Black widow spiders: dating and mating

We’ve just heard about the false widow spider which has a more famous - or should I say infamous - cousin, the black widow spider. As the name suggests, female black widow spiders don’t tend to be ‘married’ to their partners for very long until death do them part. Chris Smith found out more about the sex lives of these and other spiders is Maydianne Andrade from the University of Toronto:

Maydianne - So spiders mate in a very different way. You just heard that males have structures called palps they're kind of like boxing gloves, essentially the tops of their heads. And those are actually what they use to mate with. And so they would actually climb onto a female in most species and then insert sort of a coil at the top of this boxing glove to transfer sperm. Now, one of the other strange things about it is that there's no direct connection between the place where they make their sperm and these boxing gloves. So they actually have to fill them up before they head off to try to mate with a female.

Chris - When do they become sexually mature? I mean, how long do these spiders live for? And at what point can they begin to reproduce?

Maydianne - So in black widows, the males are actually developed much more quickly and die much more quickly than females, even if they aren't successful at mating and aren't cannibalised. So males might take a couple of months to develop. Females might take three or four months. And so what you end up having is sort of quickly developing relatively small males. They're much smaller than the female. Sometimes several hundred times less in terms of body weight. And the male may only live for a couple of weeks to a couple of months after he becomes sexually mature.

Chris - It seems strange that they should end up being devoured or becoming prey or killed by the very partner they're trying to mate with. Why did that evolve in the first place? It seems counter-intuitive and counter-productive.

Maydianne - So it's counterintuitive to us because we tend to think about collective action towards the same goal, but in terms of most other organisms, males and females are each trying to reproduce successfully. Males are trying to typically have as many offspring as possible, which means mating with multiple females in many species, but for the females, especially for a large, predatory, carnivorous female who can produce more offspring with the more she eats, once she has sperm from a male, she doesn't necessarily need that male anymore. And these species, the male does not help with parental care or anything like that. So for the female, she gets the sperm and then, well, there's a snack available on the web.

Chris - So do they actually store sperm them and just use it when they need it?

Maydianne - Yeah, that's the last part of the puzzle is that female black widows have to store sperm storage organs, and they can actually store sperm for two years after mating only once and continue to produce offspring throughout that time. If they're well fed, we're talking about 100 to 300 offspring every two weeks, for two years after mating,

Chris - Goodness. Can they top up their sperm reserves though? Because if another male comes along and, and happens to be actually a better mating choice, you wouldn't want to put all your eggs in one basket. I mean wrong analogy, but you get what I'm saying. So will the female mate multiple times and therefore have the opportunity to replace what she's already picked up with something that's better genetic stock?

Maydianne - Some females do do that. We don't know kind of their internal rules for that yet. But one of the things we do know about their mating behavior is that if the first male is not a very good quality, say, or does not court her for long enough, she will mate with him or copulate with him only once. And that means only one of her sperm storage organs is filled and the other one is available for any subsequent male who comes along who's actually better quality or invests more in that courtship.

Chris - And how does the female work out whether a male is, is good quality or not?

Maydianne - We know that they can detect a lot about males through the web. So males are vibrating throughout the courtship and they do that by plucking at the web with their palps or those boxing gloves, but also by flexing and extending their legs. So if you imagine a guitar string, it's like the male is plucking at those guitar strings that make up the web as they approach females. That can tell her things about his body size, about his energetic vigour. And then most importantly, we know that that signal is saying, 'I am not prey, I am not prey, I am not prey', at least not until they mate.

Chris - And obviously it's in the male's interest to make sure that he does father as many offspring as possible and pass his genes on to as many new spiders as he can. Is there anything he can do so that he does end up being the father and doesn't get pushed aside by a newcomer?

Maydianne - From the species we've studied, courtship is an endurance competition that really shows the female, 'I am the best father for your offspring'. So we know, for example, in a couple of the species, if the male doesn't court for at least an hour before even touching the female, the female is more likely to kill him before he completely fills up her sperm storage organ. So he will lose out in terms of paternity. In redback spiders, for example, which is an Australian black widow, males can court for up to eight hours before they actually successfully mate with a female. But the males who do that are more likely to mate twice and the females less likely to mate with a rival who shows up later.

Chris - That's some foreplay isn't it? Is there anything else the male can do to avoid being eaten, but nevertheless, manage a successful mating?

Maydianne - The other thing is that these males have actually evolved to mate with females before they become sexually mature, before the female does. So the female just before sexual maturity actually has intact organs underneath her exoskeleton. The male cuts that open and mates through that. And females are much less choosy when they're mated it in that way.