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Jumping Spiders Produce Protein-Packed Milk to Feed Their Young

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When the 18th-century biologist Carl Linnaeus classified a group of animals as “mammals,” he based the name on one key characteristic—mammary glands, from which females produce milk to feed their young.

While lactation is a common feature among mammals, it turns out that it isn’t unique. Scientists have since learned that some nonmammalian creatures also make milk to feed their young.

Cockroaches, for example, nourish their developing embryos with a milky, protein-rich fluid.

And a new study, published today in Science, reveals that at least one other invertebrate species, the ant-like jumping spider Toxeus magnus also produces milk to feed its young.

Zhanqi Chen, a postdoc at the Chinese Academy of Sciences’s Center for Integrative Conservation, and his colleagues began to investigate parental behavior in T. magnus after noticing that the spiders’ breeding nests consisted of either several adults or a single adult female and her young.

An observation that suggested that the mothers engaged in long-term care. Upon further investigation, the researchers made a puzzling finding: Baby spiders steadily grew bigger despite never leaving their nests, and their mothers did not appear to be bringing them any food.

There were several potential explanations for where the offsprings’ nutrition came from, Chen says, such as trophic eggs (unfertilized eggs stockpiled for food) or regurgitation (when a parent vomits up food they ate to feed its young).

But the team’s observations led them to another, unexpected possibility.

While recording data on the growing spiders’ body sizes one evening, Chen spotted some spiderlings attached to their mother’s body—it looked to him just like a mammal latching on to its mother’s breast.

I had many hypotheses, but this one was not included,” Chen tells The Scientist. “At that point, I was so excited, I couldn’t sleep.” 

This startling finding jumpstarted a series of additional experiments. When the researchers peered at the critters under a microscope, they discovered that mothers excreted a milk-like substance from their epigastric furrows, an abdominal opening from which they lay eggs.

The team then analyzed the milk’s contents and found it was composed of sugar, fat, and four times more protein than cow milk.

They also demonstrated that the milk was crucial for offspring survival: When the researchers blocked the mothers’ epigastric furrows, spiderlings died within 10 days of hatching.

Chen, Quan, and their colleagues have several remaining questions they hope to tackle in future experiments.

For example, they plan to investigate whether the spiders, like mammals, produce milk in mammary glands, and to probe for other invertebrate species that display similar parenting behaviors.

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