Dugong
Dugong Temporal range: Miocene–recent[1]
| |
---|---|
A dugong photographed underwater | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Order: | Sirenia |
Family: | Dugongidae |
Subfamily: | Dugonginae
|
Genus: | Dugong Lacépède, 1799[4] |
Species: | D. dugon
|
Binomial name | |
Dugong dugon (
Müller , 1776) | |
Dugong range |
The dugong (/ˈd(j)uːɡɒŋ/; Dugong dugon) is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow (Hydrodamalis gigas), was hunted to extinction in the 18th century.
The dugong is the only sirenian in its range, which spans the waters of some 40 countries and territories throughout the
Like all modern sirenians, the dugong has a
The dugong has been hunted for thousands of years for its
Evolution
Dugongs are part of the
Etymology and taxonomy
The word "dugong" derives from the
Other common local names include "sea cow", "sea pig" and "sea camel".
Dugong dugon is the only extant species of the family Dugongidae, and one of only four extant species of the Sirenia order, the others forming the manatee family.[13] It was first classified by Müller in 1776 as Trichechus dugon,[14] a member of the manatee genus previously defined by Linnaeus.[15] It was later assigned as the type species of Dugong by Lacépède[16] and further classified within its own family by Gray[17] and subfamily by Simpson.[14]
Dugongs and other sirenians are not closely related to other marine mammals, being more related to elephants.[18] Dugongs and elephants share a monophyletic group with hyraxes and the aardvark, one of the earliest offshoots of eutherians. The fossil record shows sirenians appearing in the Eocene, where they most likely lived in the Tethys Ocean. The two extant families of sirenians are thought to have diverged in the mid-Eocene, after which the dugongs and their closest relative, the Steller's sea cow, split off from a common ancestor in the Miocene. The Steller's sea cow became extinct in the 18th century. No fossils exist of other members of the Dugongidae.[19]
Molecular studies have been made on dugong populations using mitochondrial DNA. The results have suggested that the population of Southeast Asia is distinct from the others. Australia has two distinct maternal lineages, one of which also contains the dugongs from Africa and Arabia. Limited genetic mixing has taken place between those in Southeast Asia and those in Australia, mostly around Timor.[13] One of the lineages stretches from Moreton Bay to Western Australia, while the other only stretches from Moreton Bay to the Northern Territory.[18] There is not yet sufficient genetic data to make clear boundaries between distinct groups.[13]
Anatomy and morphology
The dugong's body is large with a
The dugong's
A dugong's brain weighs a maximum of 300 g (11 oz), about 0.1% of the animal's body weight.
The
An adult's length rarely exceeds 3 metres (10 ft). An individual this long is expected to weigh around 420 kilograms (930 lb). Weight in adults is typically more than 250 kilograms (550 lb) and less than 900 kilograms (1,980 lb).[29] The largest individual recorded was 4.06 metres (13 ft 4 in) long and weighed 1,016 kilograms (2,240 lb),[19] and was found off the Saurashtra coast of west India.[30] Females tend to be larger than males.[19]
Distribution and habitat
Dugongs are found in warm coastal waters from the western Pacific Ocean to the eastern coast of Africa,[22] along an estimated 140,000 kilometres (87,000 mi) of coastline[31] between 26° and 27° to the north and south of the equator.[13] Their historic range is believed to correspond to that of seagrasses from the Potamogetonaceae and Hydrocharitaceae families. The full size of the former range is unknown, although it is believed that the current populations represent the historical limits of the range,[13] which is highly fractured.[20] Their distributions during warmer periods of Holocene might have been broader than today.[32] Today populations of dugongs are found in the waters of 37 countries and territories.[18] Recorded numbers of dugongs are generally believed to be lower than actual numbers, due to a lack of accurate surveys. Despite this, the dugong population is thought to be shrinking,[13] with a worldwide decline of 20 percent in the last 90 years. They have disappeared from the waters of Hong Kong, Mauritius, and Taiwan, as well as parts of Cambodia, Japan, the Philippines, and Vietnam. Further disappearances are likely.[18]
