Got (spider) milk? Maternal care in lactating arachnids

By David Han ’21

Nursing as a part of maternal care has long been considered the exclusive domain of mammals, at least in popular media. Most invertebrate mothers, the popular wisdom goes, employ an extreme version of tough love; newly-hatched insects and spiders are lucky if they aren’t eaten by their parents! That’s why ​Zhanqi Chen, Rui-Chang Quan, and their colleagues​ were initially puzzled when they observed mother jumping spiders (​Toxeus magnus​) raising their young for over 20 days without bringing any insect prey back to their nests. How could the baby spiders, or spiderlings, grow in the absence of food? Closer observation yielded a bizarre and potentially paradigm-shifting discovery. Upon her return to the nest, the spiderlings swarmed their mother, drinking a nutritious liquid from her underbelly: spider milk! As it turns out, the mother spider secretes a nutrient-rich liquid from a specialized groove at the base of her abdomen called the epigastric furrow, around which her many spiderlings appear to cluster, almost like piglets around a mother sow. Additionally, the brood was observed to stay together well into adulthood, returning to their nest from their hunting forays each evening.

When the research team investigated the nutritional makeup of the spider milk itself, their findings were even more surprising. While the liquid was, as expected, rich in fat and sugars, it contained roughly four times the protein of cow milk, revealing just how beneficial it could be in spurring rapid development. This type of nutritional supplement is not common in arthropods, and its evolutionary past is uncertain; the ongoing hypothesis, however, is that spider milk may in fact be derived from liquified, unfertilized eggs, or trophic eggs, which are used by other invertebrates as a convenient source of pre-packaged meals for offspring.

To uncover just how important the provision of milk is to the spiderlings, Dr. Chen and his colleagues decided to test how the spiders fared under different levels of maternal care. When the mother’s milk ducts were blocked upon the hatching of offspring, long-term survivorship dropped precipitously, indicating that juveniles are almost totally dependent on milk. However, when the milk supply was cut off after 20 days, or the mother was removed altogether, survivorship did not decrease nearly as dramatically; by that point, the young spiders had learned the ropes of hunting and could fend for themselves outside the nest. In fact, it seems that the spiderlings are naturally “weaned” by about 40 days after hatching, despite the fact that they would continue to hang around their mother’s nest for more than two weeks afterwards.

During the observation of offspring survival rates and development length, an interesting, and wholly unexpected, trend presented itself. As it turns out, the absence of a mother spider drastically influences the ultimate sex ratio of her offspring. Under normal conditions, the offspring sex ratio skews female, but when mothers were removed, their progeny are more likely to be male.

As it turns out, the care provided by the mother is required for offspring survival only during the crucial first 20 days of development; their survivorship did not much change when removal occurred beyond the early to mid-juvenile stage. At the same time, the average body length of the adult spiderlings decreased when the mother was removed, which indicates that some degree of developmental stunting occurs without consistent parenting. Additionally, parasite load on the spiderlings was significantly higher on individuals lacking maternal care, indicating that, in addition to providing milk and shelter, the mothers may groom their offspring as well. From these findings, it appears that the spiders raise their offspring like coyotes raise their pups; by providing milk to spiderlings until they can successfully hunt themselves and maintaining a safe haven for their return, the mother spiders create ideal conditions for successful development.