Sexual cannibalism, where one partner kills and eats the other during or after mating, occurs across dozens of species of spiders, insects, crustaceans, and octopuses. It is overwhelmingly the female that consumes the male, though a handful of spider species show the reverse when males happen to be larger. Far from being a fluke of nature, this behavior is driven by natural selection: females that eat their mates often produce more offspring in better condition, giving them a real evolutionary edge.
Spiders: The Most Common Mate-Eaters
Spiders account for the largest and best-studied group of animals that eat their mates. The behavior has been documented across many families, from tiny orb weavers to large wolf spiders. One consistent predictor is size: when males are much smaller than females, cannibalism rates go up. A broad analysis across spider species found a strong positive relationship between the degree of size difference between the sexes and the frequency of sexual cannibalism. In species where males are closer to the female’s size, it happens far less often.
Black widow spiders are perhaps the most famous example, though the reality is more nuanced than the reputation suggests. In a South American widow species, researchers documented cannibalism rates of 70% during a female’s first mating, rising to 85% when the female had already mated before. The female initiates the attack, and most males are killed during the very first copulation. Australian redback spiders take things further: males actively somersault into the female’s jaws after mating. This self-sacrifice increases the number of offspring the female produces, and it tends to occur in highly venomous, monogamous species where a male’s entire reproductive strategy hinges on a single mating event. Brown widow spiders show the same pattern.
Not all spider cannibalism is inevitable. In nursery web spiders, males sometimes bring an insect wrapped in silk as a “nuptial gift” to the female. Males that show up without a gift are significantly more likely to be eaten. In the orb spider Leucauge argyra, smaller males face higher attack rates, suggesting females may assess a male’s quality before deciding whether he’s worth more as a meal than a mate. And in some species where males and females are similar in size, either sex can end up cannibalizing the other.
How Eating a Mate Boosts Reproduction
The payoff for females is substantial and well-documented. In a study of wolf spiders in the wild, females that cannibalized a male produced egg sacs that were 40% heavier and contained 30% more offspring per clutch. Those spiderlings were also in better body condition than offspring of non-cannibalistic females. Beyond sheer numbers, timing shifts dramatically: cannibalistic females produced their egg sacs 13 days earlier, and those eggs developed four days faster. The net result was that their young hatched a full 15 days ahead of offspring from non-cannibalistic females.
That head start matters. Earlier hatching means warmer temperatures during the critical growth period. By the end of the dispersal season, spiderlings from cannibalistic mothers had grown to a larger size than their peers. Interestingly, survival rates between the two groups were about the same, so the advantage is not about staying alive but about being bigger, earlier, and in better shape when it counts.
Praying Mantises
The praying mantis is the other iconic mate-eater. In field observations of Mantis religiosa, sexual cannibalism occurred in 31% of matings. That’s lower than many people assume, partly because lab settings (where mantises are confined together) tend to inflate the rate. Still, roughly one in three wild matings ends with the female consuming the male, typically starting with his head.
What makes mantis cannibalism especially striking is what happens next. The nerve clusters that control mating movements are located in the male’s abdomen, not his brain. In fact, a nerve center in the head actively inhibits mating until a female is clasped. When a female bites off the male’s head mid-copulation, that inhibition is removed, and the headless male begins copulating more vigorously and repeatedly. The female gets both a meal and a more effective mating session, a grim but efficient arrangement.
Octopuses
Several octopus species practice sexual cannibalism, though it looks quite different from the spider version. In the wild, a large female day octopus (Octopus cyanea) was observed in Micronesia attacking a male after mating, suffocating him, and spending two full days consuming him in her den. The common octopus (O. vulgaris) shows similar behavior: the larger animal grabs the smaller one with seven arms and uses the eighth to block the prey’s siphon, cutting off its ability to breathe. Consumption typically begins at the arm tips.
Because male octopuses are often smaller than females, they approach mating cautiously. Males of many species will mate from a distance using a specialized arm, or attempt to mate only when the female is distracted by food. The size disparity makes smaller males especially vulnerable.
Other Animals That Eat Their Mates
Beyond spiders, mantises, and octopuses, sexual cannibalism shows up in several less well-known groups. Amphipods, small crustaceans found in both marine and freshwater environments, exhibit the behavior. It has also been documented in gastropods (snails and slugs) and copepods (tiny aquatic crustaceans), though less frequently. The common thread across all these groups is that females are typically larger than males and that food resources are limited enough to make a mate’s body a meaningful nutritional contribution.
What Triggers the Attack
Hunger is a major factor. Across multiple spider and mantis species, experimentally food-limited females cannibalize males at significantly higher rates than well-fed females. But hunger alone doesn’t explain the full picture. In wolf spiders, female body condition only predicted cannibalism when the size difference between the sexes was also accounted for. In other words, a hungry female paired with a relatively small male is the highest-risk combination. A hungry female paired with a large male is far less likely to attempt it, likely because the physical risk of attacking a bigger partner outweighs the nutritional reward.
How Males Try to Survive
Males in cannibalistic species have evolved a range of countermeasures. One of the most dramatic was discovered in the communal orb-weaving spider Philoponella prominens, where males catapult themselves away from females immediately after mating. Using hydraulic pressure in a leg joint that lacks muscles, males launch at speeds up to 88.2 centimeters per second. Males that failed to catapult were invariably caught and eaten.
Other strategies are subtler. Nursery web spider males offer silk-wrapped prey as a distraction. Some male spiders approach females only when the female is already feeding. In species like the redback spider, where males have essentially no chance of finding a second mate, the strategy flips entirely: males cooperate with their own consumption because the nutritional boost to the female translates directly into more of their own offspring. The male’s body becomes his final parental investment.