Not just mammals: Some spiders nurse their young with milk

Zebras, bats, and bears do it. So do whales, tigers, and humans. These animals all nourish their newborn offspring with milk. It’s a defining characteristic of what it means to be a mammal.

According to a study published today in the journal Science, a jumping spider native to southeastern Asia does the same thing. Toxeus magnus has been found to suckle its babies with a nutritious fluid secreted by its own body. The liquid contains a solution of sugars, fats, and proteins, so the researchers, led by conservation biologist Rui-Chang Quan of the Chinese Academy of Sciences, are calling it milk.

Perhaps most surprisingly, the researchers found that spiderlings continue drinking their mother’s milk even after reaching sexual maturity. “That's super weird,” says Jonathan Pruitt, an evolutionary ecologist at McMaster University in Canada. “The fact that the parental care extends all the way until female offspring are adults is pretty surprising, eyebrow-raising.”

The evolutionary implications of this behavior are raising eyebrows as well.

Good parenting

The idea of spider milk, while strange, is actually reasonably consistent with what is known about spider parenting.

Despite their reputation as solitary creatures, various types of spiders have been seen caring for their offspring. “Many female spiders will guard their egg cases and forgo eating while they do that,” says Pruitt, who studies social behavior in spiders. “Some spiders will open their egg cases and allow their offspring to ride around on their backs like [in] The Magic School Bus.”

Other females will even regurgitate pre-digested food for their young, like birds do. Pruitt adds that some spider moms go so far as liquefying their own bodies to be consumed by their young.

But this is the first time spiders have been seen nourishing their offspring with a milk-like fluid.

Got milk?

“If a loose definition of milk is a nutritive substance that's nourishing young, then it would be considered milk,” confirms Amy Skibiel, a lactation physiologist at the University of Idaho.

“When you think about other non-mammals that produce milk-like fluids, it does become a little less surprising,” Skibiel says. Some birds, such as pigeons, doves, flamingoes, and penguins, produce a substance derived from epithelial cells called “crop milk,” which they feed to their young. And cockroaches are known to secrete a sort of milk, which they use to sustain developing embryos.

In the case of Toxeus magnus, though, Quan and his team argue that the spiders’ behavior is more akin to mammalian lactation. Like many mammals, newly hatched spiderlings are entirely dependent on milk to meet their nutritional needs. In this case, for the first 20 days of the young spiders’ lives.

In one illustrative experiment, the researchers glued shut the spider mothers' epigastric furrow, the egg-laying organ that also secretes milk. The hatchlings all died within their first 11 days.

Nursing teenagers

Even after hatchlings are old enough to find food on their own, the researchers discovered that spiderlings will continue to take advantage of their mom's milk for an additional 20 days. And daughters (but not sons) were allowed to continue nursing even after reaching sexual maturity.

At the adolescent and adult stages, being deprived of milk simply forced the spiders to spend more time foraging, indicating that milk was no longer essential for their survival. Still, the researchers suspect that in the wild, milk provisioning could still positively impact survival, since foraging outside the nest increases the risk of predation.

But while milk might be essential at certain life stages, it isn't the only thing at work here. Maternal care and milk provisioning appear to work together to ensure the long-term survival of young spiders. In experiments where the mom was allowed to remain in the nest, but was prevented from nursing her young after day 20, spiderlings still had fewer parasites than those from nests in which the mom was removed entirely.

Of the 187 spiderlings researchers observed in 19 different nests, the survival rate for those that received both maternal care and milk was 76 percent. Removing the mother at day 20 reduced the spiderlings’ survival rate to about 50 percent.

Spilled milk

Since spider mothers also deposit milk droplets into the nest itself, this suggests that milk might have a function beyond nutrition. “There's still some unanswered questions that would help us determine if this is functionally similar to mammalian lactation,” Skibiel says.

In mammals, milk probably did not initially evolve for nutrition. Instead, lactation was likely either a means for mothers to jumpstart their infants' immune systems by providing them with antibodies, or a means to keep their eggs moist, as duck-billed platypuses do.

And while the evolution of milk-like fluids outside of the mammal family remains rare, it's also unlikely that this is the only spider species to have done so. “The more I think about it, there is actually a pretty good chance this is happening all over the place,” Pruitt says. “Out of 50,000 described species of spiders, you can bet your boots this isn't the only one that does it.”