The Reality of Female Spiders Eating Males
Female spiders consuming their male counterparts after mating is a well-known concept, often associated with the black widow. This behavior, known as sexual cannibalism, is observed in many spider species. While commonly perceived as universal, it is not present in all spider species.
Sexual cannibalism can occur at various stages: before, during, or after copulation. In some cases, like the Australian redback spider (Latrodectus hasselti), the male actively contributes to his consumption, somersaulting onto the female’s fangs during mating. This extreme behavior results in the male being eaten in over 60% of cases. Other species, such as the nursery web spider (Pisaurina mira), exhibit post-mating cannibalism, where the female often attempts to consume the male for sustenance after sperm transfer.
Evolutionary Reasons for This Behavior
Several hypotheses explain why sexual cannibalism has evolved in spiders, often linking it to female reproductive success. One prominent theory is the nutritional benefit hypothesis. Consuming the male provides the female with a significant boost of nutrients, such as proteins and fats, which can be directly channeled into producing more eggs or healthier offspring. Studies on species like the Mediterranean tarantula (Lycosa tarantula) show that cannibalistic females produce more offspring per egg sac and produce progeny with better body condition.
Sexual cannibalism can also relate to paternity assurance for the male. In species like the redback spider, a male’s self-sacrifice can increase the duration of copulation and, consequently, the amount of sperm transferred, leading to a higher chance of fertilizing the female’s eggs. This ensures his genetic material is passed on, even at the cost of his life, especially if future mating opportunities are limited. Furthermore, in some species, females might be more likely to reject subsequent suitors after consuming their first mate, further securing the cannibalized male’s paternity.
Resource availability also plays a role. If food is scarce, the male becomes a readily available and nutrient-rich meal for the female. This is particularly relevant for species where females are food-limited in nature. Female dominance and aggression also contribute to this behavior. Females are often significantly larger and more dominant than males, making the male an easy target for predation. This aggressive nature can sometimes extend to mating interactions, where the male is simply treated as prey.
How Males Try to Avoid Being Eaten
Male spiders have evolved diverse strategies to survive mating encounters. Elaborate courtship rituals are common, where males perform complex dances, vibrations, or offer “nuptial gifts” like wrapped prey to appease the female and signal their intent as a mate rather than prey. For example, male wolf spiders may offer a dead fly or an egg sac to distract the female’s predatory instincts.
Timing is another crucial factor. Some male spiders attempt to mate when the female is less aggressive or vulnerable, such as immediately after her final molt, when her exoskeleton is still soft. Male redback spiders, for instance, have been observed mating with immature females, which significantly reduces the risk of cannibalism and can increase successful insemination. These immature females can store sperm until they mature, allowing the male to survive and potentially mate again.
Stealth and speed are also vital for male survival. Some males engage in quick copulation and rapid escape. In the orb-weaving spider Philoponella prominens, males use a catapulting mechanism with their front legs to launch themselves away from the female at high speeds immediately after mating. This allows them to escape being eaten, with studies showing 152 out of 155 successful escapes. Other males use silk to restrain the female during copulation. Male nursery-web spiders (Pisaurina mira) wrap the female’s legs in silk, reducing her mobility and their risk of being cannibalized.
When Cannibalism Doesn’t Occur
Sexual cannibalism is not a universal behavior across all spider species, and its occurrence can vary widely. While black widows are famously associated with sexual cannibalism, North American black widow species do not typically kill the male, and males can sometimes coexist in a female’s web without harm. Environmental factors also influence the likelihood of cannibalism. When resources are abundant, and females are well-fed, the nutritional incentive for consuming a male may decrease. Studies suggest that female black widows might only devour their mates when hungry. This indicates cannibalism can be a facultative behavior, more prevalent under specific ecological conditions like food scarcity.
The effectiveness of male avoidance strategies also plays a significant role in preventing cannibalism. Successful courtship displays, precise timing, or physical restraints employed by the male can lead to successful escapes. In some funnel-web spiders, males induce a cataleptic state in females through specific vibrational signals, rendering the female quiescent and allowing the male to mate safely. This ensures the male survives to potentially mate multiple times.