Science & Technology

Spider Milk? Jumping Spider Mothers Actually Nurse Their Young

The checklist for being a mammal is pretty straightforward: you need to be warm-blooded, you need to have body hair, and you need to nurse your young. So imagine scientists' surprise when they discovered that a spider — an invertebrate that is definitely not a mammal — produces milk and nurses its young.

Charlotte's Tots

The spider in question is already a little unusual. The jumping spider known as Toxeus magnus doesn't look much like a spider — its body shape mimics that of ants — and doesn't live much like a spider, either. Usually, once a spider hatches and gets strong enough to find its own food, it stakes out on its own as a lone hunter. But T. magnus live as happy little families in a single nest, often with two or more adults or one adult female and several baby spiderlings. That habit is what made researchers from the Chinese Academy of Sciences and China's Hubei University curious: If they lived as families, did the mother take care of her young as they matured?

To find out, the researchers observed the spiders' comings and goings from the nest over a period of weeks. What they saw was strange; for about three weeks, they didn't see any spiderlings leave the nest to find food, nor did they see the mother bring any food back to the nest. Despite this fact, the spiderlings tripled their body lengths during that period of time.

When they looked closer, they solved the mystery: the mother spider was producing a sort of milk for her offspring. For the first week, she left droplets of it around the nest for her spiderlings to find, but eventually, the babies started sucking directly from her — for lack of a better term — "teat." (Technically, the milk came out of her epigastric furrow, a genital structure in her abdomen.)

Even after three weeks when the spiders were old enough to find food on their own, they continued to nurse from their mother. They eventually stopped after about a month and a half, but even then they continued to use the nest at night. Those darn slacker teenagers, amiright?

Jumping on the Mammal-Wagon

T. magnus isn't the first non-mammal to make milk to feed its young. Cockroaches do it too, but in their case, the "milk" is simply deposited to feed their developing embryos, not produced over the long-term to nurse their young into adulthood. The tsetse fly is another bug that has mammal-like habits, giving birth to live young and feeding them with a milk-like fluid, but that fluid, too, is only given to the developing larva. Even pigeons also make a sort of "milk," which both parents produce in specialized glands in their throats while they incubate their eggs.

But this jumping spider is the only invertebrate known so far to actually nurse its young like a mammal, according to the researchers. They say it's a sign that scientists need to reevaluate how common this tendency is across the animal kingdom, both in invertebrates and other organisms. Once again, science proves to be more complex than our clean definitions can handle.

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The natural world can be sweet, but it can also be scary. For the latter, check out "Plight of the Living Dead: What Real-Life Zombies Reveal About Our World — and Ourselves" by Matt Simon (and check out our interview with him on the Curiosity Daily podcast right here). We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer November 30, 2018

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