Frogs of Borneo.

Description
Diagnosis: Microhyla borneensis is a diminutive microhylid species (SVL 10.6-12.8 mm in eight male specimens, 17.9-18.8 mm in two female specimens) that can be distinguished from its congeners by the following characters: a brown, tuberculate dorsum with scapular marking in the shape of an hourglass (though tubercles and markings are indistinct in preservative), tubercles somewhat more distinct on flanks than on its back, mid-vertebral ridge beginning at the forehead and continuing in a weak, broken line to posterior, the forehead lacking a dermal fold, no tympanic annulus or membrane, males with greatly reduced Finger I, resembling a tubercle, toe tips slightly expanded, longitudinal grooves on phalanges, resulting in two structures resembling scales, Toe IV basally webbed, narrow dermal fringing on toes and both inner and outer metatarsal tubercles present (Das and Haas 2010).

M. borneensis is morphologically similar to M. malang, but can be distinguished by its smaller snout-vent length, less developed toe webbing, the absence of a mid-dorsal black spot and lack of light lines that edge dorsal dark markings in M. malang. In terms of life history, M. malang breeds in ponds and pools while M. borneensis uses pitcher plants. Tadpoles of M. malang have darker coloration than those of M. borneensis. Calls can also be used to distinguish the two. M. malang emits a lower frequency than M. borneensis (Matsui 2011).

Description: M. borneensis has a stout, subtriangular and flattened body (Das and Haas 2010; Inger 1966). The skin is smooth or tuberculated, but the venter is smooth (Inger 1966). The head is narrow (Das and Haas 2010). The snout, obtusely pointed in both dorsal and lateral view, projects well beyond the lower jaw. The canthus rostralis is obtuse and the loreal region is vertical. Nostrils are lateral and nearer to tip of snout than to eye. The eyes are small with circular pupils. It lacks both vomerine and maxillary teeth. The mouth extends to the posterior corner of jaw. The tongue is oval-shaped, large, and is free for about half its length. The palate lacks dermal ridges. The anterior edge of the mandible has a weak W-shaped notch (symphysial knob). A weak supratympanic fold is present (Das and Haas 2010). There is no visible tympanum (Inger 1966). The tips of the three outer fingers are wider than the basal phalanges. Fingers lack fringes. The subarticular tubercles are not expanded and there is no division in the outer palmar tubercles. The webbing is broad and doesn’t reach the disks of the first and second toes (Inger 1966). Forelimbs are short. The lower arm lacks enlarged glands. Fingers lack webbing and fringe; fingertips are slightly swollen, but not to the point of being dilated. Fingers (and toes) have longitudinal grooves on the dorsal surfaces, forming two "scales". Finger I is greatly reduced (a nub resembling a tubercle in males; a single phalanx is visible in females). Subarticular tubercles are rounded and are prominently visible on Fingers II-IV. Palmar tubercles are fleshy. Hindlimbs are also short. Toes have reduced webbing and the same sort of longitudinal grooves as the fingers. Toe tips are weakly expanded. Subarticular tubercles are rounded and prominent on toes. The inner metatarsal tubercle (dark, smaller), as well as the outer metatarsal tubercle (large, pale, and elongated but not shovel-shaped) are present. Males have a median subgular vocal sac, in addition to having greatly reduced first fingers, but lack nuptial pads (Das and Haas 2010).

Coloration: The ground color is reddish brown. The lips are spotted with black. A dark round spot surrounds the anus. The hindlimbs can have dark crossbars. The throat is mottled with brown. The venter is whitish (Inger 1966).

Tadpole morphology: The tadpole has a terminal mouth with a greatly reduced oral disc. It lacks keratinized beaks. The upper jaw has a shallow labial flap. The lower jaw has a very reduced labial flap that is separated from a "chin-like" bulge by a dermal fold; this lower labium has two knob-like papillae at the corners and three vestigial papillae, producing an undulating contour of the lip. Gill filter apparatus is reduced in size and complexity compared to that of other microhylids. Eyes are lateral. Prior to metamorphosis, nostrils are not perforated (do not open externally). Vent is medial and embedded in the lower tail fin. Tail fins begin at the junction of the trunk and tail, with the ventral fin higher than the dorsal fin. The tail tip is short and acuminate to flagellar (Das and Haas 2010).

Tadpole coloration: Larvae appear moderately dark grey in life due to the presence of scattered melanocytes, but cream-colored in preservative with translucent belly and tail fins. The iris is bronze on black in life, but wholly black in preservative (Das and Haas 2010; INger 1966).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Malaysia

Malaysian region distribution from AmphibiaWeb's database: Sabah, Sarawak

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (1 records).

M. borneensis is endemic to Malaysia (western Borneo). It occurs in Kubah National Park, Matang Range, and Sarawak, Malaysia. It is thought to also be present in Bako National Park, about 25 km NE of Kubah NP. The type specimens were found along the road leading to the summit of Gunung Serapi, at approximately 300 m asl. Habitat at the type locality consists of Bornean heath forest (known locally as kerangas), with high numbers of pitcher plants (Nepenthes ampullaria) in the forest undergrowth (Das and Haas 2010).

