Best parents breathe through the skin

Today I come to speak to you about those who, for me, are one of the best parents in the animal world.

Most people associate parental care with mammals and birds, with examples such as lions, albatrosses or kangaroos. And this is normal, as this care is much more widespread in these groups.

However, there are very peculiar and breathtaking cases in amphibians. The term parental care is considered to be any behaviour by parents that increases the survival of the offspring. That is, from the oviparous vertebrates, parental care would be the nutritious food (yolk sac) present in the egg (lecitotrophy) but it is something that has evolved a lot and we will focus on some more sophisticated ones.

I would like to make a brief point by introducing the three main groups of amphibians, since two are well known to everyone, but the third is difficult to associate with this large group. The first two are the urodeles or caudata (newts and salamanders) and the anurans (frogs and toads), but the last one are the caecilians or gymnophiona, which are elongated amphibians without limbs that remind more to a snake or a giant worm than to an amphibian, but they belong to this group

Having said that, we can begin to know the great representatives of parental love in the amphibious world.

The egg caretakers (egg attendance):

This is common in frogs and toads, and there are many different kinds of caretakers. There are some species that bury themselves and their eggs as in the genus Hemisus. Others deposit the eggs in leaves and protect them, like the glass frogs (genus Centrolene), and others make foam nests (genus Leptodactylus). Sometimes it is the female that attends to the eggs, and in others the male or both.

But this does not only happen in anurans, there are also several urodeles that protect their eggs. One example are the giant salamanders (genus Andrias and Cryptobranchus), where the female deposits the eggs in an underwater cave and the male fertilizes and guards them as if he was Fluffy (Harry Potter's three-headed dog) with the Philosopher's Stone.

Some salamanders also have this dedication and lay their eggs out of the water, protecting them until the area is flooded, since the larvae are aquatic, as in the case of the marbled salamander (Ambystoma opacum).

This type of behaviour also occurs in caecilians. It will be hard to imagine how something like a giant worm protects its eggs, but in several species they coil around the eggs and protect them, as other behaviours that we will see below.

Pictures from the internet, from top to bottom:

Glass frog attending its eggs, Cochranella euknemos. Author: Jesse Delia.

Perez's snouted frog belonging to Leptodactylidae, Edalorhina perezi. Author: Andrew Gray.

Male Japanese giant salamander (Andrias japonicus) protecting the egg clutch. Author: Yukihiro Fukuda.

Female marbled salamander (Ambystoma opacum) caring its eggs. Author: Amber Hart.

I may not have convinced you that they are good parents with the aforementioned, so let's move on to the next category.

Egg transport:

There are four very distinct types here, and they're all in toads and frogs. One of them is present in Spain, and they are a good example of committed fathers, our midwife toads (genus Alytes), in which, after reproduction, the male puts the clutch of eggs between his hind legs, and will carry them with himself for a month, until the moment they can hatch, when he will take them to a pond.

In all other cases, it is the mothers who profess love and care for their offspring. A well known and somewhat macabre case is that of the aquatic surinam toad (Pipa pipa), in which the eggs are displaced onto the female's back and the female's tissue grows encapsulating them individually. There the eggs will remain until they are born as tiny toads, fully developed and able to swim freely.

Similarly, but on land, we have the example of the Hemiphractidae family, in which the eggs are also deposited on the female's back, that later develops a "pouch" like that of kangaroos, enveloping the eggs, which gives them the common name marsupial frogs.

Within this family, the development of the pouch varies depending on the species, with the genus Gastrotheca being the most complex, totally enveloping the eggs, and that skin is highly vascularized for gas exchange, in other words, helping the eggs to breathe.

The most extreme and amazing case occurs in two species that are currently (since the 1980s) extinct. They are Rheobatrachus silus and R. vitellinus. And where did they live? Of course, in Australia. These two toads are known as gastric-brooding frogs. And that's what I made you the spoiler with.

