Toxeus magnus spider. Image: CHEN Zhangqi
Got milk? If you’re a special species of jumping spider, the answer is yes. That’s what scientists discovered when they observed female Toxeus magnus spiders nursing spiderlings with a milky substance for several weeks.The finding, published Thursday in Science, suggests that despite milk’s strong association with mammals—a group named for its lactating mammary organs—nursing behaviors may be widespread in organisms that we wouldn’t think of as breastfeeders.
Though T. magnus, an Asian spider that mimics ants, does not have traditional mammary hardware like nipples or udders, it does have a small opening in its abdomen called an “epigastric furrow” from which the spider milk flows. This furrow is an organ on the underside of female spiders used to lay eggs.Samples of the spider's milk were analyzed and found to contain sugar, fat, and about four times as much protein as cow’s milk."Our findings demonstrate that mammal-like milk provisioning and parental care for sexually mature offspring have also evolved in invertebrates," said first author Zhanqi Chen, a spider expert at Xishuangbanna Tropical Botanical Garden, in a statement. "We anticipate that our findings will encourage a reevaluation of the evolution of lactation and extended parental care and their occurrences across the animal kingdom."Chen’s team first noticed there might be something unique about T. magnus spiders during field observations of the arachnids’ unusual nesting behavior. Mother spiders remained in the nest for weeks after their offspring hatched, providing time for the spiderlings to grow and learn foraging skills. This high level of parental investment contrasts with spider species that are left to their own devices as spiderlings, or the delightfully gruesome spiders that eat their parents once they hatch.To figure out why T. magnus moms seem so devoted to their spider-kids, Chen’s team studied 32 adult females as they nested in laboratory conditions. Once the spiderlings hatched, they stayed put in the nests for around 20 days, before searching for their own food. Throughout this period, the offspring grew even though the mother was not sharing foraged food with them.
Microscopic inspections revealed that the spiderlings were suckling milk droplets from epigastric furrows. Chen’s team conducted several experiments to assess how essential the milk was for spiderling development. The researchers blocked the furrows of some mothers so that newly hatched spiderlings couldn’t suckle. That proved to be universally fatal to the young spiders, with offspring all perishing within 10 days of hatching, demonstrating that this lactation adaptation is essential to T. magnus’ reproductive success.Of the spiderlings that freely suckled for some 52 days, which is around the time it takes for the species to reach adulthood, the survival rate was an impressive 76 percent. These spiderlings began to supplement their diets with foraged foods about 20 days after hatching, but still frequently nursed from their mothers for several weeks.
Interestingly, once the spiderlings became sexually mature, only the daughters were allowed back into the nest. The adult males were attacked by their mothers, and barred from returning to their birthplace. Chen’s team thinks this might be an adaptation to prevent inbreeding between mothers and sons and brothers and sisters. In the wild, the males likely seek out the nests of non-relatives.These spiders probably evolved lactation for the same reason as mammals—to give their offspring a headstart in life. “Extended parental care could have evolved in invertebrates as a response to complex and harsh living environments that require offspring skills (e.g., hunting, predator defense) to be fully developed before complete independence,” Chen’s team wrote in the study.Given that spiders can build intricate structures with silk and fly around the world with electrostatic force, it shouldn’t come a surprise that they can lactate, too. Spiders may be small, but their ability to evolve incredible abilities seems limitless and often inspires scientists to try to replicate their superpowers.For instance, scientists and startups are capitalizing on the extraordinary tensile strength and properties of spider silk by mass-producing synthetic versions of it. Perhaps this study will spark interest in manufacturing spider milk in large quantities as well—there are, after all, stranger niche markets out there.So, spider milk anyone?Get six of our favorite Motherboard stories every day by signing up for our newsletter.