Lactation is a trait historically associated almost exclusively with mammals, so the idea that a spider could produce milk challenges a fundamental concept in biology. However, a specific spider species exhibits a nurturing behavior that is functionally analogous to mammalian nursing. This species provides a nutrient-rich fluid to its offspring, expanding the understanding of parental care across the animal kingdom. This remarkable discovery forces a reevaluation of how complex parental investment can evolve in invertebrates.
The Direct Answer: Discovery of Spider Milk
Yes, one species of spider produces a milk-like fluid to feed its young. This discovery centers on the ant-mimicking jumping spider, Toxeus magnus, a species found in parts of Asia, including China and Taiwan. Researchers noticed that the spiderlings remained in the mother’s nest long after hatching and continued to grow, even when the mother did not bring prey back. This extended presence suggested a form of sustained maternal care highly unusual for spiders.
Closer observation revealed that the mother was secreting a white, nutrient-dense fluid that the spiderlings consumed. This fluid serves the same purpose as milk in mammals, providing complete nutrition for the young during their earliest stage. Because the process was so similar to lactation, scientists began referring to the substance as “spider milk,” despite the mother lacking mammary glands.
Composition and Production of the Fluid
The spider milk is a highly potent source of nutrition for the developing young. Chemical analysis showed the fluid contains sugar, fat, and a remarkably high concentration of protein. Specifically, the fluid contains 2 milligrams of sugar and 5 milligrams of fat per milliliter, alongside an extraordinary 124 milligrams of protein per milliliter. This protein concentration is nearly four times higher than that found in cow’s milk, making the secretion exceptionally rich.
The fluid is not produced by specialized mammary glands, which are unique to mammals. Instead, the mother spider secretes this milky substance from her epigastric furrow, the opening on the underside of her abdomen used for laying eggs. For the first week, the mother deposits droplets of this milk around the nest for the newly hatched spiderlings to drink. As the young grow, they transition to drinking the fluid directly by pressing themselves against the mother’s abdomen, a behavior that closely resembles suckling.
The Extended Nursing Period
The most surprising aspect of this maternal behavior is the duration of the nursing period, which is exceptionally long for an invertebrate. For the first 20 days after hatching, the spiderlings are completely dependent on the mother’s milk, which is their sole source of food. Experiments that blocked the mother’s ability to secrete the fluid resulted in the death of all young spiders within ten days, confirming the fluid’s necessity for early survival.
Around day 20, the spiderlings become large enough to leave the nest and begin foraging for solid food on their own. However, they do not wean immediately and continue to nurse from the mother for several more weeks. The mother’s care and milk provision typically lasts for about 40 days, until the offspring reach the sub-adult stage. Even after the young began hunting, those deprived of the milk supplement had a significantly lower survival rate, demonstrating that the extended nursing boosts their long-term health.
Significance of Invertebrate Lactation
The discovery of this behavior in Toxeus magnus challenges previous assumptions about intensive parental care in the animal kingdom. Prolonged provisioning of a nutrient-rich fluid to mobile, foraging offspring was thought to occur only in long-lived mammals and some social vertebrates. The spider’s behavior is functionally similar to mammalian lactation and represents a remarkable example of convergent evolution.
Most spider species offer very limited maternal care, often only guarding the egg sac before the young disperse quickly upon hatching. The extended care and provisioning of milk in T. magnus suggests this complex, high-investment strategy evolved as a response to specific environmental pressures. Factors like high predation risk or unreliable food sources may have favored the evolution of this specialized, sustained food source. This finding encourages researchers to reevaluate maternal behaviors and the evolution of lactation across a broader range of invertebrates.