You may not think of spiders as the maternal sort. But some do make good moms. Wolf spiders carry their egg sacs around. And after hatching, the spiderlings ride on mom’s back. This ant-mimicking jumping spider takes another tack— she provides milk to her babies. At first, they suck up little droplets that the mother places around the nest. As they get larger, the spiderlings suckle straight from their mom’s epigastric furrow. In lab tests, researchers found they continued to suckle until the spiders became subadults— which is about 40 days after hatching. Researchers were able to figure this out by carefully watching these families in their nests. Even though the mother spider never brought anything back to the nest to eat, the spiderlings tripled in length in the first 20 days. To look more closely at the benefit of this milk and maternal care, the researchers painted over the mom’s epigastric furrow to cut off the milk supply. Without milk, they found that the young didn’t grow and died about 10 days later. When they removed the mother after 20 days, the subadults foraged on their own– more than they would if mom were there but fewer survived over all. What is spider milk anyway? The milk comes out of this egg-laying opening the spiders have–so one idea is that the spider milk might be evolutionarily related to trophic eggs. This is when animals feed infertile eggs to their offspring. Spider milk is protein rich. An analysis of the milk droplets revealed the protein content was around 4 times that of cow’s milk While it is not usual for animals, even invertebrates like spiders and cockroaches, to give their food to their young–the length and intensity of feeding and maternal care given by these jumping spider moms was thought to only happen in cognitively advanced, social vertebrates, like orangutans, people, and and elephants. Researchers speculate that if there’s a
chance there will be no food for newborns, or all the young spiders are very likely to get eaten by predators, then it can make sense for a mother to take on the cost of parental care. This milk-giving spider suggests scientists will need to rethink the extent of maternal care and milk provisioning throughout the animal kingdom And how much cognitive power you need to take care of offspring in these ways.