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Bornean Chorus Frog

Species : Microhyla borneensis (Microhyla nepenthicola)
Size (snout to vent) : Female 1.9 cm, Male 1.3 cm

The Bornean Chorus Frog, or Bornean Narrow-mouthed Frog, is one of a handful of species of the genus Microhyla which inhabits parts of the island of Borneo. Likes its cousin the Pothole Chorus Frog Microhyla petrigena it is endemic to the island i.e. it is not known to occur on other islands or on the mainland of Southeast Asia.

This tiny frog reaches a maximum length of 19 mm, and is thus considered the smallest frog in Southeast Asia. It occurs in moist, humid lowlands and appears largely restricted to undisturbed primary forest, but can also thrive in forest edge settings.

It has long been known to breed in temporary rain pools and wallows made by wild pigs, such as those made by the Bearded Pig Sus barbatus and Eurasian Wild Pig Sus scrofa.

In 2010, however, unusual breeding behaviour of this species was described (Das & Haas, 2010): tadpoles were found thriving inside water-filled pitchers of the Flask-shaped Pitcher Plant Nepenthes ampullaria. These tadpoles were assigned to a newly described species, namely Microhyla nepenthicola, however other researchers treat the latter as being one and the same as the Bornean Chorus Frog (Microhyla borneensis).

Regardless of taxonomic placement, to find tadpoles living inside pitcher plants (and successfully metamorphosing into young frogs) is remarkable, but not unique. For example, in Singapore, tadpoles of another member of the Microhylid family, namely Kalophrynus limbooliati, have been found inside pitchers of the same species of pitcher plant (Lim & Ng, 1991, as Kalophrynus pleurostigma).

In the field, adult frogs are most easily distinguished from other members of the genus Microhyla by the colour and patterning on the back which comprises a dark brown (or purplish ?) patch with broad bands extending onto the flank: this is edged with a pale line.

The tadpoles reach 11 mm in length: a metamorph, with well developed hind legs, is illustrated in Figures 2 and 3. The tadpoles lack specialised mouthparts, but gain their nutrition wholly from their store of egg yolk. (Reference can be made to Das & Haas (2010) for a full description of the tadpoles).

Within Borneo, this small frog is known from Sarawak, Sabah and northeastern Kalimantan (the Indonesian province in Borneo).

Fig 1 : Adult frog found at Kubah National Park, Sarawak.  Photo thanks to Law Ing Sind.

Figs 2 and 3 : Metamorph, with well developed hind legs, found inside a terrestrial pitcher of Nepenthes ampullaria. Photos thanks to Law Ing Sind.

Fig 4 : A cluster of pitchers of Nepenthes ampullaria, of various sizes. Typically these pitchers are found on the forest floor amongst leaf litter and may measure up to  10 cm tall.

References : H19

Das, I., & Haas, A. (2010). New species of Microhyla from Sarawak: Old World痴 smallest frogs crawl out of miniature pitcher plants on Borneo (Amphibia: Anura: Microhylidae). Zootaxa, 2571(1), 37-52.

Lim, K.K.P. & Ng, P.K.L. (1991) Nepenthiphilous larvae and breeding habits of the sticky frog, Kalophrynus pleurostigma Tschudi (Amphibia: Microhylidae). Raffles Bulletin of Zoology, 39, 209214.

Fig 1
ゥ  Law Ing Sind
Fig 2
ゥ  Law Ing Sind
Fig 3
ゥ  Law Ing Sind

Fig 4