Milk comes from mammals. It’s an obvious mammal-like thing. Even our government knows that. And yet, Chinese scientists have documented jumping spiders that supply their young with droplets of a nutrient-rich liquid from a groove on the mother’s body. It is the only food for the spiders until they start foraging, and even then they drink it until they are slightly more mature. Results are reported in Science.
Cockroaches and pigeons also provide their young with a substance described as “milk” because it comes from their bodies and is the exclusive source of food for the young. Cockroach mothers deposit this substance in the brood sac where their embryos develop.
Deaf parents – both mothers and fathers – produce crop milk and feed it to the baby birds for their first few days of life, until the babies can actually digest food. Cereal milk consists of nutrient-filled cells that are shed in flakes from the insides of the parent birds’ crops, which are located under their necks.
Toxeus magnus are the jumping spiders studied here, and they look like ants. Their way of childcare 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 nonviable eggs that are turned into food for the offspring that survive.
These spiders also live in small nuclear family-like nests where the mother takes care of the children even after they reach adulthood. For a while, the fry go out to forage, but still keep coming home, making them look even more like mammals.
This is certainly not breastfeeding, but these researchers think it is “functionally and behaviorally comparable to breastfeeding”. It also serves a similar evolutionary need: to compensate for insecure access to food and to keep 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 higher than cow’s milk. It’s not vegan.
Science2018. DOI: 10.1126/science.aat3692 (About DOIs).