Bornean File-Eared Tree Frog

Bornean file-eared tree frog（圖片來源：James Jolokia，採用 CC BY 4.0 授權）

The Bornean file-eared tree frog (Polypedates otilophus), also known as the file-eared tree frog, is an amphibian in the family Rhacophoridae. It is primarily distributed across Borneo, including Brunei, Kalimantan (Indonesia), and the Malaysian states of Sabah and Sarawak, with occasional records from Sumatra. Its elevational range extends from lowland rainforests to mountainous regions up to approximately 1000 meters. This species inhabits secondary forests, forest edges, logging areas, plantations, and even areas near human settlements, indicating a notable tolerance to habitat disturbance.

Adults are medium to relatively large in size. Males typically measure 6 to 8 cm in body length, while females can reach 8 to 10 cm. A defining feature is the serrated bony ridge located behind and above the tympanum, giving the appearance of "ears" and inspiring its common name. Body coloration is variable, ranging from light brown and gray to yellow-brown and bright yellow. The dorsum is marked with multiple narrow black longitudinal stripes, and the limbs display distinct black transverse bands. The iris is yellow-gray with a horizontal pupil, and the tips of the digits are expanded into adhesive discs that facilitate climbing.

In the wild, this species is largely arboreal. During the day, individuals remain concealed among foliage in the canopy, while at night they become active within vegetation approximately 1 to 4 meters above ground. During the breeding season, they gather around pools or temporary puddles. Their reproductive behavior is highly specialized. Males call between April and June to attract females, producing a call composed of a prolonged harsh note followed by several sharp "chuck" sounds. After amplexus, the female uses her hind limbs to whip secretions into a foam nest, which is attached to leaves or branches above water. Upon hatching, the tadpoles drop into the water to continue development. Tadpoles can reach up to 8 cm in length, are yellow-green with golden spots, and have a silvery white ventral surface. After approximately two months in water, they complete metamorphosis. Juveniles remain near water bodies before gradually moving into the forest canopy.

Adults feed primarily on insects and spiders, especially large tree crickets. Tadpoles consume algae and organic detritus in water. This species emits a distinct musty odor, which may deter predators or make it less appealing when handled by humans. Due to its wide distribution and stable population, the International Union for Conservation of Nature (IUCN) classifies it as Least Concern, although habitat destruction and deforestation may pose long-term risks.

Housing Requirements in Captivity

When keeping the Bornean file-eared tree frog in captivity, it is essential to replicate a tropical rainforest environment. A glass enclosure is recommended, as glass helps dissipate heat and prevents excessive temperature buildup. Adult frogs should be housed in a vertically oriented terrarium of at least 45×45×60 cm, with height being more important than floor area due to their arboreal nature. Juveniles can be temporarily kept in a 30×30×45 cm enclosure before being transferred to a larger space as they grow.

The optimal daytime temperature ranges from 24 to 27°C, with a slight drop to 22 to 25°C at night. While wild individuals may experience temperatures up to 30°C, prolonged exposure above 28°C in captivity can lead to stress and dehydration, so temperature must be carefully controlled. Heating can be provided using a side-mounted heat mat with a thermostat, and a low-wattage bulb may be added if necessary. Temperatures exceeding 30°C should be strictly avoided, as they may be lethal.

Humidity should be maintained at a moderate to high level, ideally between 60 and 80%. The enclosure should be misted once in the morning and once in the evening. A shallow water dish should be provided for drinking and maintaining humidity. Water must be clean and free of chlorine; filtered or bottled water is recommended, and it should be replaced daily. As frogs often use the water dish for defecation, it must be cleaned regularly.

Although frogs are less dependent on ultraviolet light (UVB) than reptiles, this species still receives diffuse UVB exposure in the wild. Therefore, low-intensity UVB lighting (5 to 6% UVB) should be provided in captivity to support calcium metabolism and prevent metabolic bone disease.

Suitable substrates include coarse bark (such as orchid bark), coconut fiber, or flattened sphagnum moss, kept slightly moist. Fine substrates should be avoided to prevent accidental ingestion and intestinal blockage. The enclosure should include branches, vines, and either artificial or live plants to provide climbing structures and shelter, effectively simulating a forest environment. If possible, a bioactive setup can be established with live plants and microbial communities to maintain a stable micro-ecosystem.

Feeding Conditions

The Bornean file-eared tree frog is carnivorous and primarily feeds on insects, so live prey should be provided in captivity. Common food items include crickets, grasshoppers, fruit flies, and moths. Mealworms, silkworm larvae, or calcium worms can be offered occasionally for dietary variety. The staple diet should consist of high-protein, easily digestible insects.

All feeder insects should be gut-loaded with nutrient-rich vegetables, fruits, or commercial insect diets before being offered to the frogs. This enhances their nutritional value and ensures the frog receives a balanced diet. Additionally, feeder insects should be dusted with calcium powder or multivitamin supplements (especially those containing vitamin D3) prior to feeding to prevent calcium deficiency and skeletal issues. Adult frogs should be fed every two days, with 4 to 6 large crickets or an equivalent amount per feeding. Juveniles and subadults should be fed daily, primarily with smaller insects.

Feeding is best conducted in the evening, as this species is nocturnal and most active at that time. Prey can be offered using forceps or released directly into the enclosure for natural hunting behavior. Uneaten prey should not be left in the enclosure for extended periods, as insects may injure the frogs or decompose and contaminate the environment.

Hydration is equally important. In addition to the water dish, frogs will drink water droplets from misting. Because amphibian skin absorbs water, maintaining appropriate humidity and clean water sources is more effective than force-feeding water.

Breeding Conditions

Successful breeding in captivity requires simulation of the rainy season. Keepers often use a rain chamber equipped with a misting system to mimic rainfall and increase humidity, triggering reproductive behavior. Males call at night to attract females, and after successful pairing, foam nests are constructed on leaves above water.

After oviposition, fertilized eggs are enclosed within the foam, which protects them from desiccation and predators. Upon hatching, tadpoles fall into the water below. In captivity, a clean, chlorine-free shallow water container should be prepared, and tadpoles should be provided with green algae, prepared feeds, or boiled vegetable mash. Tadpoles grow rapidly and typically metamorphose into froglets within 8 to 10 weeks.

Breeding success is closely tied to how accurately environmental conditions are replicated. Disturbance should be minimized during the breeding period to avoid stress that may prevent mating. Once tadpoles complete metamorphosis, they should be transferred immediately to a more terrestrial setup and provided with small insects as food.

If breeding occurs, keepers must be prepared in advance with appropriate setups for both eggs and tadpoles, as failure to do so may result in high mortality. It is also important to consider whether one can adequately care for and rehome the offspring to prevent overbreeding and abandonment.

Amplexus behavior in Bornean file-eared tree frogs（圖片來源：Caroline Jones，採用 CC BY 2.0 授權）

Preventing Invasive Spread

The Bornean file-eared tree frog is native to Borneo, but as a species present in the pet trade, it may be kept in many regions worldwide. Releasing it into non-native environments could cause ecological harm, such as competition with native amphibians, transmission of diseases, or disruption of local ecosystems.

Although there are currently no confirmed records of this species becoming invasive, its large size, high reproductive capacity, and adaptability suggest that it could establish populations in suitable tropical or subtropical regions and pose ecological risks. Therefore, under no circumstances should individuals be released into the wild. If an owner can no longer care for the animal, it should be rehomed through other keepers or animal rescue organizations.

With responsible care and proper management, the Bornean file-eared tree frog can be maintained as a unique and highly engaging species in captivity while avoiding the risk of becoming an invasive species.

Author: Shui-Ye You