Scinax boulengeri is moderately sized. Adult males range from 36 to 49 mm, while adult females range from 42 to 53 mm snout-vent length (Savage 2002). Scinax boulengeri has a pointed, acuminate snout, tuberculate dorsum, white vocal sac, long webless fingers with truncated discs, and thighs marked by a contrasting pattern of black and yellow or green vertical bars. A dark spot is commonly found on the groin (Duellman 2001). It has a distinctive shape due to a strongly flattened body and protruding nostrils (Guyer 2005).
Dorsally, the skin is tuberculate. The ventral surfaces of the limbs have smooth skin, but the belly and the posteroventral surfaces of the thighs have granular skin. Individuals from Costa Rica (the Golfo Dulce region) are more tuberculate than those from the Caribbean lowlands of Central America (Duellman 2001). The top of the head is slightly convex and the lips are flared. The long snout is acuminate in both lateral and dorsal profiles, with the tip extending beyond the lower jaw. The nostrils point dorsolaterally and are positioned just above the leading edge of the lower jaw. A thin dermal fold runs from the posterior edge of the eye, above the tympanum, to just above the insertion of the arm. This fold covers the upper edge of the tympanum. The tongue is narrowly heart-shaped. Vocal slits extend to the angle of the jaw. There is a single, median vocal sac which is moderately distendable (Duellman 2001).
The arm lacks an axillary membrane and is moderately long and slender. The ventrolateral edge of the forearm has no tubercles and the wrist has no transverse fold. The long and thin fingers are unwebbed and have large truncate discs, with the width of the third finger's disc about equal to the tympanum. Narrow dermal fringes are present on the edges of the digits. Each digit also has small, conical supernumerary tubercles on its proximal segment. Large, round subarticular tubercles are present. There is a single flat, tripartite palmar tubercle, and an elongated, flat prepollical tubercle. Duellman (2001) states that males do not possess nuptial excrescences on the prepollex, which is only slightly enlarged, but Savage (2002) directly contradicts this and says that adult males have whitish nuptial pads. Hindlimbs are also moderately long and slender. The tibiotarsal articulation extends to a point between the eye and the nostril. There is a weak tarsal fold, and a thin transverse dermal fold on the heel, as well as one or two tubercles. The inner metatarsal tubercle is low, flat, and rounded. The outer metatarsal tubercle is small and conical when present. Toes are long and slender, with truncate discs nearly as large as those on the fingers. The toes are about three-fourths webbed, with the webbing on the first and second toes present as a fringe on the medial side of the first toe and the lateral edge of the second toe. The subarticular tubercles are large and rounded. Small, rounded supernumerary tubercles are present in a single row on the proximal segment of each digit.
Scinax boulengeri is grayish tan, brown, or dull green with darker brown markings, which usually consist of a dark triangular mark between the eyes and two or more dark blotches on the back. Forearms, thighs, shanks, and feet have transverse bars. The iris is bronze. The throat is white, spotted with small gray flecks. The venter is creamy white, while the groin is pale green with one to several black spots or mottling. Flanks are pale yellowish green. The anterior and posterior surfaces of the thighs have long, vertical black bars, separated by pale green, yellowish green, or yellow-orange interspaces. (Duellman 2001).
A typical tadpole in developmental stage 38 has a body length of 11.5 mm and total length of 33.5 mm. The larval body is slightly deeper than wide. The snout is rounded, with small, anterolateral nostrils. The eyes are moderately large and directed laterally. The spiracle is sinistral, with the opening directed posteriorly. The vent (cloacal tube) is short and dextral. The caudal musculature is somewhat shallow and tapers gradually. The caudal fins are deep, with the dorsal fin extending onto the body nearly to the posterior edge of the eye. The tail tip ends in a sharp point (Duellman 2001). The larval mouth is small and anteroventral, with massive, heavily serrated beaks. The lips have a single lateral row of blunt papillae and also have lateral folds with small conical papillae. There are two upper rows of denticles, which are of equal length and extend to the lip, and three shorter lower rows of teeth. The second upper row is narrowly interrupted medially (Duellman 2001). The lower lip has a catapult-like extension with long, pointed denticles (Savage 2002).
In the wild, the larval body is silvery yellow, the fins are transparent, and the caudal musculature is pale cream. The tail has large, closely spaced black spots on the middle two-thirds of its length. In preserved tadpoles, these spots are the only pigment visible (Duellman 2001).
