Description
Boana nigra is colloquially known as the black-flanked tree frog due to the diagnostic black coloration along the length of its flanks and hind limbs. The short and slender bodies have a snout-vent length range in adult males of 38.5 – 46.3 mm and 56.4 – 76.7 mm in adult females. Boana nigra has a short, but wide head that is wider than the body. In the lateral view, the snout slopes, and in the dorsal view, the snout is subacuminate to rounded. The nostrils are directed laterally and do not protrude. The area between the nostrils is slightly depressed. The canthus rostalis is rounded and the loreal region is concave. The distance from the eye to the nostril is shorter than the diameter of the large, protruding, circular eyes. The area between the eyes is slightly convex. The eyes also have a diameter that can be up to 1.2 times the size of the tympanic annulus. The distinct, round tympanic annulus is separated from the eye by about half the diameter of the tympanum. The tympanum membrane is large, round, and is slightly longer than high. There is an inconspicuous supratympanic fold. The skin on the dorsal side is finely granular. The cloaca is at the level of the upper thighs and is directed posteriorly. Round tubercles can be found to the sides and posterior of the cloaca. Males have a single, subgular vocal sac. There is no mental gland. The anterior portion of the throat is smooth but the skin at the posterior portion of the throat, belly and ventral surfaces of the thighs is areolate. The ventral surface of the shanks is smooth. The arm is slender and does not have an axillary membrane. There is a prepollex, but it does not project nor is it modified into a spine. There is also an inconspicuous palmer tubercle. The relative finger lengths are I < II < IV < III and end in broadly expanded, round discs that are as wide as they are long. The fingers have a webbing formula of I basal II (1⁺ – 2^–) – (2 ½ – 3^–) III (2^– – 2 ½) – (2 – 2⁺) IV. The prominent subarticular tubercles are single and round to oval. There are no supernumerary tubercles. Breeding males have large, P-shaped, rough nuptial pad on the dorsal surface of the first finger. They have long, thin hind limbs with the tibia and femur being about equal in length, and a triangular calcar on the heel. The inner metatarsal tubercle is elliptical and there is no outer metatarsal tubercle. The relative toe lengths are I < II < V < III < IV and the webbing formula is I (1 – 2^–) – (1 – 2^–) II (1 – 1⁺) – (1 – 2⁺) III (1 – 1 ½) – (1⁺ – 2⁺) IV (2^– – 2⁺) – (1 – 1⁺). The toes end in broadly expanded, round discs that are smaller than the finger discs. The large subarticular tubercles are single, round, and protuberant. The supernumerary tubercles are small and round (Caminer and Ron 2020).

DIAGNOSIS:

Unlike most species in the Boana genus, B. nigra have uniform black flanks or black vertical flank bars, the latter of which are typically found in females. This differs from other species in the genus, such as B. ventrimaculata, which instead have speckles or blotches on their flank. Juvenile B. nigra and B. appendiculata can be confused due to the similar coloration, however, they differ in habitat preference with B. nigra preferring cooler environments. Adults are easier to differentiate; B. appendiculata has bright yellow or orange ventral sides, including on their limbs. Boana nigra has paler ventrums, but can have black coloration on the ventral surface of their limbs. Boana nigra’s relative body size and tympanum size are smaller than B. ventrimaculata and B. appendiculata. Unlike B. boans, B. icamiaba, B. pombali, and B. wavrini, males of B. nigra do not have a modified prepollex or a pigmented nuptial pad during breeding (Caminer and Ron 2020)

COLORATION:

Adult B. nigra are highly variable in terms of coloration and pattern, however, the unifying feature of adult B. nigra is the dark coloration, either uniformly or dark vertical bars, along their flanks and inner thighs. Adult dorsal background coloration can be brown, reddish-brown, dark brown, or black. Patterning on the dorsum is brown and can be arranged as semi-interconnected, round marks or an X-shaped mark on the scapular region. Some individuals may also have scattered flecks of black or white. There are brown transverse bars on the dorsal surfaces of the limbs that can vary in color intensity. The dorsal surface of the upper arms, hands, and feet is also black. The triangular calcar on the heel is cream. The ventrums of adults are a lighter background color than that of the dorsum and range from cream to pale orange. The throat can be cream white or pale orange. There can be small, dark dots that can be tightly spaced, uniformly distributed, or concentrated on the anterior or posterior half of the venter. The dots can range in color from brown to bright orange. The digits and webbing color ranges from pale orange to orange to black. The palpebrum has golden yellow reticulation, and the color of their iris ranges from dark brown to brown with orange undertones to bronze (Caminer and Ron 2020).

