The competitive edge of the amphibian way (fatherly slime is provident, not primitive)

(writing in progress)

In the evolution of vertebrates from fishes to mammals, amphibians (https://en.wikipedia.org/wiki/Amphibian) seem to represent the primitive stage of transition from water to land.

Amphibians first appeared 370 million years ago, which is approximately threefold the age of mammals and birds.

To this day, amphibians remain limited by their ancestral dependence on water in the larval stage. Although the adults usually leave the water to bask and forage, the larvae usually still respire by means of gills and tend to be confined to ponds and streams.

One way in which various amphibians have reduced their reproductive ties to water-bodies is to practise diverse methods of parental care. These keep the larvae well hydrated despite hatching and developing out of the water.

Indeed, living amphibians show far more parental care than living reptiles. This may seem to be mainly as a compensation for an evolutionary hangover.

However, there is another way of interpreting this.

Far from suffering from an evolutionary hangover, living amphibians are competitively superior, having managed to usurp some of the most important niches on Earth where freshwater meets land. Where land is alternately wetted and dried, microbial decomposition produces a nutrient-pumping effect which promotes productivity at all levels in the food pyramid.

The unique adaptations of amphibians for straddling this boundary give them particular advantages in water-bodies too unreliable for fishes such as vernal pools, floodplains and marshes.

Indeed, the life histories of contemporary amphibians represent an acme of exploitation of salt-free environments in which there is enough water to boost productivity but enough drainage to maintain terrestrial vegetation. Several aspects of amphibian biology relate to the costs and benefits of living in an environment dominated by microbial activity.

For example, microbes are the staple diet of most tadpoles, but adult amphibians are prone to fungal infection. This susceptibility to pathogens perhaps partly explains why all adult amphibians frequently shed the whole skin and its microbial burden.

One important aspect of amphibian biology is that various amphibians show not only parental care but also devotion by the father as opposed to the mother.

This paternal (i.e. male parental) care of offspring – known by biologists but not previously interpreted ecologically – occurs in far more families, genera and species of frogs than of mammals or reptiles.

Superiority of amphibians in fatherly care is hardly explained by emancipation from water. However, it is consistent with the longevity of amphibians compared with mammals or reptiles of similar body sizes. Although birds are particularly long-lived among vertebrates, amphibians may also exceed birds in maximum lifespans relative to life-expectancies.

Based on their life histories, is it not true to say that amphibians emerge as having a remarkable combination of longevity and fecundity?

That amphibians are ecologically successful because of, not despite, their association with freshwater is borne out by the observation that no amphibious mammal, reptile or bird rivals tadpoles in the harvesting of microbes – the most productive food in wetlands. The lesser flamingo (Phoeniconaias minor, https://www.inaturalist.org/taxa/4259-Phoeniconaias-minor) has a staple diet of bacteria, but forages only in sodic waters unsuitable for amphibians.

In summary:
Amphibian larvae depend on water not in retention of an archaic pattern but in exploitation of ecological niches in which other Classes of vertebrates are inept competitors.

(writing in progress)

Posted on June 9, 2022 03:16 PM by milewski