A naturalist's various thoughts on the frog genus Breviceps, part 3

(writing in progress)

...continued from https://www.inaturalist.org/journal/milewski/69046-a-naturalist-s-various-thoughts-on-the-frog-genus-breviceps-part-2#

CRUX OF INTERCONTINENTAL COMPARISONS OF FROGS IN MEDITERRANEAN-TYPE CLIMATES IN WESTERN AUSTRALIA AND THE WESTERN CAPE OF SOUTH AFRICA:

My study areas are particularly similar in their climates, soils, and regimes of wildfire: Fitzgerald River National Park (https://en.wikipedia.org/wiki/Fitzgerald_River_National_Park) and its environs to the east, and Cape Agulhas National Park (https://en.wikipedia.org/wiki/Agulhas_National_Park) and its environs to the west.

The faunas and communities of frogs in these environments are complex./However, the intercontinental differences boil down to just two species that really show how different these faunas and communities of frogs really are, and the surprising failure of convergence. And this crucial difference is indicative of the whole of these continents, in a way.

These species are the Australasian treefrog Ranoidea cyclorhynchus (https://www.inaturalist.org/taxa/517081-Ranoidea-cyclorhynchus) in the Australian study area, and Breviceps rosei (https://www.inaturalist.org/taxa/24992-Breviceps-rosei) in the southern African study area.

Once we begin understand why these particular species are so different between the continents, the rest of the comparison may start to take on a conceptual framework.

Ranoidea cyclorhynchus is, in my experience, the commonest frog in the Australian study area, when one refers to frogs that occur away from the strict confines of water, i.e. frogs that occur out there in the well-drained vegetation.

This Australasian treefrog perches on proteas and typical plants of the mallee-heaths. Although it depends on pools for breeding, it is a typical treefrog in ranging tens or hundreds of metres from the nearest water (it does not need to get wet every day).

The main point to note about this most ‘typical’ of the ‘dryland’ frogs in the Australian study area is that, in being a large scansorial frog, it is categorically different from any frog occurring in the whole Fynbos Biome (https://en.wikipedia.org/wiki/Cape_Floristic_Region).

I could even argue that there is not a frog in the whole of subSaharan Africa like it. The tree frogs found in the southern African subtropics and tropics (e.g. Chiromantis, https://www.inaturalist.org/observations?place_id=any&taxon_id=26653&view=species, and Leptopelis, https://www.inaturalist.org/observations?place_id=any&taxon_id=23396&view=species) are different.

Ranoidea is an important, diverse, and widespread genus in Australasia. Therefore, R. cyclorhynchus seems to encapsulate much that is different between the two continents, despite being a relatively narrow endemic to the southern coast of southwestern Australia.

To elaborate briefly on why even those tree frogs that do occur in subtropical to tropical southern Africa are different:

The rhacophorid Chiromantis has different hands, more like those of a chameleon (https://en.wikipedia.org/wiki/Chameleon), and breeds by means of foam nests above the water, i.e. this African tree frog is far more specialised than Ranoidea, and it does not come anywhere near the mediterranean-type climate, or anything resembling fynbos vegetation.

Leptopelis (a heterogeneous genus of which only certain spp. could be called tree frogs) has vertical pupils and belongs to the Arthroleptidae, a family containing mainly frogs that would never be described as arboreal or scansorial.

My interpretation is that the predatory regime in the southern African study area – despite all my care in matching climates and soils with my Australian study area – is just too intense to allow the existence of a generalised scansorial frog of this substantial body size.

There is no niche for Ranoidea in Africa, and fine-tuning the comparison as I have done by such careful choice of study areas makes little difference to that.

The whole of Africa, including even the Fynbos Biome, is subject to predation intense enough to preclude any convergent match for Ranoidea. The adults are just too exposed, like sitting ducks. The tadpoles have no particular anti-predator adaptations. Ranoidea is ‘inferior’ in the same sense as marsupials are ‘inferior’.

For its part, Breviceps, one of the commonest frogs in the southern African study area, distinguishes this area not just within this locality-specific comparison, but also subcontinent-wide. Breviceps is widespread in southern Africa, yet has no counterpart in Australasia.

Notaden does show similarities in its adult form. However, Notaden (like Ranoidea) remains reproductively conservative (with conventional tadpoles), whereas Breviceps has extremely specialised ‘direct development’, of a kind consistent with the anti-predator strategy crucial to Breviceps.

Breviceps has at least three major features showing extreme specialisation in adaptation anti- predation. Its larvae develop without free water, underground.

The adults emerge from below ground only for only part of the time, despite the mesic climate in the Fynbos Biome. And the body form of Breviceps, particularly the face, seems designed around an anti-predator tactic of inflation to spherical shape, in conjunction with the deployment of a sticky secretion that threatens to gum up the mouth of an attacker, or even the body coils of a constricting snake./As far as I know, Ranoidea cyclorhynchus does not inflate its body when threatened by a predator.

The following show Ranoidea cyclorhynchus. This is one of the most generalised and ‘average’ of all the species in its genus. It is certainly a climbing frog, but more scansorial than arboreal:
http://static1.squarespace.com/static/525a4d11e4b030e5f06eb275/547908f1e4b028a161219075/547c292fe4b07c97883cbe48/1417424603466/spotted-thighed-frog-(Litoria-cyclorhyncha)-Oct14.jpg?format=1000w
http://m3.i.pbase.com/o2/73/838473/1/97540423.vrwY5gOl.Litoria_cyclorhyncha_1132.jpg

The following of Ranoidea cyclorhynchus shows the camouflage-pattern which sets this species apart from all tree frogs in Africa. Although certain rhacophorid and arthroleptid tree frogs do occur in subtropical and tropical Africa, none has this pattern of colouration: http://frogs.org.au/img/450/0503-JB-frog-Litoria_cyclorhyncha-131.jpg.

The following shows the slightly-developed flash colouration on the normally hidden surfaces of the hind leg in Ranoidea cyclorhynchus:
http://frogs.org.au/img/250/0503-JB-frog-Litoria_cyclorhyncha-132.jpg.

The following suggests how large Ranoidea cyclorhynchus can grow: https://c1.staticflickr.com/9/8030/8050196472_f9872209b6_b.jpg.

The following shows Breviceps rosei, a species restricted to coastal fynbos and strandveld, but similar in most ways to the typical Breviceps of the 'bushveld' areas of South Africa, namely Breviceps adspersus: http://i154.photobucket.com/albums/s272/fruitcakelou/_MG_1699Brevicepsrosei.jpg.

Whereas Ranoidea cyclorhynchus (https://www.inaturalist.org/taxa/517081-Ranoidea-cyclorhynchus) jumps well, in the usual manner of frogs, Breviceps can neither hop nor swim.

The following of B. rosei (http://www.buckhambirding.co.za/?page_id=7096) shows a pattern nobody seems to have noticed: that the dark marking that ostensibly disguises the eye also in a way makes the whole animal more conspicuous because it contrasts rather starkly with the pale area adjacent to it. The markings on the side of the head cannot, I suggest, be explained simply in terms of the usual concepts of camouflage.

The following of B. rosei (http://www.buckhambirding.co.za/?page_id=7096) shows how flat-faced this species is, much like other members of this genus. This head shape cannot be explained in terms of diet, which is not known to be specialised in B. rosei.

(writing in progress)

Posted on August 14, 2022 07:02 PM by milewski