The Guiana Shield is a range of ancient mountains covering much of French Guiana, Surinam, Guyana, and southern Venezuela. The area is one of the most biodiverse regions on Earth, particularly in the Highlands area, which comprises a series of flat-topped sandstone plateaus known as 'tepuis', which have been largely isolated from both one-another and the outside world for millions of years, giving rise to a huge number of endemic species (the area was the inspiration for Sir Arthur Conan Doyle's 'Lost World').
In a paper published in the journal Zoological Letters on 25 May 2023, Philippe Kok of the Department of Ecology and Vertebrate Zoology at the University of Łódź and Department of Life Sciences at the Natural History Museum, describes a new species of Carrying Frog from Wei-Assipu-tepui, a 2260 m high flat-topped plateau on the border between Guyana and Brazil.
Carrying Frogs, Stefania spp., are a genus of Frogs restricted to the Guiana Shield, and for the most part to the tepuis of the Guiana Highlands. Most species are endemic to a single tepui, with most tepuis having 1-2 species, while the largest can have 5-6. A single species, Stefania evansi, is found in lowland environments. The group get their common name from their reproductive behaviour, with the females carrying their eggs on their backs until they hatch producing Froglets (there is no Tadpole stage), which are also carried until they are large enough to fend for themselves. The taxonomy of Carrying Frogs can be difficult, as all members of the group tend to be morphologically similar to one another and all species are quite variable in colouration. Furthermore, genetic analysis of Carrying Frogs has revealed that apparently similar populations are genetically distinct, while apparently morphologically distinct groups are in fact the same species.
Wei-Assipu-tepui has a surface area of a only 3 km³, and is entirely surrounded by high cliffs, so that it can only be reached by helicopter of ascending a vertical rock-face. The surface of the plateau is divided by a series of fractures, the deepest of which, Sima de los Guácharos, reaches more than 100 m beneath the plateau surface. The first recorded expedition to the plateau was in July 2000, when a joint expedition by the Sociedad Espeleológica Italiana and the Sociedad Venezolana de Espeleología visited the summit with the intention of exploring its cave systems.
The 2000 expedition to Wei-Assipu-tepui made a small collection of Amphibians was made, including a previously unknown species of Bush Toad, Oreophrynella sp., which was later described as Oreophrynella weiassipuensis, as well as a Carrying Frog, Stefania sp., which was never identified. The plateau was visited again in November 2009 by Philippe Kok, who spent two weeks there, collecting Amphibian and Reptile specimens. Kok collected nine species, including 23 specimens of a Carrying Frog morphologically identical to Stefania riveroi, a species known from Yuruani-tepui in Venezuela, about 20 km to the northwest of Wei-Assipu-tepui. However, a DNA analysis carried out several years later showed that these Frogs were not closely related to Stefania riveroi, instead forming a sister group to Stefania ayangannae, a morphologically distinct species found on Mount Wokomung, and Mount Ayanganna, two large tepuis in western Guyana, both more than 80 km to the east of Wei-Assipu-tepui.
Based upon this, Kok describes the population from Wei-Assipu-tepui as a new species, which he names Stefania maccullochi, in honour of the Canadian herpetologist Ross Douglas MacCulloch, for his seminal contribution to the systematics and taxonomy of the genus Stefania in particular, and to the knowledge of the Amphibians and Reptiles of Guyana in general.
Stefania maccullochi is a large species of Carrying Frog, with the largest female found measuring 72.9 mm in length, while the largest male is 54.6 mm, with a head no longer than it is wide. Although this species appeared morphologically identical to Stefania riveroi on initial examination, a more thorough examination made after it was discovered the two populations were genetically distinct found that there were some subtle differences between the skeletons of the two species, most notably a deep indentation along the anteromedial base of the alary process of the premaxilla of Stefania maccullochi, which Stefania riveroi lacks, and the contact between the posterodorsal projection of the maxilla and the orbital/zygomatic ramus of the squamosal, which is fused in Stefania riveroi, while in Stefania maccullochi there is barely a contact.
Phylogenetically, Stefania riveroi has previously been seen as the sister species to the pairing of Stefania ayangannae and Stefania coxi, two species which are both found on the large tepuis Mount Wokomung and Mount Ayanganna, far to the east of Wei-Assipu-tepui. Both of these tepuis are noted for their high diversity of Stefania species, with six found on Mount Wokomung and five on Mount Ayanganna.
Despite being morphologically very similar to Stefania riveroi, a species found on a neighbouring tepui, Stefania maccullochi was found to be the sister species to Stefania ayangannae. Kok suggests that this implies the species have a combination of retained ancestral traits and similar environmental pressures, both inhabiting small, high platues where they are the only species of Stefania found, while Stefania ayangannae and Stefania coxi inhabit larger, lower plateaus which they share with a range of other species. he furthermore designates the four species together as the 'Stefania riveroi clade' within the genus Stefania. Interestingly, molecular clock analyses suggests that the splits between these species are not recent, with no divisions between any species likely to be more recent than the Miocene.
Wei-Assipu-tepui, the home to Stefania maccullochi, rises to 2260 m above sealevel and has a surface area of about 3 km³. Yuruani-tepui, the home to Stefania riveroi, reaches about 2400 m above sealevel, and has a surface area of 4.38 km³. The nearby tepuis of Roraima-tepui (2800 m), Kukenán-tepui (2600 m), Maringma-tepui (2100 m), and Maringma-tepui (about 2140 m) all appear to lack any species of Stefania.
The plateau of Wei-Assipu-tepui, is largely covered in vegetation, with extensive areas of coarse herbs mixed with woody subshrubs on peat soils, as well as some quaking bogs and extensive patches of dwarf forests. These dwarf forests are dominated by Bonnetia roraimae, a flowering shrub found only on tepui tops. Other species common on the plateau are the Bromeliads Brocchinia tatei and Brocchinia reducta, the Rapateaceaen Stegolepis guianensis, the Yellow-eyed Grass Orectanthe sceptrum, and the Pitcher Plant Heliamphora nutans. The highest temperature recorded on the summit in the day was 29°C, and the lowest at night was 11°C. The relative humidity varied by between a minimum of 24% in the day and a maximum of 98% at night. All measurements were taken in November.
Active Stefania maccullochi, including females with eggs, were observed at night, crawling on the ground and on low vegetation. Females with carried young were observed within Bromeliad tubes at night and by day, while another was found hiding under a rock by day. Other resting specimens were observed in the day on the ground, between rocks, and on trees as much as 2 m above the ground. On individual was observed sitting on a mossy rock at the bottom of a 30 m-deep crevice. No males were observed calling, but load single-note 'peeps' consistent with the a male Stefania.
All known individuals of Stefania maccullochi have been observed within an area of less than 10 km², and that single location, the summit plateau of Wei-Assipu-tepui, is considered to be at threat from Human-set fires in the region, and the effects of climate change. It's isolation probably currently protects it from hazards such as Human-introduced pathogens such as Chytridiomycosis, although the species cannot be presumed to be completely safe, and strict hygiene protocols would need to be followed by documentary film-makers or others visiting the plateau. For these reasons, Kok considers that Stefania maccullochi should be considered to be Critically Endangered under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.
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