Spiders regularly eat other spiders, a behavior known as cannibalism or intraspecific predation. This is a common feature across many spider families, serving as a survival strategy influenced by ecological and reproductive pressures. Cannibalism ranges from simple foraging to highly specialized reproductive acts.
Survival and Resource Scarcity
The primary driver of spider cannibalism is hunger, especially when traditional prey resources are scarce. Spiders are generalist predators, and when insect food is limited, a smaller or weaker conspecific becomes a viable, high-protein meal. Experiments show a clear link between starvation and the propensity to cannibalize, with starved spiders showing increased rates of consuming others.
This non-sexual cannibalism often functions as competition, regulating population size in a density-dependent manner. When population density is high, encounters increase, and spiders view others as competitors for limited space and food. Territoriality is a factor, particularly among burrowing species like the Mediterranean tarantula (Lycosa tarantula).
In territorial species, conflict over a burrow or hunting patch can escalate into a deadly fight, with the winner frequently consuming the loser. Consumption is heavily influenced by size asymmetry, where a larger individual is much more likely to cannibalize a smaller one. This size advantage ensures the act is a low-risk foraging strategy.
The Specialized Case of Sexual Cannibalism
Sexual cannibalism occurs when the female consumes the male before, during, or immediately after copulation. This act is common across many spider families and is driven by conflict over reproductive investment. Females, typically larger than males, gain a substantial nutritional boost from the male’s body, which is converted into resources for egg production.
The female’s nutritional gain increases her fecundity, egg case size, and offspring survival, supporting the “adaptive foraging hypothesis.” In species such as the Australian redback spider (Latrodectus hasselti), the male actively participates in his own consumption, suggesting a benefit for his genes. During copulation, the male somersaults onto the female’s fangs, prolonging copulation time.
This prolonged copulation increases sperm transfer and provides paternity assurance, effectively blocking other males from mating. By sacrificing his life, the male increases the likelihood of siring the offspring, representing “terminal investment.” Males have evolved counter-strategies, including mating with immature females, offering silken “nuptial gifts,” or restraining the female prior to approach to reduce the risk of being eaten.
Predation Among the Young
Cannibalism occurs in the early stages of a spider’s life, regulating clutch size. Fratricide, or sibling cannibalism, occurs frequently among spiderlings immediately after hatching or before they disperse from the egg sac or nursery web. This behavior is driven by intense competition for the limited resources within the clutch.
Consuming siblings ensures the surviving young are better nourished and more robust, increasing their chances of survival upon dispersal. Beyond siblings, some species exhibit matriphagy, where the mother allows her offspring to consume her body. In the desert spider (Stegodyphus lineatus), the mother’s abdomen tissues break down as she guards her eggs, and the young feed on her after hatching.
Matriphagy is an act of parental investment, providing a final nutrient transfer that boosts the offspring’s weight and survival rates. This ensures the next generation has the necessary strength and size to begin their predatory lives. The young consume nearly all of the mother’s body mass, leaving behind only the exoskeleton.