Description
Pristimantis omarrhynchus is an Ecuadorian frog with a snout-vent length range from 23.5 - 27.3 mm in females and 12.1 - 20.0 mm in males. The head is slightly wider than long. The snout is pointed to rounded when viewed from above, and rounded when viewed from the side, with a papilla on the tip of the snout. The nostrils are directed laterally. The canthus rostralis is straight and the loreal region is slightly concave. The upper eyelids each have a single conical tubercle surrounded by smaller rounded tubercles. There is also a small, subconical tubercle that is wider than the eyelid in the flat interorbital region but there is no cranial crest. The orbits are more than one eye diameter apart. The tympanum is 43 - 62% of the diameter of the eye. The tympanum and tympanic ring are visible, and a fold extends over the anterodorsal third of the tympanic ring. The supertympanic fold also has small cone-like postrictal tubercles. It has a mostly smooth dorsal side, but the flanks and pelvis are finely granular with a few round tubercles and two rounded-cone-like tubercles can be found on the scapula. The ventrum is areolate. It also has a discoidal fold that extends from the mid-flank to the ventral area, and continuous dorsolateral folds that extend from behind the eyelid to the middle of the back near the pelvic girdle. The cloaca is also surrounded by flattened warts. The arms are slender and ulnar tubercles are present. The palm has three small rounded tubercles along with an oval thenar tubercle. Finger I is shorter than finger II, and fingers II, III, and IV have slightly expanded finger pads with circummarginal grooves. The fingers lack lateral ridges, but have supernumerary tubercles and subarticular tubercles; fingers III and IV each have two distal subarticular tubercles. Males have small, slanted, nonkeratinized nuptial excrescences. The hind limbs are also slender with the tibia being slightly more than half the snout-vent length. There is a large tubercle on the heel as well as small, subconical tubercles on the distal edges of the knees and tarsus. There is no tarsal fold. The oval internal metatarsal tubercle is three to four times larger than the cone-like external metatarsal tubercles, and there are few low supernumerary tubercles. The unwebbed toes are narrow and either lack or have weak lateral ridges. Toe digit V, which reaches the subarticular tubercle of toe IV, is longer than toe III. The prominent subarticular tubercles are round. All toes also have slightly expanded toe pads, which are larger than those on the fingers. The toes have hyperdistal tubercles (Bejarana-Muñoz et al. 2022).

DIAGNOSIS:

Pristimantis omarrhynchus can be distinguished from most other nearby Pristimantis by its defined dorsolateral folds, “)(“-shaped dark brown back markings, rounded scapular tubercles, single conical eyelid tubercle, heel and tarsus tubercles, bump on the tip of the snout, slightly expanded toe tips, and small body size. More specifically, most other members of the P. boulengeri group lack dorsolateral folds except P. baiotis and P. quantus. Dorsal markings in P. boulengeri, P. brevifrons, P. cryptopictus, P. quantus and P. urani are irregular, stripped in P. angustilineatus and P. dorsopictus, and chevron-shaped spots in P. quantus. No other P. boulengeri group members have scapular tubercles. Pristimantis angustilineatus, P. myops and P. urani do not have tubercles on their eyelids. Additionally, P. angustilineatus, P. brevifrons and P. urani do not have tubercles on their heels. Pristimantis boulengeri has low, but not conical-shaped tubercles in both locations. Lastly, P. omarrhynchus can be distinguished from its sister species, P. miltongallardoi, by its lack of ventral iridophores and its pointed snout in the dorsal view of the latter (Bejarana-Muñoz et al. 2022).

The 3 - 4 click call series of P. omarrhynchus is similar to P. boulengeri, P. dorsopictus, and P. cryptopictus with the difference being the number of repeated clicks in series. Specifically, P. boulengeri has 1 - 9 repetitions, and P. dorsopictus, and P. cryptopictus have discrete click calls rather than a series of click calls (Bejarana-Muñoz et al. 2022).

COLORATION:

In life, the dorsal coloration ranges from light greenish-brown to dark reddish-brown to light yellowish-orange. Most specimens have a dark brown “)(“-shaped dorsal mark. There are also cream-colored dorsolateral folds. The limbs, and flanks are covered with alternating brown and cream-colored bars. The stripes over the tympanum and on the canthus are dark brown to black. The anterior region of the thighs and shins are brown; the posterior region of the thighs have dark brown and yellowish bars. The hands and feet have a background color of reddish brown with dark brown to black spots while the digits are yellowish cream. The ventral side and throat are brown to yellowish-cream with faint black speckling. The groin and ventral axillae are yellowish to reddish cream. The irises are silvery with greenish-gold reticles and a coppery-brown midline. Nuptial pads are white (Bejarana-Muñoz et al. 2022).

