The incongruous dexterity of frogs as 'primitive' vertebrates, part 1

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Frogs (Amphibia: Anura, https://en.wikipedia.org/wiki/Anura) seemingly remain at a lower evolutionary level than lizards.

Although certain frogs (e.g. Hylidae: Phyllomedusa, https://www.inaturalist.org/observations?taxon_id=23714) possess an opposable thumb, this could be explained as compensation for the fact that no frog possesses horny claws on its forelimbs.

Indeed, none of the six thousand living species of frogs possesses true keratinous claws on the fore digits, despite the facts that

• supernumerary tubercles, partly analogous with claws, occur on the hands and/or feet of many species, and
• bony ‘claws’ occur on the hands of a few species.

However, the full dexterity of frogs - which seems incongruous with the 'primitive' image of amphibians - is a phenomenon likely to be under-appreciated by naturalists.

There is no literature comparing dexterity in frogs and lizards. However, a certain categorical difference between these two groups remains unexplained by the presence or absence of claws. The clawless digits of frogs achieve dexterity by a combination of adhesion to objects and the movement of one finger or toe relative to others on the same hand or foot.

I define dexterity as an ability to make differential movements of the digits of hand (manus), foot (pes), or both, in signalling and in manipulating objects or materials.

A typical example of dexterity is movement of the thumb (pollex) relative to other digits. Amphibians retain the thumb despite having lost the fifth digit on the hand. However, dexterity can be achieved by any movement of one digit relative to others on the same limb.

If dexterity is thus defined as differential deployment of the digits in prehending objects other than perches, no lizard shows dexterity.

Although chameleons (Squamata: Lacertilia: Chamaeleonidae, https://en.wikipedia.org/wiki/Chameleon) grasp perches by means of opposable digits, this does not qualify as dexterity by my definition. Furthermore, certain tree frogs (Rhacophoridae: Chiromantis, https://www.inaturalist.org/observations?taxon_id=26653) use a similar grasp.

One lineage of arboreal monitor lizards, the Varanus prasinus species-group (https://www.inaturalist.org/taxa/114888-Varanus-prasinus) of Australia and New Guinea, is capable of using the whole forelimb to reach into holes to extract prey such as larvae by means of the claws. However, this does not qualify as dexterity because all the claws grapple in unison and the digits are not deployed relative to each other.

By contrast, many lineages of frogs – including truly primitive ones – show dexterity.

Prime examples are as follows:

• Frogs in various arboreal (e.g. Hylidae: Phyllomedusa spp. and Pelodryadidae: Ranoidea aurea, https://www.inaturalist.org/taxa/517069-Ranoidea-aurea), aquatic (e.g. Pseudidae: Pseudis, https://www.inaturalist.org/observations?taxon_id=24406), and terrestrial (e.g. Pelodryadidae: Ranoidea australis, https://www.inaturalist.org/taxa/517070-Ranoidea-australis, and Ranoidea novaehollandiae, https://www.inaturalist.org/taxa/517099-Ranoidea-novaehollandiae) lineages have opposable digits - particularly the thumb - allowing them to use a precision grip to prehend prey and to place it in the mouth manually.
• Climbing frogs in at least two families (Hylidae, e.g. Phyllomedusa, and Hyperoliidae, e.g. Afrixalus, https://www.inaturalist.org/observations?taxon_id=23449) hide their clutch by using the hind digits to wrap a leaf around sticky eggs deposited on the foliage.
• Many or most frogs use their digits to rub off their skins during ecdysis (skin-shedding, which occurs in all frogs that leave the water as adults), and certain perching frogs (Hylidae, Hyperoliidae, Rhacophoridae and possibly Arthroleptidae) use their digits to spread sunscreen-like secretions over their bodies.
• Various frogs whip albuminous secretions into foam nests, which demands subtle strokes of the digits rather than gross beating of the whole foot or hand. For example, in Limnodynastes (Limnodynastidae, https://www.inaturalist.org/observations?taxon_id=22027) the breeding female temporarily develops foam-beating fringes on only the first two digits on her hand.
• Various frogs lure prey by differential gesturing with the hind digits. Not only do lizards fail to emulate such subtle movements of the feet, but no lizard is known to lure prey at all.
• Male surinam toads (Pipidae: Pipa, https://www.inaturalist.org/observations?taxon_id=25460) and marsupial frogs (Hemiphractidae: Gastrotheca, https://www.inaturalist.org/observations?taxon_id=24126) use their hind digits to place the clutch on the back of the female, where the eggs hatch in special pouches. Female marsupial frogs later use the tips of the hind digits to extract the offspring from the narrow opening of the pouch.
• Male midwife toads (Alytidae: Alytes, https://www.inaturalist.org/observations?taxon_id=24472) stroke or probe the female cloaca with the tips of the hind digits during courtship. Although not particularly dexterous, this is remarkable in a lineage ancient enough to be called ‘living fossils’.

Certain aquatic frogs are dexterous, as follows.

Aquatic frogs that grasp prey detected by their fingertips in muddy water are not only Pseudidae (Pseudis, mentioned above) but also the particularly primitive family Pipidae (e.g. Xenopus, https://www.inaturalist.org/observations?taxon_id=25443). The latter routinely uses its hands to place food in its mouth, showing unmistakeable dexterity.

African dwarf frogs (Pipidae: Hymenochirus, https://www.inaturalist.org/taxa/25439-Hymenochirus-boettgeri, which is primitive enough to be called a ‘living fossil’) are noteworthy as the only animals known to achieve dexterity with fully webbed hands. Because no frog swims forward by using its hands propulsively, it is even possible that the main function of webbing on the hand of Hymenochirus is the enhancement of dexterity.

Admittedly, salamanders (Amphibia: Urodela, https://en.wikipedia.org/wiki/Salamander) are not as dexterous as frogs. However, even they are more dexterous than lizards.

For example, the female of a typical newt (Lissotriton, https://www.inaturalist.org/observations?taxon_id=64375) uses the short, clawless digits of her hind foot to fold the leaves of aquatic plants around each of her individual eggs.

In conclusion, what deserves to be acknowledged for the first time is the following:

Frogs are the only vertebrates – with the partial exception of a few species of otters – that have evolved a combination of dexterity and clawlessness. Clawless otters (Aonyx, https://www.inaturalist.org/observations?taxon_id=41780) retain claws on the hind feet, and their manual dexterity is limited by the absence of a thumb.

I infer that frogs are clawless partly because they are dexterous, rather than being dexterous partly because they are clawless.

The important implication of all this dexterity is that the brains of amphibians are, in certain respects, more advanced than those of lizards - despite Amphibia being ancestral to Reptilia.

to be continued in https://www.inaturalist.org/journal/milewski/68218-the-incongruous-dexterity-of-frogs-as-primitive-vertebrates-part-2#...

Posted on June 10, 2022 04:18 PM by milewski