Description
Rhacophorus helenae, commonly called Helen’s Tree Frog, is part of a group of arboreal frogs called flying frogs. The snout-vent length range of this frog is large compared to other related species, with males at 72.3 - 85.5 mm and females at 89.4 - 90.7 mm. The head is just slightly longer in length than width. In the dorsal view the snout is truncate and in the profile it is sloping with a slight point at the tip that can be seen from the ventrum. The canthus rostralis is distinct and rounded. The oval nostrils are raised and located closer to the snout tip than to the eyes. The loreal region slopes and is slightly concave. The interorbital region is convex. The eye pupil is orientated horizontally. The tympanum of the ear can be viewed externally and is slightly elevated relative to the skin. The species has no pineal ocellus and the skin is not co-ossified on the forehead. The vomerine teeth are present in oblique groups. The tongue is attached anteriorly and deeply notched. The body is dorsoventrally compressed. The forelimbs are robust with a dermal fringe or skin flap on the outer edge of the forearm, which narrows along the tarsus. The relative finger lengths are I < II < IV < III. The fingers have wide discs with circummarginal grooves and webbing between all four fingers with a webbing formula of I 1½ - 1½ II 0 – 0 III 0 - 0 IV. There is one prominent, oval subarticular tubercle on fingers I and II and two on fingers III and IV. There are no palmar tubercle, accessory palmar tubercles, or thenar tubercles. There is a prominent, oval prepollex. There is a square-shaped protrusion on the tibiotarsal articulation. The relative toe lengths are I < II < III < V < IV and end in smaller discs with circummarginal grooves than the fingers. The complete webbing on the toes has a formula of I 0 – 0 II 0 – 0 III 0 – 0 IV – 0 V. The distinct subarticular tubercles are round. One subarticular tubercle can be found on fingers I and II each, two on fingers III and V, and three on finger IV. The foot has a low, oval inner metatarsal tubercle but lacks an outer metatarsal tubercle or supernumary tubercles. The skin on the dorsum, throat, and chest is smooth and the skin on the ventral surface of the thighs and belly is granular. Above the cloaca, there is a low, single-lobed dermal ridge. Nuptial pads are present on males and are located on the outer margins of the prepollex and first finger. Males also have medial subgular vocal sacs with openings located at the edge of the mouth. The skin in the gular region of males is slightly loose skin (Rowley et al. 2012).

Froglets have less developed webbing and only rudimentary skin flaps on the forearms (Vassilieva et al. 2016).

Rhacophorus helenae can be distinguished from most other large Rhacophorus species in Southeast Asia, except for R. burmanus, R. dennysi, R. duboisi, R. feae, R, kio, R. georgii, R. maximus, R. nigropalmatus, and R. norhayatii, by its predominantly or immaculate green dorsum. From R. burmanus, the focal species can be distingushed by the presence of a white underside, black patches on axilla, and tibiotarsal projections that are found in R. helenae. Rhacophorus helenae can be distinguished from R. dennysi by the white underside, black patches on the axilla, tibiotarsal projections, and a dermal ridge of R. helenae. Rhacophorus helenae is distinguished from R. duboisis by having a green dorsum, a white underside, a bluish-green background coloration with pale yellow marbling on the posterior region of thigh, black axillary patches, dermal ridge, and tibiotarsal dermal projection. Rhacophorus helenae is distinguished from R. feae by having black axillary patches, a dermal ridge, and tibiotarsal dermal projection. From R. georgii, the focal species is distinguished in having a green dorsum and a lack of optical knobs that are present in R. georgii. Rhacophorus helenae is distinguished from R. maximus by having a bluish-green background coloration with pale yellow marbling on the posterior region of thigh, black axillary patches, a dermal ridge, and tibiotarsal dermal projection. Rhacophorus helenae is distinguished from R. niropalmatus by having a green dorsum, a white underside, black axillary patches, and a bluish-green background coloration with pale yellow marbling on the posterior region of thigh. Rhacophorus helenae is distinguished from R. norhayatii by having a larger body size, a white underside, a single-lobed dermal ridge, and a bluish-green background coloration with pale yellow marbling on the posterior region of thigh. Rhacophorus helenae is distinguished from R. kio, its sister species, by having a white underside, a low single-lobed dermal ridge, a white eye sclera, pale green or yellow webbing, and a bluish-green background coloration with pale yellow marbling on the posterior region of thigh. Rhacophorus helenae is also generally larger than R. kio (Rowley et al. 2012). Tadpoles of R. helenae can be distinguished from most other Rhacophorus species by their range. However, from the sympatric R. annamensis, it can be distinguished by the labial tooth row formula. Tooth row formula also distinguishes R. helenae from the sympatric Polypedates megacephalus as do the features of the latter having a medial gap in the double row of papillae of the lower lip, laterally positioned eyes, and a silver spot on the snout tip (Vassilieva et al. 2016).

