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Lombard, R. E., & Wake, D. B. (1976). Tongue evolution in the lungless salamanders, family plethodontidae I. Introduction, theory and a general model of dynamics. Journal of Morphology, 148(3), 265–286. https://doi.org/10.1002/jmor.1051480302

Soares D, Adams R, Hammond S, Slay ME, Fenolio DB, Niemiller ML (2017) Evolution of Coprophagy and Nutrient Absorption in a Cave Salamander. Subterranean Biology 24: 1-9. https://doi.org/10.3897/subtbiol.24.15013

Lombard, R. E., & Wake, D. B. (1976). Tongue evolution in the lungless salamanders, family plethodontidae I. Introduction, theory and a general model of dynamics. Journal of Morphology, 148(3), 265–286. doi:10.1002/jmor.1051480302

Rebouças, R., & Solé, M. (2015). Diet ofAdenomera thomei(Almeida and Angulo, 2006) (Anura: Leptodactylidae) from a rubber tree plantation in southern Bahia, Brazil. Studies on Neotropical Fauna and Environment, 50(2), 73–79. https://doi.org/10.1080/01650521.2015.1048613

Secor, S. M. (2005). Evolutionary and Cellular Mechanisms Regulating Intestinal Performance of Amphibians and Reptiles. Integrative and Comparative Biology, 45(2), 282–294. https://doi.org/10.1093/icb/45.2.282

NAYA, D. E., & BOZINOVIC, F. (2004). Digestive Phenotypic Flexibility in Post-Metamorphic Amphibians: Studies on a Model Organism. Biological Research, 37(3). https://doi.org/10.4067/s0716-97602004000300002

Regal, P. J., & Gans, C. (1976). Functional Aspects of the Evolution of Frog Tongues. Evolution, 30(4), 718. https://doi.org/10.2307/2407812

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Eurycea wallacei

A small, permanently gilled salamander.

has little pigmentation, being mostly pink or whitish with scattered black spots on the back and sides

The limbs are slender limbs and the eyes are very reduced.

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Amphiuma tridactylum

A large, eel-like aquatic salmander.

three-toed amphiumas have tiny limbs, a single gill slit, no external gills, and no eyelids

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Plethodon shermani

lungless

relies on cutaneous respiration

possess naso-labial grooves, which aid in chemoreception

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Eurycea spelaea

cave-dwelling salamander

this is the only known blind, troglobitic salamander which undergoes a complete metamorphosis

the reduced eyes are dark spots visible through the partially fused eyelids

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Tongue evolution in the lungless salamanders, family plethodontidae. I. Introduction, theory and a general model of dynamics

Plethodontid salamanders catch their prey by extending their tongue from their mouth. To construct a working theory for tongue motions, researchers looked at the theoretical mechanics of the hyobranchial skeleton. Salamanders protrude their tongue when beheaded in a certain way, and it does not retract. Examining these heads after they've been repaired and sectioned verifies the predictions. These findings are then utilized to fine-tune the working hypothesis and develop a generic model of plethodontid tongue dynamics. The primary projection (subarcualis rectus I) and retraction (rectus cervicis profundus) muscles have been found. The skeleton is folded inactively along a tractrix-shaped morphological pathway. The geometry of the track and the precise layout of the folded skeleton are validated by sectioned material analysis. During retraction, the skeleton unfolds along the track and spreads into the resting condition. The model established here will be utilized as a foundation for making predictions about family selection patterns and for analyzing comparative and evolutionary research.

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Evolution of coprophagy and nutrient absorption in a Cave Salamander

The transition from carnivory to omnivory is a difficult one to comprehend. In theory, the capacity to eat at many trophic levels improves an animal's fitness in a new habitat. Because most underground ecosystems lack light and photosynthesis, they have a limited number of trophic levels, making food shortage a problem in many subterranean environments. Expanding the range of one's diet is one way to avoid famine. Grotto Salamanders (Eurycea spelaea (Stejneger, 1892) have been observed voluntarily ingesting bat guano, defying the belief that salamanders are purely carnivorous. They investigated whether grotto salamanders had widened their diet as a result of cave adaption and discovered that, despite coprophagous activity, salamanders cannot get enough nutrition from bat guano alone. The findings indicate that coprophagy developed before physiological or intestinal biome adaptations.

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Megophrys monticola

pelobatid frog of Malaysia

have umbrella-like mouths with which to suspend themselves from the water's surface

the tongue is shaped like a Ping-Pong paddle with the long cylindrical "handle'"

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Rhinophrynus dorsalis

peculiar burrowing toad from Mexico

It is toothless, has a very small head, tiny eyes, a pipidlike tadpole

had a tongue like a mammal, free in front rather than behind

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Leptodactylus ocellatus

Large nocturnal frog

Inhabits all varieties of ponds, rivers and even lakes and can often be found in strongly anthropogenized areas.

Least diverse species of Leptodactylus

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Rana temporaria

Has a squat body

no tails

They have a wide, flat head connected to a short, solid body

observed to have presumptive hair cells in the epithelium of the common macula soon after it forms

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Functional Aspects of the evolution of Frog Tongues

The tongue has been employed as a "taxonomic trait" in the categorization of frogs since the 19th century and continues to be used today (d. Lynch, 1973). Amphibians are notable for having a tongue that is propelled from the mouth to impact on, adhere to, and pull prey into the buccal cavity. Although this is sometimes listed as a defining feature of the class, the tongue appears to be flipped out rather than moved incrementally only in certain salamanders and frogs. Its movement is therefore similar to that of chameleons' projectile tongue, rather than anteaters', birds', and snakes' just extendable tongue. Frog tongues are propelled anteriorly by a forward shift of the hyobranchial skeleton, whereas salamander tongues are propelled anteriorly by rotation of "soft" tissues around the mandibular symphysis (Regal, 1966; Ozeti and Wake, 1969; Lombard and Wake, 1976; Larson and Guthrie, 1976).

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Morphological changes in the alimentary canal of the common frog ( Rana temporaria L.) in the annual cycle

Juszczyk et al. (1966) studied the seasonal fluctuation in gut mass and histology (the anatomy of the stomach and intestinal mucosa) in five age groups and both sexes of Rana temporaria. The study's main findings are as follows: a) individuals have marked seasonal variation in gut development, which is correlated to the annual feeding cycle; b) female gut attains a much greater mass than male gut during the period of high feeding activity; c) the beginning of gut regression in the middle of terrestrial life (i.e. the active period) suggests endogenous control of gut changes. Naya et al. (2003b) assessed seasonal variation in the intestine length of the South American common frog based on this previous work (Leptodactylus ocellatus). Their findings not only confirmed the occurrence of seasonal variations in eating and reproductive activities but also revealed a considerable sex influence. Indeed, in men, intestinal length flexibility appears to be linked to the yearly eating cycle, but in females, it appears to be primarily linked to the reproductive cycle. Furthermore, females' seasonal fluctuations in stomach length were more noticeable than men's.

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The figure above depicts the digestive system of a frog which is composed of digestive tract or alimentary canal along with the associated digestive glands.