Comparison between burrowing frogs on sand dunes: Arenophryne (Western Australia) and Breviceps (Namib/Namaqualand of southern Africa)

(writing in progress)
 
Consider Arenophryne (https://en.wikipedia.org/wiki/Arenophryne), a myobatrachid genus of frogs in Western Australia, and Breviceps (https://en.wikipedia.org/wiki/Breviceps), a brevicepitid genus of frogs in southern Africa.

Despite being as unrelated as any two lineages of frogs, these are probably each other’s closest intercontinental counterparts in

• dumpy body form, with inability to hop or swim (https://australian.museum/learn/animals/frogs/northern-sandhill-frog/),
• short face and small mouth (https://www.frogid.net.au/frogs/arenophryne-rotunda), and
• terrestrialised breeding habits.

Neither Arenophryne nor Breviceps has any counterparts, to speak of, on any other continent. There is no frog in South America, let alone the Northern Hemisphere, that resembles either of these genera.

Two of the 20 spp. of Breviceps are the focus of this comparison. This is because they, like both of the two spp. Arenophryne, are narrowly restricted to a dry far-west coast, where they are associated with bare sand dunes under arid to semi-arid climates.

Both Arenophryne and Breviceps combine direct development (with no free-living tadploes) with the laying of eggs deep underground. In this combination they are extremely unusual among frogs worldwide. I do not know of any South American frog that is similar in this respect.

Direct development is, of course, common among frogs worldwide. However, most such lineages are associated with forests, not semi-deserts. Furthermore, most lay their eggs on, or close to, the surface of the ground.

The taking of direct development into semi-deserts is possibly the strongest element of evolutionary convergence between Arenophryne and Breviceps. The morphological convergence of the adults is rather ambivalent, because the Australian genus is only superficially similar to the southern African genus.

There are two species of Arenophryne, which occur between Shark Bay (https://en.wikipedia.org/wiki/Shark_Bay) and Geraldton (https://en.wikipedia.org/wiki/Geraldton), on the far-western coast of Western Australia.

Both spp. live in what are essentially calcareous sand dunes. They emerge at night to eat ants, and spend the day resting underground. As far as I know, neither of these frogs can swim.

They lay their large eggs underground, where the larvae develop based on yolk provided by the mother.

In both cases, the parents perform amplexus (https://www.dictionary.com/browse/amplexus) above ground, but then submerge together into the sand. The eggs are fertilised in chambers underground, that are dug mainly by females.

In terms of habitat, there are two species of Breviceps that are most similar, occurring in coastal dunes in Namaqualand (https://en.wikipedia.org/wiki/Namaqualand) and near the mouth of the Orange River (https://en.wikipedia.org/wiki/Orange_River).

These are Breviceps namaquensis (https://www.inaturalist.org/taxa/24991-Breviceps-namaquensis) and B. macrops (https://www.inaturalist.org/taxa/25004-Breviceps-macrops), both of which have extremely large eyes.
 
The following are the differences between Breviceps (particularly the two spp. mentioned above) and Arenophryne.
 
Breviceps

• is not myrmecophagous, and there is no evidence that B. namaquensis or B. macrops are particularly dependent on termites, let alone ants;
• inflates its body to an extreme degree and with little provocation, whereas Arenophryne has not been recorded inflating its body;
• has (in these two species) larger eyes than those of Arenophryne, proportional to body size (eye size is variable in Breviceps according to species, but the two spp. that occur in environments similar to that of Arenophryne have particularly large eyes);
• digs mainly with its hindlimbs;
• has an even flatter face than Arenophryne;
• has a distress call, whereas no such call has been recorded for Arenophryne; and
• secretes a noxious and possibly sticky goop when threatened, while remaining inflated (Arenophryne behaves similarity, but seems to do so only in extremis).

There is certainly some convergence between the myobatrachid genus Arenophryne of coastal semi-arid Western Australia and the geographically parallel Breviceps namaquensis and B. macrops.

However, some of the differences are obvious. For example, Arenophryne is among the few lineages of frogs worldwide that dig with their forelimbs. Arenophryne has well-developed forelegs, because it is one of the few genera of frogs worldwide that digs forward, in contrast to Breviceps which digs backward.

Some of the above differences between Breviceps and Arenophryne can be related to intercontinental differences in pressures of predation. An example is in degree of inflation of the body as an anti-predator tactic.

Breviceps has extreme tactics vs predators, whereas Arenophryne has only moderate tactics. This comparison supports the idea that the most important factor shaping Breviceps is the intense predation of southern Africa.

Although I have not seen inflation of the body recorded in the literature for Arenophryne, photos suggest some degree of inflation as an anti-predator tactic. Arenophryne may perhaps inflate its body to some extent (as do many lineages of frogs worldwide) but this tactic is poorly developed in the Australian genus, which also lacks a distress call.

Photos suggest that Arenophryne sometimes ‘plays dead’, something of which I have seen no evidence in Breviceps. Furthermore, the black-and-white pattern on the belly of Arenophryne seems quasi-aposematic. I know of no aposematic patterns in Breviceps.

This does not necessarily mean that Arenophryne is more toxic than Breviceps, but it indicates a different anti-predator strategy. Certainly I have not read of adhesive secretions in Arenophryne.
 
Arenophryne is no more similar to Breviceps that, for example, Australian Ranoidea is to southern African Chiromantis and Pyxicephalus. There are large differences in inflation, distress call, eye size, method of digging, face shape, body proportions, colouration, etc.

