Have milk? Of course, you do it; few things are as mammals as our milky childhoods. Of course, we all have spine (just like lizards), warm blood (birds too) and hair (plants too) – but it is the mammary glands from which mothers breastfeed their offspring that set us apart really from the rest of the tree of life.
That's why researchers announced today that another species is providing milk to their young. The last member of the milk-producing family … is a spider.
This, of course, depends on the definition you give to "milk". According to certain restrictive definitions, milk refers explicitly to the nutritive liquid secreted by mammals to feed their young. Ask the dairy industry and she is a firm believer that this definition is the only one acceptable – and has even tried to sue the companies that, in her view, use the term abusively. But in the course of history, the word "milk" usually refers to a number of milk-like substances, including coconut milk, almond milk and some plant juices. There is no general consensus on the definition of milk.
There are already some non-mammals known to feed their young substances produced by their bodies. Doves and pigeons, for example, produce "harvest milk," a very nutritious substance, but not particularly similar to milk, that parents regurgitate for their young. Flamingos and some penguins also secrete a similar substance in their esophagus. There is even a species of cockroach that gives birth to live young people after feeding them "milk" inside the mother's hatch – a substance that has been declared among the "most nutritious substances on Earth" .
But these substances are quite far from milk and, although these parents give them to their young, they certainly do not feed in the most literal sense of the term. That's why it's so dramatic to see the image of a spider, surrounded by his little head, like a dog with a new litter of puppies. Yes, this image exists. You asked him:
Mother spider milk
Researchers at the Chinese Academy of Sciences made this discovery by following an observation about Toxeus magnus, a jumping spider species that mimics ants. Spiders are usually solitary, but scientists have noticed that groups hang out in nests, including groups of an adult female and several youngsters.
They suspected that there was more to learn about the interactions in these nests. Why did not these spiders scatter unless they stayed together?
To find out, the team, led by researcher Zhanqi Chen, studied spiders in a lab located at the Xishuangbanna Tropical Botanical Garden in Yunnan. After the hatching of new baby spiders, the scientists observed whether the spiders would leave the nest to feed and if the spider mother would bring food to the nest. They did not do it. It lasted 20 days and all the while, the spiders grew up and developed normally.
A closer look reveals the extraordinary: just after the hatching of the spiders, the mother spider dropped droplets of a nutrient into the nest and the babies ate from the droplets.
After the first week, the spiders have graduated actually suck the milk directly from the mother spider. This continued even after the spiders left the nest for their first hunt, completing their nutrition until the age of about 40 days.
The milk is secreted by the epigastric furrow of the spider, a spot under the abdomen, near the nipples of a dog or cat.
Spider milk is most mammals yet
If you are not yet stunned enough, the researchers were able to get a spider, under a microscope, to secrete a few drops of milk "after gentle finger pressure on the abdomen". They of course analyzed the liquid and found that it had sugar, fat and about four times more protein than cow's milk.
Finally, to be really sure to see what they thought they saw, they performed a number of tests on spiders. At different stages of spider development, they either closed the mother's epigastric furrow to block access to the milk (by covering it with correction fluid), or removed the entire mother from the nest. This allowed them to see how the spiders depended on the milk itself and what care the mother lavished on the milk.
Blocking the milk or removing the mother after 20 days, when the spiders were old enough to leave the nest, was detrimental, but not devastating to young spiders. But preventing breastfeeding of newly hatched spiders resulted in 100% mortality. They need their mother's milk to live, just like little humans.