Seen a brown frog clinging onto a surface in an urban area?
Heard a loud, nasal 'quack' in the evening?
You have likely encountered a Four-lined Tree Frog!
FOUR-LINED TREE FROG
Polypedates leucomystax (Gravenhorst, 1829)
Figure 1. A Four-lined Tree Frog, with a plain back, clinging onto the wall of a building at the Singapore Botanic Garden. (Source: Everlyn Julya Koh) Figure 2. A Four-lined Tree Frog, with four distinct brown stripes on its back, holding onto the stalk of a plant. (Source: Thomas Brown, Wikimedia Commons, used in accordance to terms of Fair Use)
1. INTRODUCTION
An ubiquitous and widespread frog species across Southeast Asia, including Singapore, the Four-lined Tree Frog (Polypedates leucomystax) is one of the most commonly-encountered frog species in Singapore. This nocturnal native species can be found in both natural and urban landscapes, typically heard or seen near pools of water in the evening where they congregate and produce loud, characteristic calls.
TABLE OF CONTENTS
2. BIOLOGY
2.1. Diet
The Four-lined Tree Frog is generally considered an opportunistic predator and has a diet differing between the tadpoles and adult life stages. Within water bodies where food is scarce, exotrophic tadpoles are known to feed on almost any food items that are available, and larger tadpoles may feed on almost any organism that they can overpower. This include both live and dead tadpoles belonging to the same or different species, as well as decomposing plant material such as fallen leaves and bark.
2.2. Predation & Parasite
The Four-lined Tree Frog faces changing predation pressures as it metamorphoses from a tadpole to an adult, transitioning from largely facing aquatic predators to terrestrial ones. The main predator of tadpoles are nymphs of Order Odonata (dragonflies and damselflies) that dwell in the same water body as tadpoles.
The Four-lined Tree Frog may also carry various protozoan parasites (e.g. Cepedea dimidiata, Entamoeba ranarum, Trypanosoma sp.) in its guts, rectum and blood.
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| Figure 3. A Four-lined Tree Frog being held at the nape by a Painted Bronzeback in Singapore. (Source: Tan & Low (2018), used in accordance to terms of Fair Use) |
3. LIFE CYCLE & REPRODUCTION
3.1. Mating
In Singapore which has a tropical rain forest climate (based on Köppen climate classification), the Four-lined Tree Frog breeds year-round as there is no distinct dry season which may halt breeding. Locally, female Four-lined Tree Frogs have been found with an enlarged oviduct throughout the year. Eggs have also been sighted in almost all months across the year.
Mating begins with the male frogs congregating on the vegetation near a stagnant water body to form a calling group.
Mating in the Four-lined Tree Frog has also been said to be largely driven by female mate selection. While females do not produce advertisement calls, females may produce a weak reciprocal call in response to advertisement calls emitted by a male.
When a male successfully chases an approaching female, amplexus ensues which involves the male frog hitching securely onto the back of the female by clasping tightly at the female's arms or armpit regions (Figure 4).
3.2. Oviposition
Much like many frog species from the Family Rhacophoridae, the Four-lined Tree Frog constructs foam nest at the oviposition site where ivory-white eggs will be deposited into during a breeding event. The foam nest can be created on overhanging vegetation above a stagnant water body (Figure 5) or attached to a surface at the edge of a water body (Figure 6).
During a breeding event, other males may continue to contest for a chance to breed by attempting to dislodge the clasped male of the amplecting pair. In some cases, one female may simultaneously engage in amplexus with multiple males (Figure 5), a polyandry phenomenon said to be rare among frogs.
Eggs may hatch after four days into tadpoles with external gills. These tadpoles will then maneuver out of the foam nest and fall into the water beneath where they live until metamorphosis.
| Video 1. Documentation of Four-lined Tree Frog calling in Singapore, 3rd February 2012 evening. (Source: Leong Tzi Ming, used in accordance to terms of Fair Use) |
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| Figure 4. Smaller male (top) and larger female (bottom) adult Four-lined Tree Frogs in amplexus. (Source: Nick Baker, Permission pending; Annotation: Everlyn Julya Koh) |
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| Figure 5. Multiple male Four-lined Tree Frogs clasping onto one female during oviposition. (Source: Feng & Narins (1991), used in accordance to terms of Fair Use) |
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| Figure 6. A foam nest constructed on the inner walls of an artificial structure. (Source: W.A. Djatmiko, Wikimedia Commons, used in accordance to terms of Fair Use) |
4. HABITAT & RANGE
4.1. Habitat
The Four-lined Tree Frog is notable for being an adaptable opportunist and a successful human commensal, with its range intimately following human activities which have been said to provide suitable living and breeding habitat for the frog.
