Inside the Wild Ways Many Creatures Make Milk

Milk seems to set mammals apart. The very name of our beastly family, after all, comes from the word mammae, a term for the chest glands that produce milk. The feature has bound mammals together through hundreds of millions of years, including egg-layers like the duck-billed platypus that secrete milk from pores in their skin. As zoologists have continued to explore life on Earth, however, they keep finding unexpected milk-producers among creatures as distantly related as jumping spiders and penguins. Milk, of one form or another, is not actually synonymous with mammals.

Earlier this year, in the journal Science, biologist Carlos Jared of the Butantan Institute, in São Paulo, Brazil, and colleagues announced the discovery of one such unprecedented milk provider. Females of the worm-like amphibian Siphonops annulatus exuded fatty fluid—milk—for their new offspring over the course of two months. The milk didn’t come through a nipple, the researchers found, but from within the amphibian’s cloaca in a case of convergent evolution. The babies even made clicking sounds as they approached their mother for milk, not unlike hungry puppies eager for their next meal.

Zoologists knew something was up with these amphibians, called caecilians, for decades. “Almost 70 years ago, a study was published showing that female viviparous caecilians kept embryos inside their oviducts at unfavorable times for their birth,” Jared says. The embryos were nourished by a milk-like substance secreted in the oviduct. And that was hardly all. Over time, zoologists kept finding examples of baby caecilians feeding on the mucus and skin of their mothers—a strategy quite different from the lay ’em and leave ’em behavior many frogs employ to kick-start the next generation. Caecilians showed remarkable parental care for amphibians that superficially looked like noodles.

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In the case of Siphonops annulatus, babies of the species spent a lot of time around the cloaca of their mothers, which seemed to ooze a sticky substance. “The puppies are excited and are constantly caught competing for places around it, placing their heads almost entirely inside it,” Jared says. When the researchers studied the makeup of the fluid, as well as the behavior, they found the offspring were feeding on milk specifically released by the mother—in response to the high-pitched sounds of the babies.

Surprising as the finding was, it was far from the first time researchers have found non-mammals that have evolved their own forms of milk. The substance is closely associated with parental care, providing offspring with essential food and keeping them close when they’re at their most vulnerable. What became standard for mammals has nevertheless evolved multiple times among animals, including those that we normally don’t associate with doting parenthood.

Naturalists have long known that some birds—such as pigeons, penguins and flamingos—produce their own form of milk for their offspring. In 1786 the English surgeon John Hunter observed that pigeons of any sex form a fatty, protein-rich milk in the lining of the crop in their throat. Studies and observations since then have underscored that these birds are not just regurgitating food but are forming an avian milk to feed their hatchlings while the babies are nestbound. Given that birds evolved their own form of milk several times, and birds inherited many abilities from their dinosaur ancestors, researchers suggest that some dinosaurs may have cared for their young in a similar way.

Studies and observations about non-mammal milk has often placed the word milk in quotes, as it is not exactly the same as mammalian milk and doesn’t share the same evolutionary origin. When speaking of such liquids in non-mammals, says University of Cincinnati entomologist Joshua Benoit, “we usually call it a milk-like substance.” Milk, proper, is usually restricted to mammals. Nevertheless, he notes, “for all intents, it does the same thing, and even some of the proteins present are similar.” Various organisms secrete fluids that are directly analogous to milk and used to nourish offspring in exactly the same way. “Tsetse flies produce a milk-like substance that is white and rich in fat, secreted by females, and nourishes the young,” Benoit says, noting “the only thing it lacks is being the right animal.”

Invertebrates, in particular, appear to have adapted milk-like substances multiple times. Spiders are not exactly the first animals we think of when it comes to parental care. But in 2018, again in Science, biologists announced that they’d found a jumping spider that provides milk to its offspring until the spiderlings are grown enough to fend for themselves. In this case, the jumping spider produces droplets of a milk-like fluid from a furrow on its abdomen. The parent spiders then continue to give milk to their babies even after the spiderlings have grown to adult size. Until the youngsters consistently catch their own food, they have milk available to them.

“Milk-like substances have evolved among arthropods a few times,” Benoit says. Flies evolved the fluids at least three times, cockroaches once or twice, spiders have gotten in on it, and even male ants did the same. The question is why milk continues to originate time and again.

Working out when different organisms evolved milk and close parental care is a more challenging task. The whole point of milk is to be ingested, and the soft tissues that produce milk are rarely preserved in the fossil record. Working out the prehistory of milk among mammals is relatively straightforward—all mammals make milk, and so it must have been possible in the last common ancestor of ourselves and the duck-billed platypus—but when it happened for the other animals that have evolved the same behaviors is not so straightforward.

In most cases, Benoit says, some form of parental care comes first and then opens the possibility for parents to offer their offspring nutritious substances like mucus, skin or milk. The same amphibian recently discussed in Science, for example, also allows its babies to eat some of its skin, and some baby frogs seem to suckle in a similar way from their parents. If an orifice or pores secrete fatty or protein-rich fluids that nourish said offspring, over time it can become modified into an alternative milk.

The close connection between milk and parental care might give paleontologists some direction in where to look for when different animals began producing milk. In 2021, researchers described a 99-million-year-old spider that was preserved with an egg sac filled with spiderlings. Modern spider species that carry such egg sacs often guard their offspring for an extended period of time, and carrying the egg sac at all is a form of parental care. Looking for such associations among fossilized spiders and other invertebrates might help researchers find good candidates for species or evolutionary groups that were making milk-like substances in the deep past. The interrelationships of modern milk-producers can help refine the search, as well. All tsetse flies create a milk-like substance, Benoit notes, and so the ability probably goes back to the group’s origin about 50 million years ago.

Parental care and milk provisioning have undoubtedly helped mammals thrive from the time of the earliest dinosaurs to the present day, yet mammals are not unique in assisting their offspring. Many forms of life have evolved to do as the beasts do, making parental sacrifices to help the next generation thrive. The next time you see a pigeon on a sidewalk—or a caecilian in a tropic forest, if you’re so lucky—consider the care we share in common.