Wednesday, July 13, 2011
First, I've gotta say, if spiders can be cute then jumping spiders are the cutest. For today's entry, I'm redoing an older video post about jumping spiders, expanding on it and including the papers that go along with it.
Jumping spiders belong are classified as Arachnida: Araneae: Salticidae. They are easily distinguished from other spiders by the four big eyes and four small eyes on the top of their head. They hunt by spotting prey from a distance, sneaking up on it, and then pouncing. Hence, there name. They are also known for their creative, and often entertaining, courtship dances. The males dance before the females, displaying their dancing talents, their coloration, and their overall sexiness. If the female likes the dance then she will choose that male to mate with. A good dance will also inhibit her predatory tendencies, meaning she won't eat her suitor.
Before we get to the actual study, first take a look at this short video of a male jumping spider (Habronattus tarsalis) performing a courtship dance (sorry, embedding has been disabled for this video so you'll have to click the link):
http://youtu.be/UTbHpV_zFjE (or click HERE)
There are a probably few things that you noticed right away: (1) he moves his pedipalps throughout his dance, (2) he moves around a lot while quickly moving his front legs, (3) "touchdown!" arms, and (4) there is no sound on this video.
The article that is the focus of today's post takes a look at #4 as it relates to all the other numbers. It is known that many types of spiders use substrate-propagated vibration as a kind of signalling. A good example of this are the vibrations propagated through the webs of orb-weaving spiders. Other spiders use water, soil, leaf litter, or plants. There are three types of substrate-born vibration production mechanisms that have been described in spiders: percussion (drumming body parts against a substrate), stridulation (rubbing two rigid body structures together), and vibration (oscillation of body parts coupled to the substratum). Jumping spiders are unique in that they are "visual specialists." Their very good sight coupled with the vibration makes for a good display.
Before reading about the study, take a look at the courtship display, this time with sound:
Male and female Habronattus dossenus were the test subjects of this study. Females were anesthetized with CO2 and tethered to a wire with wax on a substrate of stretched nylon fabric. Then they dropped the males onto the nylon and allowed them to court the female freely. The courtship was video recorded, and they recorded the substrate vibrations with a system that allowed them to measure the velocity of the moving surface.A total of 20 different males were recorded, using the same tethered female. Then the researchers analyzed the videos frame by frame and created power spectra for the analysis of the vibratory signals. Next, they put together the signal manipulation experiments. For this the substrate floor for courtship was a sheet of graph paper attached to a square cardboard frame. Again, the females were tethered and the males allowed to dance. The courtship ritual was videotaped, and the males seismic signals were recorded by placing a piezo-electric sensor directly underneath the female. That way they could test exactly what she was "hearing."
From the video they were able to break down the courtship into four distinct phases:
Phase 1 - Sidling movements where the male approaches in a "zigzag" visual display, waving his forelegs and spreading his pedipalps
Phase 2 - The male comes to within one body length of the female and produces rapid "downbeat" gestures
Phase 3 - Multiple bouts of prolonged signalling
Phase 4 - The male attempts to mount the female
Additionally, seismic displays were found to only occur in phases 2 to 4. These seismic displays were broken down into three broad categories: thumps, buzzes, and scrapes. Thumps occur when the forelegs are raised high above the body in a nearly vertical position and are then rapidly slapped down onto the substrate, producing a percussive impulse, and also producing an air-borne sound. Then the forelegs return to vertical and the abdomen is pulled back and then released, causing it to "ring," a brief, high intensity signal. Scrapes occur in groups, between thumps. They consist of up-and-down movements of the tips of the forelegs followed by dorso-ventral oscillation (rocking motion) of the abdomen. Buzzes occur alone in Phase 3 and are always preceded by 2 to 5 thumps. Here, the front legs come down in slow continuous movement while the abdomen produces a sustained, rapid, low-amplitude oscillation. These signals are last a long time and their frequencies are temperature dependent.
Overall, when the researchers combined the seismic and visual display data they found the courtship dances to be very complex. They found that the seismic signals produced by the male corresponded to the movements of his abdomen but not of his forelegs. If they prevented the abdominal movements they found that it effectively "silenced" the males with out affecting their visual or percussive displays. These abdominal signals match the frequency characteristics of the buzz but not the thump or the scrape. Therefore, there is no timed coordination of the visual and seismic signals. That means there is not a common production mechanism, seismic signals are produced independently of visual signals.
Why evolve multiple signals? The authors pose two "quality-based" hypotheses. Females are looking for strong, healthy males that carry good genes that they can pass along to their offspring. Because they can't see genes directly they must find another way to assess the males' condition. Signals. The first hypothesis is that of the "backup signal." This is when different signals provide the same information about the sender but allow for a more accurate assessment of the sender's condition. In the case of jumping spiders, this would mean that the visual and seismic signals are alternative media for the same signal information. If visual signals are obscured then seismic signals become very important. This would be more plausible if the spider were courting at night (which it does not), if the female were far away (which she is not), or if chemical cues were produced (which they are not). Perhaps the three seismic signals are backups for each other, if the substrate the spider is dancing on does not carry one form of signal it will carry the others. The second hypothesis is that of "multiple signals." This is where different signals code for different aspects of the sender's condition. This is, perhaps, the better hypothesis. Male jumping spiders are strikingly ornamented, especially the body parts they use for courtship. Coloration is something that females are often attracted to as well as predators. If a male can survive even when he is easy to see then he must have good genes right? In this case, the three different seismic signals could also be relaying multiple messages, especially since they are produced by different anatomical structures. Likely telling the female that these structures are working like they are supposed to.
See what I mean? Complicated.
This is the study:
Elias, Damian O. et al. (2003) Seismic signals in a courting male jumping spider (Araneae: Salticidae). The Journal of Experimental Biology: 206, 4029-4039. (DOI: 10.1006/anbe.2003.2245)
If you like this one here's another study with web building spiders:
Maklakov, Alexei A., Trine Bilde, and Yael Lubin (2003) Vibratory courtship in a web-building spider: Signalling quality or stimulating the female. Animal Behaviour: 66(4), 623-630. (DOI: 10.1006/anbe.2003.2245)
Some websites where you can learn more about jumping spiders:
Salticidae (Aranae) of the World and Monograph of the Salticidae (Araneae) of the World by Jercy Proszynski
Tree of Life page on jumping spiders by Wayne Maddison
Jumping Spiders of the World
Some movies of jumping spiders HERE (click on "videos" on the left side of the screen) and jumping spider courtship HERE
And a fun story about "Dirty Dancing" spiders from over at NPR:
(first image from the linked NPR story, credited to Thomas Shahan)