4.2. Range
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| Figure 7. Global distribution map of Four-lined Tree Frog showing its extant native range (highlighted in orange) and areas where it has been introduced (circled in purple). (Source: Diesmos et al. (2004), used in accordance to terms of Fair Use) |
The Four-lined Tree Frog is widely distributed and abundant throughout Southeast Asia and has a native extant range stretching from Bangladesh, Thailand to the many islands of Indonesia and the Philippines (Figure 7).
In Singapore, the Four-lined Tree Frog is also widespread and abundant across the island in both natural and built-up environments. In green spaces, the frog can be commonly seen in urban parks and gardens such as in Singapore Botanic Garden and Lower Peirce Reservoir Park, and in forest reserves and green buffer areas such as Bukit Timah Nature Reserve and various nature parks (e.g. Hindhede, Dairy Farm). They may even inhabit wetland and swamp habitats such as the Sungei Buloh Wetland Reserve and Nee Soon Swamp Forest respectively. Populations have also been reported on offshore islands, such as Pulau Ubin, Pulau Tekong and Semakau Island.
5. CONSERVATION, THREATS AND USES
According to the IUCN Red List of Threatened Species, the Four-lined Tree Frog has been listed as 'least concerned' with a stable population that is not critically fragmented.
Locally, this species has been considered 'common' by the Lee Kong Chian Natural History Museum and is hence not listed within the Singapore Red Data Book. While this frog species is not exposed to disturbances at an extent that could lead to its decline, the supposedly heavy use of pesticides around residential areas might endanger local sub-populations. This is especially problematic should individuals consume prey coated with pesticides or when pesticide is absorbed through the skin of the frog. This frog species is also involved in the international pet trade and is a popular captive frog species in many European and American regions.
The primary conservation concern for the Four-lined Tree Frog is the cryptic diversity within the species complex since the phylogenetic relationship within the species complex still remains largely unresolved. This can pose serious conservation implications as some taxa within the complex might be in a predicament which requires conservation efforts. However, there needs to be trustworthy identification of each species within the complex before any at-risk species may be identified and conservation methods can be crafted. Hence, further research into the taxonomy, population size and distribution trends of this species is definitely needed.
6. MORPHOLOGY
6.1. Description
TADPOLE & METAMORPH
The tadpoles of the Four-lined Tree Frog can grow to 40–50 mm in total length (Figure 8). The body of the tadpole is slightly flattened from the top, and is elongated in shape with eyes on each side of the body (Figure 9).
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| Figure 8. Lateral view of the Four-lined Tree Frog tadpole with emerged hind limb. (Source: Meegaskumbura et al. (2011), used in accordance to terms of Fair Use; Annotations: Everlyn Julya Koh) | Figure 9. Dorsal view of the Four-lined Tree Frog tadpole with emerged hind limbs. (Source: Nick Baker. Permission pending; Annotations: Everlyn Julya Koh) |
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| Figure 10. A Four-lined Tree Frog metamorph that has fully-emerged and developed front and hind limbs but has not fully reabsorbed tail. (Source: Jonathan Hakim, permission pending; Annotation: Everlyn Julya Koh) |
ADULT
The adult Four-lined Tree Frog is of a small to moderately large size, measuring 30–75 mm in snout-vent length (SVL) (Figure 11).
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Figure 11. Dorso view of the Four-lined Tree Frog (striped morph). (Source: Pavel Kirillov, Wikimedia Commons, used in accordance to terms of Fair Use; Annotation: Everlyn julya Koh) | Figure 12. Lateral view of the Four-lined Tree Frog (striped morph). (Source: Thomas Brown, Wikimedia Commons, used in accordance to terms of Fair Use; Annotation: Everlyn Julya Koh) |
There are generally three colour morphs in the Four-lined Tree Frog: striped (Figure 12), mottled (Figure 13), and plain (Figure 14) morphs.
