Why are amphibians so cannibalistic?

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Many amphibians show cannibalistic tendencies, with

• larvae eating unfertilised eggs (https://news.mongabay.com/2016/01/rediscovered-tree-frogs-get-their-own-genus-eat-their-own-eggs/),
• larvae eating fertilised eggs (https://onlinelibrary.wiley.com/doi/10.1111/eth.12995),
• larvae eating larvae (https://phys.org/news/2017-05-tadpoles-unknown-adults-cannibalistic-siblings.html),
• larger adults eating smaller adults (https://www.nature.com/articles/d41586-021-02317-9#:~:text=Tadpole%20terror&text=Although%20adult%20cane%20toads%20are,in%20a%20single%20clutch2. and https://www.zmescience.com/science/biology/cane-toad-cannibal-01092021/),
• adults eating larvae (https://www.dailymail.co.uk/sciencetech/article-6527131/Cannibal-frogs-eat-tadpoles-prefer-taste-relatives-offspring.html), and even
• larvae eating adults (https://www.jstor.org/stable/1445301).

The details of this cannibalism are intriguing.

For example, in certain frogs (e.g. Scaphiopodidae: Spea bombifrons, https://www.inaturalist.org/taxa/26699-Spea-bombifrons) and salamanders (e.g. Ambystomatidae: Ambystoma tigrinum, https://www.inaturalist.org/taxa/26749-Ambystoma-tigrinum), the larvae are polymorphic with certain morphs practising cannibalism particularly often (also see https://zooatlanta.org/frogs-and-their-diets-some-interesting-exceptions/).

And, in certain salamanders, cannibalism can be piecemeal, with larvae surviving partial consumption by regenerating the lost body-parts (https://news.northeastern.edu/2019/10/21/northeastern-biology-professor-studies-axolotl-regeneration-to-further-understand-human-stem-cell-therapy/).

It could be argued that, despite these intrigues, cannibalism in amphibians is unsurprising given the extreme fecundity of the species involved.

Indeed, most amphibians produce so many eggs that some of the offspring are superfluous, particularly where seasonal pools dry so rapidly that whole cohorts risk death. Cannibalism consequently may make little difference to overall survivorship.

However, there are two unrecognised questions about the consumption of conspecifics by amphibians.

Firstly, how can certain frogs combine cannibalism with parental care?

And secondly, how can certain salamanders combine cannibalism with a reliance on toxicity as their main defence against predators? Many salamanders (https://en.wikipedia.org/wiki/Salamander) are toxic enough to depend on warning colouration (aposematism, https://www.pnas.org/doi/10.1073/pnas.2133521100#:~:text=Aposematism%20is%20a%20widespread%20trait,of%20warning%20coloration%20and%20toxicity. and https://www.sciencedirect.com/science/article/abs/pii/S1084952113000700 and https://www.nature.com/articles/s41598-021-97206-6), yet some of these species are cannibalistic, at least at the larval stage.

The puzzles are as follows.

• How does it make sense for survival of offspring to be boosted by parental care but depressed by cannibalism within the same species? And
• How does it make sense for flesh to be toxic and yet worth cannibalising?

My new explanation invokes a fundamental aspect of amphibian biology, namely the risk of microbial (particularly fungal) infections where permanently moist skin is frequently exposed to the air.

Given that microbes can mutate rapidly, it stands to reason that a regular adjustment of immune responses is crucial for amphibian survival.

The lymphatic system is extremely developed in frogs, with extensive sacs of lymphatic fluid located just under the skin over much of the body, and an intricate system of ‘lymph hearts’ (https://en.wikipedia.org/wiki/Lymph_heart) for circulation of this fluid. The greater development of the lymphatic system in amphibians than in other vertebrates (https://pubmed.ncbi.nlm.nih.gov/23640588/) suggests that microbial attack is as important as predation in this class.

Most frogs eat their own skins during skin-shedding (ecdysis, https://en.wikipedia.org/wiki/Ecdysis), which occurs frequently throughout adult life. This can be seen as tantamount to partial ‘self-cannibalism’, in which the frog samples the microbes on its skin, while at the same time removing them in a tactic combining hygiene with self-immunisation.

Cannibalism provides not only food but also potential information for the immune system about pathogens and antigens. Therefore, its antimicrobial benefits may outweigh certain conflicts of interest with respect to parental care and food value.

For example:
In various species of amphibians, one or both parents continue to guard eggs, larvae or – in the case of the frog Stefania (https://www.inaturalist.org/taxa/24207-Stefania-evansi) – even metamorphosed juveniles.

Part of the benefits of such guarding may be secretion by parental skin of appropriate antibiotics into the watery medium of the offspring. Indeed, certain toxins produced by the skins of amphibians may perhaps have an antimicrobial as well as an anti-predator function. Such mechanisms would potentially compensate for any losses through cannibalism.

My interpretation is that amphibians may be immunising or even inoculating themselves by means of cannibalism.

Might this explain why an ostensibly self-defeating practice has evolved to be so common across this class of vertebrates?

Posted on June 14, 2022 06:46 PM by milewski