The tender art of tadpole parenting

14 Min Read

This article originally appeared on Well-known magazine.

Most amphibians They are not exactly loving parents; they simply find a mate and release as many eggs or sperm as possible, hoping that at least some of the fertilized eggs will produce viable larvae, and that at least some of those larvae will survive to adulthood.

But in as many as one in five amphibian species, one or both parents remain present to care for their offspring, using a dizzying variety of strategies.

The most famous amphibian parents are brightly colored poisonous frogs, a group of about 200 species that will appear repeatedly in this article. Yet their parenting skills may not be as exceptional as once thought, says biologist Jennifer Stynoski of the University of Costa Rica, who decided to study this group when she saw them on a field trip as a student years ago. “I think they just got a lot of attention because they are so beautiful. They are very cute to study.”

So: what makes an exemplary amphibian parent? There is still much to discover, but some common principles have emerged.

Stay away from the water

Unlike reptiles and the birds that evolved from them, the ancestors of today’s amphibians never evolved eggs with a hard, waterproof shell. This means that their eggs need water to survive, as do the gill larvae that usually wriggle out.

Yet the ponds where many amphibians lay their eggs are full of other animals, many eager to supplement their diet by slurping down a mouthful of eggs. “This must be a reason why so many species have evolved ways to lay their eggs outside the water,” says behavioral ecologist Eva Ringler of the University of Bern in Switzerland.

In a recent one article in the Annual overview of ecology, evolution and systematicsRingler, Stynoski and colleagues describe the different places where amphibians have evolved to lay their eggs. Some species build bubble nests to keep their eggs afloat; others place them on plants above the water so that the larvae fall in immediately after hatching. Some have gone even further: the wonderfully bizarre Surinamese toad (Pipa pipe), for example, stores eggs in special brood pouches on its body.

Keeping eggs out of water is especially common in the humid tropics, where the eggs, larvae and parents are less likely to dry out. “Yet amphibian parentage is not limited to the tropics,” says Ringler. “The male of the midwife toad (Alytes midwives) here in Switzerland, for example, protects the eggs by carrying them between his hind legs.”

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The male midwife toad (Alytes midwives, top) protects the eggs it has fertilized by carrying them between its hind legs. Females of the marbled snoutbill (Hemisus marmoratus), also known as the shovel-nose frog, can build a slide to help tadpoles get to the water from an underground breeding chamber. CREDITS: BERNARD DUPONT (ABOVE), KELLY ABRAM (BELOW) / WIKIMEDIA COMMONS via Knowable Magazine

Collaborate (if necessary)

Depending on the species, parental care in amphibians is provided by father, mother, or both. The developed strategy is linked to the way in which a species reproduces. Salamanders and salamanders are an interesting group to illustrate the importance of this, says evolutionary biologist Balázs Vági of the University of Debrecen in Hungary, who has conducted a number of studies comparing parenting styles in amphibians to understand how they came about.

In certain salamanders – thought to behave most similarly to the common ancestor of the entire group – the eggs are fertilized outside the body, with the female laying eggs and the male releasing his sperm on top. In these species, males do most of the care, which largely involves protecting the eggs from predators. “This makes sense,” says Vági, “because they are sure they are the father, and the females might escape before fertilization is complete.”

But in most salamander species and also salamanders, the eggs are fertilized in the female’s body, albeit somewhat indirectly: the male deposits a sperm packet that the female then picks up with her cloacal opening. In this case it is often the male who leaves and the female who takes care of the offspring. “The lineage is more obscure and the female is actually bonded to the developing larvae,” Ringler explains, so males can get away to mate with other females. “Therefore, paternal care is very unlikely in internally fertilizing species.”

In poison frogs, fertilization takes place outside the female’s body, but usually outside the water. This makes quite a difference in the evolution of rearing strategy: it is easier for a female to be sure that her chosen mate is the only one to fertilize her eggs – which can be difficult in a crowded pond – and for a male to be sure that the offspring are his.

This could be why coparenting evolved in these species. Males will defend a territory very vociferously, and females will look for a male with prime real estate to lay eggs in relative privacy. Each gender plays its role in care activities, but experiments have shown this if one parent disappears, the other can take over. Assured of their paternity, men especially want to take care of the eggs they find them in their territory, while females are more choosy.

