The act of breast-feeding is so fundamental to being a mammal that we named ourselves after it. (“Mammalis” translates to “of the breasts.”) But over time, scientists have discovered that other animals also produce nutrient-rich elixirs to feed their young, including flamingos, cockroaches and male emperor penguins.
The latest addition to the cast of organisms that lactate — or something like it — is a species of jumping spider.
Researchers in China have discovered that females of the Toxeus magnus spider secrete a milk-like fluid to feed their offspring. The study, published Thursday in the journal Science, also found the arachnid mothers continue to provide the fluid, which contains about four times as much protein as cow’s milk, well after their spawn had become young adults.
Though the spiders aren’t using mammary glands to produce the fluid, and hence are “lactating” in name only, the findings should prompt scientists to reconsider what they know about nursing and how it evolved, the researchers said.
“Finding such mammal-like behavior in a spider, or in any invertebrate for that matter, was a surprise,” said Richard Corlett, a conservation biologist at the Chinese Academy of Sciences and an author of the study.
Jumping spiders are the single largest group of spiders in the world, with more than 5,000 species and a presence on nearly every continent. The tiny T. magnus, also known as the black ant mimicking jumper, looks like an ant, walks like an ant and even waves its front legs in the air like a pair of antennas (it jumps when threatened or hunting). The species is found mostly in Southeast Asia.
The study came about after the lead author, Zhanqi Chen, also of the Chinese Academy of Sciences, noticed that young T. magnus seemed slow to leave the breeding nest, suggesting the mothers were providing some sort of extended child care. That hypothesis received a boost when he and his colleagues observed newborns in the lab and found that neither they, nor their mother, left the nest to find food for the first 20 days.
Looking closer, they found that during the first week, the mother was depositing droplets of fluid from her underside onto the nest that the hatchlings would come and drink. After the first week, the offspring would drink the fluid directly from the mother’s body.
Adding to their surprise, the researchers found that the mother continued to provide the fluid even after her young began leaving the nest to forage at about 20 days old. The suckling finally ceased at 40 days, though the offspring still used the nest at night for another 20 days.
The extended nursing may be an evolutionary response to the creatures’ tiny size and vulnerability.
“We think that it may reflect the high risk of juvenile spiders becoming prey themselves if they have to hunt for their own food,” said Dr. Corlett.
It’s also unclear how the T. magnus generates the fluid, which contains 2 milligrams of sugar per milliliter of liquid, 5.2 milligrams of fat and a whopping 124 milligrams of protein. The researchers suspect it may have evolved from trophic eggs — unviable eggs that some insects produce to feed their young — because the fluid emerges from the same opening that produces eggs.
When the researchers blocked the mother’s ability to produce the milk by covering the opening with correction fluid (a.k.a White Out), all the hatchlings died within 11 days, showing their complete dependence on the substance.
Though further study is needed to understand how and why the T. magnus generates the fluid, the study should further challenge the assumption that lactation is a uniquely mammalian trait, said Dr. Corlett. “Our findings suggest that ‘lactation’ may arise in non-mammals when it provides a significant advantage in offspring survival.”