In life, recently metamorphosed individuals have dorsums with irregular, black spots and blotches. The side of their heads, flanks, dorsal surface of the thighs, hidden surfaces of the thighs, and webbing is black. The ventrum is grey and can have scattered brown spots (Caminer and Ron 2020).

In preservative, the dorsal coloration ranges from brown, dark brown, cream, to grey. The larger patterning remains dark brown and the minute flecks retain their color. The transverse bands on the limbs are dark brown or grey. The characteristic black coloration on the flanks, hidden surfaces of the thighs, and shanks also retain their color in the uniform morph, but the barred variant fades to grey and dark bars. The heels can become cream colored. The ventrum has a creamy white to yellowish white background color with some individuals having dark brown marks on the belly. The webbing and discs become yellowish white, cream, black, or grey (Caminer and Ron 2020).

VARIATION:

Boana nigra is a sexually dimorphic species, with males being darker in overall color and smaller than females. Specifically, males often have noticeably uniform black flanks, whereas females have vertical dark bars at the flanks and it is more common in females for dorsal patterns to extend into the coloration of the flanks. However, adults in general exhibit varied patterning like dots, reticulations, banding, and faint striping on their dorsum. Individuals can also vary in the shape of their snout, which ranges from subacuminate to rounded in the dorsal view and truncated to short and rounded in the lateral view. In most specimens, the head is slightly wider than its length and than the body width. Calcar size can also range from inconspicuous to distinct. And lastly, webbing of the hands and feet (see above) can also vary (Caminer and Ron 2020).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Ecuador

View distribution map in BerkeleyMapper.

Boana nigra can be found in the provinces of Morona Santiago, Napo, Pastaza, Sucumbíos, and Zamora Chinchipe in Ecuador on the eastern slopes of the central Andes at elevations between 910 and 1847 m a.s.l. Specifically, they are found in tropical rainforests and montane forests of the central Andes, and in the eastern foothills or montane cloud forest, which are characterized by trees with mosses and abundant epiphytes (Caminer and Ron 2020).

Life History, Abundance, Activity, and Special Behaviors
Boana nigra is an arboreal species and can be found on different types of vegetation 30 to 400 cm above the ground near streams, pools, swamps, and lakes. However, the species can also be found in leaf litter (Caminer and Ron 2020).

The call from one male was recorded in an air temperature of 14.8 °C. That vocalization consists of a single groan-like note with a mean duration of 0.33 seconds per note and a dominant frequency range of 996 to 1063 kHz. Each note has 21 - 37 discrete pulses (Caminer and Ron 2020).

In at least one location in Pastaza Province, B. nigra is sympatric with B. appendiculata but the two species do not appear to hybridize there (Caminer and Ron 2020).

Larva
Boana nigra undergoes indirect development and has tadpoles (Caminer and Ron 2020).

Trends and Threats
Boana nigra is listed as “Vulnerable” on the IUCN Red List of Threatened Species because it has a relatively small Extent of Occurrence (16,911 km²) and is only known from 5 - 10 locations that are threatened by habitat loss or degradation from the expansion of farming (both agricultural and livestock), logging, and road development. However, the species is found in some protected areas, namely Parque Nacional Sangay, Reserva Zanjarajuno, Bosque Protector Abanico, and Parque Nacional Cayambe Coca (IUCN 2023).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intensified agriculture or grazing
Mining
Climate change, increased UVB or increased sensitivity to it, etc.

Comments
Maximum Likelihood and Bayesian analyses of 12S, 16S, ND1, COI, and CytB mtDNA indicate that B. nigra is sister to B. appendiculata. The next closest clade is composed of B. diabolica and B. geographica, the latter of which both B. nigra and B. appendiculata were once considered. The species in this study were also delimited based on morphology and call data (Caminer and Ron 2020).

The phylogeny and distribution of species in this genus suggest an origin in the Amazon basin. Because B. nigra is the only species that is exclusively found above 1000 m a.s.l., the species authority believes this is a recent dispersal from lowland tropical rainforest to Andean montane forests (Caminer and Ron 2020).

References
Caminer, M. A., and Ron, S. R. (2020). Systematics of the Boana semilineata species group (Anura: Hylidae), with a description of two new species from Amazonian Ecuador, Zoological Journal of the Linnean Society, 190(1), 149–180. [link]

IUCN SSC Amphibian Specialist Group. (2023). Boana nigra. The IUCN Red List of Threatened Species 2023: e.T184672405A186752109. https://dx.doi.org/10.2305/IUCN.UK.2023-1.RLTS.T184672405A186752109.en. Accessed on 07 March 2025.

Species Account Citation: AmphibiaWeb 2025 Boana nigra: Black-flanked Treefrog (English) <https://amphibiaweb.org/species/9258> University of California, Berkeley, CA, USA. Accessed Apr 29, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 29 Apr 2025.

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