This spider feeds a type of milk to its babies
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.”
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
biology The study of living things. The scientists who study them are known as biologists.
coauthor One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.
component Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).
diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.
echidna Also known as the spiny anteater, this is an egg-laying mammal native to Australia, Tasmania and New Guinea. The generally solitary animal has small eyes and a long beak-like nose. It has poor vision but a keen sense of hearing and smell. The short-beaked species have dark fur largely hidden by hollow spines. Spines on long-beaked echidnas are much longer — about 5 centimeters (2 inches) long — and help to give it camouflage.
egg The unfertilized reproductive cell made by females.
evolutionary An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.
fat A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.
forage To search for something, especially food. It’s also a term for the food eaten by grazing animals, such as cattle and horses.
fruit flies Tiny flies belonging to the species Drosophila melanogaster.
gland A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.
invertebrate An animal lacking a backbone. About 90 percent of animal species are invertebrates.
lactation (v. lactate) A release of milk by female mammals after giving birth. This high-protein, fatty fluid gives their young the nutrients needed to fuel their postnatal growth.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
microscope An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.
nurse (in biology) A term for suckling a young mammal so that it can feed on its mother’s milk.
physiologist A scientist who studies the branch of biology that deals with how the bodies of healthy organisms function under normal circumstances.
platypus Sometimes known as the duckbill, this shy Australian egg-laying mammal (Ornithorhynchus anatinus) has a streamlined body and flat bill. Its waterproof fur allows it to comfortably navigate in rivers and other waterways, where it feeds on invertebrate animals that live in the sediment. It uses electrical signals given off by the muscles of its prey to find its food. Males have a spur on the inner side of each ankle releases venom for use in their defense.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.
secrete (noun: secretion) The natural release of some liquid substance — such as hormones, an oil or saliva — often by an organ of the body.
sibling An offspring that shares the same parents (with its brother or sister).
social (adj.) Relating to gatherings of people; a term for animals (or people) that prefer to exist in groups. (noun) A gathering of people, for instance those who belong to a club or other organization, for the purpose of enjoying each other’s company.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
spider A type of arthropod with four pairs of legs that usually spin threads of silk that they can use to create webs or other structures.
taste One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.