Callagur borneoensisPainted Terrapin, Biuku

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Geographic Range

Painted terrapins, Callagur borneoensis, are found in Southeast Asia on the Sundaland Archipelago. Their range extends from southern Thailand, through Malaysia and Borneo, and also includes the island of Sumatra of Indonesia. Painted terrapins migrate up to 3 km. Due to live trade, painted terrapins may now be found globally. ("Batagur Borneoensis", 2000; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Dunson and Moll, 1980; Ernst, et al., 1997; Holt, 2010; Jensen and Das, 2006; Moll, et al., 1981; Praschag, et al., 2007; "Painted terrapin (Callagur borneoensis)", 2010)

Habitat

Painted terrapins inhabit estuaries and the tidal portions of medium to large rivers as well as mangrove swamps and creeks. They may also inhabit other areas that are influenced by tides. Painted terrapins migrate during the mating season. Females nest on sea beaches similar to those used by sea turtles. ("Callagur borneoensis", 2006b; "Batagur Borneoensis", 2000; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Dunson and Moll, 1980; Ernst, et al., 1997; Holt, 2010; Praschag, et al., 2007; "Painted terrapin (Callagur borneoensis)", 2010)

Physical Description

Painted terrapins are medium- to large-sized chelonians (superorder Chelonia). Females may reach lengths of over 60 cm, whereas males are much smaller, reaching only 30 to 40 cm. The carapace is usually under 60 cm in length, though they may measure as long as 76 cm.

As painted terrapins are part of the sub-order Cryptodira, they exhibit many characteristics unique to this grouping. They do not contain free nasal bones, and the descending process of the prefrontals are also connected with the vomer. The stapes is found in an open groove and is entirely covered by the quadrate behind. The pterygoid processes do not contain wing-like lateral expansions and the dentary bones are also united. The sacral ribs are well developed, and the pelvis is also free from both the carapace and plastron.

The plastron is well developed but smaller than the shell opening, and it contains a slight anal notch. The bridges are wide and thick with the plastron well sutured to the carapace. The color of the plastron is a uniform light yellow or cream/grey. On the bridge, the inguinal scutes are larger than the axillary scutes, and the anterior/posterior lobes are shorter than the bridges. There is no plastral hinge present, but the plastral buttresses are quite large and extend almost to the neurals. The head is small to moderate in size with a pointed snout containing prominent, round nostrils at the tip. They also lack a salt excreting gland. The edges of the jaw are finely serrated, and there is a shallow medial notch on the upper jaw. The back of the head is covered in small scales but the limbs have large scales present on their anterior side. Each forelimb contains five claws, and all toes are webbed. In both sexes, the limbs are usually olive or grey in color.

The shell is oval, relatively flattened and very smooth in adults. Vertebrals 1, 2, 3 and 5 are wider than they are long. The 4th vertebral is as wide as it is long and is smaller than the 3rd vertebral. A projection that stems from the 4th vertebral's anterior border fits into the concave portion on the posterior side of the 3rd. Vertebral shapes vary among species of turtle, and the projection on the 4th vertebral is unique to this species.

Painted terrapins are colorful turtles that exhibit both sexual and seasonal dichromatism. Shell color varies with sex and season. In males, the shell is either green or brown/grey. Three bold, black longitudinal stripes run down the shell along the vertebrals and costal areas. These stripes may be continuous or broken up slightly. Males also have black markings on the marginals of the shell. The head of male painted terrapins is black or grey with a broad, mid-saggital dull orange stripe running between the eyes to the snout. Males also have longer, thicker tails than the females. In females, the shell is typically a drab brown color, and is slightly more domed than the shells of males. Females also have black longitudinal stripes on their shells, which may become obscured with age. The head of the female is brown and becomes more grey in color on the neck and limbs. Females are very similar in color to females of the species Batagur baska. However, the species are easily distinguished, as, unlike Batagur baska, Callagur borneoensis has five claws on their forefoot.

