Description
Synapturanus mesomorphus is a medium-sized Synapturanus frog with a male snout-vent length averaging 24.7 mm (ranging from 22.0 - 26.0 mm, n = 6) and a female snout-vent length averaging 27.9 mm (ranging from 26.3 - 29.4 mm, n = 11). The head is slightly wider than it is long, and in the lateral view, it is convex. The snout, which is rounded in the dorsal and lateral views, is long and protrudes far beyond the lower jaw by about 2.0 mm. The nostrils are placed laterally and located closer to the tip of the snout than to the eyes. The canthus rostralis is rounded. The loreal region is very concave and there is a groove between the naris and the eye. The eyes are smaller than the eye-nare distance. Posterodorsally, the concealed tympanum is blocked by the supratympanic fold, and it is only distinct anteroventrally. The supratympanic fold runs from the posterior corner of the eye toward the axilla, and is continuous with the occipital and gular folds. The forearm is robust. The thenar tubercle is barely distinct while the palmar tubercle is indistinct, and there are no subarticular tubercles. The relative finger length formula is III > IV > II > I. The fingertips are rounded. Fingers II and III have preaxial fringes that reach the base of the fingers. The hind limb is robust, and there is a small, ovoid inner metatarsal tubercle. The outer metatarsal tubercle is indistinct. The relative toe length formula is IV > III > V > II > I. The toe tips are expanded (Fouquet et al. 2021a).

DIAGNOSIS:

Prior to this species’ description, Synapturanus mirandaribeiroi, S. rabus, and S. salseri were the only three named species that made up the genus. Synapturanus mesomorphus is similar to S. salseri. Unlike S. salseri, S. mesomorphus has a convex head (flat in S. salseri) and a longer call (0.160 - 0.173 s in S. mesomorphus compared to 0.079 in S. salseri). Synapturanus mesomorphus can be differentiated from S. mirandaribeiroi because the former is smaller (snout-vent length 22.9 - 26.0 mm in males compared to 26.6 - 30.8 S. mirandaribeiroi males), has a brown dorsum with sparse speckles and blotches (compared to a diffuse mottled pattern in S. mirandaribeiroi), and has a call with tonal notes (pulsed in S. mirandaribeiroi). Compared to S. rabus, S. mesomorphus is larger (snout-vent length 22.9 - 29.4 mm compared to 16.2 - 19.0 mm in S. rabus), has smaller eyes (5.1% of snout-vent length compared to 7.3% in S. rabus), has preaxial fringes on Fingers II and III (compared to no fringes in S. rabus), has a convex head in the lateral view (flat in S. rabus), has a brown dorsum with sparse beige speckles and blotches in life (compared to a uniformly brown dorsum in S. rabus), and has a call with longer notes (0.160 - 0.173 s compared to 0.039 in S. rabus) (Fouquet et al. 2021a).

Synapturanus mesomorphus was described in the same paper as S. zombie, from which it can be distinguished by its smaller size (snout-vent length 22.9 - 26.0 mm in males compared to 37.0 - 40.6 mm in S. zombie males), having a light to dark brown dorsum with sparse beige speckles and blotches (compared to a medium brown dorsum with many orange spots and blotches in S. zombie), a stripe from the canthus rostralis and upper eyelid to midway between the eye and axilla (absent in S. zombie), and having a call that does not have downward frequency modulation (104 - 194 Hz decrease in S. zombie) (Fouquet et al. 2021a).

COLORATION: (In life and/or in preservative)

In life, the dorsum is dark to light brown and has small beige speckles and blotches, sparsely scattered throughout the back, head, arms, and legs in life. In preservative, the speckles and blotches are cream-colored, and all colors are generally faded. There is a somewhat continuous stripe that is often broken into small flecks or spots that extends from the snout along the canthus rostralis and upper eyelid to midway between the eye and axilla. The venter is pearl white and has melanophores. The throat is similarly-colored to the dorsum in both males and females (Fouquet et al. 2021a).

VARIATION: (if known)

Synapturanus mesomorphus have clear sexual dimorphism in size, as the snout-vent length of males and females do not overlap (males have an average snout-vent length of 24.7 mm with a range from 22.0 - 26.0 mm, while females have an average snout-vent length of 27.9 mm with a range from 26.3 - 29.4 mm). Males have a supracarpal pad. In gravid females, the ovaries are visible through the skin. Individuals show extensive variation in color, which ranges from dark to light brown and spots varying from white or beige to orange. The stripe from the upper eye and canthus rostralis may be discontinuous, or it may extend from just a few millimeters posteriorly past the eye to as long as to the axilla. This stripe can be narrow or broad (Fouquet et al. 2021a).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Guyana

View distribution map in BerkeleyMapper.

