Low amplitude vibrations produced by ‘twerking’ of the abdomen may prevent a premature and grisly end to the courtship efforts of male black widow spiders, are the findings of new research published in Frontiers in Zoology today. Female black widow spiders, which are notorious predators, with a low tolerance for intruders, are wooed by hopeful suitors who whisper these vibrations on entrance to their webs and thus may avoid being confused with prey items.
In order to investigate how male spiders signal their presence to a female, without appearing as prey, researchers from Simon Fraser University recorded the vibrations made by the black widow spider (Latrodectus hesperus) and the hobo spider (Tegenaria agrestis) as well as prey items (house flies and crickets) when entering a female’s web. BioMed Central spoke with Catherine Scott, one of the researchers involved in this study, to find out more.
Can you tell us a little bit about the spiders you studied?
Female web-building spiders are accomplished predators that are extremely sensitive to vibrations. The web functions as an extension of the spider’s exquisitely tuned sensory system, allowing her to very quickly detect and respond to prey coming into contact with her silk. This presents a prospective mate with a real challenge when he first arrives at a female’s web: he needs to signal his presence and desirability, without triggering the female’s predatory response.
To investigate how male spiders handle this problem, we studied two rather different web-building spider species. The western black widow (in the family Theridiidae) builds a tangle-web, and the hobo spider (family Agelenidae) builds a sheet-web. Female black widows are much larger than males and are highly aggressive, and although sexual cannibalism is rare in this species, courting males are potentially risking their lives. Hobo spider females, on the other hand, are not much bigger than males, and are not aggressive during courtship.
We expected that the first vibrations that the male transmits after entering a female’s web would be critical for avoiding a deadly case of mistaken identity, and that black widow males in particular might produce vibrations that are very distinct from those of prey.
What did you find?
We recorded vibrations produced by males, crickets, and flies on both types of webs, and found that, indeed, black widow courtship signals differed more from prey vibrations than hobo spider courtship signals did. One behaviour performed by male black widows (abdomen tremulation) resulted in very stereotyped vibrations that were particularly distinct from prey vibrations in terms of their waveform, and also were especially low-amplitude. When we played these whisper-like vibrations back to females, they were much less likely to elicit a predatory response than louder, prey-like vibrations.
What surprised you about the findings?
One surprising thing about our results was the lack of very stereotyped motifs in the courtship vibrations of either species. This contrasts with the more complex songs of spiders that court on other substrates, like leaf litter or plants.
This might be because of the complexity and irregularity of the tangle-webs and sheet-webs that must transmit the vibratory signals in our study species. In the black widow, it seems that it’s simply the low-amplitude of male vibrations (especially abdomen tremulation), right at the beginning of courtship that may be the key feature distinguishing them from the vibrations produced by struggling prey.
What are the implications of the study for future work?
With the exception of orb-web weaving species, very little is known about the kinds of vibratory courtship signals that male web-building spiders use to communicate with females through their webs. We’re excited to have added new information about vibratory communication in tangle-web and sheet-web spiders, which will contribute to a better overall understanding of the function and evolution of web-borne vibratory courtship signals. We have also been studying the way that the different types of webs built by hobo spiders and black widows transmit vibrations, to try to gain insight into how these properties might shape the spiders’ communication systems. In this article we’ve provided some basic groundwork that will inform future studies to further investigate the function of specific courtship signals and what kind of information they transmit to the female.
One of the most exciting things we saw was that sometimes, female black widows actually responded with abdomen twitches (similar to the male’s abdomen shaking, but much more emphatic) when we played low-amplitude vibrations to them through their webs. These twitches undoubtedly transmit their own vibrations through the web. While this study focused on male courtship signalling, it would be very interesting to look more at the female’s behaviour, and any signals she may be transmitting back to the male.