Why Do Wolf Spiders Carry Their Babies? Egg Sacs and Spiderlings

How a wolf spider carries her egg sac

After mating, a female wolf spider spins a round silk sheet, lays her eggs on it, covers them with more silk and rolls the whole package into a ball. This egg sac, often pale gray, cream or greenish, is then fastened to her spinnerets, the silk-producing organs at the tip of her abdomen.

From that moment she carries the sac wherever she goes. She holds her abdomen slightly raised so the sac does not drag along the ground, and she can still run, hunt and retreat into her burrow. This is a key difference from nursery web spiders, which look similar but carry their egg sac in their jaws and pedipalps.

In the Carolina wolf spider, the largest wolf spider in North America, the egg sac holds about 200 eggs and is carried for roughly two weeks. Females are often seen at the top of their burrows with the sac held up toward the warmth, a behavior researchers call sunning, which probably speeds up development.

Why carry the eggs instead of leaving them?

Carrying eggs is costly, so it must bring big benefits. The main ones are:

• Protection from predators and parasites. An unguarded egg sac is an easy meal for ants, beetles and other spiders, and certain small wasps lay their eggs inside spider egg sacs. A mother can fight off or flee from many of these threats.
• Control of temperature and moisture. A mobile mother can move her eggs into the sun to warm them or down into a cool, moist burrow to stop them from overheating or drying out.
• A wandering lifestyle. Many wolf spiders have no permanent home. Carrying the sac lets the mother keep hunting over a wide area instead of staying tied to one spot.

Wolf spider mothers are strongly attached to their sacs. In classic experiments the French naturalist Jean-Henri Fabre took egg sacs away from females of a large European wolf spider, and found that mothers searched for them and would even accept a substitute ball of other material. A mother that loses her sac often searches the ground for it.

What happens when the spiderlings hatch?

When the young are ready to emerge, the mother often helps by biting open the tough silk of the egg sac. The spiderlings then climb up her legs and crowd onto the top of her abdomen. With a large species, the result looks like a living fur coat of dozens or even hundreds of tiny spiders, sometimes piled several layers deep.

The babies grip special knobbed hairs on the mother’s back, which give them something to hold on to while she moves. If one falls off, it usually climbs back up one of her legs, often helped by a fine silk safety line.

How long do wolf spiders carry their babies?

The riding period is short compared with the time spent as eggs. In the Carolina wolf spider the spiderlings stay on their mother for about the first six days of life. In other species it can be one to two weeks or longer, depending on temperature and the size of the brood.

What do the babies eat?

While they ride on their mother, the spiderlings do not hunt and are not fed by her. They live on the remains of the yolk from their eggs, which provides enough energy for the first days of life. This is why the riding period lasts only until the spiderlings have molted and are able to catch their own tiny prey, such as springtails and small insects.

Research on the Carolina wolf spider suggests that the time spent with the mother matters. Spiderlings that stayed with their mother for longer developed better hunting skills and larger brains than those separated early.

When and how do the spiderlings leave?

Eventually the spiderlings climb down and scatter into the surrounding grass and leaf litter. Some disperse on foot, while others release a thread of silk into the breeze and float away, a behavior called ballooning. This helps explain why wolf spiders are found almost everywhere, from gardens to mountain meadows.

Each young spider is now on its own. It hunts, grows and molts several times before becoming an adult. For more on their lives and lifespans, see how long do spiders live?

Is this behavior unique to wolf spiders?

Among spiders, carrying the egg sac on the spinnerets and the babies on the back is the trademark of the wolf spider family (Lycosidae). Most other spiders hide their egg sacs in webs, crevices or curled leaves and leave the young to fend for themselves. Nursery web spiders carry the sac in their jaws and then build a silk “nursery” tent around the young.

Their scorpion relatives show a strikingly similar habit: newborn scorpions also ride on their mother’s back for their first days of life. If you spot a large spider with a white ball at the back end or a fuzzy back full of babies, you are almost certainly looking at a female wolf spider. She is harmless if left alone, as explained in are wolf spiders dangerous?