Dugongs are generally found in warm waters around the coast[22] with large numbers concentrated in wide and shallow protected bays.[13] The dugong is the only strictly marine herbivorous mammal, as all species of manatee utilise fresh water to some degree.[13] Nonetheless, they can tolerate the brackish waters found in coastal wetlands,[33] and large numbers are also found in wide and shallow mangrove channels and around leeward sides of large inshore islands, where seagrass beds are common.[13] They are usually located at a depth of around 10 m (33 ft),[20] although in areas where the continental shelf remains shallow dugongs have been known to travel more than ten kilometres (6 mi) from the shore, descending to as far as 37 metres (121 ft), where deepwater seagrasses such as Halophila spinulosa are found.[13] Special habitats are used for different activities. It has been observed that shallow waters are used as sites for calving, minimizing the risk of predation. Deep waters may provide a thermal refuge from cooler waters closer to the shore during winter.[13]
Australia
Persian Gulf
The Persian Gulf has the second-largest dugong population in the world, inhabiting most of the southern coast,[13] and the current population is believed to range from 5,800 to 7,300.[37] In the course of a study carried out in 1986 and 1999 on the Persian Gulf, the largest reported group sighting was made of more than 600 individuals to the west of Qatar.[38] A 2017 study, for instance, found a nearly 25% drop in population since 1950.[37] Reasons for this drastic population loss include illegal poaching, oil spills, and net entanglement.[38]
East Africa and South Asia
In the late 1960s, herds of up to 500 dugongs were observed off the coast of
There are less than 250 individuals scattered throughout Indian waters.[52] A highly isolated breeding population exists in the Marine National Park, Gulf of Kutch,[53] the only remaining population in western India. It is 1,500 kilometres (800 nautical miles) from the population in the Persian Gulf, and 1,700 kilometres (900 nmi) from the nearest population in India. Former populations in this area, centered on the Maldives and the Lakshadweep, are presumed to be extinct. A population exists in the Gulf of Mannar Marine National Park and the Palk Strait between India and Sri Lanka, but it is seriously depleted. Recoveries of seagrass beds along former ranges of dugongs, such as the Chilika Lake have been confirmed in recent years, raising hopes for re-colorizations of the species.[54] The population around the Andaman and Nicobar Islands is known only from a few records, and although the population was large during British rule, it is now believed to be small and scattered.[13]
Southern Pacific outside of Australia
A small population existed along the southern coast of China, particularly the Gulf of Tonkin (Beibu Gulf), where efforts were made to protect it, including the establishment of a seagrass sanctuary for dugong and other endangered marine fauna ranging in Guangxi.[55][56] Despite these efforts, numbers continued to decrease, and in 2007 it was reported that no more dugong could be found on the west coast of the island of Hainan.[57] Historically, dugongs were also present in the southern parts of the Yellow Sea.[58] The last confirmed record of dugongs in Chinese waters was documented in 2008. In August 2022, an article published on the Royal Society Open Science concluded that dugongs were functionally extinct in China, which was based on a large-scale interview survey conducted across four southern Chinese maritime provinces (Hainan, Guangxi, Guangdong, and Fujian) in the summer of 2019.[59][60]
In Vietnam, dugongs have been restricted mostly to the provinces of
In Thailand, the present distribution of dugongs is restricted to six provinces along the Andaman Sea,[66] and very few dugongs are present in the Gulf of Thailand.[67] The Gulf of Thailand was historically home to a large number of animals, but none have been sighted in the west of the gulf in recent years,[13] and the remaining population in the east is thought to be very small and possibly declining.[68] Dugongs are believed to exist in the Straits of Johor in very small numbers. The waters around Borneo support a small population, with more scattered throughout the Malay Archipelago.[13]