During the mating season, both sexes exhibit a change in color. The head of females changes to a reddish color. In males, the change in coloration is more drastic. The shell of males changes to a light gray or a creamy white color. Their black stripes are thus much more visible during the mating season. The head of males turns stark white, and the stripe on their head becomes bright red, edged in black. The coloration of males give them a "painted" appearance, giving the species its common name. ("Callagur borneoensis", 2006b; Baur, 1890; Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Ernst, et al., 1997; Moll, et al., 1981; Praschag, et al., 2007; "Painted terrapin (Callagur borneoensis)", 2010)

  • Sexual Dimorphism
  • female larger
  • sexes colored or patterned differently
  • male more colorful
  • Range mass
    25 (high) kg
    55.07 (high) lb
  • Range length
    76 (high) cm
    29.92 (high) in
  • Average length
    50 to 60 cm cm
    in

Development

The embryological development of turtles, including painted terrapins, occurs within the egg where embryos receive nutrients from the yolk. Embryos develop in the oviduct and enter diapause until after oviposition. This period between ovulation and oviposition typically lasts about 14 days for freshwater turtles. While embryos develop during this period, albumininization and shell production also occur in order to produce the final egg. The head, somites, pharyngeal clefts, eye-spots and limb buds soon form. As development continues, species-specific formation occurs, such as the type of foot, the scale pattern, coloration, etc. Embryos hatch after a few months. As a turtle struggles to emerge from its shell, circulation in the extra-embryonic membranes shut down.

It is unknown what factors contribute to sex determination in this species. One possibility is temperature sex determination (TSD), which may have been exhibited in a zoo in the United States that obtained various juveniles from Malaysia, 90% of which were male. The eggs from which these turtles hatched were incubated in captivity, and it has been speculated low incubation temperatures led to a strongly unequal male-to-female hatching ratio. This suggests more males are produced at cooler temperatures. Other environmental factors can influence TSD. Because nests of Callagur borneoensis are made upon sea beaches, exposure to different degrees of sunlight, and differing nest depth could affect sex determination. Position of eggs in a nest could also contribute to this phenomenon; eggs laid at the bottom of the nest experience cooler temperatures than those laid at the top, which could affect sex ratio of hatchlings. however, no direct studies have confirmed or refuted the presence of TSD in this species.

In juvenile painted terrapins, the carapace is almost circular with a light grey, brown, or grey-brown coloration and is marked with dark spots on the costals. The head is brown to yellow-brown on the dorsal part and becomes grayish on the neck and limbs. Females retain this juvenile coloration throughout their life, whereas males develop an adult coloration. Juveniles also have a well-developed continuous median keel and an interrupted lateral keep on the pleural scutes on either side. A light line runs along this keel and along serrations on the posterior marginals. These keels become lower and eventually disappear with growth.

Turtles grow very slowly, especially relative to mammals and birds. As with most vertebrates, juveniles grow rapidly, and growth rates decrease considerably as they reach maturity. Although it is unknown if painted terrapins experience indeterminate growth, some other species of turtles do. Painted terrapins may display this trait under certain environmental conditions. (Blanco, et al., 1991; Bonin, et al., 2006; Ernst, et al., 1997; Gibbons and Greene, 2009; Moll, et al., 1981; "Painted terrapin (Callagur borneoensis)", 2010; Wyneken, et al., 2007)

  • Development - Life Cycle
  • neotenic/paedomorphic
  • temperature sex determination

Reproduction

Painted terrapins are polygynangrous with both males and females having multiple mates during the mating season. Males compete for the opportunity to reproduce with a female. This results in combative interactions. Male-female courtship is similar to these male-male interactions, though males do not attempt to drive off the female.