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| Figure 12. Striped morph of the Four-lined Tree Frog. (Source: Wildlife Singapore, permission pending) | Figure 13. Mottled morph of the Four-lined Tree Frog. (Source: W.A. Djatmiko, Wikimedia Commons, used in accordance to terms of Fair Use) | Figure 14. Plain morph of the Four-lined Tree Frog, encountered at the National University of Singapore (Source: Everlyn julya Koh) |
The sex of the Four-lined Tree Frog may often be determined by body size (Figure 15). Females are much larger in size than males, measuring 57–75 mm and 37–50 mm respectively.
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| Figure 15. Male (striped morph) and female (mottled morph) Four-lined Tree Frog in amplexus. Abdomen of female frog is swollen with eggs. (Source: W.A. Djatmiko, Wikimedia Commons, used in accordance to terms of Fair Use; Annotation: Everlyn Julya Koh) |
6.2. Diagnosis
The genus Polypedates consist of 24 different species, but only the Four-lined Tree Frog may be found locally.
Outside of Singapore, such as in Borneo, the Four-lined Tree Frog may occur in the same regions as other members of the same genus.
| Species | Photograph labelled with diagnostic features | Distribution |
| Four-lined Tree Frog (Polypedates leucomystax) |
Figure 16. Four-lined Tree Frog (Source: Pavel Kirillov, Wikimedia Commons, used in accordance to terms of Fair Use; Annotation: Everlyn julya Koh) | Throughout Southeast Asia, including Singapore |
| Collette's Tree Frog (Polypedates colletti) |
Figure 17. Collette's Tree Frog has a pointed snout while the Four-lined Tree Frog has a blunt snout. (Source: Inger et al. (2017), used in accordance to terms of Fair Use; Annotations: Everlyn Julya Koh) | Endemic to Borneo |
| Green-eyed Tree Frog (Polepedates chloropthalmus) |
Figure 18. Green-eyed Tree Frog has green eyes while the Four-lined Tree Frog do not has green eyes. (Source: Das (2005), used in accordance to terms of Fair Use; Annotations: Everlyn Julya Koh) | Endemic to Borneo |
| File-eared Tree Frog (Polypedates otilophus) |
Figure 19. File-eared Tree Frog has serrated projections above its tympanum and a sharply-pointed projection at the edge of jaw while the Four-lined Tree frog do not have these projections. (Source: Inger et al. (2017), used in accordance to terms of Fair Use; Annotations: Everlyn Julya Koh) | Throughout most parts of Southeast Asia (Absent in Singapore) |
| Dark-eared Tree Frog (Polypedates macrotis) |
Figure 20. Dark-eared Tree Frog has a thick dark stripe extending from behind its eyes to over its tympanum and shoulder while the Four-lined Tree Frog has light brown flanks. (Source: Inger et al. (2017), used in accordance to terms of Fair Use; Annotations: Everlyn Julya Koh) | Throughout most parts of Southeast Asia (absent in Singapore) |
7. TAXONOMY
7.1. Taxonavigation
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Amphibia |
| Order | Anura |
| Family | Rhacophoridae |
| Genus | Polypedates |
| Species | Polypedates leucomystax |
7.2. Etymology
The species name 'Polypedates leucomystax' has a Greek origin. The word 'Polypedates' is derived from the Greek noun 'pēdētēs' which means a leaper or a dancer, or from the Greek verb 'pēdanto' which means to leap or jump. Added with the English prefix 'poly-' which means 'many' in Greek, 'Polypedates' may mean multiple leapers or dancers which aptly described the arboreal lifestyle of members in the genus where members leap from branches to branches. The word 'leucomystax' is derived from the Greek adjective 'leucos' which means white, and the Greek noun 'mystax' which means an upper lip or a moustache. The species epithet 'leucomystax' is hence attributed to the extended white stripe present above the upper lips of this frog species.
| Term | Origin and meaning |
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| Poly | Greek noun for 'many'. |
| Pedates | Greek noun 'pēdētēs' meaning 'a leaper or dancer', or Greek verb ' pēdanto' meaning 'to leap or dance'. |
| Leuco | Greek adjective 'leucos' meaning 'white'. |
| Mystax | Greek noun 'mystax' meaning 'an upper lip' or 'a moustache'. |
7.3. Original description & Type information
Despite being presently known as Polypedates leucomystax, this species was first described as Hyla leucomystax by Johann Ludwig Christian Gravenhorst in 1829 from a specimen obtained from Java, Indonesia. His original description of the species in Latin may be loosely translated into the species having a 'rust coloured dorsal skin', 'granular skin at the underside of the abdomen and hip' and 'the presence of an extended white line at the lateral edge of the upper jaw' (Figure 21).