All the islands of the Philippines once provided habitats for sizeable herds of dugongs. They were common until the 1970s when their numbers declined sharply due to accidental drownings in fishing gear and habitat destruction of seagrass meadows. Today, only isolated populations survive, most notably in the waters of the Calamian Islands in Palawan, Isabela in Luzon, Guimaras, and Mindanao. The dugong became the first marine animal protected by Philippine law, with harsh penalties for harming them.[69][70][71] Recently, the local marine trash problem in the archipelago remained unabated and became the biggest threat to the already dwindling population of Dugongs in the country. Litters of plastic waste (single-use sachets, plastic bottles, fast food to-go containers, etc.) and other non-biodegradable materials abound in the coastal areas. As these materials may be mistaken as food by dugongs, these may lead to death due to plastic ingestion. Overpopulation and lack of education of all coastal fisherfolk in the Philippines regarding marine trash are harming the coastal environment not only in Palawan but also across the islands of the Philippines.[72]
Populations also exist around the Solomon Islands and New Caledonia, stretching to an easternmost population in Vanuatu. A highly isolated population lives around the islands of Palau.[13]
A single dugong lives at Cocos (Keeling) Islands although the animal is thought to be a vagrant.[73][74]
Northern Pacific
Today, possibly the smallest and northernmost population of dugongs exists around the
Populations around Taiwan appear to be almost extinct, although remnant individuals may visit areas with rich seagrass beds such as Dongsha Atoll.[82] Some of the last reported sightings were made in Kenting National Park in the 1950s and 60s.[83] There had been occasional records of vagrants at the Northern Mariana Islands before 1985.[84] It is unknown how much mixing there was between these populations historically. Some theorize that populations existed independently, for example, that the Okinawan population was isolated members derived from the migration of a Philippine subspecies.[85] Others postulate that the populations formed part of a super-population where migration between Ryukyu, Taiwan, and the Philippines was common.[86]
Extinct Mediterranean population
It has been confirmed that dugongs once inhabited the water of the
Ecology and life history
Dugongs are long-lived, and the oldest recorded specimen reached age 73.
Although they are
Dugongs are
Feeding
Dugongs, along with other
Most dugongs do not feed on lush areas, but where the seagrass is more sparse. Additional factors such as protein concentration and regenerative ability also affect the value of a seagrass bed.[18] The chemical structure and composition of the seagrass are important, and the grass species most often eaten are low in fiber, high in nitrogen, and easily digestible.[13] In the Great Barrier Reef, dugongs feed on low-fiber high-nitrogen seagrass such as Halophila and Halodule,[18] to maximize nutrient intake instead of bulk eating. Seagrasses of a lower seral are preferred, where the area has not fully vegetated. Only certain seagrass meadows are suitable for dugong consumption, due to the dugong's highly specialized diet. There is evidence that dugongs actively alter seagrass species compositions at local levels. Dugongs may search out deeper seagrass. Feeding trails have been observed as deep as 33 metres (108 ft), and dugongs have been seen feeding as deep as 37 metres (121 ft).[13] Dugongs are relatively slow-moving, swimming at around 10 km/h (3 m/s).[90] When moving along the seabed to feed they walk on their pectoral fins.[23]
Dugong feeding may favor the subsequent growth of low-fibre, high-nitrogen seagrasses such as Halophilia and Halodule.[94] Species such as Zosteria capricorni are more dominant in established seagrass beds,[95] but grow slowly, while Halophilia and Halodule grow quickly in the open space left by dugong feeding.[94] This behavior is known as cultivation grazing and favors the rapidly growing, higher nutrient seagrasses that dugongs prefer.[94] Dugongs may also prefer to feed on younger, less fibrous strands of seagrasses,[95] and cycles of cultivation feeding at different seagrass meadows may provide them with a greater number of younger plants.
