Female spiders eat their mates because, like, they (drum roll) EVOLVED that way ... or because size matters?

We learn from ScienceDaily (Sep. 11, 2008) that female spiders do not necessarily eat their mates. Now, before we move on, let us pause to think of all the just-so Darwinian sexual selection stories we have heard that explain why they do. (They add to their energy stores, they prevent the male from mating again, they ... )

Researchers Shawn Wilder and Ann Rypstra from Miami University in Ohio found that, in general,

Males are more likely to be eaten if they are much smaller than females, which likely affects how easy they are to catch. In one species of spider, Hogna helluo, large males were never consumed while small males were consumed 80% of the time. This result was also confirmed when Wilder and Rypstra examined published data from a wide range of spider species. Males are more likely to be eaten in species where males are small relative to females.

Much research on sexual cannibalism has focused on a few extreme cases involving sexual selection and sperm competition. However, by looking at data on a wide range of spiders, Wilder and Rypstra discovered that the size of the male relative to the female (often referred to as sexual size dimorphism) determines how often sexual cannibalism occurs in a species.

This sounds like a polite way of saying that previous researchers have focused on the few cases that would confirm Darwin's theory of sexual selection and its theoretical heirs, without looking at fundamental facts like, how does a usually unintelligent creature like a spider know when to attack and consume another life form and when not to. This calculation may well be made irrespective of mating, as Wilder and Rypstra's research suggests.

Does a spider even know that it is having sex? Or that that matters?

They go on to say,

"We were surprised to find that such a simple characteristic such as how small males are relative to females has such a large effect on the frequency of sexual cannibalism," states Shawn Wilder. In many cases, sexual cannibalism may not be a complex balancing act of costs and benefits for males and females but rather a case of a hungry female eating a male when he is small enough to catch.

In an interesting twist, evolution does not appear to be driving this relationship. ...

No surprise there. "Evolution" need not drive the relationship. Once spiders have neural circuits (however, exactly, they acquired them) which determine whether a given life form is too big to attack, they probably don't need "evolution" to drive the subsequent relationship, whether or not it involves reproduction.

Our local spiders pounce on insects that get trapped in their webs but flee humans that accidentally break them. So, in the absence of neuroscience studies on spiders, I will assume that the spider has a system for judging size. In that case, we might predict that neural circuits urging spiders to flee will override those urging them to attack - when the size of the possible object of attack exceeds certain boundaries.

In that case, it would be more useful for researchers to study the spider's nervous system and find the relevant circuit than to speculate on how Darwinian sexual selection might explain why spiders attack or do not attack.

See also:

O'Leary meets an intelligent spider

Peacocks and sexual selection (another situation in which famed Darwinian sexual selection does not explain anything)

Journal reference: Wilder et al. Sexual Size Dimorphism Predicts the Frequency of Sexual Cannibalism Within and Among Species of Spiders.. The American Naturalist, 2008; 172 (3): 431 DOI: 10.1086/589518