microecos

Daring Jumping Spider (Phidippus audax) dines on a Praying Mantis (Mantis religiosa) hatchling. Trans-class paedophilic cannibalism or just a hungry spider? Photo by the author.

Yesterday the Times ran an article by Carl Zimmer on sexual cannibalism, accompanied by some fantastic Catherine Chalmers photos.

Sexual cannibalism is the actual term used by biologists to describe the consumption of a male conspecific by a female during or just after mating. This behavior is most infamously associated with mantids, although field studies suggest it may be rather uncommon in most wild mantids.

Cartoon by self-described “round, purple lynx”, Rahball

Sexual cannibalism is also infamous in spiders, think “black widow”, and is probably rather more common in many types of spiders (including some species of Phiddipus) than in mantids. Elsewhere in the animal kingdom sexual cannibalism is quite rare, reported only in amphipods, nudibranchs and copepods. Oh yeah, and humans of course.

Few things fascinate people more than violence or sex (pace MPAA) and when you combine the two you’ve got blockbuster potential. This no doubt accounts for the sensationalized treatment of the subject from the very first accounts.

Placing them in the same jar, the male, in alarm, endeavoured to escape. In a few minutes the female succeeded in grasping him. She first bit off his front tarsus, and consumed the tibia and femur. Next she gnawed out his left eye…it seems to be only by accident that a male ever escapes alive from the embraces of his partner (Howard 1886).

American entomologist Leland Ossian Howard

Sexual cannibalism isn’t just sensational, it’s also scientifically contentious. Zimmer’s article reviews the history of the scientific debate in light of a recent paper by Lelito and Brown in the August issue of American Naturalist. In a follow up blog post, Zimmer examines sexual cannibalism within the broader “Adaptationist/Exaptationist” divide.

The central argument around sexual cannibalism is to what extent sexual cannibalism might actually be adaptive for males. In the “extreme paternal investment” model it’s supposed that offspring may get a big enough boost from a dad-fed mom that males are actually complicit partners in their own death. In the twenty years since this was postulated very little scientific evidence has been found to support willing paternal sacrifice.

Others (most famously Steve Gould in an essay entitled “Only His Wings Remained” 1984) have argued that sexual cannibalism is simply a byproduct of the general voracity of the female, one not particularly troubled by the ethical implications of mariticide.

Will tomorrow’s people be sexual cannibals? Image © Freemantle Media

The fact that sexual cannibalism appears almost exclusively among aggressive generalist predators, often in species with moderate to strong sexual dimorphism, suggests that sexual cannibalism is primarily, exaptive. Furthermore, both spiders and mantids are known to be cannibalistic in other circumstances, eating siblings, un-related juveniles etc. Lelito and Brown report that in the Chinese Mantis (Tenodera aridifolia sinensis), up to 63% of the diet of adult females is made up of male conspecifics.

An inherent tendency for females to make lunch out of anything smaller than themselves also sheds light on the complicated courtship practices of spiders and mantids. Males use complicated sensory cues to signal “MATE NOT FOOD” and nimble feet or novel positioning to allow the male to avoid or restrain the pointy parts of the female. The Lelito and Brown study finds that the caution-level and mating behaviors of males are strongly affected by the hunger level of their potential mates.

Safe sex, spider style. Evarcha falcata (left) and the bondage-inclined Xysticus cristatus (right). Originally published in the marvellous The Book of Spiders and Scorpions by Rod Preston-Mafham. Excerpted from PZ Myers’ classic post Spider Kama Sutra.

A few species, notably red-backed spider males who famously “somersault” into the jaws of the female at the climax of the nuptial act, do seem to display a degree of male complicity. However the selective benefit in this case (and others like it) seems to come in the form of extended matings and/or exclusion of rivals, not well nourished mates or offspring.

Zimmer draws the conclusion that sexual cannibalism is a selectively important phenomenon,

But the paper is more Dawkins than Gould. The male mantises have some way of telling how hungry the females are, and take lots of precautions–jumping on from further away, taking longer to dismount, and so on..

I see things rather differently. Evidence for the paternal investment model (the subject of Gould’s original criticism) remains slim. In light of Lelito and Brown and the last 20 years of work on the subject it seems clear that Gould’s skepticism was vindicated. Sexual cannibalism is molded by a suite of complex adaptive and exaptive factors far more intricate than the simple “just-so story” of extreme paternal investment.

In keeping with our new motto (see previous post), this post owes a debt of gratitude to Coturnix, Michele Doughty and Kenwyn Blake Suttle.