Spider cannibalism
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[edit] Sacrificial 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. Most males get consumed at this stage (Maydianne CB Andrade reports 65%).[1] Males that 'sacrifice' themselves gain the benefit of increasing their paternity relative to males who do not get cannibalized.[1]
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. Foelix writes, "The supposed aggressiveness of the female spider towards the male is largely a myth... only in some exceptional cases does the male fall victim to the female." [2] Michael Roberts says, "It is rare for a fit male to be eaten by the female."[3] 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.[3]
[edit] Other kinds of 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.
[edit] References
- ^ a b Andrade, Maydianne C.B. Behavioral Ecology (2003), 14:531–538
- ^ Foelix, Rainer F. Biology of Spiders, 1982.
- ^ a b Roberts, Michael J. Spiders of Britain and Northern Europe, Collins, London, 1995.
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