Scientists typically define mammals as animals that have hair, produce live young, and produce milk. It’s a simple definition, but nature is very good at defying simple definitions. Platypuses, for instance, lay eggs. And plenty of non-mammals produce milk for their young. One group of researchers from China have discovered that , and it’s so nutritious that their offspring eat it for a surprisingly long time.
The researchers were studying a species of jumping spider, hoping to learn more about how these spiders cared for their young. To do that, they set up a nest in the lab with a spider mother and a collection of her young, observing them continually while taking notes. To their surprise, they noticed that the young stayed inside that nest for over a month, which is a very long time for a spider.
Upon closer examination, the researchers found that the mother spider was producing some kind of substance, secreting it onto the floor of the nest. The young spiders seemed to eat that substance, and after about a week they began latching onto their mother directly to get easier access to it.
The researchers collected some of that substance and analyzed it, and they realized that the substance was, essentially, milk. In fact, it was apparently delicious milk—at least for spiders. The spider young spent around 38 days inside the nest, drinking that spider milk, even though they were technically able to leave the nest and fend for themselves after about 20 days.
So is this one species of spider the only lactating arachnid, or is this behavior more common than scientists thought? There’s really no way to tell just yet. But the earth has more than a million different species of invertebrate, so it wouldn’t be surprising to find more species that make milk of their own. With the incredible diversity that the animal kingdom offers, we might have to rethink mammals as the lactation experts.