Another odd aspect of parental care in amphibians: the lack of cloacal gestation

@alexanderr Would you care to comment on this at the draft stage?
@tonyrebelo

(writing in progress)
  
Frogs (https://www.inaturalist.org/observations?place_id=any&taxon_id=20979&view=species) and salamanders (https://www.inaturalist.org/observations?place_id=any&taxon_id=26718&view=species) have some remarkable ways of caring for eggs and larvae.

They manage to insert offspring into the most improbable nooks of their bodies, in many cases, these are crannies invented specially for this purpose.

To describe even some of these tactics would fill up a whole page: mouth pouches, stomach, cavities in the back, pouches on the back, etc.

It would be no exaggeration to say that the pouch system of marsupials looks unimaginative by comparison.
 
But what I noticed for the first time recently is how odd the following is:
No frog takes up the offspring, after external fertilisation, into perhaps the obvious place: into the cloaca and the urinary or reproductive tracts.

Virtually all frogs have external fertilisation. However, this has not stopped various frogs from fabricating ‘substitute-wombs’ on all sorts of body parts. This includes males (fathers).

So, one would think that the reproductive tract itself would be one of the first options to be taken.

But not so: of the 4800 spp. of frogs worldwide, I have not heard of a single one that reinserts eggs or larvae into the cloaca, whether female or male.
 
Please bear in mind that various frogs and salamanders do retain offspring in the uterus. In some cases they remain there until metamorphosis is complete. This qualifies as ovovivipary (https://en.wikipedia.org/wiki/Ovoviviparity).

So, it would theoretically be possible to use the uterus to gestate offspring by having the offspring re-enter the mother’s body via the cloaca. But such is not the case.
 
Even the male could theoretically adopt such a system, despite lacking a uterus. This is because the anuran bladder is much more than a short-term reservoir for urine.

In some frogs, e.g. Ranoidea (Cyclorana, https://www.inaturalist.org/observations?taxon_id=517030), the bladder stores an extremely large amount of water during aestivation. Virtually no frog drinks. Therefore, in many frogs the water in the bladder is derived from water absorbed through the skin, including a special water-absorbing patch in front of the cloaca on the ventral side of the body.

Why has some lineage of frogs not evolved such that males take up offspring through the cloaca, and ‘gestate’ them in the bladder? After all, mouth pouches and the stomach have indeed been evolutionarily drafted into ‘gestation’.
 
There is another twist to this tale. Although external fertilisation is typical for frogs, salamanders instead practise internal fertilisation in a strange way.

Males of salamanders do not copulate, despite complicated and dedicated courtship. Instead they deposit a large spermatophore on the substrate and then encourage or maneuver the female to position her cloaca above it. The female then uses ‘cloacal lips’ to take hold of the spermatophore (which is a substantial object, far bigger than any phallus would likely be), and laboriously draw it up into her reproductive tract. 
 
What is remarkable about this rigmarole is that the trouble taken by both sexes is actually more, not less, than that required for copulation.

There must be some good reason why salamanders insist on external fertilisation by means of a ‘laid’ spermatophore that is exposed to the outside atmosphere and the substrate, before being self-inserted into the female’s reproductive tract.

One species of salamander has even developed a quasi-phallus on the dorsal base of the male’s tail, which is used to stimulate the cloaca of the female before she self-inserts the spermatophore. It is as if salamanders will do everything sexually except actually copulate.
 
It is evident that, for salamanders and indeed frogs, to pick up eggs or larvae into the cloaca and then ‘brood’ them in the reproductive or urinary tracts would not be as far-fetched as it might at first seem.
 
Bottom line:
One of the most remarkable aspects of amphibians is that they ‘brood’ their externally fertilised young in what seems like any cavity they can find or fabricate on, or in, their bodies. Except, that is, for the obvious ones: the cavities to which access would be gained through the cloaca.
 
(writing in progress)

Posted on 15 de julho de 2022, 04:26 AM by milewski