Do Black Widows Eat Their Mates? The Truth About Cannibalism

Why are they called widow spiders?

Eating a partner during or after mating is called sexual cannibalism. It is known in many spiders, in praying mantises and in a few other invertebrates. Widow spiders, genus Latrodectus, earned their common name because females were widely reported to kill and eat their mates, leaving them “widows”.

The behavior is real, but it varies a lot between species. In some widows it is common, even expected. In others, including the southern black widow and its North American relatives, it is the exception rather than the rule.

What really happens in the wild?

Male black widows are tiny, with a body only about 3–6 mm (0.1–0.25 in) long, compared with 8–13 mm (0.3–0.5 in) for the female. Once mature, they spend their short adult lives wandering in search of females.

Field and laboratory studies show that many North American males mate and then leave. Western black widow males typically mate with more than one female in their lifetime, which would be impossible if they were always eaten. In some widow species, males even stay for a while at the edge of a female’s web.

So why the famous reputation? Much of the early evidence came from spiders kept in small jars or cages. There a male has no room to retreat after mating, and a hungry, confined female is more likely to treat him as food. Some males also die naturally soon after mating, sometimes with damaged palps, the leg-like feelers they use to transfer sperm, and a female may later eat a dead male found in her web. Both situations make cannibalism look more common than it is.

How male black widows avoid being eaten

Males have several ways to lower the risk:

• Careful courtship. A male announces himself by plucking and vibrating the threads of the female’s web in a species-specific pattern, so she recognizes a suitor rather than trapped prey. Courtship can last from minutes to hours.
• Reading the silk. Female widows coat their webs with chemical signals. A 2011 study found that male western black widows courted well-fed females more than starved ones, apparently judging hunger from cues on the silk.
• Taking down the web. During courtship, western black widow males cut and bundle up large parts of the female’s web, which researchers think helps keep rival males away.
• Mating with immature females. In laboratory trials, male western black widows mated with subadult females that were about to make their final molt. These males copulated for longer and fathered more offspring than males that mated with adults, although approaching an immature female too early can still be deadly.

The redback: the male that offers himself

The Australian redback (Latrodectus hasselti), a close relative of the black widows, is the extreme case. During mating the male performs a somersault that places his abdomen directly against the female’s fangs. In roughly two out of three matings, she starts eating him while mating continues.

Research by biologist Maydianne Andrade, published in 1996, showed why this suicidal move can pay off. Males that were eaten copulated for longer and fertilized more eggs, and females that had eaten a mate were more likely to reject later suitors. Since most redback males die while searching for a female and never mate at all, giving everything in one encounter can be a male’s best strategy. Males that escape being eaten usually die of their injuries soon afterward anyway.

The brown widow (Latrodectus geometricus), now common in the southern US, behaves in a similar way: mating with an adult female typically ends with the male’s somersault and death. But brown widow and redback males can also mate with subadult females, which skips the sacrifice altogether.

Why would a female eat her mate?

For a female, a male is a small but useful meal at a time when she needs energy to produce eggs. A female widow can lay several egg sacs, each holding a few hundred eggs, so extra food matters. Biologists also see cannibalism as a form of mate choice: a female can reject an unwanted suitor by attacking him, and a female that has already mated may gain little from tolerating another one.

Cannibalism in widows is not limited to mates. Spiderlings often eat their siblings, especially when food is scarce, and adults of some species will attack each other. This general readiness to eat their own kind is one reason widows are almost always solitary.

Other animals that eat their mates

Widows are far from alone. Female praying mantises sometimes eat males, even beginning with the head while mating continues, a behavior explained in why do female praying mantises eat males? Among spiders, sexual cannibalism has been recorded in many orb weavers, including the European garden spider, and in wolf spiders and fishing spiders.

In most of these species, as in the black widows, cannibalism is an occasional risk rather than an inevitable ending. A widow’s mating habits are also unrelated to how risky she is to people, which is covered in how dangerous is a black widow?