Description
Barbourula busuangensis is a large, heavy-bodied, highly aquatic frog found in the fast-moving waters of mountain streams on the Philippine islands of Busuanga and Palawan. Adaptations to this life-style include a dorsoventrally flattened body shape, dorsally placed nostrils and eyes, robust and muscular hind limbs and forelimbs, and webbed digits on the forelimbs as well as the hind limbs. The body is covered with numerous very fine warts that increase in size laterally from the midline. This species is dark in color, from dark green to black, with faint lighter green blotches and crossbars on the hind limbs. There is no external tympanum.

Taylor and Noble (1924) described the Philippine Flat-headed frog, B. busuangensis, as a new genus and species from a single preserved specimen. The authors noted the similarity of this species to some of the most basal lineages of anurans, and especially members of the genus Bombina. Currently, Barbourula is grouped with Bombina in the Bombinatoridae (Ford and Cannatella 1993). Iskandar (1978) described a second species of the genus, Barbourula kalimantanensis, from neighboring Borneo.

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Philippines

View distribution map in BerkeleyMapper.

B. busuangensis is a rare and poorly known species of frog first described from the Philippine island of Busuanga, the largest island of the Calamianes island group (Taylor and Noble 1924). The species has since been reported only a handful of times in the scientific literature (Myers 1943; Inger 1954; Brown and Alcala 1970; Alcala and Brown 1987). In 1961, a survey of the herpetofauna of the Philippines produced the first record of B. busuangensis from the island of Palawan (Brown and Alcala 1970), reinforcing the association of this species with a fauna more similar to that of Borneo than the rest of the Philippine archipelago (Taylor and Noble 1924). This impression was reinforced by the later discovery of B. kalimantanensis on northern Borneo (Iskandar 1978).

Within Palawan and Busuanga, B. busuangensis is restricted to clean, cool, mountain streams in undisturbed forest. In these streams, this species can be found floating at the water surface with only its eyes and nostrils visible, or amongst rocks in fast flowing water (Alcala and Brown 1987). However, this species is very wary and quick to hide underneath submerged rocks when disturbed, which may explain why this frog remains so elusive.

Life History, Abundance, Activity, and Special Behaviors
Many aspects of the biology of this interesting species of frog are poorly known and remain largely unstudied because of its extreme rarity and wariness. Most of the basic facts of the ecology and life history of B. busuangensis are still a mystery.

For almost a century, much of the reproductive mode of this species was unknown (see News of the Week below). Since tadpoles of B. busuangensis had not been discovered until Miñarro et al. (2024), it was presumed it was a direct developer. Additionally, Inger (1954) noted that eggs in the ovaries of gravid females were large, unpigmented, and few in number. This egg morphology and coloration (or lack thereof) lead Brown and Alcala (1983) to speculate further that this species may have a specialized mode of reproduction, such as direct development. To investigate this possibility, Alcala and Brown (1987) attempted to induce breeding in the laboratory.

Larva
The tadpoles of B. busuangensis are unique being rheophilous (occurring in fast flowing rivers), endotrophic (obtaining nutrients from remaining yolk), and nidicolous (being reared in a nest). Tadpoles are translucent-white with scattered melanophores in coloration and have a large sucker-like oral disc without keratinized mouthparts that may help them adhere to rock surfaces. All these characteristics account for a new reproductive mode in anurans (Miñarro et al. (2024).

Trends and Threats
The secretive nature, low abundance, and strict habitat requirements of B. busuangensis mean that very little is known about the ecology and natural history of this species. In fact, the difficulty associated with finding this species on Busuanga Island led Taylor and Noble to remark in their original description that the species may have already been nearing extinction almost eighty years ago (Taylor and Noble 1924).