Although it is assumed that all species of turtles have a ritualized courtship process, this behavior has been observed in very few species in the wild. In captivity, some courtship processes have been recorded for painted terrapins such as head swaying and throat pumping. In Emydids, courtship behavior includes the male titillating the female cheeks with his front claws. Copulation by most turtles occurs when the tail mounts the carapace of the female. He curls his tail under hers until both cloacas are touching then inserts his penis. In Emydids, the male also clasps onto the female carapace with two of his feet.

It is not known whether painted terrapins copulate on land or in water. Among some species of turtles which mate in water, males have longer tails than females. This characteristic occasionally apparent in painted terrapins. (Blanco, et al., 1991; Gibbons and Greene, 2009; Wyneken, et al., 2007)

The breeding season of painted terrapins is seasonal and is thought to be associated with the wet and dry seasons. Reproduction occurs from June to August on the East coast of the Malay Peninsula and from October to January/February on the west coast. Painted terrapins express breeding colors during this time.

Females migrate as much as 3 km from the mouth of their home river in order to lay their eggs. In extreme southwest Thailand, females migrate up rivers to exposed sand bars to nest. This also occurs in mangrove swaps where no beaches are present. In peninsular Malaysia, nesting occurs on sea beaches that are shared with sea turtles such as Dermochelys coriacea and Chelonia mydas.

Females nest up to 2 or 3 times per season and produce 10 to 25 large eggs at a time. Average clutch size is usually between 10 and 12 eggs. These eggs have pliable shells and are ellipsoidal in shape, measuring 68 to 76 mm by 36 to 44 mm in size. Nesting occurs at night at low tide. Females emerge from the sea, select a location and dig a 30 cm deep hole in the sand. She then lays her eggs, covers them, and returns to the sea.

Incubation generally lasts 70 to 90 days. The process of hatching is similar in all chelonians (superorder Chelonia). Once embryos receive an unknown stimulus to hatch, they struggle to crack open their shell by using a caruncle found on their snout. They also use their legs, which creates other cracks in the egg. Once free of the egg, a newly hatched turtle proceeds to dig itself out of the nest chamber by struggling against the upper surface. This causes sand to trickle down from the top of the nest and settle at the bottom. Once free of the nest, hatchlings proceed to migrate from their beach nest to the mouth of the parent river containing fresh water.This journey may be as much as 3 km for the newborns, the same distance migrated by the mother.

In captivity, painted terrapins reach sexual maturity around 6 to 8 years of age. ("Callagur borneoensis", 2006b; "Batagur Borneoensis", 2000; Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Dunson and Moll, 1980; Ernst, et al., 1997; "Painted terrapin (Callagur borneoensis)", 2010)

  • Breeding interval
    Painted terrapins breed once or twice yearly.
  • Breeding season
    Painted terrapins breed seasonally from June to August and from October to January/February.
  • Range number of offspring
    10 to 25
  • Average number of offspring
    12
  • Range age at sexual or reproductive maturity (female)
    6 to 8 years
  • Range age at sexual or reproductive maturity (male)
    6 to 8 years

Female turtles, including painted terrapins, store large quantities of fat before ovulation so embryos will have sufficient nutrients inside the egg. These stored energy reserves help to provision and protect offspring for several weeks or months. Females also build a nest, which partially protects eggs from predators and environmental conditions. Like all turtles, painted terrapins do not care for their young throughout the incubation period. (Gibbons and Greene, 2009; Wyneken, et al., 2007)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

The lifespan of painted terrapins is currently unknown. The estimated lifespan for turtles in general ranges from 40 to 80 years.

One zoo obtained a number of painted terrapins in 1985. At this time, the turtles were approximately 2 to 4 years of age. However, it is not known if these specimens are still living today. (Blanco, et al., 1991; Gibbons and Greene, 2009)

Behavior

Turtles are generally quite solitary, and observed behaviors of painted terrapins are limited to mating, feeding, and basking.

Painted terrapins have been observed eating Berembang fruit thrown down by leaf monkeys into the water. In captivity, they have also responded in a similiar manner to humans that throw food to them in the water.

