Description
Mantidactylus radaka is a very large, stout-bodied frog with a male snout-vent length range of 88 - 93 mm and a female snout-vent length range of 85 - 103 mm. The head is somewhat wider than body and is wider than long. The snout is blunt, with dorsolaterally, slightly protuberant nostrils near the tip. The canthus rostralis is indistinct and the loreal region is concave. The tympanum is small, with a horizontal diameter that is about a third of the horizontal eye diameter. The upper edge of the tympanum is concealed by a supratympanic fold that is straight above the tympanum and then gently curves to the axilla. The hand does not appear to have an outer metacarpal tubercle, but does have a distinct inner metacarpal tubercle. The fingers have a relative length of I < II < IV < III, do not have webbing, and have single subarticular tubercles. The fingers end in enlarged discs. Nuptial pads are absent. The tibia is longer than the foot. Like the hand, the foot does not appear to have an outer metatarsal tubercle, but does have an inner metatarsal tubercle. The relative toe lengths are I < II < III = V < IV. These frogs have fully webbed feet with a webbing formula of 1(0), 2i(0.5), 2e(0), 3i(1), 3e(0), 4i(1), 4e(0.5), 5(0). The skin on the dorsum is rugose with small warts that become larger on the flanks. There are large, distinct femoral glands. The ventrum is smooth (Rancilhac et al. 2020).

DIAGNOSIS:

There are two similar species within the same genus: M. guttulatus and M. grandidieri. Mantidactylus radaka can be differentiated from M. guttulatus by having a longer relative hind limb length (hind limb to snout-vent length ratio of 1.46 - 1.60 versus 1.28 - 1.43). They also have larger and ovoid femoral glands in males (versus round ones in M. guttulatus and M. grandidieri) and indistinct femoral glands in females (versus always recognizable in M. guttulatus and M. grandidieri). In life, M. radaka typically has a hidden and smaller tympanum when compared to the tympanum of these other two species. Lastly, Mantidactylus radaka individuals in Besariaka have been observed to favor running water while M. grandidieri appears to prefer stream banks (Rancilhac et al. 2020).

COLORATION:

In preservative, the dorsal coloration is brown with tiny yellow-ish dots on the warts. Their venter is beige with indistinct mottling on the throat. In life, the coloration is similar but with larger yellowish dots and uniformly green-brown irises. They also have light tips on the inner three fingers. These frogs lack a dorsolateral color border as well as a distinct frenal stripe. They are also characterized by the absence of large yellowish markings in the inguinal region or between coloration of flanks and belly (Rancilhac et al. 2020).

VARIATION:

There is no significant difference between the tympanum diameter of males and females. In terms of femoral pores, females in life have tiny black patches whereas females in preservative lack them altogether. Young males typically have externally and internally distinct femoral pores that are ovoid. Some females also have a very dark throat coloration (Rancilhac et al. 2020).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Madagascar

View distribution map in BerkeleyMapper.

Mantidactylus radaka are found in northern Madagascar, between the Manongarivo Special Reserve and Bealanana district (Razafison 2020). These frogs have been observed to inhabit riparian regions, living very close to or near calm stretches of streams. They seem to prefer small and shallow streams with sandy bottoms in rainforest (like in Tsaratanana) more than shallow streams with rocky bottoms (like near Bemanevika), though this species can be found in both regions. The localities of M. radaka span an elevational range from 690 m up to at least 1,590 m (Rancilhac et al. 2020).

Life History, Abundance, Activity, and Special Behaviors
Mantidactylus radaka individuals in Besariaka have been observed to favor running water (Rancilhac et al. 2020).

Observations on M. guttulatus recorded by Schulze et al. (2016) are believed to actually be for M. radaka. As such, the reproduction period for M. radaka is at the beginning of the warm-rainy season and peaks towards the beginning of the cool-dry season.

They have been observed to lay very large clutches of eggs (Rancilhac et al. 2020, Razafison 2020).

Larva
Tadpoles seem to be Type IV, with large and muscular tails and low fins. Morphological adaptations for inhabiting streams (i.e. generalized body and tail shape) are present in tadpoles of this species (Schulze et al. 2016).

Trends and Threats
The M. radaka population has declined as a result of anthropogenic pressures. These frogs are a popular food source in the regions of Madagascar that they are found. Additionally, deforestation and uncontrolled hunting may play a role in their decline. The extent that their population has been affected is still being debated because they have been observed to lay very large clutches of eggs (Rancilhac et al. 2020, Razafison 2020).

Relation to Humans
Mantidactylus radaka is a popular food source that is consumed throughout Madagascar. In towns, restaurants use them to make frog fritters and in villages, people either hunt them or breed them in wells or private ponds (Razafison 2020).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Urbanization
Habitat fragmentation
Intentional mortality (over-harvesting, pet trade or collecting)

Comments

PHYLOGENETIC RELATIONSHIPS:

The Maximum Likelihood analysis conducted on 16S mtDNA and Rag-1 nDNA sequences indicated that M. guttulatus is the sister taxon to the clade composed of M. radaka and M. grandidieri. Four other undescribed clades were also identified and were placed basally to the three named Mantidactylus species (Rancilhac et al. 2020).

ETYMOLOGY:

The species epithet, “radaka,” comes from the Malagasy word for large frogs in reference to the size of the species and other members of the genus (Rancilhac et al. 2020).

References

Rancilhac, L., Bruy, T., Scherz, M. D., Pereira, E. A., Preick, M., Straube, N., Lyra, M. L., Ohler, A., Streicher, J. W., Andreone, F., Crottini, A., Hutter, C. R., Randrianantoandro, C., Rakotoarison, A., Glaw, F., Hofreiter, M., Vences, M. (2020). "Target-enriched DNA sequencing from historical type material enables a partial revision of the Madagascar giant stream frogs (genus Mantidactylus). Journal of Natural History, 54(1-4), 87-118. [link]

Razafison, R. (2020). “Madagascar giant frog is a new species, but also a deep-fried delicacy.” Mongabay Environmental News, 1 Sept. 2020. https://news.mongabay.com/2020/09/madagascar-giant-frog-is-a-new-species-but-also-a-deep-fried-delicacy/. Accessed in April 2022 [link]

Schulze, A., Randrianiaina, R.-D., Perl, B., Glaw, F., Vences M. (2016). “The unexpectedly dull tadpole of Madagascar’s largest frog, Mantidactylus guttulatus.” Acta Herpetologica, 11(2), 119-125. [link]

Species Account Citation: AmphibiaWeb 2023 Mantidactylus radaka: Madagascar Giant Stream Frog <https://amphibiaweb.org/species/9313> University of California, Berkeley, CA, USA. Accessed Jun 3, 2023.

Citation: AmphibiaWeb. 2023. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 3 Jun 2023.

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