to the 2.3 metres (7.5 ft) and 220 kilograms (490 lb) Dall's porpoise. Overall, they tend to be dwarfed by other cetartiodactyls. All species have female-biased sexual dimorphism, with the females being larger than the males.
Odontocetes possess teeth with cementum cells overlying dentine cells. Unlike human teeth, which are composed mostly of enamel on the portion of the tooth outside of the gum, whale teeth have cementum outside the gum. Porpoises, like other odontocetes, possess only one blowhole.
All porpoises have a thick layer of blubber. This blubber can help with protection to some extent as predators would have a hard time getting through a thick layer of fat and energy for leaner times; the primary usage for blubber is insulation from the harsh underwater climate. Calves are born with only a thin layer of blubber, but some species compensate for this with thick lanugos.
Porpoises have two flippers on the front, and a tail fin. These flippers contain four digits. Although porpoises do not possess fully developed hind limbs, they possess discrete rudimentary appendages, which may contain feet and digits. Porpoises are fast swimmers in comparison to seals, which typically cruise at 5–15 kn, or 9–28 kilometres per hour (5.6–17.4 mph); Dall's porpoise, in comparison, can travel at speeds up to 74 kilometres per hour (46 mph). The fusing of the neck vertebrae, while increasing stability when swimming at high speeds, decreases flexibility, which means they can't turn their heads. When swimming, porpoises rely on their tail fin propel them through the water. Flipper movement is continuous. Porpoises swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Some species log out of the water, which may allow then to travel faster, and sometimes they porpoise out of the water. Their skeletal anatomy allows them to be fast swimmers. They have a very well defined and triangular dorsal fin.
Porpoises are adapted for diving to great depths. In addition to their streamlined bodies, they can slow their heart rate to conserve oxygen; blood is rerouted from tissue tolerant of water pressure to the heart and brain among other organs; hemoglobin and myoglobin store oxygen in body tissue; and they have twice the concentration of myoglobin than hemoglobin.
The porpoise ear has specific adaptations to the marine environment. In humans, the middle ear works as an impedance equalizer between the outside air's low impedance and the cochlear fluid's high impedance. In whales, and other marine mammals, there is no great difference between the outer and inner environments. Instead of sound passing through the outer ear to the middle ear, porpoises receive sound through the throat, from which it passes through a low-impedance fat-filled cavity to the inner ear. The porpoise ear is acoustically isolated from the skull by air-filled sinus pockets, which allow for greater directional hearing underwater. Odontocetes send out high frequency clicks from an organ known as a melon. This melon consists of fat, and the skull of any such creature containing a melon will have a large depression. The large bulge on top of the porpoises head is caused by the melon
The porpoise eye is relatively small for its size, yet they do retain a good degree of eyesight. As well as this, the eyes of a porpoise are placed on the sides of its head, so their vision consists of two fields, rather than a binocular view like humans have. When porpoises surface, their lens and cornea correct the nearsightedness that results from the refraction of light; they contain both rod and cone cells, meaning they can see in both dim and bright light, but they have far more rod cells than they do cone cells. Porpoises do, however, lack short wavelength sensitive visual pigments in their cone cells indicating a more limited capacity for color vision than most mammals. Most porpoises have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas and a tapetum lucidum; these adaptations allow for large amounts of light to pass through the eye and, therefore, a very clear image of the surrounding area.
Porpoises are not thought to have a good sense of taste, as their taste buds are atrophied or missing altogether. However, some have preferences between different kinds of fish, indicating some sort of attachment to taste.
Captive cetaceans have occasionally been known to mimic human speech. Scientists have suggested this indicates a strong desire on behalf of the cetaceans to communicate with humans, as they have a very different vocal mechanism, so producing human speech likely takes considerable effort.
Porpoises emit two distinct kinds of acoustic signals, which are called whistles and clicks:
- Clicks are quick broadband burst pulses, used for sonar, although some lower-frequency broadband vocalizations may serve a non-echolocative purpose such as communication; for example, the pulsed calls of belugas. Pulses in a click train are emitted at intervals of ~35–50 milliseconds, and in general these inter-click intervals are slightly greater than the round-trip time of sound to the target.
- Whistles are narrow-band frequency modulated (FM) signals, used for communicative purposes, such as contact calls.
Porpoises are known to teach, learn, cooperate, scheme, and grieve. The neocortex of many species is home to elongated spindle neurons that, prior to 2007, were known only in hominids. In humans, these cells are involved in social conduct, emotions, judgment, and theory of mind. Porpoise spindle neurons are found in areas of the brain that are homologous to where they are found in humans, suggesting that they perform a similar function.
Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the ⅔ or ¾ exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient that can be used as another indication of animal intelligence. The brain to body mass ratio in some odontocetes is second only to humans.
Porpoises are known to engage in complex play behavior, which includes such things as producing stable underwater toroidal air-core vortex rings or "bubble rings". There are two main methods of bubble ring production: rapid puffing of a burst of air into the water and allowing it to rise to the surface, forming a ring, or swimming repeatedly in a circle and then stopping to inject air into the helical vortex currents thus formed. They also appear to enjoy biting the vortex-rings, so that they burst into many separate bubbles and then rise quickly to the surface. They are also known to produce bubble-nets for the purpose of foraging. They sometimes also take part in an activity called porpoising, in which a porpoise travelling at high speeds make long jumps out of the water or skim the surface.