In preservative, the dorsal side is dark brown, light brown, or cream, with light brown dorsolateral folds; the “)(“-shaped dorsal mark is retained. There are also irregular spots on the dorsum and the cloaca is dark brown. The tubercles on the body have white tips. The supratypmanic and canthal stripes are dark brown to black. Most individuals exhibit dark brown flanks with cream barring. The forelimbs have dark brown to black marks and the fingers are light brown with black marks on fingers III and IV. The dorsal hind limbs have alternating light and dark brown barring. The anterior region of the legs are light brown, and the digits are cream-colored. The ventrum, including the hidden surfaces of the limbs is light brown to cream. The palmar and plantar surfaces are light brown (Bejarana-Muñoz et al. 2022).

VARIATION:

Pristimantis omarrhynchus exhibits sexual size dimorphism, with females on average being 1.5x larger than males, males possessing vocal sacs and nuptial pads, and coloration and patterning may differ between sexes in preservative. More specifically, in females, the dark brown back pattern may look like an “X”, a “^”, or a “)(“. Males may be dark reddish brown, light brown, or greyish cream with yellowish or whitish forearms and thighs. Throat color ranges from pure cream to brown. One individual uniquely has its dorsolateral stripe going fully onto the head and lacks other dorsal patterns (Bejarana-Muñoz et al. 2022).

There is also some individual variation in life in color, patterning, and tubercle morphology. Specifically, some individuals may have a dark brown middorsal bar, a cream-colored patch on the head, or cream-orange dorsolateral folds. The eyelid tubercles, heel tubercles, and snout bump on vocalizing males are more developed than non-vocalizing males. Furthermore, the prominence of any individual’s tubercles and snout bump may vary widely over the course of the day (Bejarana-Muñoz et al. 2022).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Ecuador

View distribution map in BerkeleyMapper.

Pristimantis omarrhynchus has been found in two sites in the upper basin of the Napo River (near the Quijos and Coca tributaries) in the Ecuadorian provinces of Napo and Sucumbíos on the slopes of the Reventador Volcano. The known sites are in the Cayambe-Coca Ecological Reserve between 1700 and 1900 meters above sea level. The species could be found at the limits of the subtropical and temperate zones, which consisted of mature and secondary forests and evergreen forests in the slopes and foothills of the volcano Reventador, respectively (Bejarana-Muñoz et al. 2022).

Life History, Abundance, Activity, and Special Behaviors
Specimens of P. omarrhynchus can be found up to 1.5 m from the forest floor in mature forests. Some specimens were also found in the forest edges, clearings, and disturbed areas. The type series was collected both before and after volcanic activity of ash fall from the Reventador volcano (Bejarana-Muñoz et al. 2022).

Based on gravid females being found in both December and April, and males calling in September, P. omarrhynchus appears to exhibit continous breeding (Bejarana-Muñoz et al. 2022).

Males produced a clicking call with five to six repetitions and two harmonics in series of 3 - 4 calls. The first has a dominant frequency of 4040 Hz (standard deviation 60 Hz) and the secondary harmonic is around 7970 Hz (standard deviation 110 Hz). The single note calls had notes that lasted an average of 66.2 ms (standard deviation 12.0ms), followed by an average intercall interval of 9.2 ms (standard deviation 2.9ms). The intervals between call series were irregular and lasted between 7.2 - 71.5 seconds (Bejarana-Muñoz et al. 2022).

Males may call antiphonally, or alternating between males (Bejarana-Muñoz et al. 2022).

Larva
As a member of the Strabomantidae, the species is assumed to be direct-developing (Jaime and Angiolani-Larrea 2023).

Comments
Based on Maximum Likelihood analysis of 12S, 16S, and COI mtDNA and RAG-1 nDNA sequences, P. omarrhynchus is part of the Pristimantis boulangeri group, and is part of a clade containing P. brevifrons, P. angustilineatus, P. urani, P. boulengeri, P. cryptopictus, P. dorsopictus, and P. miltongallardoi. Its sister taxon is P. miltongallardoi, but it has the lowest uncorrected genetic p-distance to P. dorsopictus (Bejarana-Muñoz et al. 2022).

OTHER INTERESTING INFORMATION:

An albino individual may have been observed, but the researchers were unable to determine whether the individual was P. omarrhynchus or its sister species P. miltongallardoi (Jaime and Angiolani-Larrea 2023).

References
Bejarano-Muñoz, P., Ron, S. R., Navarrete, M. J., and Yánez-Muñoz, M. H. (2022). Dos nuevas especies del grupo Pristimantis boulengeri (Anura: Strabomantidae) de la cuenca alta del río Napo, Ecuador. Cuadernos de Herpetología, 36(2), 125-154. [link]

Jaime, C., and Angiolani-Larrea, F. N. (2023). First record of complete albinism in a species of the genus Pristimantis Jiménez de la Espada, 1870 (Anura: Strabomantidae) in the Ecuadorian Andes. Revista Latinoamericana de Herpetología, 6(1), 32-35. [link]

Species Account Citation: AmphibiaWeb 2025 Pristimantis omarrhynchus: English: Omar´s Rain Frog. Spanish: Cutín de Omar Torres. <https://amphibiaweb.org/species/9571> University of California, Berkeley, CA, USA. Accessed May 11, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 11 May 2025.

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