Hyperolius nimbae, Laurent, 1958

Gongomin, Basseu Aude-Inès, Kouamé, N’Goran Germain, Agoh, Konan Laurent, Kanga, Kouassi Philippe & Rödel, Mark-Oliver, 2022, New biological data for two rare reedfrog species, Hyperolius nimbae Laurent, 1958, and H. chlorosteus (Boulenger, 1915) (Anura: Hyperoliidae), Amphibian & Reptile Conservation (e 318) 16 (2), pp. 88-103 : 91-93

Observations on H. nimbae

Habitat, population size, and activity of H. nimbae . The Mount Nimba Reedfrog was found only in the Yéalé village (07°31.928’N, 008°25.401’W; 425 m asl) at the periphery of MNINR. Our efforts to confirm the species around village forests at Dagbonpleu, Danipleu, Kouan-Houlé, and Zéalé were unsuccessful. The forest areas formerly at these sites were destroyed and replaced by roads and new settlements. At Yéalé, the habitat comprised altered forest bordered by patches of bamboo, intact forests, degraded forests with large clearings, and thick grassy and shrubby vegetation around the settlement. Large parts of the village’s surroundings were dominated by small-scale subsistence farming, mainly plantings of corn, cassava, plantains, cocoa, and coffee, as well as rice in swamps. During the entire study, the vegetation around swamps was checked at seven sites without detecting any sign of H. nimbae ’s presence .

Specimens of H. nimbae were found only in a mixed cocoa and coffee plantation (including plantains) edging a large and deep pond (greater than 100 x 70 m; Fig. 2 ), where males started calling at dusk, usually around 1810 h GMT. Calling males were active in each month of our study and congregated in large choruses, particularly during the rainy season. Calling activity never stopped, but was low during the dry season, as indicated by the much lower numbers of frogs caught during that time ( Fig. 3 ). The calling males were mostly well concealed, perching between thick branches and leaves of cocoa and coffee trees at ~ 1.80–2.10 m above the ground. Througout the study, a total of 305 frogs were captured ( Fig. 3 ; Appendix 1). They comprised 277 individuals, including one couple, caught in the rainy season and 28 males caught in the dry season. Generally, additional calling males were inaccessible, calling from high up in tall trees close to the deep pond.

Sympatric amphibian species in Yéalé were the White-lipped Frogs Amnirana sp. “albolabris west”, Afrixalus dorsalis , A. fulvovittatus , Hyperolius concolor , H. fusciventris fusciventris , H. picturatus , Phrynobatrachus gutturosus , and P. latifrons , thus representing a typical composition of a farmbush anuran community ( Schiøtz 1967).

Morphology of H. nimbae . We retained 13 males and one female H. nimbae as vouchers. Snout-urostyle lengths of the voucher males ranged from 28.8–35.0 mm (32.3 ± 1.8 mm), thus slightly exceeding the known range of H. nimbae ( Schiøtz 1967) . The head was slightly longer (12.0 ± 1.1 mm; range: 10.0– 13.5 mm) than broad (10.8 ± 0.8 mm; range: 9.9–12.5 mm); interorbital space ranged from 5.0– 7.5 mm (6.4 ± 0.7 mm); the distance between eye and nostril (2.1 ± 0.3 mm; range: 2.0–3.0 mm) approximated the distance from nostril to tip of snout (2.2 ± 0.3; range: 1.5–2.0 mm); eye diameter (4.6 ± 0.4 mm; range: 4.0– 5.1 mm) was larger than internarial space (3.2 ± 0.3 mm; range: 3.0–4.0 mm) and tympanum diameter (2.3 ± 0.3 mm; range: 2.0–3.0 mm); crus length (16.5 ± 1.0 mm; range: 15.1–18.9 mm) slightly exceeded thigh length (15.0 ± 0.8 mm; range: 13.5–16.1 mm); and foot length including the longest toe ranged from 21.0– 26.9 mm (23.1 ± 1.6 mm).

