Why is a spider like a Derringer?

The Derringer pistol:

Pros: Easy concealment. Stylish.

Con: You’ve only got one shot.

Argiope bruennichi of the male variety are rather like Derringer pistols. Because, you see, their sex organs tend to break during mating.

Yes. Break off.

You almost can’t help but wonder if being eaten by the female – which is the fate for a large percentage of them – isn’t almost a relief to the poor boys at that point.

When you only one shot at genetic glory, you would expect to aim at the target carefully. You might expect the males to be incredibly selective about what females they mate with. In many invertebrates, there’s an easy measure of female quality: size. Bigger bodies mean more eggs.

Shulte and colleagues tested this by experiments in the field and the lab. The males had a choice between two females that differed in size. They didn’t just measure which female was mated with, but the level of the male’s interest, from “Just looking” (male comes to female’s web) to “Sure I’ll come in to see your etchings” (male enters web) to fake yawn (male courts female) to squeaking bedsprings (copulation).

The prediction is that as the differences in the two females get bigger, so too should the differences in the males’ behaviours.

The reality was... not much difference in the males’ behaviour. The males tended to go to one female very quickly and stay with that female regardless of the size, particularly in the lab. In the field, there’s more evidence for choosiness: half the males

What’s going on here? It’s not terribly clear. Maybe the cost of searching for females is so high (predation, maybe?) that the pressure is on for males to mate with the first female they meet. Maybe the male of this species doesn’t have the sensory ability to discriminate sizes, but it would be darned unusual, because lots of arthropods can do such tasks.

The males aren’t complete oblivious to the females, though. Males select quite strongly for unmated females. Because, you see, if there had been a previous male... well, let’s just say that working around the broken sex organs of a previous suitor can really cramp your style. The likelihood of siring any little spiderlets goes way down for previously mated females. But even then, some males would reject unmated females, and it’s not clear why.

Reference

Schulte, K., Uhl, G., & Schneider, J. (2010). Mate choice in males with one-shot genitalia: limited importance of female fecundity Animal Behaviour DOI: 10.1016/j.anbehav.2010.07.005

Derringer photo by remixoverdrive on Flickr. Spider photo by Joachim S. Müller on Flickr. Both used under a Creative Commons license.

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