The holotype of P. leucomystax, obtained from Java, Indonesia, is currently stored at the Breslau Museum, presumably the Museum of Natural History University of Wroclaw.
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| Figure 21. An excerpt from the original description of Polypedates leucomystax as Hyla leucomystax by Johann Ludwig Christian Gravenhorst in 1829. (Source: Gravenhorst (1829), used in accordance to terms of fair use) |
7.4. Synonyms
A synonym is a scientific name given to a species that currently has a different scientific name. Synonyms may come about when taxonomists describe a species that has already been described. Through the years, the phylogenetic position of P. leucomystax has been reconsidered many times, leading to the species having faced multiple genus and epithet changes. Just nine years after being originally described in the genus Hyla by Gravenhorst, this species was moved to genus Polypedates by Tsudi in 1838. For the next 30 years, this species has been moved back and forth through Hyla and Polypedates. However, for most of its history, P. leucomystax had been places into the genus Rhacophorus. It is only until 1987 that the species has been placed back into genus Polypedates. The synonyms of the species are as follow:
- Hyla leucomystax Gravenhorst, 1829
- Hyla sexvirgata Gravenhorst, 1829
- Hyla quadrilineata Wiegmann, 1834
- Polypedates leucomystax Tschudi, 1838
- Hyla leucopogon Tschudi, 1838
- Hyla quadrivirgata Tschudi, 1838
- Polypedates rugosus Duméril and Bibron, 1841
- Polypedates quadrilineatus Günther, 1859 "1858"
- Limnodytes celebensis Fitzinger, 1861 "1860"
- Rhacophorus maculatus var. quadrilineata Boulenger, 1882
- Hylorana longipes Fischer, 1885
- Polypedates maculatus quadrilineatus Fischer, 1885
- Rhacophorus leucomystax Boulenger, 1889
- Rhacophorus leucomystax leucomystax Mocquard, 1890
- Rhacophorus leucomystax quadrilineatus Mocquard, 1890
- Rhacophorus leucomystax var. sexvirgata Boettger, 1894
- Rhacophorus leucomystax quadrilineata Werner, 1903
- Rhacophorus maculatus leucomystax Annandale, 1912
- Rhacophorus maculatus himalayensis Annandale, 1912
- Hyla wirzi Roux, 1927
- Rhacophorus (Polypedates) leucomystax Bourret, 1927
- Rhacophorus (Polypedates) quadrilineatus Bourret, 1927
- Rhacophorus kampeni Ahl, 1927
- Rhacophorus (Rhacophorus) kampeni Ahl, 1931
- Rhacophorus (Rhacophorus) leucomystax Ahl, 1931
- Rhacophorus (Rhacophorus) himalayanus Ahl, 1931
- Rhacophorus (Rhacophorus) leucomystax leucomystax Wolf, 1936
- Rhacophorus (Rhacophorus) wirzi Forcart, 1946
- Rhacophorus leucomystax quadrilineatus Inger, 1954
- Polypedates teraiensis Dubois, 1987
- Polypedates leucomystax Dutta, 1997
- Rhacophorus teraiensis Ao, Bordoloi & Ohler, 2003
7.5. Common names
As the striped morph of P. leucomystax is most commonly seen in Singapore, most Singaporeans primarily adopt the common name 'Four-lined Tree Frog' when referring to this species. However, other common names such as 'Common Tree Frog', 'Java Whipping Frog', and 'White-lipped Tree Frog' are also popularly used in neighbouring countries such as Malaysia and Indonesia where this species also commonly occur. It may also be interesting to note that while most common names shown are English names, there are also non-English common names attributed to this species. The common names of the species are as follow:
- Bamboo Tree Frog
- Brown Tree Frog
- Common Tree Frog
- Four-lined Tree Frog
- House Tree Frog
- Jar Tree Frog
- Java Whipping Frog
- Malayan House Frog
- Malayan Tree Frog
- Stripe Tree Frog
- White-lipped Tree Frog
- Shiroagogaeru (Japanese)
- Palakang Saging (Tagalog)
7.5. Species complex
Many lineages of animals that occupy a wide geographical range may sometimes encompass cryptic species, and Polypedates leucomystax is one of such species.