Distribution and Habitat
Country distribution from AmphibiaWeb's database: Colombia, Costa Rica, Honduras, Nicaragua, Panama, Venezuela
Caribbean slopes of Nicaragua, Costa Rica, and Panama; Pacific slopes from Costa Rica to Ecuador. Scinax boulengeri lives in humid, lowland tropical forests with rainfall evenly distributed throughout the year (Duellman 2001).
Life History, Abundance, Activity, and Special Behaviors
Scinax boulengeri is nocturnal and arboreal. It can be found near the forest floor perching on bushes, stumps, logs, and low trees, as well as in secondary growth and isolated trees and shrubs in pastures and open areas (Scott 1983).
This species is a prolonged breeder, with peaks occurring in May/June and again in August. In humid areas males call throughout the year but in drier areas calling occurs from May to November (Savage 2002). Bevier (1997) states that calling was not correlated to the amount of rainfall. However, this statement differs from other accounts, some of which assert that calling males are more numerous on rainy nights (Guyer 2005).
Scinax boulengeri males begin congregating at pond breeding sites soon after the first heavy rains of the season (Bevier 1997). The males generally begin their 3 to 4 hour calls at dusk in areas bordering bodies of water, rarely moving from their initial areas during the same night. Males call from secluded spots on stumps or logs while hidden under vegetation, as well as from dense bushes, depressions in logs, and stumps at pondside or in the water (Duellman 2001). Calling rates reduce greatly after the first hour, as do the physical aggressions from establishing territory. Calls consist of a single low and guttural note, emitted over intervals lasting between ten seconds to several minutes, usually with the head down and cocked away from the substrate (Savage 2002; Guyer 2005). Call notes have a fundamental frequency of around 71 cycles/second and two harmonics of 1600 and 2800 cycles/second, with a duration of 0.24 to 0.27 seconds and a pulse rate of 80-120 pulses/second (Duellman 2001).
Amplexus occurs out of water, and the eggs are laid in shallow water (Savage 2002). The female lays 600-700 eggs in water, each ranging from 1.5 to 1.6 mm. in diameter (Jungfer 1987). Eggs hatch in 1 to 1.5 days, while tadpoles take 40 to 88 days to reach metamorphosis (Jungfer 1987).
Tadpoles feed on small crustaceans and other aquatic insect larvae, as well as developing conspecific and heterospecific tadpoles (Savage 2002). The catapult-like, toothed extension of the larval lower lip aids in flipping prey into the mouth (Savage 2002). The heaviest predation on Scinax boulengeri tadpoles is by other tadpoles, most likely including cannibalistic predation by this species (Roberts 1994).
The adult diet consists of arthropods (Guyer 2005).
A Spanish-language species account can be found at the website of Instituto Nacional de Biodiversidad (INBio).
Bevier, C. R. (1997). ''Breeding activity and chorus tenure of two neotropical hylid frogs.'' Herpetologica, 53, 297-311.
Duellman, W.E. (1970). The Hylid Frogs of Middle America. Monograph of the Museum of Natural History, University of Kansas.
Guyer, C., and Donnelly, M. A. (2005). Amphibians and Reptiles of La Selva, Costa Rica and the Caribbean Slope: A Comprehensive Guide. University of California Press, Berkeley.
Jungfer, K.-H. (1987). ''Beobachtungen an Ololygon boulengeri (Cope, 1887) und anderen 'Knickzehenlaubfröschen'.'' Herpetofauna, 9, 6-12.
Roberts, W.E. (1994). ''Evolution and ecology of arboreal egg-laying frogs.'' Ph.D. dissertation, University of California, Berkeley
Savage, J. M. (2002). The Amphibians and Reptiles of Costa Rica:a herpetofauna between two continents, between two seas. University of Chicago Press, Chicago, Illinois, USA and London.
Scott, N. J. (1983). ''Hyla boulengeri (Ranita de Boulenger, Boulenger's Hyla).'' Costa Rican Natural History. D. H. Janzen, eds., University of Chicago Press, Chicago.
Originally submitted by: Stephen Chu (first posted 2008-03-19)
Edited by: Kellie Whittaker (2009-11-02)
Species Account Citation: AmphibiaWeb 2009 Scinax boulengeri: Boulenger's Treefrog <https://amphibiaweb.org/species/1094> University of California, Berkeley, CA, USA. Accessed Sep 24, 2021.
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Citation: AmphibiaWeb. 2021. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 24 Sep 2021.
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