Several kinds of female animals, including spiders, are known to devour their male counterparts, often right after they finish having sex. But every so often, the female isn't deadlier than the male, as South America's wolf spider will tell you.
The nocturnal arachnid is found all along South America's Atlantic coast. Researchers from the Clemente Estable Institute of Biological Research observed male wolf spiders taking the almost unprecedented step of cannibalizing their female counterparts. In females, sexual cannibalism is known to confer a serious reproductive advantage, as the devoured male is often able to spend far more time than he would otherwise fertilizing as many eggs as he can. This sacrifice goes a long way to improving his long-term evolutionary fitness.
But why are male wolf spiders now eating their females, as there obviously can't be a reproductive benefit? The answer is fairly brutal: for female wolf spiders, it's either have sex or be devoured. The males apparently like to mate with the younger, virgin females who are still at the height of their fertility, but as they grow older and lose some of their reproductive prowess they become prey for the males. It appears their benefit for the males is purely nutritional.
Why have the wolf spiders taken up this incredibly rare strategy? It's probably because their environment is highly unstable, and there's no consistent guarantee that prey will be available. Under the circumstances, actually eating one's fellow spiders becomes a grimly efficient way to both get some much-needed nutrition and keep the overall population numbers down.