Sexual cannibalism, where one partner (typically the female) consumes the other during or after mating, is a perplexing behavior observed in spiders. While it appears brutal, this interaction represents a complex biological strategy shaped by evolutionary pressures, rather than simple aggression.
Understanding Sexual Cannibalism
Sexual cannibalism involves an animal consuming its mate either before, during, or after copulation. While this behavior is not exclusive to spiders, it is particularly well-documented within this group of arachnids. It occurs among various invertebrates, including insects and crustaceans, and even some snake species.
Its prevalence varies significantly between spider species, often correlating with the degree of sexual size dimorphism, where females are considerably larger than males. Misconceptions often surround this behavior, particularly the idea that it is a standard outcome for all spider matings. In reality, cannibalism occurs in some species more frequently than others, such as those within the Latrodectus genus, which includes black widows. However, even in black widow species, mate cannibalism is not always the rule, especially in North American varieties where males often survive to mate again.
The Driving Forces Behind the Behavior
Nutritional Benefit
The nutritional benefit hypothesis suggests females gain a significant nutrient boost. Consuming the male provides high-quality protein and energy, directly enhancing reproductive success by improving egg production and offspring condition. This additional food can lead to earlier breeding, more offspring per egg sac, and healthier progeny.
Aggressive Spillover
The aggressive spillover hypothesis proposes that a female’s general predatory aggression, beneficial for foraging and growth, can extend to mating encounters. If a female is naturally more aggressive, she might perceive the male as prey, especially if he is smaller. This aggression, cultivated for efficient prey consumption, can “spill over” into adulthood.
Mistaken Identity
Mistaken identity can also play a role if the female fails to recognize the male as a potential mate, perceiving him as prey. This can occur if the male approaches clumsily or fails to perform species-specific courtship rituals that signal his identity.
Adaptive Suicide
Adaptive suicide suggests males gain an evolutionary advantage by allowing themselves to be consumed. By providing nutrients directly to the female, the male indirectly invests in his offspring, potentially increasing the number or quality of eggs fertilized by his sperm. This self-sacrifice can also prolong copulation, ensuring more sperm transfer, especially if the male’s chances of finding another mate are low.
Spiders Known for This Behavior
The black widow spider (Latrodectus mactans) is a famous example. While North American black widows do not always kill the male, the male Australian redback spider (Latrodectus hasselti) frequently sacrifices itself. After inserting his second palpus, he flips into the female’s fangs, often resulting in consumption. This act can prolong copulation, increasing the male’s paternity success.
Orb-weaving spiders also exhibit sexual cannibalism. In species like Cyrtophora citricola, females often consume males, leading males to prefer well-fed, fertile mates. Some orb-weavers, such as Philoponella prominens, have males perform rapid escape maneuvers after mating. In Nephila plumipes, many males do not survive copulation, and virgin females in poor condition are more likely to cannibalize larger males.
Dark fishing spiders (Dolomedes tenebrosus) present an extreme case where males consistently die during their first and only copulation. Females invariably cannibalize these males post-copulation. This obligate death does not significantly prevent subsequent copulations by other males.
Male Strategies for Survival
Nuptial Gifts
Male spiders have evolved diverse strategies to navigate mating risks. One tactic is offering nuptial gifts, such as a silk-wrapped prey item, during courtship. While the female consumes the gift, the male can transfer sperm, and this distraction protects him from being eaten. Some males even offer “worthless gifts” like empty silk wraps, demonstrating a deceptive strategy.
Mate Guarding or Tying
Mate guarding or tying is another survival mechanism. Male nursery-web spiders (Pisaurina mira) use silk to bind the female’s forelegs during copulation, significantly increasing their survival rate. This restraint protects the male and can allow for multiple palpal insertions, potentially increasing sperm transfer.
Courtship Rituals
Precise courtship rituals are crucial for male survival. Complex dances, vibrations, and web-plucking signals help males communicate their species and intentions, reducing the chance of being mistaken for prey. Male wolf spiders, for instance, may play dead (thanatosis) during courtship, a highly effective tactic for successful copulation.
Other Strategies
Mating during specific periods, such as when the female is molting, can reduce cannibalism risk, as she is less aggressive and vulnerable. Body size differences also influence the risk, with smaller males potentially being overlooked as prey or able to escape more easily. Some male widow spiders even mate with sexually immature females by piercing their exoskeleton to access sperm storage structures, thereby avoiding cannibalism from a fully mature female. These diverse strategies highlight the ongoing evolutionary arms race between male survival and female reproductive success.