The incongruous dexterity of frogs as 'primitive' vertebrates, part 2
...continued from https://www.inaturalist.org/journal/milewski/67087-the-incongruous-dexterity-of-frogs-as-primitive-vertebrates-part-1#
Here I discuss the opposability in digits of the frog Chiromantis.
It is frequently stated that the climbing frogs in the genus Chiromantis, a genus shared between Africa and Asia, possess an opposable thumb.
I have found that the arrangement of the digits of the manus in Chiromantis is actually different from those in the genus Phyllomedusa of the Neotropics, which belongs to a different family.
It is also different from that in chameleons.
It turns out that the arrangement of the fingers in Chiromantis is 2 versus 2 rather than the 1 vs 3 seen in Phyllomedusa.
So, the opposability shown by Chiromantis is one in which not just the thumb but also digit no. 2 are opposed to digits 3 + 4. This kind of opposability is reminiscent of what is seen in chameleons and koalas, and is not particularly relevant to the opposability of the thumb alone, as epitomised by humans.
Whereas Phyllomedusa belongs to the Hylidae, Chiromantis belongs to the Rhacophoridae. The recent taxonomic changes within the Rhacophoridae (319 spp. in 14 genera) are reviewed here: http://labs.eeb.utoronto.ca/murphy/PDFs%20of%20papers/2011_rhacophorids.pdf
The bottom line:
While Chiromantis has odd hands for a frog, it is not a good example of the opposability of the thumb. This is for the simple reason that the thumb does not grip perches independently of digit no. 2, in this genus.
The following drawing shows the arrangement of fore digits in genus Chiromantis. The digits are grouped into 1+2 versus 3+4, the two pairs being opposable to each other when the frog perches on thin stems.
http://africanamphibians.lifedesks.org/pages/27465
However, I have seen no evidence that the thumb by itself can be made opposable to the rest of the hand, as happens in Phyllomedusa
The following species, Chiromantis kelleri, is odd among Rhacophoridae in penetrating semi-arid climates. It lives in thorn scrub in Somalia, and breeds in temporary pools after rain in the semi-desert. In this photo, you can see how Chiromantis, even on flattish substrates, tends to direct digits nos. 3 and 4 forwards, while digits nos. 1 and 2 are apparently directed backwards.
There are many genera of tree frogs worldwide, in various families. However, I know of no other genus that tends to split the fore digits into this 2 versus 2 arrangement.
Rhacophoridae: Chiromantis kelleri:
http://www.herpetology-africa-ethiopia.com/Chiromantis_kelleriV.jpg
The following photo, of an East African species, shows again how the fore digits of Chiromantis tend to be split into 2 vs 2.
Rhacophoridae: Chiromantis petersi:
http://africanamphibians.myspecies.info/sites/africanamphibians.myspecies.info/files/chiromantis_petersii_6.png
In the following species of Chiromantis from the Himalayan lowlands, the pattern is not particularly noticeable. Just looking at this photo (which resembles the African frogs in family Hyperoliidae), I would not have suspected that the pattern of opposition in the hand is particularly odd for a tree frog.
Rhacophoridae: Chiromantis vittatus:
https://www.pixoto.com/images-photography/animals/amphibians/golden-lined-reed-frog-chiromantis-vittatus-133548231
The following map shows the world distribution of family Rhacophoridae, a family best known for the flying treefrogs beloved of Attenborough-type documentaries.
Please note that the family, although usually associated with the rainforests of southeast Asia, actually penetrates the semi-arid Horn of Africa in the form of Chiromantis kelleri. Also note the absence from Madagascar and the fact that the family has crossed Wallace’s Line to reach Sulawesi. The species reaching Kruger National Park and Zululand is Chiromantis xerampelina, which perches out in the scrub as a ghostly pale frog clinging to perches.
http://labs.eeb.utoronto.ca/murphy/PDFs%20of%20papers/2011_rhacophorids.pdf
The following map illustrates the semi-arid tolerance of one species of rhacophorid, a species of Chiromantis.
Rhacophoridae: Chiromantis kelleri:
http://maps.iucnredlist.org/map.html?id=58798
The following photo is a reminder of the digital arrangement in Neotropical Phyllomedusa. The point to note is that even in Phyllomedusa, which relies on its opposable grip more than on the expanded discs on the finger tips, there is still a tendency to use a pattern of opposition on the hands, in which two of the digits are directed forward and the other two directed backward. I am unsure which two are directed forward, the choice being 2+3, or 3+4.
Also note how the ‘big toe’ is used opposably in Phyllomedusa. This is something I have never heard of in Chiromantis or other Rhacophoridae, which seem to rely more than Phyllomedusa on expanded digit tips and webbing between the digits, for grip on perches.
Hylidae: Phyllomedusa sp.:
http://amazingdata.com/the-beautiful-complexity-of-animal-hands/
Comentários
Clear statement in literature that amphibians do possess thumbs:
There is effectively a controversy as to whether amphibians possess thumbs or not. This is only an ‘effective’ controversy, as opposed to an argument, because I have not seen any actual debate of the question. All I have seen is contrary statements that just seem to ignore each others’ viewpoints.
Some writers say that digit no. 1 of the manus (i.e. the pollex = thumb) has been lost in all amphibians. Others say that this digit has been retained, and it is instead digit no. 5 of the manus that has been lost.
The following book takes it for granted that the pollex remains in amphibians: Hofrichter R (ed.) 2000. Amphibians: the world of frogs, toads, salamanders and newts. Firefly Books.
Co-author Alfred Goldschmid, in his section titled “Skeleton and musculature”, pp. 66ff, states: “While up to seven fingers and toes were present in early fossil amphibians, various palaeozoic amphibian groups has already evolved five-digit extremities – i.e. the basic pattern for all subsequent tetrapods...all recent amphibians have only four fingers and it is assumed that the outermost fifth finger has regressed...In the Anura the most common phalangial formula in the fingers is 2-2-3-3...the South American genus Atelopus has a first digit that is no longer visible...sex differences, especially in the thumb segments of the Anura, have also been noted, since males often have claspers underlaid with an additional bone crest.” [the latter refers to the pre-pollex]
Goldschmid, of the University of Salzburg, seems to be a recognised authority in this field. Since he mentions no controversy in this essentially technical section of this semi-popular (picture-rich, coffee-table format) book, I assume that he does not see his fact as controversial, even though it is assumed.
It may ultimately be unprovable that the lost digit on the manus of frogs is no. 5, as opposed to no. 1. However, I accept Goldschmid’s view. This is not only because of his authority, but also because of the following logic.
I see it as unparsimonious to assume that the pollex would have been lost in all frogs, at the same time as the pre-pollex being retained in many frogs - and in some cases being used to grow a large keratinous claw. This claw represents the only true fore-claw found in frogs (including the African clawed toad, Xenopus, which has claws on its hind foot, not fore foot).
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