Description
Ptychadena robeensis is a small, robust frog, with a maximum snout-vent length of 31.8 mm in males and 34.5 mm in females. The head is about as long as wide. The snout slightly rounded, projecting beyond the lower jaw. The interorbital distance is equal to the eye diameter and to the internarial distance and the nostrils are half-way between the tip of the snout and the eyes. The tympanum is 0.54 - 0.66 times the eye diameter in females and 0.57 - 0.79 times the eye diameter in males. Individuals typically have four continuous dorsal ridges and one cream lateral ridge on each side, either continuous or interrupted. The hand is free of webbing with the following finger formula: 1< 2 < 4 < 3. The finger tips are not expanded into discs, but rounded. The hind limbs are short, with the tibia shorter than half the snout-vent length in both males and females. The lengths of the tibia and thigh are equal, and the foot is of equal or slightly longer length than the tibia. The toe tips are rounded, and the inner metatarsal tubercle is present, while the outer metatarsal tubercle is absent. The toe formula is: 1 < 2 < 5 < 3 < 4, and the foot webbing formula (toe internal/external sides, number of phalanges webbed) is: 1e(1), 2i/e(1–1), 3i/e(1–1), 4i/e(1–1), 5i(1) (Goutte et al. 2021).

DIAGNOSIS

Ptychadena robeensis is distinguishable from all Ethiopian Ptychadena, except for P. nana, by a considerably smaller size and shorter limbs. Compared to P. nana, it has relatively longer hands, greater eye-tympanum distance, and longer feet. Ptychadena robeensis also presents a more or less distinct cream or golden blotch on the tympanum, which is absent in P. nana. Finally, the bodies of adult male P. robeensis are not covered in warts as in P. nana (Goutte et al. 2021).

The call of P. robeensis can be distinguished from the calls of all other members of the P. neumanni species complex, except for P. delphina , by its regularly spaced pulses and long inter-pulses intervals (57 ± 5 ms). The call of P. robeensis is distinguishable from the call of P. delphina by a higher frequency (Goutte et al. 2021).

COLORATION

As the other members of the P. erlangeri species group, P. robeensis shows color polymorphism. In life, dorsum coloration varies from grey-brown to reddish or yellowish brown, or bright lime green. Dark blotches are disposed more or less symmetrically on the dorsum, vary in size and number, and are either dark brown or black. A vertebral stripe from the tip of the snout to the vent may be present. The vertebral stripe, when present, may be thin or wide and cream, sand, yellow or bright lime green. Wide stripes may be doubled with a thin, lighter line. A few individuals completely lack melanization on the dorsum, resulting in an almost uniform light brown or bright lime green coloration. The limbs of these individuals, however, have melanization patterns comparable to other individuals (Goutte et al. 2021).

The iris is bicolored, the upper third cream, silver or golden and lower two thirds brown to dark brown. Upper and lower jaws are cream, golden or light green, often with irregular light grey or light brown markings but no barring. Most individuals feature a dark brown canthal stripe from the tip of the snout to the back of the upper lip, sometimes dusted with small green spots. Individuals lacking dorsal melanization have a golden canthal stripe covered with small dark grey spots. Interestingly, the nostrils of these individuals are outlined in black. The tympanum is dark brown with a more or less defined cream to golden vertical blotch. Throat white or very pale yellow in females and pale to deep yellow in adult males. The vocal sacs are most often dark grey, but can also be bicolored dark grey anteriorly and light grey posteriorly in some individuals, and, more rarely light grey. One or a few dark blotches are present on the anterior part of the flank of some individuals. A thin cream or light green longitudinal stripe on the tibia extending to half or the whole length of the thigh is present in some individuals. Brown barring on the tibias, thighs and feet may be very distinct or almost completely absent. Thighs posteriorly marbled with dark brown and light brown, yellow or light green. Ventrum uniformly white (Goutte et al. 2021).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Ethiopia

View distribution map in BerkeleyMapper.

Ptychadena robeensis occupies a very small range around the town of Robe in Oromia, Ethiopia. It is found at elevations ranging from 2,375 m to 3,120 m a.s.l. The westernmost individuals were found near Dinsho, which is also the highest elevation point of the species range. In the south, P. robeensis is found between Robe and Goba, while it is known up to just west of Ali in the north. The species presumably occurs further north on the plateau as elevation and habitat seem rather homogeneous up to the Shebelle River (Goutte et al. 2021).

Life History, Abundance, Activity, and Special Behaviors
Individuals are found active at night in grassy meadows and cultivated fields near small water bodies. During the day and in dry periods, adults and juveniles can be found under rocks (S. Goutte personal comments).

Males are found calling at night from shallow grassy puddles or flooded cultivated fields. Calling activity typically starts after 22:00, even though males may be at calling sites earlier. Males have been found calling in syntopy with P. cooperi and P. levenorum. Genetic analyses have shown that P. levenorum and P. robeensis hybridize where their ranges overlap (Freilich et al., 2014; Goutte et al. 2021).

The call of P. robeensis (5 males, 101 calls) is composed of a single pulsed note of 534 ± 53 ms in duration, containing 9.8 ± 1 regularly-spaced pulses. Pulse amplitude increases gradually until 292 ± 19 ms, after what it decreases. Call peak frequency is 2876 ± 74 Hz with a slight increase in frequency within the call (Goutte et al. 2021).

Possible reasons for amphibian decline

General habitat alteration and loss
Intensified agriculture or grazing
Urbanization

Comments

PHYLOGENETIC RELATIONSHIPS

References
Freilich, X., Tollis, M., and Boissinot, S. (2014). Hiding in the highlands: Evolution of a frog species complex of the genus Ptychadena in the Ethiopian highlands. Molecular Phylogenetics and Evolution, 71, 157–169. [link]

Goutte, S., Reyes-Velasco, J., Freilich, X., Kassie, A., and Boissinot, S. (2021). Taxonomic revision of grass frogs (Ptychadenidae, Ptychadena) endemic to the Ethiopian highlands. ZooKeys, 1016, 77–141. [link]

Reyes-Velasco, J., Goutte, S., Freilich, X., and Boissinot, S. (2021). Mitogenomics of historical type specimens clarifies the taxonomy of Ethiopian Ptychadena Boulenger, 1917 (Anura, Ptychadenidae). ZooKeys, 1070, 135–149. [link]

Species Account Citation: AmphibiaWeb 2024 Ptychadena robeensis: Robe's grass frog <https://amphibiaweb.org/species/9322> University of California, Berkeley, CA, USA. Accessed Nov 21, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 21 Nov 2024.

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