The female of this species would swallow her eggs, like a female version of the myth of Chronos. And you might ask... how can she do that? Doesn't she digest the eggs? Well, she doesn't. This happens because the eggs secrete hormones (prostaglandins) that inhibit acid secretion in the mother's stomach. In this way, the eggs develop and hatch in the stomach (evolving into another type of parental care that we will see below, the transport of tadpoles), a process that lasts about 8 weeks, in which their mother does not eat, until, on the day of birth, like Chronos with the gods, she "vomits" them.

Surinam toad (Pipa pipa) hatching.

From top to bottom:

Male common midwife toad (Alytes obstatricans), carrying the eggs.

Female Hemiphractus fasciatus with the eggs. Author: Edgardo Griffith.

Female Gastrotheca cornuta with the pouch full of eggs. Author: Brad Wilson.

Rheobatrachus silus, one of the few images of the "birth" that exist.

Tadpole/ froglet transport:

The most common case of this are the poison arrow frogs (family Dendrobatidae), which usually carry the tadpoles from the place where they lay their eggs on the ground, to bromeliads with water inside and are like bathtub-nurseries, or to different points with water, depending on the species. Other species from different families carry the froglets, but it is less common.

But as before, there are also weirder examples here. One of them is Darwin's frog (Rhinoderma darwinii), whose male swallows the eggs shortly before they hatch and stores them in its vocal sac (the part that is inflated when calling), where they receive nutrients through the membrane, keeping them there for up to 52 days, when they complete their metamorphosis and emerge into the outside world.

And of course, there's also the bizarre example from Australia, Assa darlingtoni. From the same family as the gastric-brooding frogs in this case the male sits on top of his children at the moment the tadpoles hatch, and they move and go into two pouches that he has in the inguinal region, where they will stay for 80 days until they come out of there as toads.

"Birth" of Darwin's frog (Rhinoderma darwinii)

Parental care in Assa darlingtoni.

Offspring caring:

Of course, these dedicated parents don't just take care of eggs and transportation, they also take care of the offspring. There are some species like Cophixalus parkeri where, like humans, the young take time to become independent and stay several months in the "nest" with the parents. In other cases, as in the African bullfrog (Pyxicephalus adspersus), the male takes care of the tadpoles in the ponds, preventing them from being preyed upon, and when the small ponds begin to dry out, he builds a canal with his hind legs that allows the tadpoles to pass to the larger pond.

Parental care in a male African bullfrog (Pyxicephalus adspersus).

A curious example is that of some species of the aforementioned poison arrow frogs. The strawberry poison frog (Oophaga pumilio), shows great care for its young. The female deposits her eggs in the ground, which are cared for by the male, moistening them. When they hatch, the tadpoles are carried one by one on the mother's back to the bromeliads with water, to which she is guided by the male by calls, where she deposits them individually. The female, days later (in some species guided by the male in the same way) returns to the same bromeliads and deposits an unfertilized egg to be eaten by the tadpole concerned, which in fact only feeds on those eggs. This behavior is present in several places in the world in different species with some variations, such as laying many unfertilized eggs in ponds where there are several tadpoles.

Parental care in strawberry poison frog (Oophaga pumilio).

And I've kept one of the most astonishing and unexpected cases to the end. Remember the caecilians? Those fat worms that are actually amphibians that we talked about at first. It seems that salamanders and caecilians don't have very sophisticated parental care behaviours, as we haven't talked about them since egg attendance. Well, no, caecilians are exceptional mothers. In some viviparous species, the young feed on an outer layer of the mother's oviduct (uterus). Not only that, several oviparous species have been discovered in which a very unique phenomenon occurs. The mother, after an early development of the offspring, generates an external skin layer with a very different composition (rich in proteins and fats), which serves as food for the offspring. Although this may seem almost cannibalistic, the young eating the mother, is much more similar to lactation.

Offspring feeding on the special skin layer of their mother.

And with this I conclude the post of the great parents of the amphibian world, with behaviors so particular and unique that it is difficult to imagine how they have developed in evolution. I hope you have liked it and have discovered a new side of the frogs, salamanders and caecilians. The animal world is amazing, and sometimes the smallest thing is even more stunning than what we see with the naked eye.