Terrapins may also crawl onto logs or vegetation mats to bask in the sun. They are quite shy on land and often move quickly for the water when disturbed. They are also quite irascible, and males attempt to bite when picked up or disturbed. Painted terrapins display aggression toward one another,especially among males during the mating season. Combative behavior over mates has also been observed among males.

Turtles can learn from previous experience, especially when pertaining to survival and/or food. For example, they approach feeding stations in captivity when someone splashes the water. Turtles also consistently navigate up to 3 km to seasonal nesting sites. (Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Gibbons and Greene, 2009; Holt, 2010; "Painted terrapin (Callagur borneoensis)", 2010)

Home Range

Little information is available regarding the home range of painted terrapins.

Communication and Perception

Little is known regarding the communication and perception of painted terrapins. It is presumed that they can feel vibrations in the water, which can be associated with the arrival of food in captivity or objects falling into the water in the wild. Vision is also presumed to be a key factor to the survival of painted terrapins. The bright read streak located on the head of males may make it easier for a female to recognize her mate in the shade of the mangrove zone. (Bonin, et al., 2006; Gibbons and Greene, 2009; Holt, 2010)

Food Habits

Painted terrapins are primarily herbivorous and feed mainly on mangrove fruits, fallen vegetation, shoots and greenery from riverside plants. River grass is a major food source on the Duncan River. They also occasionally feed on village refuse, especially fruit scraps, when discarded into the water. Certain mollusks and shellfish are also consumed, although the consumption of sea creatures may be accidental.

In captivity, painted terrapins feed on kale, spinach, bok choy, dandelion, romaine, and water hyacinth (Eichhornia crassipes). Additionally, they may feed on Mazuri Turtle Diet, mangoes, bananas, cantaloupe, Purina turtle chow, earthworms, and warm water stunned fish. They also eat many leaves that fall into the water. They do not, however, show interest in strawberries or apples, which most herbivorous turtles and tortoises seem to prefer. ("Callagur borneoensis", 2006b; Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Ernst, et al., 1997; Holt, 2010; "Painted terrapin (Callagur borneoensis)", 2010)

  • Animal Foods
  • fish
  • mollusks
  • terrestrial worms
  • aquatic crustaceans
  • Plant Foods
  • leaves
  • wood, bark, or stems
  • fruit
  • flowers

Predation

Natural predators of painted terrapins are unknown. However, they are hunted by humans for their meat and eggs.

Aquatic turtles generally avoid predators by swimming under logs or vegetation or beneath the bank to stay hidden. A turtle's shell also provides protection from predators. Their brown, green, and black coloration outside of the mating season may help them blend into the environment. The silt-filled rivers of Southeast Asia may also help these turtles stay hidden. (Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Gibbons and Greene, 2009; "Painted terrapin (Callagur borneoensis)", 2010)

Ecosystem Roles

The role of painted terrapins in their ecosystem is greatly unknown. Non-marine turtles on the island of Borneo collectively play an important role in the dynamics of freshwater ecosystems, though they are not specifically described. Painted terrapins eat a variety of plant materials in their habitat. (Jensen and Das, 2006)

Economic Importance for Humans: Positive

The painted terrapin is hunted for its eggs and meat, which are both widely consumed. The eggs are highly prized, as they are worth over five times as much as chicken eggs and can be sold for human consumption in many parts of Asia. The physical beauty of this turtle also makes it extremely appealing to hobbyists, and they are prized among the international live animal trade as a pet. They are also bred in captivity and raised in zoos. ("Callagur borneoensis", 2006b; Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Holt, 2010; "Painted terrapin (Callagur borneoensis)", 2010)

Economic Importance for Humans: Negative

There are no known adverse effects of painted terrapins on humans.

