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 Calming Shudder
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.
