Spiders are cannibals, but the behavior is far more complex than simple aggression. Cannibalism, the act of consuming another of the same species, is common across the arachnid family and serves various biological roles. This practice is governed by factors like reproductive state, hunger levels, and the age of the individuals involved. The context of the interaction determines whether the act is opportunistic feeding, a reproductive strategy, or resource management, making it an intricate part of a spider’s life cycle.
Sexual Cannibalism
The most widely known form is sexual cannibalism, typically involving the female consuming the male due to her much larger body size. The female views the male as potential prey, especially if she is hungry or if the male is significantly smaller. Studies on the wolf spider Hogna helluo show that smaller males are consumed about 80% of the time, indicating that size and ease of capture are major factors.
Consumption can occur before, during, or immediately after copulation. The Australian redback spider, Latrodectus hasselti, exhibits male sacrifice where the male somersaults his abdomen onto the female’s fangs during mating. This voluntary sacrifice, which results in the male being consumed approximately 65% of the time, prolongs copulation and increases the male’s paternity success.
While the American black widow, Latrodectus mactans, is the namesake for the behavior, cannibalism is less frequent in this species. Male-biased sexual cannibalism has also been observed in species like the wolf spider Allocosa brasiliensis, where males may cull older, less fertile females.
Cannibalism in Young Spiders
Cannibalism is a frequent occurrence among juvenile spiders, driven primarily by competition and resource scarcity. Sibling cannibalism, or siblicide, happens when spiderlings prey on their siblings, often while still clustered in the egg sac or the mother’s web. For juvenile redback spiders, this behavior is common immediately after hatching and serves to eliminate competition in a resource-limited environment.
The propensity for this sibling-on-sibling predation can be a heritable trait. In the false widow spider Steatoda grossa, the rate of cannibalism is strongly influenced by prey availability, suggesting it is a survival mechanism triggered by hunger.
Beyond siblings, an extreme form of filial cannibalism known as matriphagy occurs, where the offspring consume their mother. In the desert spider Stegodyphus lineatus, the mother hoards nutritional resources for her young, who eat her alive one to two weeks after hatching. The black lace-weaver Amaurobius ferox first lays nutritional “trophic eggs” before the spiderlings consume her body days later, ensuring a massive nutritional boost for the next generation.
The Evolutionary Reasons for Eating Kin
The persistence of cannibalism is explained by the significant evolutionary advantages it confers, framing the behavior as a calculated survival strategy. The primary driver is the substantial nutritional benefit gained from consuming a conspecific, particularly for females who require a large energy supply for egg production. Cannibalistic females gain protein and nutrients that translate into heavier egg cases, higher energy density in the eggs, and increased hatching success for their offspring.
In the wolf spider Lycosa tarantula, females that consumed a single male produced offspring with better body condition and laid their egg sacs sooner. This timing promoted more rapid development due to warmer temperatures. Even when the male is smaller, the nutritional investment acts as a form of paternal investment, where the male’s body directly contributes to the fitness and survival of his progeny.
For juvenile spiders, cannibalism serves as resource management, where stronger individuals eliminate weaker competitors. This ensures that limited food resources are concentrated in the individuals most likely to survive and reproduce. This self-thinning mechanism guarantees that the next generation begins with a higher baseline of health and size, maximizing the overall reproductive success.