The phrase mother's milk conjures up idyllic images of mother and child. Mammals practice breast feeding, of course, but other non-mammals have a similar nutrient-rich elixir that serves a similar purpose.
And now we've found a new one — the jumping spider. The females of the species Toxeus magnus produce something so close to milk that we might as well call it that. They produce a milky substance that offspring like so much, they'll keep coming back for well past sexual maturity.
This discovery, reported in Science, was made by researchers at the Chinese Academy of Sciences. They were studying T. magnus spiders, a jumping spider species that looks more like an ant than a spider. Researchers were interested in why these spiders could often be found hanging around their nests for extended periods of times, sometimes past 20 days or or so, by which time they should have left home and struck out on their own as adult spiders contributing to spider society.
Parental care among spiders isn't unheard of. Plenty of mother spiders guard their eggs or leave behind snacks for the babies to feast upon when they hatch, so a doting T. magnus spider wasn't odd in the grand scheme of things. What was odd to researchers was that the mother never seemed to leave the nest to get the babies any food, and yet the babies still grew at a consistent rate.
One of the researchers, Zhanqi Chen, noticed something strange one day. A young spiderling was seemingly attached to its mother's underside. When Chen looked closer, it seemed as if the spiderling was actually suckling. Chen took one of the adult females and brought it under a microscope. With a little push on its abdomen, a little drop of white liquid seeped out. They analyzed the liquid and found that it contained sugar, fat and about four times the amount of protein as cow's milk.
It was everything a growing spider needs. It was spider milk.
Milk: It does an ant body good
Armed with this new information, researchers took a closer look at nests. It turned out that the mother was secreting this milk at points along the nest for about a week, and the babies spiders would gobble it up. This explained why no one was leaving the nest for food. After a week, the spiderlings would suck the milk directly from the mother's underside, from the epigastric furrow, a fold on the female's underside.
Offspring were allowed to nurse for almost 40 days, well past the point when they could've left and gotten food on their own. After this 40-day period, mom would get fed up with the young male and attack them, forcing them to flee. Daughters were allowed to stay and continue to feed if they desired. Researchers suspect this behavior is intended to prevent inbreeding. To determine how vital the milk was, Chen painted over the epigastric furrow, essentially blocking it. Spiders younger than 20 days old died. Those 20 days or older grew more slowly, left the nest sooner and were more likely to die before they reached full adulthood.
It's probably the closest to mammalian lactation researchers have seen in non-mammals. Of course, there are still plenty of questions about the spiders and their process. Is there a gland that produces the milk? Does this milk also provide the spiders with hormones and bacteria that help immunity, like mammalian milk does? And the biggest question is this: What other creatures are helping their young with milk that we simply don't know about?