Spider cannibalism

Nephila sp. eating a conspecific

Spider cannibalism is a special case of cannibalism in which a female spider kills and eats a male of the same species before, during, or after copulation. Cases in which males eat females are rare.

Females eating males

It is often said that the male (usually significantly smaller than the female, down to 1% of her size as seen in Tidarren sisyphoides) is likely to be killed by the female after the coupling, or sometimes even before intercourse has been initiated. This supposed propensity is what gave the black widow spider, Latrodectus mactans, its name. However, the three species of North American black widows do not seem to usually kill the male (although they have been known to). Males can sometimes even live in the web of a female for a while without being harmed in any way. The male Australian redback spider Latrodectus hasselti is killed by the females after it inserts its second palpus in the female genital opening; in over 60% of cases the female then eats the male.

Although the male Latrodectus hasselti may sometimes die during mating without the female actually consuming it, this species represents a possible strategy of "male sacrifice". The male redback, while copulating, "somersaults" and twists its abdomen directly onto the fangs of its mate. Approximately 65% of males get consumed at this stage. Males that "sacrifice" themselves gain the benefit of increasing their paternity relative to males who do not get cannibalized.

However, despite these examples and many other similar reports, the theory of the "sacrificial male" has become greater than the truth. Mating of spiders is not always followed by cannibalism. Indeed, scholars have noted that the "supposed aggressiveness of the female spider towards the male is largely a myth" and that cannibalism only occurs in exceptional cases. And yet, spider cannibalism has been proved to occur in some species more than in others, mainly species belonging to Latrodectus.

There has always been speculation on why this sacrifice of male mates might occur despite the fact that there is an obvious disadvantage to the sacrificial males. One theory is that once the male has mated, he is unlikely to mate again and so any further extension of his life is of lesser evolutionary benefit than his indirectly contributing nutrition to the eggs. Having more offspring would give the male the advantage of having his genes passed on over other males that might avoid being eaten. This scenario would be consistent with the hypothesis of Roberts that old or unfit males get eaten, whilst younger and fitter ones may survive to mate again.

Males eating females

Reversing the traditional roles, species that have males consuming females are relatively unknown; the few that do show a male-biased sexual dimorphism. Examples include Allocosa brasiliensis, Evarcha culicivora, and Argyronetia aquatica. Male water spiders, Argyronetia aquatica, show a predilection for mating with larger females, while cannibalizing females smaller than themselves.

As is the case with Allocosa brasiliensis, males cull older females who are less fertile than their young counterparts.

Sacrificial mothers

• Offspring of the species Stegodyphus lineatus eat their mother
• Females of Segestria florentina sometimes die while guarding her eggs, and the hatched spiders will later eat her

Non-reproductive cannibalism

Some spiders, such as Pholcus phalangioides, will prey on their own kind when food is scarce. Also, females of Phidippus johnsoni have been observed carrying dead males in their fangs. This behavior may be triggered by aggression, where females carry over hostility from their juvenile state and consume males just as they would prey. Sih and Johnson surmise that non-reproductive cannibalism can occur due to a remnant of an aggression trait in juvenile females. Known as the “aggressive spillover hypothesis”, this tendency to unselectively attack anything that moves is cultivated by a positive correlation between hostility, foraging capability, and fecundity. Aggression at a young age leads to an increase in prey consumption and as such, a larger adult size. This behavior “spills over” into adulthood, and shows up as a nonadaptive trait that manifests itself through adult females preying on males of their same species.