Conservation Status

Painted terrapins are considered critically endangered by the IUCN and are listed under Appendix II by CITIES. Because of their rapid disappearance throughout their entire range, there has been some push to move this species to Appendix I. They are now rare throughout much of their former range, with one or two rivers housing more than 100 nesting females. They are almost extinct in Thailand, and painted terrapins are one of the most seriously threatened river turtles in Southeast Asia.

Hunting for eggs and meat has stressed populations of this species. Painted terrapin eggs are worth five times as much as chicken eggs, making this species vulnerable to over-exploitation. The predictable feeding and nesting patterns of painted terrapins allows hunters to collect nearly all of a female's eggs at one time. Low egg productivity also puts this species at risk. Populations are also threatened by hobbyists wishing to collect individuals. Members of this species however, do not survive well in captivity and are difficult to raise.

Habitat destruction has also contributed to reduced numbers of painted terrapins. Their mangrove habitat is heavily impacted by the creation of shrimp ponds. Pollution and the creation of channels also can negatively effect the rivers in which they live. Sand mining, beach protection, and beachfront development can disrupt the breeding habits of painted terrapins. The construction of dams, sea walls and jetties as well as the removal of sand and vegetation also threaten the survival of the species. This can prevent replacement sand from entering nesting beaches, limiting nesting sites. On occasion, painted terrapins are accidently caught in fish nets and drown.

International trade of painted terrapins has been banned by its listing in CITES as well as protective legislation put into place by most of Peninsular Malaysia. Export quotas were limited for this species in 2002 to prevent excess loss of individuals. Egg harvesting can only be carried out by licensed collectors, who must sell 70% of their collection to the Malaysian Fisheries Department to be incubated. However, these prices are not comparable to those on the black market. Only a very small percentage of eggs sold on the black market can be recovered.

In 2006, a hatchery opened in Kedah, Malaysia with the objective to help conserve this species. Painted terrapins are also listed as endangered by the Turtle Conservation Fund. Additionally, a Wildlife Protection Ordinance in the state of Sarawak on northern Borneo lists this species as "Totally Protected." ("Callagur borneoensis", 2006b; "Batagur Borneoensis", 2000; Blanco, et al., 1991; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Holt, 2010; Jensen and Das, 2006; United Nations Environment Programme and World Conservation Monitoring Centre, 2004; "Painted terrapin (Callagur borneoensis)", 2010)

Other Comments

There is some confusion over the classification of painted terrapins. They are often classified as both Callagur borneoensis and Batagur borneoensis. Some argue that, because painted terrapins live naturally in Asia, they should be classified under the family Geoemydidae. This family is composed of a sister group of land tortoises that are found in Asia, North Africa, and Europe. It has also been suggested that, based on DNA sequencing, Callagur borneoensis should be reclassified under a new clade also containing species in the genera Batagur and Kachuga. These species would constitute one entire new genus. Under this new classification, painted terrapins would be called Batagur borneoensis and would therefore also reside in the family Geoemydidae.

Painted terrapins cannot reside in excess of 50% sea water for an extended period of time due to its seeming lack of salt-excreting gland. Because painted terrapins can not excrete salt out of their system, remaining in high percentages of salt water can cause a net loss of water from their body and blood. When experimentally placed in 100% sea water, members of this species also lost 1% of their body weight per day. Because they cannot properly osmoregulate the fluids in their body while in sea water, it is likely that high exposure to salt water for an extended period of time can cause problems in system functioning.

Hatchlings can survive for at least two weeks in 100% sea water directly after birth. This adaptation allows hatchlings to migrate away from the nesting site as they follow the declining salinity gradient to fresh water streams. Although at this time hatchlings have sufficient tolerances to salt water, they must eventually settle in water that is comprised of at least 50% fresh water in order to survive. ("Callagur borneoensis", 2006b; Bonin, et al., 2006; "Callagur borneoensis", 2006a; Dunson and Moll, 1980; Ernst, et al., 1997; Praschag, et al., 2007)

Contributors

Brittnee Kegler (author), University of Alberta, Augustana Campus, Doris Audet (editor), University of Alberta, Augustana Campus, Gail McCormick (editor), Animal Diversity Web Staff.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

frugivore

an animal that mainly eats fruit

herbivore

An animal that eats mainly plants or parts of plants.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

References

Asian Turtle Trade Working Group. 2000. "Batagur Borneoensis" (On-line). In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4.. Accessed October 03, 2010 at http://www.iucnredlist.org/apps/redlist/details/163458/0.

