As humans, we generally think that we should be somewhat choosy when picking a mate. However, we are lucky in that making the wrong choice rarely results in being eaten by said partner. However, in spiders like the western black widow, females commonly engage in ‘precopulatory sexually cannibalistic behaviour’ – or in other words, a bit of pre-sex snacking on their male partner.
This is an interesting phenomenon for a number of reasons, but one that might not immediately spring to mind is that it reverses the usual sex roles in terms of which sex is the ‘choosy’ one. In a lot of species, females are the ‘choosy’ sex, meaning that they have the upper hand in choosing who to mate with, as they normally have to invest more into making babies than do males.
However, adding the risk of becoming lunch into the mix rather shakes things up. Usually for a male getting a lot of mates might be considered a good thing, but for the black widow, more mates also means more chance of being eaten. So, it pays for him to be choosier. He’s aiming to pick a mate who is not only in good condition, but also unlikely to eat him before he gets a chance to mate with her.
So, how might a male black widow go about finding this ideal female for himself? Well, we know that spiders can use chemical cues transferred through the air to convey types of information. They use these chemicals in finding prey to eat, avoiding predators, and also for finding a mate. But do males use these chemicals in discriminating the good from the bad females? Johnson and colleagues this month published a paper1 reporting a set of experiments finding out just how the male chooses a good mate.
A male wants to choose a well-fed female, as she is likely to be in better condition for fathering his children, but also less likely to eat him (I bet you never thought you’d read that sentence). One way to tell how hungry a female might be is to get up close and have a good look at her, but this is not always the most appealing option for obvious reasons. Another good way would be to have a look at her web: hungry females build more sticky webs better for catching prey, whilst less hungry females build larger, less sticky webs.
To work out how a male choose between females, the researchers took seven well-fed females and seven starved females and let them build webs. First, females were removed from these webs, and a male was introduced to have a look around. It was found that he was much more likely to show courting behaviour if he was on the web of a well-fed female rather than a starved one, even though the actual female wasn’t there. This indicates that the web itself if telling the males important information about what type of female built it.
Unsurprisingly, when a different group of males were then shown to webs where the females were still there, they similarly courted the well-fed females more than the starved ones. As expected (unfortunately not by the males in question), the starved females attacked, and killed in some cases, their male suitors a lot more often than the well-fed females did.
The researchers now set up a clever new experiment where females were swapped, so that hungry females were put on the webs of well-fed females, and well-fed females were put on the webs of hungry females. A group of males were introduced to them as before to see who they would choose to court now. Without wanting to anthropomorphise, it’s hard not to feel bad for the male happily sauntering up to a web of a well-fed female, only to discover a ravenous one waiting to attack. However, this time males tended to choose webs where the females were well-fed, even though the web was giving the opposite message. However, this finding wasn’t as strong as in the previous experiment, indicating that although males do seem to look at the female when deciding whether she’s worth courting, what her web looks like is still important when making this decision.
The final and most exciting piece to this puzzle is provided by the last experiment. Here a new bunch of males were given just the silk from a web, wrapped around a toothpick, without any female present. Now, in order to decide on the quality of a female, the male has no female to inform him, and no web with its tell-tale structure, only chemicals contained in the silk. Here the males truly demonstrated the extent to which their senses were tuned to not being eaten: they courted the silk from the well-fed females more than the silk from the starved females, showing that they could tell what type of female they potentially would soon be courting from the chemical cue alone.
So, it seems that male black widows will use pretty much every signal available to them when trying to decide which female is the best (and safest) to mate with. At first glance it might seem that such safety measures are over the top, yet where the cost of making the wrong decision is so high, we can see that natural selection has truly been at work.
1 Johnson, J.C., et al., Male black widows court well-fed females more than starved females: silken cues indicate sexual cannibalism risk, Animal Behaviour (2011), doi:10.1016/j.anbehav.2011.05.018