In life, the dorsum of adults is evenly green, but may have a scattering of small white spots. There is a large black splotch on the axilla. The flanks, upper arms, ventral side of the lower arms, and both the anterior and posterior sides of the thighs are bluish-green and marbled with pale yellow that appears as flecks on the flanks. The dermal projections on arms and legs and the supracloacal dermal ridge are lined with white. The background coloration of the dorsal surface of the digit webbing is pale green with pale yellow or green margins; there may be black pigmentation spreading from the base of the fingers that fades to pale green and yellow. The throat, chest, and belly are immaculately white with some pinkish tinge at the posteriorlateral edge. The ventral surfaces of the hands and feet are pinkish. The iris of the eyes is golden yellow with faint flecks of dark gold and a black periphery. The sclera is white. In preservative, specimens look similar to live coloration except that the green fades to violet and the yellow portions fade to white (Rowley et al. 2012).

In life, dorsal coloration of the tadpoles ranges from olive-yellow to dark brown depending on the lighting and water coloration. Tadpole coloration in preservative is a uniform yellowish-grey with more coloration on the dorsum and tail and a more transparent coloration on the belly. The tail has a fine marble-like pattern. The eyes and the mouthparts are black. Faint lines of neuromats are visible on the dorsal and lateral surfaces of the head and body (Vassilieva et al. 2016).

In life, the coloration of juveniles is light bluish-grey dorsally with fine, dark speckling and bright yellow ventrums. The dark spots on the flanks were absent (Vassilieva et al. 2016).

The type series for this species show sexual dimorphism and variation in patterning and coloration. Females are generally larger than males and have pale green rather than pale yellow margins on their digit webbing. There is slight variation in the location and concentration of white specks or spots found over the whole body. The amount of black coloration on webbing also varies slightly between individuals (Rowley et al. 2012).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Viet Nam

View distribution map in BerkeleyMapper.

Rhacophorus helenae has been found in four populations at Nui Ong Nature Reserve, Tan Phu Forest, Binh Chau–Phuoc Buu Nature Reserve, and in Ma Da Forest in Vietnam (Rowley et al 2012, Vassilieva et al. 2016). The first two locations are surrounded by farmland, but they contain low-elevation mixed-evergreen, deciduous, and bamboo forest habitat (Rowley et al. 2012). In the latter two locations, the populations were found near small or temporary water sources on the edge of disturbed and fragmented semideciduous forest (Vassilieva et al. 2016). It is speculated that the species may also occur in other lowland forest fragments and may even also occur in eastern Cambodia (Rowley et al. 2012).

Life History, Abundance, Activity, and Special Behaviors
Rhacophorus helenae has an extended breeding season lasting from at least May to September. Breeding pairs have been observed in trees above temporary pools, as have foam nests (Rowley et al. 2012). (Vassilieva et al. 2016).

Males of R. helenae vocalize following rain events at sunset or early in the evening from 1 - 7 meters above the ground, sitting on trees and bushes adjacent to small pools of water. Calls can be divided into five main categories: 1) simple tonal, 2) tonal with pulses, 3) simple wideband, 4) wideband with pulses, and 5) pulsed. The call duration for each of the 5 groups is 39, 120, 90, 100, and 170 ms respectively. The pulse rate for groups 2, 4, and 5 is 42.8, 47.9, 52.6 pulses per second respectively. The frequency peak for each of the 5 groups is 875.7, 978, 960, 996.7, 1164.4 Hz respectively (Vassilieva et al. 2016).

Simple tonal calls are the most commonly expressed call out of the five different call groups, contributing 63% of total calls. While the great diversity of calls exhibited by R. helenae is not unheard of, it is quite surprising, as many other species within the Rhacophorus genus - and other related genera - are characterized by fairly simple, pulsed calls. Single males (not in choruses), exhibit only a single call; however, when males participate in choruses at breeding sites, each male exhibits many calls. In choruses, male call intensity and rate increase as a result of inter-male competition for females, since females prefer high call rates. Since there are so many different call types, it is thought that some call types function to communicate with females, while other calls function to communicate with other males, as is the case in Polypedates leucomystax, a tree frog in the Rhacophoridae family (Vassilieva et al. 2016).

Females lay eggs in foam nests on rocks or 10 - 12 m above ground on large, wide green leaves that slightly cover a non-permanent rain-filled pool/pond. This is so the larvae have easy access for the next stage. In captivity, two females laid clutches of about unfertilized 200 - 230 small eggs that had no pigment. The eggs measured 2.3 ± 0.1 mm in diameter (Vassilieva et al. 2016).

Tadpoles are found in temporary and seasonal ponds. Other species that can be found in the ponds are Polypedates megacephalus and Rhacophorus annamensis (Vassilieva et al. 2016).

Juveniles, in captivity, took 11 – 14 days to develop from stage 40 to stage 46 (Vassilieva et al. 2016).