To examine the ambiguity in phylogenetic relationship between cryptic species within the P. leucomystax species complex, a phylogenetic study was conducted by Kuraishi et al. in 2013 which analysed the phylogenetic relationship of groups within the species complex using Polypedates specimen collected from diverse areas of Southeast Asian and South Asian countries. In this study, several mitochondrial genes (12S rRNA, tRNAval and 16S rRNA) and concatenated nuclear DNA genes (BDNF, NCX, POMC, Rag1, Rhod, SCF and Tyr) of Polypedates specimen were used for Maximum Parsimony (MP), Maximum Likelihood (ML) and Bayesian analyses. Bayesian trees were then generated to show relationship between Polypedates species.
Based on the Bayesian tree generated using mtDNA genetic sequences (Figure 22), the P. leucomystax species complex can be said to consist of six major clades with each clade represented by one species: South China Clade (P. cf. mufus 1), North China Clade (P. brauen), Indochina Clade (P. megacephalus), Sunda Clade (P. leucomystax), Laos Clade (P. cf. mufus 2) and Malay Clade (P. sp.). However, the Bayesian tree (Figure 22) still suggest substantial ambiguity amongst species within the complex, with many nodes where these six major clades originate from having relatively low bootstrap support values, hence suggesting that these clades are not particularly stable. This ambiguity may also be highlighted by the Malay Clade being embedded within a polytomy, suggesting that phylogenetic position of this clade within the tree has not yet been resolved. The Bayesian tree combining analysis of both mitochondria DNA and nuclear DNA (Figure 23) provided a largely similar relationship and conclusion. However, instead of a polytomy as highlighted by analysis using mitochondra DNA, analysis using concatenated nuclear DNA shows that P. sp. from the Malay clade had diverged prior to P. macrotis. Although this relationship may be substantially supported by a BPP of 1.0, this support value may drop with additional data and specimen used since the Malay clade had a small number of samples used. Thus, there needs to be further work carried out on the taxonomy on P. leucomystax species complex to better resolve such cryptic relationship between the species.
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| Figure 22. A Bayesian tree constructed using 2005 bp mitochondrial 12S rRNA, tRNAval and 16S rRNA genes for specimen used in the study. The numbers at the end of each branch represent sample numbers which the scientists have assigned to each of the specimen. Numbers above the branches are bootstrap support (BS) values for the maximum parsimony (MP), maximum likelihood (ML) and Bayesian posterior probability (BPP) (MPBS/MLBS/BPP). Asterisks mark nodes with MPBS and MLBS ‡ 70% and BPP ‡ 0.95. (Source: Kuraishi et al. (2013), used in accordance to terms of fair use) | Figure 23. Bayesian trees constructed using 2005-bp sequence of the mitochondrial 12S rRNA, tRNAval and 16S rRNA genes (left) and a 4696-bp concatenated sequence of the nuclear BDNF, NCX, POMC, Rag1, Rhod, SCF and Tyr genes (right). Numbers above the branches are bootstrap support (BS) values for the maximum parsimony (MP), maximum likelihood (ML) and Bayesian posterior probability (BPP) (MPBS/MLBS/BPP) in the mtDNA tree (left) and BPP in the nuDNA tree (right). (Source: Kuraishi et al. (2013), used in accordance to terms of fair use) |
7.5. Genetic sequence/barcode
To create a phylogeny between samples or specimens, universal marker genes are typically used. For amphibians, commonly used genes in constructing a phylogeny include mitochondria genes such as the Cytochrome Oxidase Subunit 1 (COI) gene, 16S rRNA and tRNAval. For Polypedates leucomystax, the DNA sequence of COI gene, and 16S rRNA and tRNAval may be obtained from Genbank database here and here respectively.