Synapturanus mesomorphus is known to occupy eight localities in Guyana, South America: Mabura Hill Forest Reserve, Iwokrama Forest Reserve, Kaieteur National Park, Bay Camp, Meamu River, Kuribrong, Konawaruk Camp, and Kartabo. These are located at the eastern edge of the Pantepui ecoregion, between Pantepui and the lowlands of the Eastern Guiana Shield. Much of these areas are affected by gold mining. The species can be found in habitats like clearings in white sand forests, white sands in well-drained mixed forests, or ferralic arenosols near riverine floodplain forests on alluvial soils. They occupy unflooded areas (Fouquet et al. 2021a).

Life History, Abundance, Activity, and Special Behaviors
Synapturanus frogs are nocturnal and fossorial. However one individual was found at dusk on the ground (Fouquet et al. 2021a).

The call of S. mesomorphus consists of a tonal note that is 0.160 - 0.173 s in length, having a dominant frequency of 1.06 - 1.13 kHz (n = 2). Calling is thought to be triggered by the sound of rain drops on the ground. Males call from burrows below leaf litter only when it is drizzling or there is heavy rainfall (Fouquet et al. 2021a).

In gravid females, the ovaries are visible through the skin. Eggs are thought to be laid in underground burrows, and tadpoles develop within the burrow (Fouquet et al. 2021a).

The conserved morphology of the posterior part of the body across the Synapturanus genus and the morphology of the head and humerus suggest that adaptations to a fossorial lifestyle mostly affected the anterior region (Fouquet et al. 2021a).

Synapturanus mesomorphus feeds on ants and termites (Fouquet et al. 2021a).

Larva
Synapturanus larvae are endotrophic, and larvae develop within underground burrows (Fouquet et al. 2021a).

Trends and Threats
Much of the small species range is affected by illegal activities including gold mining and deforestation. Due to the small number of known populations across a small range and likelihood of decline, the species authority suggested an IUCN Red List status of “Vulnerable” (Fouquet et al. 2021a).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Mining

Comments

PHYLOGENETIC RELATIONSHIPS:

Based on Maximum Likelihood and Bayesian analyses of a 577 bp fragment of 16S rDNA sequences, Synapturanus is split into three monophyletic clades: the Eastern Clade, the Western Clade, and the Central Clade. Synapturanus mesomorphus, previously referred to as Synapturanus sp. “Guyana,” is part of the Eastern Clade and sister to S. mirandaribeiroi (Fouquet et al. 2021b).

References
Fouquet, A., Leblanc, K., Fabre, A.-C., Rodrigues, M.T., Menin, M., Courtois, E.A., Dewynter, M., Hölting, M., Ernst, R., Peloso, P.L.V., and Kok, P.J.R. (2021a). Comparative osteology of the fossorial frogs of the genus Synapturanus (Anura, Microhylidae) with the description of three new species from the Eastern Guiana Shield. Zoologischer Anzeiger, 293, 46–73. [link]

Fouquet, A. Leblanc, K., Framit, M., Réjaud, A., Rodrigues, M.T., Castroviejo-Fisher, S., Peloso, P.L.V., Prates, I., Manzi, S., Suescun, U., Baroni, S., Moraes, L.J.C.L., Recoder, R., de Souza, S.M., Dal Vecchio, F., Camacho, A., Ghellere, J.M., Rojas-Runjaic, F.J.M., Gagliardi-Urrutia, G., de Carvalho, V.T., Gordo, M., Menin, M., Kok, P.J.R., Hrbek, T., Werneck, F.P., Crawford, A.J., Ron, S.R., Mueses-Cisneros, J.J., Rojas Zamora, R.R., Pavan, D., Simões, P.I., Ernst, R., and Fabre, A-C. (2021b). Species diversity and biogeography of an ancient frog clade from the Guiana Shield (Anura: Microhylidae: Adelastes, Otophryne, Synapturanus) exhibiting spectacular phenotypic diversification. Biological Journal of the Linnean Society, 132(2), 233–256. [link]

Species Account Citation: AmphibiaWeb 2024 Synapturanus mesomorphus <https://amphibiaweb.org/species/9395> University of California, Berkeley, CA, USA. Accessed Jan 22, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 22 Jan 2025.

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