Life History, Abundance, Activity, and Special Behaviors
Due to its tiny size, this frog is heard but rarely seen. Collected specimens are thus primarily males (Das and Haas 2010). Males aggregate in and around pitcher plants (Nepenthes ampullaria) (Das and Haas 2010). N. ampullaria is carnivorous, as are all pitcher plants, and it grows only under forest canopy (Clarke and Lee 2004). It is thought that the extremely small size and reduced foot webbing of the frog may be necessary for it to successfully move around the inside of the pitcher plant without slipping into the waxy zone below the plant’s peristome, which traps arthropods (Das and Haas 2004; Clarke and Lee 2004).

Choruses of calling males can be heard throughout the year, with the exception of very dry nights. Calling begins at dusk, with the most active calling heard in the early evening. The chorus is sporadic, with group calling lasting a few minutes and punctuated by intervals of silence or where only a single male is calling. Calls consist of either a series of pulsed notes, or pulsed notes with additional notes (Matsui 2011).The pulse series consists of harsh rasping notes, with each note containing two short pulse groups and one longer pulse group, of 1-2 pulses or 5-9 pulses respectively. Dominant frequencies are in the range of 3.0-5.5 kHz (Das and Haas 2010).

Eggs are deposited within pitcher plants (Nepenthes ampullaria), sometimes attached to the sides of the pitchers (Das and Haas 2010) or often lying freely at the bottom of the pitcher (A. Haas, pers. comm.). Multiple clutches can be deposited within a single plant; tadpoles at different developmental stages were found together in a single pitcher. Pitchers contained up to fourteen larvae, with adult males calling from inside the pitchers next to the larvae. DNA barcoding was used to confirm that adults and tadpoles were of the same species. Tadpoles are tiny and pale and do not move about unless disturbed. When undisturbed, the tadpoles hang relatively motionless with their heads up at an angle of about 30 degrees and the tail tip constantly vibrating. When disturbed, they rapidly burrow into the detritus of undigested arthropod parts at the bottom of the pitcher plant liquid. Metamorphosis takes about two weeks from oviposition. Metamorphs measure 3.5 mm SVL on average, and are nearly translucent (Das and Haas 2010).

Although a number of amphibians breed in phytotelmata (small water bodies contained in plants), very few so far are known to breed or even to take shelter in pitcher plants. Other frogs found to breed or live in Nepenthes plants include Kalophrynus pleurostigma and K. cf. heterochirus, which have free-living tadpoles. Frogs with direct development (genus Philautus) have also been found in various species of Nepenthes plants, including Philautus aurifasciatus, P. kerangae, P. mjobergi, and P. saueri (Das and Haas 2010). One North American frog, Pseudacris crucifer, may also associate with pitcher plants (Russell 2008).

Trends and Threats
This species occurs within a protected area, Kubah National Park, in Sarawak, Malaysia (Das and Haas 2010). It is probably also present in Bako National Park, based on a specimen that appears from sequence analysis to have been mistakenly identified as M. borneensis (Das and Haas 2010). Threats to M. borneensis are not known but the species is most likely susceptible to logging and deforestation (Iskandar 2004).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities

Comments
M. borneensis was known as “Old World’s smallest frog”. However, other species are smaller when females are considered (Matsui 2011).

M. borneensis is most closely related to M. malang (Matsui 2011).

References

Clarke, C., and Lee, C. (2004). A Pocket Guide. Pitcher Plants of Borneo. Natural History Publications (Borneo) Sdn. Bhd. and Science and Technology Unit, Kota Kinabalu.

Das, I., and Haas, A. (2010). ''New species of Microhyla from Sarawak: Old World’s smallest frogs crawl out of miniature pitcher plants on Borneo (Amphibia: Anura: Microhylidae).'' Zootaxa, 271, 37-52.

Djoko Iskandar, Robert Inger 2004. Microhyla borneensis. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2010.4. www.iucnredlist.org. Downloaded on 26 April 2012.

Inger, R. F. (1966). ''The systematics and zoogeography of the Amphibia of Borneo.'' Fieldiana Zoology, 52, 1-402.

Matsui, M. (2011). ''Taxonomic revision of one of the Old World’s smallest frogs, with description of a new Bornean Microhyla (Amphibia, Microhylidae).'' Zootaxa, 2814, 33-49.

Russell, R. W. (2008). ''Spring peepers and pitcher plants: a case of commensalism?'' Herpetological Review, 39, 154-155.

Species Account Citation: AmphibiaWeb 2014 Microhyla borneensis: Borneo Rice Frog <https://amphibiaweb.org/species/2170> University of California, Berkeley, CA, USA. Accessed Apr 16, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 16 Apr 2024.

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