Today, the known range of B. busuangensis has been extended to include the larger island of Palawan, greatly increasing the area of suitable habitat for this species compared to what was thought by Taylor and Noble. However, the strict habitat requirements of the frog mean that its future is still in peril due to the increasing destruction of lowland forest on Palawan associated with logging and agriculture (Infante et al. 2002). Populations of B. busuangensis on Busuanga Island may be in particular danger because of the low availability of suitable habitat and the increasing pressure from migrant settlers in the form of pollution and habitat modification (Banks et al. 1999).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intensified agriculture or grazing
Subtle changes to necessary specialized habitat
Habitat fragmentation
Local pesticides, fertilizers, and pollutants

Comments
This species was featured in News of the Week 23 December 2024:

Miñarro et al. (2024) had the privilege of discovering the reproductive mode of a poorly known river-dwelling frog, Barbourula busuangensis, from Palawan, Philippines. Knowledge of life history is scarce for this ancient family Bombinatoridae and key to understanding evolution of such traits. Long hours of field work and dissections of gravid females from scientific collections revealed that this species had large, unpigmented eggs and were relatively few in proportion to their large body size. Thus, pointing to a K-strategist, a species that produces few, large offspring and provides some type of parental care, versus an R-strategists, which produce many offspring but offer no care. Due to its secretive behavior, hiding under rocks of fast-flowing streams, the reproductive behavior of B. busuangensis remained elusive to other researchers who only hypothesized that they laid their eggs under rocks and the embryos might follow direct development (see Inger 1954, Brown and Alcalá 1983). But lo and behold, one night —more than 100 years after its description— Marta Miñarro (of unmatched perseverance) introduced an endoscope camera into a rock wall crevice in the middle of the river and found a nest of B. busuangensis! An adult was present presumably guarding a nest composed of new metamorphs and advanced tadpoles (about 20 in total), revealing that this species does not bypass the larval stage as predicted. Because the sex of adults is indistinguishable in the field, the authors could not determine if parental care is provided by the male or the female. The unique tadpoles of B. busuangensis are rheophilous (occurring in fast flowing rivers), endotrophic (obtaining nutrients from remaining yolk), and nidicolous (being reared in a nest). Tadpoles are translucent-white with scattered melanophores in coloration and have a large sucker-like oral disc without keratinized mouthparts that may help them adhere to rock surfaces. All these characteristics account for a new reproductive mode in anurans. While they have partially solved a century-old mystery, additional field research is required to fully understand the reproduction of this secretive species. (Written by Patricia A. Burrowes)

References

Alcala, A. C., and Brown, W. C. (1987). ''Notes on the microhabitats of the Philippine discoglossid frog Barbourula busuangensis.'' Silliman Journal, 34, 12-17.

Banks, C. B. (1999). ''Philippine amphibians assessment.'' Froglog, 33, 1.

Brown, W. C., and Alcala, A. C. (1970). ''The zoogeography of the herpetofauna of the Philippine islands, a fringing archipelago.'' Proceedings of the California Academy of Sciences, 38, 105-130.

Brown, W. C., and Alcala, A. C. (1983). ''Modes of reproduction of Philippine amphibians.'' Advances in Herpetology and Evolutionary Biology. A. G. J. Rhodin and K. M. Miyata, eds., Museum of Comparative Zoology, Cambridge, MA.

Ford, L.S., and Cannatella, D.C. (1993). "The major clades of frogs." Herpetological Monographs, 7, 94-117. [link]

Inger, R.F. (1954). ''Systematics and zoogeography of Philippine Amphibia.'' Fieldiana: Zool., 33, 184-531.

Iskandar, D. T. (1978). ''A new species of Barbourula: first record of a discoglossid anuran in Borneo.'' Copeia, 1978, 564-566.

Miñarro, M., Burrowes, P. A., Lansac, C., Afuang, L., & De la Riva, I. (2024). "Mystery solved: unravelling the life history of the enigmatic and ancient Philippine frog Barbourula busuangensis (Anura: Bombinatoridae)." Salamandra, 60(4). [link]

Myers, G. S. (1943). ''Rediscovery of the Philippine discoglossid frog, Barbourula busuangensis.'' Copeia, 1943, 148-150.

Taylor, E. H., and Noble, G. K. (1924). ''A new genus of discoglossid frogs from the Philippine Islands.'' American Museum Novitates, 121, 1-4.

Species Account Citation: AmphibiaWeb 2024 Barbourula busuangensis: Philippine Flat-headed Frog <https://amphibiaweb.org/species/2038> University of California, Berkeley, CA, USA. Accessed Jan 25, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 25 Jan 2025.

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