Description
Rhombophryne proportionalis is a miniaturized frog that was previously listed as part of the Stumpffia genus. It has a snout vent length range in males of 11 - 12.3 mm. Only three males were examined for the species description and no female specimens have been observed. The head width is around one third of the snout vent length, and is wider than the body. The head is wider than it is long, and head length is more than 50% of the head width. From a dorsal view the snout is rounded, while from a lateral view the snout is squared. The nostrils are directed laterally, not protuberant, and closer to the eyes than to the tip of the snout. The canthus rostralis is rounded and concave. The loreal region is flat and vertical. The tympanum is more than half the eye diameter, and its shape is round and indistinct. The supratympanic fold is weakly raised and indistinct; it curves slightly from the posterior corner of the eye then goes over the tympanum and toward the axilla (Scherz et al. 2019).

The forelimb is slender and its length (sum of the hand, upper arm, and lower arm lengths) is 49% of the average snout vent length. The subarticular tubercles are single and faint. The outer metacarpal tubercle is paired and faint, while the inner metacarpal is distinct and elongated. The hands are not webbed. The first finger is reduced, and the relative length of fingers is 1 < 2 = 4 < 3. The finger tips are not expanded. The hind limbs are robust with a total length (sum of foot length, thigh length, and tibiofibular length) that is 125% of the snout vent length. The tibiofibular length specifically is 34% of the snout vent length. The lateral metatarsalia is strongly connected. The inner and outer metatarsal tubercles are thin and small, respectively, with both being indistinct. The toes have no webbing, and the first toe is highly reduced. The relative toe length is 1 < 2 < 5 < 3 < 4. The dorsal skin is smooth (both in preservative and in life), but in life there are also scattered tubercles and a distinct ridge above the scapular region. The ventral skin is granular in live frogs (Scherz et al. 2019).

Rhombophryne proportionalis has no clavicles, which distinguishes it from most other miniature frogs. However, S. achillei, S. analanjirofo, S. contumelia, S. davidattenboroughi, S. makira, S. obscoena, and some specimens of S. tridactyla similarly have no clavicles. Rhombophryne proportionalis can be further distinguished by a dark colouration on the lateral surface of the head with a distinct color border, as well as fewer reduced fingers and toes. Other adult species in its genus, Rhombophryne, can be distinguished by their greater size. Rhombophryne proportionalis also has a unique call, different from other frogs in Madagascar. Its call is a rapid series of high pitched tonal notes (Scherz et al. 2019).

In alcohol, the dorsum is a chocolate brown color that is darker on the head, with a faint dark brown line that runs from the inguinal region anteriorly toward the eye. Laterally, the head has a distinct color border to the dorsum. The head is a darker brown and its border is defined by the supratympanic fold. The dorsal skin of the leg has a dark brown crossband on the shank. The color border between the flank and venter is indistinct. The venter is brown with cream flecks, getting slightly darker and having fewer flecks on the chin. The legs and arms have the same color ventrally as the abdomen. The bottom of the foot is dark brown down the medial half. Coloration is the same in life, just more vibrant (Scherz et al. 2019).

Morphologically, there is little variation between the three specimens studied. However, sometimes the supratympanic fold is less curved and more of a straight line from the eye to above the arm. The coloration of the three specimens is fairly consistent as well. Dorsolateral lines are sometimes not present, and instead can be broken into spots above the suprascapular region and lines in the inguinal region (Scherz et al. 2019).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Madagascar

View distribution map in BerkeleyMapper.

Rhombophryne proportionalis has been found in Bepia and Andranomadio on the Tsaratanana massif in northern Madagascar. These two sites have an elevation of 959 and 949 meters respectively. They are also located in Tsaratanana National Park, which was formerly a Strict Nature Reserve. This is a fairly small distribution, as R. proportionalis inhabit an area of less than 2000 square kilometers (Scherz et al. 2019).

Life History, Abundance, Activity, and Special Behaviors
Rhombophryne proportionalis is a terrestrial species that lives in the leaf litter (Scherz et al. 2019).

Rhombophryne proportionalis call during the day on the ground in open, shrubby landscape. In some areas, several R. proportionalis can be heard calling in a chorus. Its call is a rapid series of high pitched tonal notes. The calls are repeated at long and irregular intervals, with each call consisting of a series of 9 – 17 notes (Scherz et al. 2019).

Trends and Threats
The habitat in which the species lives is potentially in decline. As a result, the species is likely to be classified as "Endangered." However, the reserve, Tsaratanana National Park, that R. proportionalis inhabit is closed to the public, making for limited human interaction (Scherz et al. 2019).

Comments
Maximum Likelihood and Bayesian Inference analyses were conducted using the 16S and 12s rRNA mitochondrial genes. Rhombophryne proportionalis is sister to a clade consisting of R. alluaud, R. laevipesi, other Rhombophryne species, and possibly Plethodontohyla species (Scherz et al. 2019).

The species is named with the epithet “proportionalis,” meaning “proportional” in Latin. This is in reference to the comparatively proportional dwarfism of R. proportionalis (Scherz et al. 2019).

References

Scherz, M. D., Hutter, C. R., Rakotoarison, A., Riemann, J. C., Rödel, M. O., Ndriantsoa, S. H., et al. (2019). "Morphological and ecological convergence at the lower size limit for vertebrates highlighted by five new miniaturised microhylid frog species from three different Madagascan genera." PLoS ONE, 14(3). [link]

Species Account Citation: AmphibiaWeb 2022 Rhombophryne proportionalis <https://amphibiaweb.org/species/8990> University of California, Berkeley, CA, USA. Accessed Dec 21, 2024.

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

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