8. SYSTEMATICS
The family Rhacophoridae, or "Old-World Tree Frog", is an exceptionally diverse amphibian family in Asia. This family is a well-supported monophyletic clade, with 18 genera and 413 species spreading all across Asia with a disconnected presence in Africa.
In this study, Chan et al. (2018) had carried out a comprehensive multi-loci phylogeny of Rhacophoridae using molecular data from 327 specimen representing an estimated 247 species from all 18 genera of the family. For all specimen, two mitochondria genes (16S rRNA and Cytochrome-b) and two nuclear DNA genes (RAG-1 and Tyrosinase) were analysed. In particular, the 16S gene was used because it is said to an universal DNA barcoding marker gene in amphibians, and it is also popularly used in species delimitation.
Proc. Natl. Acad. Sci., 106, 8267-8272.]]>
Both the Bayesian and maximal likelihood phylogenies constructed by this study show largely the same topologies with most nodes having high support values. From the Bayesian phylogeny (Figure 24), the genus Taruga is shown to be the sister genus to Polypedates with maximum bootstrap support (100/100/1.0). This topology may also be observed with high bootstrap support in the maximum likelihood phylogeny (Figure 25). This topology has also been shown to be well-supported by Meegaskumbura et al. (2010) with Bayesian posterior probability of 1.0 and maximum parsimony bootstrap support of 100% when the authors newly described the genus Taruga.
Within genus Polypedates, the topologies within the P. leucomystax species complex is relatively well-supported with all BS > 90. However, this study has shown P. leucomystax to be a well-supported sister species to P. cf. mutus (Figure 25) instead of P. megacephalus (Figure 22 & 23). This discrepancy in topology further highlights the controversy mired within the taxonomy of P. leucomystax complex and that more data should be analysed in order to clarify the topology between these cryptic species. Furthermore, unlike Kuraishi et al. (2013), the authors for this study have also considered P. impresus and P. teraiensis to be part of the species complex with the topologies of these two species having high BS > 90. Since P. impresus has been included in phylogenetic studies of the species complex as early as that of Kuraishi et al. (2013), this discrepancy may be due to different sampling efforts and not due to species being newly added into the complex.
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| Figure 24. Bayesian phylogenetic tree at the generic level. Node support is denoted as: IQ-TREE SH-aLRT bootstrap support/Ultrafast bootstrap support/Bayesian posterior probability. The genus Polypedates is boxed up using an orange rectangle while the node bootstrap support value for monophyly has been underlined in orange. (Source: Chan et al. (2018), used in accordance to terms of fair use) | Figure 25. A section of maximum likelihood phylogeny of the family Rhacophoridae estimated using 3483 bp of sequence data from four genes. Circles at nodes denote ultrafast bootstrap support values from the IQ-TREE analysis (blue = BS > 90; red = BS < 90). Polypedates leucomystax has been boxed up using an orange rectangle.(Source: Chan et al. (2018), used in accordance to terms of fair use) |
9. GLOSSARY
| Word | Meaning |
|---|---|
Nocturnal | Active beyond dusk and before dawn. |
Cryptic species | Individuals that are morphologically very similar to one another but belong to different species. |
Species complex | A group consisting of closely-related species that are so similar in morphology and genetics to the extend that the boundaries between them are ambiguous. |
Opportunistic predator | A predator that feeds whenever a prey is available. |
Exotrophic tadpole | A tadpole that needs to feeds on other organism to grow and metamorphose. |
Commensalism | An interaction between two organisms by which one benefits and the other acquire neither benefit nor harm. |
Advertisement calls | The vocalisation created for the purpose of attracting a mate. |
Amplexus | The mating posture of a frogs and toads, where the male clasp onto the back of a female. |
Oviposition | The laying of eggs. |
Cloaca | A short tube at the vent of the frog which connects internally to the genital, bladder and rectum. |
Polyandry | A form of mating structure whereby a female organism has more than one male mate. |
Ventral | The underside or abdominal side of an organism. |
Dorsal | The upperside or back of an organism. |
Species delimitation | The process by which species boundaries are decided. |
Holotype | A single type specimen which the new species is being described and named upon. |
Monophyletic clade | A group of organism that encompasses a common ancestor and all of its descendant. |
