Why Are Black Widow Males So Destructive?

 We often hear about animals where the males mate with multiple females. However, many animals have the opposite system, where a single female courts and mates many males. This type of mating system (polyandry) can lead to some pretty interesting dynamics between males and females, because the males constantly attempt to out-compete each other to ensure that it’s their genes that make it into the next generation. This competition can happen in ways you might expect, like guarding a female to prevent her from mating with other males, but it can also express itself in more unusual and weird forms. In short, if you can think of a way that a male might try to stop another males’ sperm reaching a female’s egg, then natural selection has probably done it.

For example, the males of some species release chemicals during or after mating that make the female less desirable to other males. If that weren’t weird enough, a handful of animals use ‘mating plugs’ where a male deposits a plug inside a female while mating. This then blocks any sperm from potential rival males entering her.
I recently read about another unusual behaviour of this type in black widow spiders. These spiders (along with many others) show a peculiar behaviour where, while courting a female, a male will dismantle parts of her web, bundling it up and even wrapping up sections of it with his own silk. He might go as far as packing up around half of the intricate web she has built. But why do the male black widows do this?
   
Catherine Scott and her colleagues from Simon Fraser University in Canada recently published a paper investigating this question. In many web-building spiders, the females’ webs contain pheromones that are attractive to males. Therefore, it seemed plausible that the black widow males were earnestly tidying up females’ webs into bundles to remove the enticing smell attracting other males. Even though equivalent behaviour in humans would probably be possessive and controlling, in spiders it is somehow adorable.
To look into whether males were in fact removing the webs of females they were courting to deter future suitors, the researchers sought to find out whether this behaviour did indeed make the females’ webs less attractive to other males.

First they collected 50 female black widows from a beach in British Columbia, and brought them into the lab. As these spiders had already mated, they then had offspring which the researchers reared to maturity. This meant that they could tightly control what experiences the new generation of spiders had, and know that they hadn’t had any experience prior to what the experimenters gave them.
The new generation of female spiders were then allowed to build webs. The researchers took their webs and put each one in a small mesh cage back on to the beach from which their mothers were from. Each mesh cage had a small trap around it, meaning that any males trying to visit a females’ web would be caught before being able to reach it.
To see if the dismantling of webs by males made the webs less attractive, the experimenters had four different types of web that they put back on to the beach. The first type was just a female’s web, unaltered. The second was a female’s web, but with half of it removed by a male spider. The third was a female’s web, but with half of it removed by a human. Finally, the researchers put a few mesh cages which contained no web at all. This final group acted as a ‘control’, to see how many males fall into traps near these cages regardless of the presence of a web.

Over the next 24 hours, the researchers collected males that were caught in traps. Unsurprisingly, the cages with the unadulterated webs had lots of males trapped near them, while the empty cages had hardly any trapped males at all. However, the webs that had been reduced by male spiders had much fewer trapped males around them than either the intact webs or the webs that had been experimentally reduced by humans.
This exciting finding implies that it is not simply having a smaller surface area of web that makes the reduced females’ webs less attractive to males. Instead, it seems likely that the destructive courting male actually did something to the web to make it less attractive. To explore this possibility, the researchers then carried out a second experiment. This one was similar to the first, with the exception that this time the webs placed in the cages were either intact webs or intact webs with male silk added to them. The researchers did this to see whether it was the addition of the males’ silk that made the web unattractive to other males. However, this time around there was no difference in the number of males that were trapped desperately attempting to reach the females’ webs.
So what do these findings mean? It seems that males do reduce females’ webs to make them less attractive to other males, but exactly how they do this still isn’t clear. The experiments show that neither simply reducing the surface area nor adding the males’ own silk is sufficient to deter other males.
Instead, it is possible that males may selective excise areas of the females’ webs that are particularly pheromone-laden and carefully add their silk to specific areas. However, it will take more experiments to conclusively show this.
One outstanding question is why it is that females put up with this behaviour from males at all. You would think that they both wouldn’t want their web destroyed by a visiting male, and that they might like to have the option to have more visiting male suitors in the future. We know that female spiders can definitely hold their own, so it would seem that something about having their web reduced probably benefits the female as well. Perhaps being less harassed by males makes it easier for females to get on with the business of laying eggs and rearing offspring.
         
Reference
Scott, C., Kirk, D., McCann, S., & Gries, G. (2015). Web reduction by courting male black widows renders pheromone-emitting females' webs less attractive to rival males. Animal Behaviour, 107, 71-78.

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