Female praying mantises famously eat their mates while getting it on (although not as often as many believe), but at least female spiders have the decency to wait until the male is finished before snacking.
Strangely, in some species the male doesn't resist becoming post-sex dinner. A study of the dark fishing spider (Dolomedes tenebrosus) has concluded the behavior represents the ultimate in parental sacrifice; offspring of fed-upon fathers are larger and more numerous than those whose mother missed out.
Female spiders are often far larger than the males (14 times heavier in the case of D. tenebrosus), so it is unsurprising the females consider them a tasty morsel. More puzzling is that as Dr Steven Schwartz from Gonzaga University in Washington puts it in the study in Current Biology, “some males actually appear to facilitate their own cannibalism”.
In the course of his PhD, Schwartz observed that male dark fishing spiders literally curl up and die after mating, and do so close enough to the female that consumption is easy. Death is triggered after sperm is delivered when a bulb inside the pedipalp, an organ used to transfer sperm, explodes.
Schwartz's latest study explains why the males do this – it perpetuates their genes by almost doubling the number of offspring the female gives birth to, as well as increasing the spiderlings' size and lifespan.
Nutrition during pregnancy usually produces more or healthier young, but the males seem to make a particularly effective meal. Schwartz experimented with depriving the female of her mate's corpse, and replacing it with a cricket. Crickets are a common meal for female dark fishing spiders, and of a similar size to the males, so might be expected to provide a similar boost to the offspring.
This, however, was not what Schwartz found. "It's only when a female eats the male that we see these benefits," he said in a statement. "There might be a nutrient, or maybe a cocktail of nutrients, that is somehow concentrated in the males' bodies. We don't know what that is, but there is something going on there."
Presumably natural selection has worked to enhance this effect, with the most nutritious males having the most successful offspring.
Schwartz attributes the development of the male self-sacrifice both to the huge difference in size between the sexes, and the fact that mature male spiders are much more common. The chance of a male finding a second female, who has not already been impregnated, are so small he may as well invest everything he has, body included, in maximizing the chances of the one set of offspring he is likely to get.
Schwartz acknowledges that the small number of similar studies conducted in other species that practice post-copulatory cannibalism didn't find the same benefits, suggesting we have plenty more to learn about how this behavior developed and is maintained.
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