Mom nurses her young for weeks on a milk-like fluid that has four times the protein of a cow’s milk. Yet this mother is no mammal. She’s a jumping spider, an invertebrate with eight legs and a taste for fruit flies.
Mammals like us are named after our mammary glands. Those are the tissues that produce milk. Yet some other animals also secrete a type of milk to feed their babies. The diverse list includes tsetse flies and pigeons. But the newly discovered nursing in Toxeus magnus could be the closest to mammal moms’ yet. That’s what a research team from China proposed in the November 30, 2018, Science.
Biologists have recognized T. magnus as a species since 1933. However, the small spider’s mothering habit was easy to miss. The spiders are tiny. The beasts they hunt are fruit flies and other little prey. They raise a family in little nests, perhaps attached to leaves.
Coauthor Zhanqi Chen studies spider behavior at the Chinese Academy of Sciences in Menglunzhen. He noticed several T. magnus sharing a nest in 2012. At the time, he wondered if the species had some sort of extended parental care. It was another five years before he spotted the nursing behavior. That’s when a spiderling clamped itself against mom’s underside one exciting July night in 2017.
To see what’s really going on, someone needs to view a female under a microscope. A gentle finger push on the underside of her abdomen will squeeze a tiny bead of white liquid out of a crease. That crease is known as an epigastric furrow. For the first week or so after her eggs hatch, a spider mom will leave milk droplets around the nest for the crawling dots — her young — to drink. Then nursing turns more mammalian. The little ones now press themselves against their mom’s body.
Just how the spiderlings might take up the milk intrigues Wendy Hood. She’s an evolutionary physiologist at Auburn University in Alabama who studies lactation. This spider’s nursing habits have similarities to two mammals from Australia — the platypus and echidna (Ee-KID-nuh). Females of these species have no nipples. Babies lap milk that exudes from the mammary glands to the skin’s surface. It comes out “much like water from sweat glands,” Hood says. (In the majority of mammals, including humans, a baby suckles rather than laps. That suckling triggers milk to flow into its mouth.)
A hearty meal
T. magnus milk has some basic ingredients in common with a mammal’s milk. For instance, spiderlings get about 2 milligrams of sugar and 5 milligrams of fat in every milliliter of milk. There’s also 124 milligrams of protein. Baby spiders live on nothing else for the first 20 days of their lives, the researchers report. In the lab, mothers occasionally hunted fruit flies provided by researchers. However, they never brought any of that prey back to the nest to feed to their young.
After about three weeks, baby spiders start to forage on their own. But they continue nursing for nearly three weeks more, the team found. With that combined diet, 76 percent of youngsters in lab nests lived long into adulthood. Without mom’s milk, they died in about 10 days. (The researchers stopped the milk by sealing mom’s epigastric furrow as soon as spiderlings hatched. Their glue was a typists’ correction fluid.)
Researchers wondered whether milk mattered much once the youngsters started hunting. It did. Depriving these older spiderlings by sealing mom’s furrow at day 20 lowered their survival rate to around 50 percent.
Spiders are usually solitary predators. With such extensive care from mom, T. magnus spiders end up sharing a nest for an unusually long time, notes Linda Rayor. She studies spider behavior at Cornell University in Ithaca, N.Y. Only about 120 among the nearly 48,000 known spider species are known to tolerate company for more than three weeks, including siblings. And only about 30 of those live lifelong social lives. So an example of spiders sharing a nest for 40 days, Rayor says, is “a big deal.”