Ultimately, about all that is similar between Arenophryne and Breviceps is that they are both dumpy, digging frogs that have subterranean direct development. The overall picture, then, is that anti-predator defences are better-developed in Breviceps than in Arenophryne.
 
In summary, a case of ‘so near and yet so far’?

AUSTRALIA:

See https://www.sharkbay.org/publications/fact-sheets-guides/sandhill-frog/ and https://www.facebook.com/bushheritageaustralia/photos/a-southern-sandhill-frog-arenophryne-xiphorhyncha-these-cool-little-frogs-were-o/10160574985330316/.
 
The two species of Arenophryne are restricted to narrow distributions near Shark Bay (http://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Arenophryne_rotunda_distibution.PNG/220px-Arenophryne_rotunda_distibution.PNG and http://museum.wa.gov.au/sites/default/files/imagecache/wam_v2_shop_full/species/distribution/Arenophryne_xiphorhyncha_edit.jpg).

In the case of A. rotunda, the distribution includes the westernmost point of the Australian continent.

Also please note that, although Arenophryne is myrmecophagous (eating mainly worker ants), its mouth is not particularly narrow, so it seems morphologically unspecialised as a myrmecophage.

Arenophryne rotunda:
http://m0.i.pbase.com/o3/99/954699/1/135360030.RDYKu6mb.IMG_0748_wmCopy.JPG/http://calphotos.berkeley.edu/imgs/512x768/1111_1111/1111/6910.jpeg
http://museum.wa.gov.au/sites/default/files/imagecache/wam_v2_page_full_gallery/photo-galleries/318.jpeg
http://m9.i.pbase.com/o3/99/954699/1/135360029.SmTJNI64.IMG_0718_wmCopy.JPG
http://museum.wa.gov.au/sites/default/files/imagecache/wam_v2_page_full_gallery/photo-galleries/318.jpeg
http://calphotos.berkeley.edu/imgs/512x768/1111_1111/1111/6910.jpeg

Arenophryne xiphorhyncha:
http://museum.wa.gov.au/sites/default/files/imagecache/wam_v2_page_full_gallery/photo-galleries/Fig.2A.A.xiphy_.jpg
http://museum.wa.gov.au/sites/default/files/imagecache/wam_v2_page_full_gallery/photo-galleries/536.jpeg
https://museum.wa.gov.au/explore/frogwatch/frogs/southern-sandhill-frog
http://m9.i.pbase.com/o6/99/954699/1/136049959.RvD7WLci.IMG_1057_wmCopy.JPG
 
What is shown particularly well in the following photo is the degree of development of the forelimbs in Arenophryne, consistent with forward digging. If you compare this with the photos below of Breviceps, you’ll see how much lass massive the forelimbs of the southern African frog are.
 
Arenophryne xiphorhyncha:
http://museum.wa.gov.au/sites/default/files/imagecache/wam_v2_page_full_gallery/photo-galleries/536.jpeg

SOUTHERN AFRICA:

The two west-coast species of Breviceps, B. macrops and B. namaquensis, are as similar to each other as Arenophryne rotunda is to A. xiphorhyncha.

It is immediately apparent that the southern African frogs have larger eyes, less massive forelimbs, a smoother skin, and a flatter face than are the case in Arenophryne.

These two ‘Namib’ species of Breviceps have larger eyes and less flat faces than their congeners elsewhere in southern Africa.

The following specimen of B. macrops is hardly inflated, but even in this relaxed mood shows greater inflation than can be seen in any photo of Arenophryne that I have found.

Breviceps namaquensis is known to make a distress call when interfered with by potential predators, and the same is probably true for B. macrops.

Also, other species of Breviceps are known to produce milky ooze on the skin when defensive, and the same is probably true for these western species.

Neither B. macrops nor B. namaquensis seems to specialise on ants or termites, despite the specialisation of certain congeners, particularly B. adspersus, on alate termites.

Breviceps lacks any thickening of the skin on the snout, because it never digs forwards.
  
The following photos illustrate the large eyes and poorly-developed forelimbs.

When Breviceps feels threatened, it inflates its body to an extreme degree, so that the body becomes nearly spherical and the snout hardly projects beyond the silhouette of the sphere.

Breviceps is known to produce adhesive secretions from the skin during amplexus but it’s unknown whether the toxic secretions, deployed against predators, can also function adhesively.
 
The following map shows the distribution of Breviceps macrops; B. namaquensis overlaps with this but extends farther south.
 
As readers can see from maps below, the distributions of Breviceps macrops and B. namaquensis are analogous, relative to the southern African subcontinent.

Breviceps macrops:
http://maps.iucnredlist.org/map.html?id=3070

Breviceps namaquensis:

https://whyevolutionistrue.files.wordpress.com/2013/02/breviceps_namaquensis.jpg

https://whyevolutionistrue.files.wordpress.com/2013/02/018-20120930wcnp-namaquarainfrog.jpg?w=560&h=374

Breviceps macrops:

https://www.youtube.com/watch?v=ahHhF9a-swA
https://animals.fandom.com/wiki/Desert_Rain_Frog?file=Desert-rain-frog-walking.jpg
http://www.iucnredlist.org/details/3070/0
http://frogmap.adu.org.za/Species_text.php?sp=190
https://www.inaturalist.org/observations/66582610

(writing in progress)

Publicerat 14 augusti 2022 08.35 av milewski