The female measured 34.0 mm SUL ( Fig. 4A ). Its head was slightly longer (14.0 mm) than broad (12.1 mm); the interorbital space reached 8.0 mm; the distance between eye and nostril (3.4 mm) was longer than the distance from nostril to tip of snout (1.8 mm); eye diameter (5.0 mm) was larger than internarial space (3.0 mm) and tympanum diameter (2.9 mm); crus length (18.0 mm) slightly exceeded thigh length (17.1 mm); and foot length including the tarsus and the longest toe reached 24.5 mm.

Mount Nimba reedfrogs have a brief and truncated snout and rough to warty dorsal skin with a varied color pattern. The basic dorsal pattern varied from silver-grey to chocolate brown. The dark pattern on the back varied in intensity, but the general pattern was always similar and recognizable ( Fig. 4 ). The female coloration was within the variation of the males ( Fig. 4 ), but the discs on toes and fingers were more reddish than in most males ( Fig. 4A ). Likewise, the ventral color differed slightly between individuals, but the female was not different from the males ( Fig. 5 ). However, instead of a dark vocal sac and yellowish gular gland, the female had a white throat, with the edges beset by orange and black speckles. The armpits and inner parts of limbs including webbing were blood red ( Fig. 5C ). Some males had similarly reddish skin parts ( Fig. 5A ), areas which are used for water uptake, see Rödel (2000), whereas others had blue-grey skin instead ( Fig. 5B ).

Reproduction of H. nimbae . At Yéalé, a couple of H. nimbae in amplexus was captured on a Raphia Palm in deeper water (> 1 m). The couple was placed into a terrarium, and the following morning, 227 eggs were found attached to a leaf above the water surface. The eggs had a dark and light green pole that was surrounded by a thin transparent jelly ( Fig. 5D ). Egg diameter varied from 1.6‒2.5 mm, with a mean value of 1.99 mm (SD: ± 0.18 mm).

Vocalization of H. nimbae . The acoustic properties of nine advertisement calls of one H. nimbae male were analyzed ( Fig. 6 ). Hyperolius nimbae call in a long, fast series of pulse groups (notes), thus confirming the call descriptions provided by Schiøtz (1967). The call duration averaged 2.77 ± 0.56 s (range: 1.89‒ 3.25 s). The duration intervals between calls averaged 0.44 ± 0.23 s (range: 0.22‒ 0.87 s, N = 8). Intervals between notes within each call were 0.15 ± 0.03 s (range: 0.09– 3.48 s, N = 131). The mean fundamental frequency was 1,359.37 Hz (N = 9), while the dominant frequency reached 2,718.75 Hz (N = 9). The maximum frequency intensity was at 2,906.25 Hz (N = 9), and the minimum frequency intensity averaged 2,718.75 Hz (N = 9).

Threats to H. nimbae . We failed to confirm the species presence at all sites where we had recorded it only a few years before (see above). In addition, we observed various signs of habitat degradation in the habitats of H. nimbae at Yéalé. Aside from the close proximity to a human settlement, the frogs’ habitats were dominated by cocoa and coffee plants, and used for small-scale subsistence farming, i.e., plantains and cassava. Large parts of the breeding area of H. nimbae were used to establish rice paddies. Bamboo patches were steadily harvested by the local human population as construction materials. The rainforest edges were impacted by logging ( Fig. 2 ).

Figures

Fig. 2. Habitats of Hyperolius nimbae at the foothills of the Ivorian part of the Nimba Mountains. In Yéalé, the species was found in heavily degraded habitats at the edge of large swamps.

Fig. 3. Number of Hyperolius nimbae individuals recorded from 2 August 2019 to 8 July 2020 at Yéalé.

Fig. 4. Dorsolateral views of the Hyperolius nimbae female (A) and males (B–H) from Yéalé, western Ivory Coast. Note the variation in coloration.

Fig. 5. Ventral views of Hyperolius nimbae males (A and B) and female (C) adults, and the eggs (D) from Yéalé, western Ivory Coast.

Fig. 6. (A) Hyperolius nimbae usually calls perched between branches and leaves of trees. (B) Waveform (above), spectogram (center), and powerspectrogram (below) of the advertisement calls of a H. nimbae male from Yéalé, western Ivory Coast.