Larva
The body of tadpoles is oval and slightly compressed dorsoventrally. The total length of tadpoles at stage 37 is between 39.0 – 42.8 mm and the body length ranges between 14.6 – 27.1 mm. The total lengths of a single tadpole at stages 35, 38, 39, and 41 measured 36.6, 41.0, 43.6, and 46.9 mm respectively and the body lengths were 14.5, 15.0, 15.0, and 16.2 mm. The bodies are wider than high. The snout is broad and rounded. The small nostrils are round, directed rostrolaterally, have a slightly raised rim, and are farther away from the eyes and closer to the snout. The dorsolateral eyes are generally large but smaller during the less developed stages. The pupils are positioned laterally. The anteroventral mouth parts consists of a moderately wide, elliptical oral disc. Soft, unpigmented, conical papillae can be found starting from the upper corners of the mouth and across the lower labium arranged as a double row until the middle of the labium where it becomes a single row. The mouth sheath has finely serrated cutting edges with the upper sheath having a wide arch-shape and the lower sheath being smooth and V-shaped. The upper lip has flat, unpigmented, horizontal ridges and keratinous, spike-like labial denticles along the labial edge arranged in uniserial rows. The most anterior row of the upper labium is continuous, the second row has an inconspicuous gap in the middle, and rows 3 – 5 are divided by the upper mouth sheath. The lower lip is composed of three rows, the first of which has an indistinct medial isthmus while the other two are continuous. The labial tooth row formula is 5 (2-5)/3 or 1:4+4/3 for most individuals examined, however, one has an asymmetrical formula of 1:4+3/3. Tadpoles have a short left-sided (sinistral) spiracle that is wide, fused to the body and opens posterodorsally with even margins. The short dextral vent tube is connected to the lower fin with an oblique, ventrocaudally directed opening. The tail is moderately long with well-developed musculature. The upper fin does not extend onto the dorsal area, is higher than the lower fin. Both fins reach their maximum height on the back third of the tail (Vassilieva et al. 2016).

Trends and Threats
Rhacophorus helenae has an IUCN Red List status of "Endangered" and is only known to be in four isolated areas in southern Vietnam: Nui Ong Nature Reserve, Tan Phu Forest, Binh Chau–Phuoc Buu Nature Reserve, and Ma Da Forest (Vassilieva 2016). Major threats facing R. helenae include habitat loss and degradation due to habitat encroachment, as well as habitat fragmentation. Another possible concern includes collection for pet trade, as a closely related species is commonly collected. There is very little known about the species and its population sizes, including whether or not the population is in decline. More research is needed in order to determine the stability of the species (IUCN 2014).

Relation to Humans
While this species is not commonly collected for pet trade, a closely related species, Rhacophorus kio, is collected, resulting in concern that R. helenae will eventually be used in pet trade (IUCN 2014).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intensified agriculture or grazing
Loss of genetic diversity from small population phenomena
Intentional mortality (over-harvesting, pet trade or collecting)

Comments
The species authority is: Rowley, J.J., Tran, D.T.A., Hoang, H.D., Le, D.T.T. (2012). “A new species of large flying frog (Rhacophoridae: Rhacophorus) from lowland forests in southern Vietnam.” Journal of Herpetology 46(4): 480-487.

Based on uncorrected pairwise analysis of 16S ribosomal RNA, R. helenae and R. kio have a genetic distance of 4.38 – 4.74 % (Rowley et al. 2012). Further analysis of 843 bp of 16S ribosomal RNA using Maximum Likelihood and Bayesian Inference supported the notion that R. helenae and R. kio are sister species with the next most closely related species being R. borneensis (Vassilieva et al. 2016).

The species epithet, ”helenae”, is in honor of Helen M. Rowley, mother of the first author of the species description, for the support Helen gave to Jodi Rowley in her career (Rowley et al. 2012).

References

IUCN SSC Amphibian Specialist Group. 2014. Rhacophorus helenae. The IUCN Red List of Threatened Species 2014: e.T47144519A47144530.

Rowley, J.J.L., Tran, D.T.A, Hoang, H.D., Le, D.T.T. (2012). ''A New Species of Large Flying Frog (Rhacophoridae: Rhacophorus) from Lowland Forestsin Southern Vietnam.'' Journal of Herpetology, 46(4), 480-487.

Vassilieva, A.B., Gogoleva, S.S., Poyarkov Jr., N.A. (2016). ''Larval Morphology and Complex Vocal Repertoire of Rachophorus helenae (Anura: Rhacophoridae), a Rare Flying Frog from Vietnam.'' Zootaxa, 4127(3), 515-536.

Species Account Citation: AmphibiaWeb 2022 Rhacophorus helenae: Helen’s Tree Frog <https://amphibiaweb.org/species/7957> University of California, Berkeley, CA, USA. Accessed Nov 21, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 21 Nov 2024.

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