Milk comes from mammals. It’s kind of a distinctly mammalian thing. Even our government knows that. And yet, Chinese scientists have documented jumping spiders that provide their young with droplets of a nutrient-rich fluid from a furrow on the mother’s body. It is the sole nourishment for the spiderlings until they start foraging, and even then they still drink it until they get slightly more mature. Results are reported in Science.

Cockroaches and doves also provide their young with a substance described as “milk,” because it comes from their bodies and provides the exclusive source of sustenance to the young. Cockroach moms deposit this substance into the brood sac where their embryos are developing.

Dove parents—mothers and fathers both—generate crop milk and feed it to the baby birds for their first few days of life, until the babies can digest real food. Crop milk consists of nutrient-filled cells sloughed off in flakes from the inside of the parent birds’ crops, which are under their necks.

Toxeus magnus are the jumping spiders studied here, and they look like ants. Their method of child care also seems to be different from the two examples above. They have a specialized organ that makes the milk, which the researchers suggest is made from unviable eggs that are turned into food for the offspring who survive.

These spiders also live in smallish, nuclear family-like nests where the mom takes care of the kids even after they reach maturity. For a time, the young venture out to forage but still keep coming home, making them even more like mammals.

So this is definitely not lactating, but these researchers think that it “compares functionally and behaviorally to lactating.” It also serves a similar evolutionary need: compensating for uncertain food access and keeping the young in the nest for as long as possible to protect them from predators.

Jumping spider milk has a protein content of 123.9 mg/ml, four times that of cow’s milk. Vegan it is not.

Science, 2018. DOI: 10.1126/science.aat3692 (About DOIs).

https://arstechnica.com/?p=1420471