Some amphibian parents cooperate with others of their kind. In some salamander species females care for the eggs and young in a shared nest, possibly without knowing which of the little ones are theirs. And in species like the foam-nesting African tree frogs, a female will work with several males to knock her secretions into a foam nest where she can lay her eggs. While the males avoid the physical skirmishes common to other frogs, they all release their sperm into the same nest, and these sperm compete to fertilize the eggs.

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Animal photo
Amphibians have evolved a wide variety of parental care behaviors, both in water and on land. Credit: Knowable Magazine

Keep going

Some species of amphibians that lay their eggs on land skip the larvae stage altogether and emerge fully formed from the mother. But many species do release larvae, and these usually need help finding water. Females of the marbled snout frog, also called the shovelnose frog (Hemisus marmoratus), lay eggs in underground breeding chambers and can later build a slide to help move the larvae to the water.

Some frogs carry tadpoles on their backs. “We see enormous diversity in the way different poison frogs perform this task,” says Ringler. Some species, such as the brilliant-thighed poison frog (Allobates femoralis) who she studies in French Guiana, carry many tadpoles at once to a large body of water. Others, such as the strawberry poison frog (Oophaga pumilio) that Stynoski is researching in Costa Rica, wear them one by one to small pools of standing watersuch as those that collect between the leaves of bromeliad plants high in the trees.

Bring snacks

These small pools of rainwater on plants often contain little or no food. Therefore, some frog parents have evolved to feed their offspring. Female strawberry poison frogs regularly visit the pools with their young to lay unfertilized eggs that their tadpoles then eat. “Research has shown the eggs are essential for the survival of the offspring,” says Stynoski, “and also transmit protective poison to the larvae.” In other frog species, eggs are not essential, but act as complementary food.

Perhaps the most remarkable ways of feeding offspring are found in the least known group of amphibians, the limbless, worm-like caecilians, which spend much of their lives underground, feeding on worms and insects. “Like other amphibians, caecilians support their young in different ways,” says biologist Alex Kupfer of the State Museum of Natural History in Stuttgart, Germany. “But in some species we have found something truly unique: females store lipids and sugars in the outer layers of their skin, which they then colanders as food for their young.”

As if that wasn’t enough, a recent study shows Science reports that in the egg-laying ringed caecilian (Siphonops annulatus), the mother secretes a milky substance from her cloaca that the young eat quickly. It appears she does this in response to the physical touch and perhaps sound signals of her offspring.

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Animal photo
The young of the ringed caecilian (Siphonops annulatus) feed on a milky substance released from the mother’s cloacal opening. CREDIT: PHOTO BY CARLOS JARED via Knowable Magazine

Why not invite them?

Even stranger perhaps are some caecilian species, such as the Rio Cauca caecilian (Typhlonectes natans), as well as some salamanders, which lay eggs hatch in the female. In some caecilians, the young then feed on cells or secretions from a nutritious structure in her oviduct. Such an arrangement has not been found in other groups of amphibians, although a number of species are viviparous and live young emerge from the reproductive tract. While this limits the number of offspring a female can produce, it also increases their chances of survival.

This is often seen in species that live in very harsh conditions, such as alpine salamanders in areas that can be covered in snow for 10 months of the year, Ringler says. “The only viable strategy there is to keep the larvae in the body for as long as possible and to give birth to fully developed young.”

Some amphibian species even ingest their offspring. The male of Darwin’s frog (Rhinoderma darwinii) holds eggs in its vocal sac until they hatch. “If you’ve never seen a video of this, you should check it out,” says Stynoski. “Him actually vomits little baby frogs.”

Don’t let yourself become extinct, please

Unfortunately, there is a closely related species from Chile, Rhinoderma rufum, which regurgitates tadpoles, is critically endangered and possibly even extinct. And two unique Australian frog species in which females harbor tadpoles in their temporarily inactivated stomachs are now undoubtedly gone. “It’s always tragic to lose a species,” says Vági, “but to lose such a unique behavior feels even worse.”

In light of habitat destruction and climate changeSome amphibian parents may be able to help their offspring survive by keeping them safely indoors, moving them, or providing extra food. But many of these fantastically complex behaviors, and the species that depend on them for survival, are likely very vulnerable to environmental change.

“Because of their different life stages, amphibians require different habitats and different types of food throughout their lives,” says Ringler, and the loss of just one of these could put them at risk. “That’s the most worldwide threatened vertebrate group. We really need to take better care of them.”

This article originally appeared in Well-known magazine, an independent journalistic initiative of Annual Reviews. Register for the newsletter.

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