Pop stars and teenage girls no longer have a monopoly on twerking — male black widows have joined their ranks.
Research in animal communication at SFU has led to the discovery of the importance of vibratory signals to the black widow courting process. The male must gently shake his thorax, or abdomen, to alert the female that he is, indeed, a potential mate and not her next meal.
Courting a female black widow is no easy task. They are approximately twice the size of the males and have immediate predatory reactions to anything that approaches them. Unfortunately for the males, their size puts them right into the females’ prey category.
SFU biology professor, Gerhard Gries, and graduate students Samantha Vibert and Catherine Scott have determined how the female spiders differentiate between prey and prospective mates who land on their web.
Gries explains that the female’s web is not only for catching prey — it has a second function. “The web is the dance floor for the males,” Gries said. They come to the web in response to a chemical signal, a pheromone, that the female releases onto the silken strands as she spins her web. He explains that this pheromone is essentially advertising to all males: “I’m a virgin female, if you’re interested in being my mate, respond to this chemical message.”
Once a male spider lands on the female’s web, the vibrations of his movements will indicate whether he is a suitor or a victim. Male vibrations are significantly more subtle than the percussive vibrations of a struggling fly, for instance, and therefore do not trigger a predatory response in the female.
Gries uses the analogy of a first date to show the precariousness of the situation. The male can’t afford to “get off on the wrong foot” with the female. If the male makes a misstep, not only is his date over, he will inevitably become her dinner.
The professor and his fellow researchers put this to the test. Using a sophisticated playback device with an attached rod that makes contact with the web, they played their recordings of male spider signals and of struggling prey at opposite amplitudes.
If played at its regular amplitude, the fly’s vibratory signal would elicit a predatory response from the female, but if played at a lower amplitude (like the male spider’s signal), it would not. In contrast, if the male spider’s signal was played at the same high amplitude as the fly’s, the female would rush out in search of prey.
According to Gries, this demonstrates that “the amplitude of the vibrations, or the displacement of the strands, they tell her [the female spider], ‘Okay it’s prey, I’ll rush out to have a meal, or, it’s a male that wants to court me.’”
Reflecting on his findings, Gries says, “It’s quite fascinating to see that such a seemingly small [thing] makes all the difference in the world.”
