Amphibian Milk

It does the job

An amphibian of a sort you’ve probably never heard of

Provides parental care by

Offering highly nutritious skin for her babies to eat, and

Making something highly analogous to mammal’s milk, which is

Solicited by the babies when they click and chirp, and then,

The babies collect their mother’s milk from around and within her cloaca.

---

These findings are based on brand new research¹ that most people probably don’t care a bit about, but I think it provides a lovely opportunity to point out a few of the many unusual goings-on in the animal kingdom. Let’s go.

An amphibian of a sort you’ve probably never heard of

The first vertebrates to come on to land were tetrapods—the four footed ones—and the first tetrapods were amphibians. Today, there are three groups of living amphibians, but one of them, the one under discussion today, is unknown to most people.

The three clades of extant amphibians are:

• Anura (frogs and toads): Leggy, tail-less (as adults), and jumpy, this is by far the largest group of amphibians, with more than four thousand species. They are most speciose in the tropics, but can be found in most parts of the world except near the poles, and on some oceanic islands which have proven difficult for them to access. All toads are frogs; not all frogs are toads.

• Caudata (salamanders and newts): Superficially lizard-like in form, with front and hind legs of roughly equal size, there are only several hundred species of salamanders. They are generally small, and most speciose in the southeastern United States, a center of likely origin from which they have radiated out. A few east Asian species reach almost six feet in length and are, some say, delicious. All newts are salamanders; not all salamanders are newts.

• Gymnophiona (caecilians): The species in this clade are pronounced very much like people from Sicily, although there are no caecilians among the Sicilians. Entirely tropical, these burrowing, legless amphibians appear superficially like worms, but they are no more worms than salamanders are lizards. There are more than two hundred species of caecilians, they’re reclusive and cryptic, and not much is known about them outside of their own families.

Caecilian mother and babies (Siphonops annulatus), photo by Carlos Jared

An amphibian of a sort you’ve probably never heard of

Provides parental care

As mammals, we take parental care for granted. Mammals have obligate parental care in the form of gestation (for almost all of us) and lactation. The conditions that drive the evolution of parental care are varied, though, and if you’re wondering if a particular species has it, one quick and dirty rubric is to ask how many offspring they tend to have. If an individual broadcasts hundreds or thousands or yes, tens of thousands, of eggs or sperm into the environment and hopes for the best, there is little chance that they’re sticking around to offer support to their babies afterwards. If, on the other hand, you have one or two or even just a few babies at a time, the chances go up that you and your children will come to know each other, at least for a while. Most amphibians don’t provide parental care, but a surprising number of them do, including, as it turns out, some caecilians.

---

Offering highly nutritious skin for her babies to eat

There are lots of things that happen in nature that humans don’t like the sound of, and this is probably one of them. You’re having a few babies, and they start out helpless, and they’re going to be hungry, so what do you do? One solution—one that is not widely adopted across the animal kingdom, but it does work—is to imbue your skin with extra nutrients, notably lipids, after your kids hatch out of their eggs, and let them dine on you, without actually eating you alive².

Also? Given the phylogeny and biogeography of caecilians that are known to engage in skin-feeding, this behavior has probably been going on for more than one hundred million years³.

---

Making something highly analogous to mammal’s milk

Now wait a minute. Isn’t milk a mammalian innovation? Yes, and no. The particular liquid with which mother mammals feed their babies after birth, which contains not just calories but also immunological and likely circadian information as well, and which, further, facilitates the bond between mother and child that encourages even more parental care…that is indeed new with mammals. But several other organisms outside of mammals have also innovated ways to feed their young with maternal secretions which, if we think broadly about what milk is, do seem to be a sort of milk. It’s not mammal’s milk. But it is a kind of mother’s milk, full as it is of lipids and carbohydrates at a bare minimum. And caecilian mothers of the species in question—Siphonops annulatus, which lives in the Atlantic forests of Brazil, including in cacao plantations⁴ from which these particular individuals were collected, a species which is now rather famous after this week’s research publication, not that it cares a bit about that—these caecilian mothers not only feed their young by allowing their babies to munch on their own skin, but also by secreting nutritious liquid from their oviducts for their babies to eat.

---

Solicited by the babies when they click and chirp

Human mothers who have breastfed their children know that while milk sometimes emerges unbidden, more often—and more conveniently—it needs to be requested by the baby. Some such requests are quite subtle, even the smell of one’s infant being sufficient, sometimes, to produce milk; a hungry infant latching on and beginning to suck furiously is a rather less subtle cue.

In caecilians, the babies nuzzle around the back end of their mother (more on that next), but they also emit high-pitched clicking sounds, sounds both softer and more fragile seeming than the utterances of baby birds as they cry for food. And these sounds of the baby caecilians appear to facilitate the production of milk.

---

The babies collect their mother’s milk from around and within her cloaca.

The cloaca of female amphibians—indeed of reptiles as well, including birds of both sexes—is the opening at the rear end of the body which serves both reproductive and excretory purposes. (Male caecilians have a cloaca that is split, one half becoming the phallus. If you want to know more, including on the subject of ornamentation in the phallus of caecilians, here’s a little light reading.)⁵

The oviducts of a mother caecilian are not on the outside of her body, of course, so the oviducal secretions need to find their way to her babies⁶. The secretions do find their way down towards the mother’s cloaca, once solicited by both auditory and tactile cues from the babies, and then the babies, in their eagerness, sometimes stick their heads up inside the mother’s cloaca and lap up the milk from there.

It may not be elegant, but it does seem to do the job.

---

Once again:

An amphibian of a sort you’ve probably never heard of provides parental care by offering highly nutritious skin for her babies to eat, and making something highly analogous to mammal’s milk, which is solicited by the babies when they click and chirp and then, the babies collect their mother’s milk from around and within her cloaca.

All in a day’s work for hardworking caecilian mothers.

Red-eyed tree frog (Agalychnis callidryas), which most definitely does not make milk. Photo by me.

1. Original research: Mailho-Fontana et al 2024. Milk provisioning in oviparous caecilian amphibians. Science, 383(6687): 1092-1095. And here’s a lay summary in Nature: https://www.nature.com/articles/d41586-024-00686-5

2. Kupfer et al 2006. Parental investment by skin feeding in a caecilian amphibian. Nature, 440(7086): 926-929.

3. Wilkinson et al 2008. One hundred million years of skin feeding? Extended parental care in a Neotropical caecilian (Amphibia: Gymnophiona). Biology Letters, 4(4): 358-361.

4. Cacao plantations—orchards where chocolate is grown—are also excellent places in which to find the young of dart-poison frogs, whose parents deposit tadpoles in the discarded husks of cacao fruits.

5. Gower & Wilkinson 2002. Phallus morphology in caecilians (Amphibia, Gymnophiona) and its systematic utility. Bulletin of the Natural History Museum: Zoology Series, 68(2): 143-154.

6. In some viviparous species of caecilians (that is: giving live birth, as opposed to laying eggs) the oviducal secretions that we are calling milk are easily accessed by the young while still inside their mother’s body.