Due to their poor eyesight, dugongs often use smell to locate edible plants. They also have a strong
Reproduction and parental care
A dugong reaches sexual maturity between the ages of eight and eighteen, older than in most other mammals.[96] The way that females know how a male has reached sexual maturity is by the eruption of tusks in the male since tusks erupt in males when testosterone levels reach a high enough level.[97] The age when a female first gives birth is disputed, with some studies placing the age between ten and seventeen years, while others place it as early as six years.[13] There is evidence that male dugongs lose fertility at older ages.[19] Despite the longevity of the dugong, which may live for 50 years or more, females give birth only a few times during their lives and invest considerable parental care in their young.[96] The time between births is unclear, with estimates ranging from 2.4 to 7 years.[13]
Females give birth after a 13- to 15-month gestation, usually to just one calf.[96] Birth occurs in very shallow water, with occasions known where the mothers were almost on the shore.[19] As soon as the young is born the mother pushes it to the surface to take a breath.[22] Newborns are already 1.2 metres (4 ft) long and weigh around 30 kilograms (65 lb).[18] Once born, they stay close to their mothers, possibly to make swimming easier.[19] The calf nurses for 14–18 months, although it begins to eat seagrasses soon after birth.[13] A calf will only leave its mother once it has matured.[20]
Importance to humans
Dugongs have historically provided easy targets for hunters, who killed them for their meat, oil, skin, and bones. As the anthropologist A. Asbjørn Jøn has noted, they are often considered the inspiration for mermaids,[22][98] and people around the world developed cultures around dugong hunting. In some areas, it remains an animal of great significance,[19] and a growing ecotourism industry around dugongs has had an economic benefit in some countries.[20]
There is a 5,000-year-old wall painting of a dugong, apparently drawn by
Dugongs feature in Southeast Asian, especially
Conversely, dugong "tears" are considered aphrodisiacs in other parts of Indonesia, Singapore, Malaysia, Brunei, Thailand, Vietnam, and Cambodia.[103] Dugong meat is considered a luxury food and is also believed to have aphrodisiac properties. They are actively hunted in these regions, in some places to near-extinction.[100]
In Palau, dugongs were traditionally hunted with heavy spears from canoes. Although it is illegal and there is widespread disapproval of killing dugongs, poaching remains a major problem. Dugongs are also widely hunted in Papua New Guinea, the Solomon Islands, Vanuatu, and New Caledonia; where their meat and ornaments made from bones and tusks are highly prized in feasts and traditional rituals. However, hunting dugongs is considered taboo in some areas of Vanuatu.[100] Dugong meat and oil have traditionally been some of the most valuable foods of Australian Aboriginals and Torres Strait Islanders. Some Aboriginals regard dugongs as part of their Aboriginality.[18]
Local fishermen in
Dugongs have also played a role in legends in Kenya, and the animal is known there as the "Queen of the Sea". Body parts are used as food, medicine, and decorations. In the Gulf states, dugongs served not only as a source of food but their tusks were used as sword handles. Dugong oil is important as a preservative and conditioner for wooden boats to people around the Gulf of Kutch in India, who also believe the meat to be an aphrodisiac.[13]
Conservation
Dugong numbers have decreased in recent times. For a population to remain stable, 95 percent of adults must survive one year. The estimated percentage of females humans can kill without depleting the population is 1–2%.
The
Human activity
Despite being legally protected in many countries, the main causes of population decline remain
Vessel strikes have proved a problem for manatees, but the relevance of this to dugongs is unknown.
Environmental degradation
If dugongs do not get enough to eat they may calve later and produce fewer young.[18] Food shortages can be caused by many factors, such as a loss of habitat, death and decline in the quality of seagrass, and a disturbance of feeding caused by human activity. Sewage, detergents, heavy metals, hypersaline water, herbicides, and other waste products all negatively affect seagrass meadows. Human activity such as mining, trawling, dredging, land reclamation, and boat propeller scarring also cause an increase in sedimentation which smothers seagrass and prevents light from reaching it. This is the most significant negative factor affecting seagrass.[13]
Halophila ovalis—one of the dugong's preferred species of seagrass—declines rapidly due to lack of light, dying completely after 30 days. Extreme weather such as cyclones and floods can destroy hundreds of square kilometres of seagrass meadows, as well as wash dugongs ashore. The recovery of seagrass meadows and the spread of seagrass into new areas, or areas where it has been destroyed, can take over a decade. Most measures for protection involve restricting activities such as trawling in areas containing seagrass meadows, with little to no action on pollutants originating from land. In some areas, water salinity is increased due to wastewater, and it is unknown how much salinity seagrass can withstand.[13]
Dugong habitat in the
Capture and captivity
The Australian state of
Worldwide, only three dugongs are held in captivity. A female from the Philippines lives at
Gracie, a captive dugong at
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{{cite web}}
: CS1 maint: bot: original URL status unknown (link) - ^ The Asahi Shimbun Company (18 August 2014). "Protesters kept at bay as Okinawan seabed survey for relocating U.S. air station gets under way". 朝日新聞デジタル. Archived 19 August 2014 at the Wayback Machine
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External links
- Media related to Dugong dugon at Wikimedia Commons