Marlon Lopez MMG1 Design / Vandame / Shutterstock / Katie Martin / <i>The Atlantic</i>

From the start, Zhanqi Chen realized that something was odd about the spiders.

He had first spotted the species, known as Toxeus magnus, in a park in Singapore, and whenever he’d peer into their silken nests, he’d usually find a centimeter-long adult female surrounded by several smaller youngsters. That was weird. Most spiders are solitary, and even cannibalistic toward their own kind. There are a few kinds of sociable spiders that live in colonies, but Toxeus magnus shouldn’t have been one of them. It’s a jumping spider, a group generally known for being loners. And yet, there it was, apparently living in family groups, where the mothers cared for their young—another rarity among spiders.

The mystery deepened when Chen collected several of the spiders and reared them in his lab at the Xishuangbanna Tropical Botanical Garden. He noticed that after hatching, the spiderlings would stay in their home nests for at least three weeks. During that time, they never left, and their mothers never ventured out to bring back food. And yet, the spiderlings would quadruple in size. What were they eating?

Chen discovered the answer when he noticed a spiderling that seemed to have fixed itself to its mother’s underside. It wasn’t just hanging out around her. It looked like it was, for lack of a better word, suckling. Chen took one of the females, looked at it under a microscope, and squeezed its abdomen. A white droplet oozed out, and soon yellowed in the air.

It looked like milk. And for all intents and purposes, it was.

When a brood of spiderlings first hatch, their mother starts secreting the liquid from her epigastric furrow, a fold on her underside that she also uses to lay eggs. For the first week, she dabs the droplets onto the walls of her nest, and the youngsters scurry over to suck these up. After that, they drink from the furrow directly. During their crucial early period, the spiderlings rely on the milk as their only source of sustenance. It doesn’t have a lot of fat or sugar, but it’s loaded with proteins—four times as much as the equivalent amount of cow milk. When Chen stopped the spiderlings from drinking this fluid, by blocking their mother’s epigastric furrow with a dab of correction fluid, all of them died within 10 days. (The correction fluid itself didn’t affect them.)

Whether the liquid truly counts as milk depends on how you define the term. Traditionally, milk is defined as a nutritious liquid secreted by the mammary gland, and mammary glands are found only in mammals such as ourselves. But if you stretch the description to include any parental secretion that nourishes and provides for the young, then milklike stuff starts cropping up in many unexpected corners of the animal kingdom.

The tsetse fly is an insect that does a good impression of a mammal: It gives birth to live young, which it feeds within the womb with a milklike fluid. The parasitic bat flies do something similar. One species of Pacific cockroach also gives birth to live young, which it nourishes with a yellow milk, full of glittering protein crystals. Pseudo-scorpion mothers carry their hatchlings in a sac attached to their belly, and feed them with a nutritious liquid secreted from their ovaries. And pigeons (fathers included) feed their relatively helpless chicks by coughing up a chunky, fatty liquid that they secrete from their throats.

Compared with these creatures, the jumping spiders are arguably closest to mammalian lactation, in that they produce milk from a specialized organ, from which the youngsters drink over a very long time. “It would be really interesting to dissect the spiders [to see if there] was some kind of identifiable gland or something like that,” says Laura Hernandez of the University of Wisconsin at Madison, who studies lactation. And Katie Hinde, another lactation expert at Arizona State University, wants to know if the spider’s liquid contains other components that are found in mammal milk, including hormones, immune chemicals, and bacteria.

But in the meantime, “we can call it spider milk,” Hinde says. “I’m not hung up on it coming from a mammary gland. I’m interested in how it supports development.”

Twenty days after hatching, the spiderlings make their first forays out of the nest, and start hunting for small flies. But they still return to their mother to drink her milk, until finally weaning at 40 days of age. Even then, most of them stay in the nest for many weeks more, while the mother continues to care for them. She’ll throw out their molted exoskeletons, repair the nest, and evict parasites such as mites. Even when Chen dammed up the milk-producing furrow, he found that older spiderlings still benefit from their mother’s fastidiousness, and are less likely to survive in her absence.

“She feeds them well past the period when they can forage on their own, and the nursing enhances their survival,” says Linda Rayor, an entomologist at Cornell University. “That is really quite interesting, and I don’t know of any comparable data for other spiders. [It’s also] exciting that the spiders stay together long past the period where 99.9 percent of spiders have dispersed independently.”

If this unusual setup exists in other spiders, Chen hasn’t found it yet. Even close relatives of Toxeus magnus don’t produce milk, or show such prolonged parental care. So why did these traits evolve in this one species?

“We have no idea!” Chen says. He guesses that they arose because these spiders have to cope with a difficult environment, in which food is hard to get and predators are rife. The spiderlings are barely a millimeter long, even smaller than the fruit flies that they initially hunt. “Fruit flies are also good fliers, and I don’t think it’s easy for new spiderlings to catch them,” Chen says. Far better, then, for the mothers to give them a head start in life with a steady food source. Additionally, the females, for whatever reason, might find it hard to breed. If they don’t get a lot of chances at raising a new generation, it would pay to invest more heavily in the current one.

“These rare variants across the animal kingdom give us really exciting insights into the evolution of parental care,” Hinde says. All mammals make milk, so to understand why we evolved to do so, scientists have to look back in time, using genes or fossils. But species such as Toxeus magnus—an oddball milk producer within an entire family of nonlactators—make it easier to “look at what evolutionary pressures led to this huge scale-up of parental investment,” she says.

As Chen and his colleagues write, “We anticipate that [our] discoveries will encourage a reevaluation of the evolution of lactation.”

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