Spiders are generalist predators, hunting and consuming a wide variety of insects and other arthropods. The simple answer to whether they eat each other is a definitive yes, and this behavior is a common occurrence in the arachnid world. This spider-on-spider predation is a complex, evolved strategy driven by factors beyond simple hunger, including resource competition and reproductive success.
Defining Spider-on-Spider Predation
Spider-on-spider predation can be cleanly separated into two distinct ecological categories. The first category, known as cannibalism, involves a spider consuming another individual of the same species. This type of predation includes events like a mother eating her offspring, or a larger adult consuming a smaller sibling.
The second category is termed intraguild predation (IGP), which occurs when a spider kills and eats a member of a different, competing species. Since nearly all spiders are generalist predators, they often share the same insect prey, placing them within the same ecological “guild.” An example of IGP would be a wolf spider preying upon a funnel-web spider, as both species hunt similar insects in the same habitat.
Key Triggers for Cannibalism
The primary drivers for non-sexual cannibalism are the immediate biological needs of the individual spider and its environment. Resource scarcity is a major trigger, as spiders will turn to consuming conspecifics when other prey items are limited. Size disparity is another strong predictor of cannibalistic events, with the larger spider almost always preying on the smaller one.
This is frequently observed among juveniles, where recently hatched spiderlings will consume each other in the confined space of the egg sac or immediate vicinity. This early-life cannibalism helps to regulate cohort size and ensures that only the strongest individuals survive to disperse, especially in species that produce large numbers of offspring.
Sexual Cannibalism: The Reproductive Imperative
The most famous form of spider predation is sexual cannibalism, where the female consumes the male before, during, or immediately after copulation. This behavior is an evolutionary strategy that influences the reproductive success of both sexes. The female is almost always the aggressor, and species like the black widow (Latrodectus) are well-known for this trait.
Nuptial Gift Hypothesis
One explanation is the “Nuptial Gift” hypothesis, which suggests the male provides a direct nutritional benefit to the female. By consuming the male, the female gains a significant protein boost that can be invested immediately into producing more eggs or ensuring healthier offspring. For the male, this self-sacrifice can sometimes be an adaptive behavior, ensuring his genes are passed on.
Paternity Assurance Hypothesis
A second theory, the “Paternity Assurance” hypothesis, is particularly relevant when the male is complicit in his own demise, such as the male Australian redback spider (Latrodectus hasselti). This male will perform a somersault during mating, placing his abdomen directly over the female’s fangs. By allowing himself to be eaten, the male prolongs the duration of copulation and sperm transfer, which increases his certainty of paternity by reducing the likelihood that the female will mate again.
Specialized Intraguild Predators
Beyond the general acts of cannibalism, some spider species have evolved to specialize in intraguild predation, making other spiders their primary food source. The jumping spiders of the genus Portia are a prime example, exhibiting complex, almost intelligent hunting strategies to ambush other spiders.
Portia often invades the webs of other species and uses a tactic called aggressive mimicry. The predator will pluck the silk threads in specific patterns, mimicking the vibrations of a trapped insect or even the courtship signals of the resident spider’s mate. When the resident spider approaches to investigate, Portia strikes. This specialized hunting skill allows them to prey on other spiders much larger than themselves. These specialized intraguild predators play an important role in local ecosystems by controlling the populations of other coexisting spider species.