Self-awareness is seen, by some, to be a sign of highly developed, abstract thinking. Self-awareness, though not well-defined scientifically, is believed to be the precursor to more advanced processes like meta-cognitive reasoning (thinking about thinking) that are typical of humans. Research in this field has suggested that cetaceans, among others, possess self-awareness. The most widely used test for self-awareness in animals is the mirror test in which a temporary dye is placed on an animal's body, and the animal is then presented with a mirror; they then see if the animal shows signs of self-recognition. Some disagree with these findings, arguing that the results of these tests are open to human interpretation and susceptible to the Clever Hans effect. This test is much less definitive than when used for primates, because primates can touch the mark or the mirror, while cetaceans cannot, making their alleged self-recognition behavior less certain. Skeptics argue that behaviors that are said to identify self-awareness resemble existing social behaviors, and so researchers could be misinterpreting self-awareness for social responses to another individual. The researchers counter-argue that the behaviors shown are evidence of self-awareness, as they are very different from normal responses to another individual. Whereas apes can merely touch the mark on themselves with their fingers, cetaceans show less definitive behavior of self-awareness; they can only twist and turn themselves to observe the mark. So far, the only cetacean to pass is the dolphin.
Porpoises are fully aquatic creatures, which means that birth and courtship behaviors are very different from terrestrial and semi-aquatic creatures. Since they are unable to go onto land to calve, they deliver the baby with the fetus positioned for tail-first delivery. This prevents the baby from drowning either upon or during delivery. To feed the new-born, porpoises, being aquatic, must squirt the milk into the mouth of the calf. Being mammals, they, of course, have mammary glands used for nursing calves; they are weaned off at about 11 months of age. This milk contains high amounts of fat which is meant to hasten the development of blubber; it contains so much fat that it has the consistency of toothpaste. Females deliver a single calf with gestation lasting about a year, dependency until one to two years, and maturity around seven to ten years, all varying between the species. This mode of reproduction produces few offspring, but increases the survival probability of each one. Females, referred to as "cows", carry the responsibility of childcare as males, referred to as "bulls", play no part in raising calves.
Unlike most animals, porpoises are conscious breathers. All mammals sleep, but porpoises cannot afford to become unconscious for long because they may drown. While knowledge of sleep in wild cetaceans is limited, porpoises in captivity have been recorded to sleep with one side of their brain at a time, so that they may swim, breathe consciously, and avoid both predators and social contact during their period of rest.
Interactions with humans
|This section requires expansion. (September 2015)|
Porpoises, as opposed to their dolphin counterparts, don't thrive in captivity, and it is very difficult to maintain a porpoise in captivity. Currently, there are five porpoises in captivity, two in Vancouver, and three in Hiroshima. Among their collection, the reopened Miyajima Aquarium in Hiroshima houses many sea creatures, among them are three finless porpoises. In the Vancouver Aquarium, there are two harbor porpoises on display: Jack, who beached himself in 2011 onto Horseshoe Bay, Daisy, who beached herself into Horseshoe Bay in 2008.
Porpoises and other smaller cetaceans are hunted in an activity known as drive hunting. This is an accomplish by driving a pod together with boats and usually into a bay or onto a beach. Their escape is prevented by closing off the route to the ocean with other boats or nets. Porpoises are hunted this way in several places around the world, including the Solomon Islands, the Faroe Islands, Peru, and Japan, the most well-known practitioner of this method. By numbers, porpoises are mostly hunted for their meat, though some end up in dolphinariums. Despite the controversial nature of the hunt resulting in international criticism, and the possible health risk that the often polluted meat causes, thousands of porpoises are caught in drive hunts each year.
Porpoises are highly effected by bycatch. Many porpoises, mainly the vacquita, is subject to great morality due to gillnetting. The world's most endangered marine cetacean, the vaquita (Phocoena sinus), continues to be caught in small-mesh gillnet fisheries throughout much of its range. The total estimated incidental mortality caused by the fleet of El Golfo de Santa Clara was 39 vaquitas per year (95% CI = 14, 93), over 17% of the most recent estimate of population size. El Golfo de Santa Clara is one of three main ports that support gillnet fisheries throughout the range of the vaquita. Preliminary results indicate that fishing effort for San Felipe, Baja California, is comparable to that of El Golfo de Santa Clara, suggesting that this estimate of incidental mortality of vaquitas represents a minimum.
The increasing pollution for the marine mammals is also a serious problem. Heavy metals, residues of many plant and insect venoms and plastic waste Flotsam are not biodegradable. Sometimes cetaceans consume these hazardous materials, mistaking them for food items. As a result, the animals are more susceptible to diseases and have fewer offspring.
Similar effects can be, at least in a long-term sense, acidification of the oceans due to increased uptake of carbon dioxide, an effect that global warming counteracts because it heats up the atmosphere while decreasing the amount of carbon. CO2 reacts with water to form carbonic acid. The acidic water interferes with the construction of the calcium carbonate skeletons of various algae and micro-organisms. This in turn decreases the amount of plankton that baleen whales depend on as it represents the main source of food for many species.
Above all, the military and the geology employ strong sonar and produce along with blasting operations and vessel traffic which increases noise in the oceans. Marine mammals that are characterized for their use of biosonar for orientation and communication are not only hindered, but regularly causes panic surfacing. This leads to bubble out of bound in blood gases, and the animal then died because the tubes are blocked, so-called decompression accidents (known in humans as a "serious diving accident").
Naval exercises with sonar regularly result in stranded porpoises, who have gas bubbles in the blood vessels. The sound is very extensive and develops its disastrous effect in more than 100 kilometres (62 mi) radius. Depending on the frequencies used, different species are more or less affected than others. It is charged on the requirement that prior corresponding expanded operations of sonar technology first, possibly to be with sonar, ruled also that many marine mammals in the area are located.
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