Although sexual cannibalism is discouraged among humans, if not outright illegal, it's not uncommon in the spider world, as well as among mantises. Female spiders are known to eat male spiders both before and after mating, though researchers point out in a new paper published in American Naturalist that it's not crucial to the spider mating process. So the question is: What makes a female spider eat a male? Apparently there are a lot of complicated theories.

The thing that's great about this new study, however, is that its authors have determined that there is no need to develop a complicated evolutionary theory to explain spider sexual cannibalism. In fact, it's as simple as how small a male is. The smaller the male, the easier he is to catch and eat. U.S. researcher Shawn Wilder, an author of the paper, explained:

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.

It turns out, in other words, that sexual cannibalism isn't some complicated evolutionary system. Female spiders just eat men who are small enough to catch. According to the authors:

Females would not become larger to consume more males because each male would then be a smaller meal to the larger female and males would not become smaller to be eaten more often because they would not get to mate as often. Rather, sexual cannibalism may be a byproduct of the evolution of large females and small males in a predatory species.

So there you have it. Sexual cannibalism is not a mating strategy. It's because the female spider is hungry and the male spider is within range. Image via Frank Starmer.
Sexual Size Dimorphism Predicts the Frequency of Sexual Cannibalism [via American Naturalist]