For some male spiders, sex can come with a very high price: Death. How, then, do they get it on without becoming their cannibalistic mate's snack? Scientists have found that male orb-web spiders save their skin by performing a smooth move called the "shudder," which has the amazing ability to reduce female aggression.
Numerous animals — humans included — go through an elaborate courtship ritual before mating. This get-to-know-you phase provides information about an individual's identity, availability and quality. In various invertebrate species (those lacking backbones), particularly insects and arachnids, courtship is extremely important because of the female's cannibalistic tendencies — if the males aren't impressive enough, they become food.
This danger is greatest for web-building spiders. To get to his potential mate, the male must cross the female's predatory trap: Her web. What makes this feat all the more difficult is the fact that the females often have poor eyesight, so mistaking a male spider for a delicious insect is quite easy.
Earlier this year, biologists Anne Wignall and Marie Herberstein of Macquarie University in Australia studied the mating behavior of orb-web spiders (Argiope keyserlingi), a species known for sexual cannibalism. They found that males have a repertoire of moves they use during courtship. The male first cuts out a section of the female's web and builds his own "mating thread" over the hole. He then begins a courtship ritual that involves the shudder (below), where he quickly rocks back and forth to vibrate the web, abdominal wags and another move called the "mating thread dance," which involves plucking the web and bouncing on it.
These three moves influence whether the female decides to mate with the male, but the shudder has an added importance: It determines whether the female will eat him after sex. And to females, there is a right way to shudder. "The females really like males that shudder at a highest possible rate without dropping their shudder duration," Anne Wignall told io9. That is, males that shudder fast and long are less likely to be eaten after sex.
Surprisingly, however, males don't only shudder during courtship — they also shudder right when they get on the female's web. This fact led Wignall and Herberstein to wonder if maybe the shudder has a more general, aggression-reducing effect on females.
The researchers began their study by recording the web vibrations produced by a male orb-web spider's shudder using a technique called laser vibrometry. They shined a laser on to the web as it vibrated, and measured the reflected laser beam — the change in frequency of the reflected beam as the web vibrated provided the details of its motion.
Wignall and Herberstein then tested how the shudder vibrations affected female spiders while in the presence of food. They first hooked up a female spider's web to an electromagnetic shaker, which can reproduce different vibrations. They then placed an insect on to the female's web and played the shudder vibration through the shaker. They repeated the experiment using white noise vibrations and no vibrations at all. With every female tested, they got the same result: The white noise and silent experiments didn't affect the female spiders, but the shudder delayed their attack by an average of 300 seconds.
Importantly, the shudder vibrations only delayed the females from attacking — it didn't stop them altogether. "We think that the delay is just long enough for the male to go across to the female," Wignall said. So when the male gets on to the female's web, he shudders to calm her and give him time to get close to her without her attacking him. Once he's close, he can use other techniques, such as releasing pheromones and brushing against her legs and abdomen, to let her know who he is and that he wants to get down to business. He then performs his courtship ritual to entice her, and includes the shudder in hopes of further controlling her aggression and preventing her from eating him after sex.
What's more, the researchers found that the shudder is kind of a generic move among orb-web spiders. They played A. keyserlingi shudder vibrations for A. aetherea females, and got the same results as when the male and female's species was the same. When they looked at the shudder waveforms, they saw that they were very similar, and both very different from the vibrations of struggling prey and males walking across the web. This suggests that the shudder vibration may have actually developed a long time ago in the orb-web spider's lineage.
The researchers are now interested determining if the shudder technique is used or understood by other related web-building spiders. "We want to test if the Argiope shudder will work when we play it back to a female spider of a different family," Wignall said. "It just so beneficial and amazing that the males have this potentially fine-tuned control of the female's behavior before copulation — before he can show how sexy he is."
Check out the study in journal Scientific Reports.
Top image: female Argiope keyserlingi. Inset image: male Argiope keyserlingi. All media via Anne Wignall.