2006. "Callagur borneoensis" (On-line). Asian Turtle Conservation Network. Accessed October 03, 2010 at http://www.asianturtlenetwork.org/field_guide/Callagur_borneoensis.htm.

CITES. 2006. Callagur borneoensis. Twenty-second meeting of the Animals Committee, AC22 Doc. 10.2: 40-46. Accessed October 03, 2010 at http://www.cites.org/eng/com/aC/22/E22-10-2-A4.pdf.

Wildscreen. 2010. "Painted terrapin (Callagur borneoensis)" (On-line). ARKive Images of Life on Earth. Accessed October 03, 2010 at http://www.arkive.org/painted-terrapin/callagur-borneoensis/.

Baur, G. 1890. On the Classification of the Testudinata. The American Naturalist, 24(282): 530-536.

Blanco, S., J. Behler, F. Kostel. 1991. "Propagation of the Batagurine Turtles Batagur baska and Callagur borneoensis at the Bronx Zoo." (On-line). Accessed October 11, 2010 at http://www.tortoise.org/archives/batagur.html.

Bonin, F., B. Devaux, A. Dupré. 2006. Turtles of the World. Baltimore: Johns Hopkins University Press.

Dunson, W., E. Moll. 1980. Osmoregulation in Sea Water of Hatchling Emydid Turtles, Callagur borneoensis, from a Malaysian Sea Beach. Journal of Herpetology, 14(1): 31-36.

Ernst, C., R. Altenburg, R. Barbour. 1997. "Callagur borneoensis" (On-line). Turtles of the World. Accessed October 03, 2010 at http://nlbif.eti.uva.nl/bis/turtles.php?menuentry=soorten&id=248.

Gibbons, W., J. Greene. 2009. Turtles: the animal answer guide. Baltimore: Johns Hopkins University Press.

Holt, E. 2010. "Callagur borneoensis" (On-line). Empire of the Turtle. Accessed October 03, 2010 at http://www.empireoftheturtle.com/callagur_borneoensis.htm.

Jensen, K., I. Das. 2006. "Biological Observations on the Asian Soft-Shell Turtle In Sarawak, Malaysian Borneo, With Notes On the Biology and Conservation of Other Non-Marine Turtles" (On-line). For the Care and Conservation of Tortoises, Terrapins and Turtles Worldwide. Accessed November 12, 2010 at http://www.britishcheloniagroup.org.uk/testudo/v6/v6n3jensen.htm.

Moll, E., K. Matson, E. Krehbiel. 1981. Sexual and Seasonal Dichromatism in the Aisan River Turtle, Callagur borneoensis. Herpetologica, 37(4): 181-194.

Praschag, P., A. Hundsdorfer, U. Fritz. 2007. Phylogeny and taxonomy of endangered South and South-east Asian freshwater turtles elucidated by mtDNA sequence variation (Testudines: Geoemydidae: Batagur, Callagur, Hardella, Kachuga, Pangshura). Zoologica Scripta, 36(5): 429-442.

United Nations Environment Programme, , World Conservation Monitoring Centre. 2004. "Annex C: Reptiles and Amphibians" (On-line). Accessed October 03, 2010 at http://www.unep-wcmc.org/species/sca/pdfs/E20-08-5-AC.pdf.

Wyneken, J., M. Godfrey, V. Bels. 2007. Biology of Turtles. Boca Raton: CRC Press.