Page 1 of 1 | by Stephanie Shorter, Ph.D.
Animals communicate with each other in a multitude of ways. When signals inVolve multiple components, these parts may happen sequentially or simultaneously, and can inVolve multiple sensory modalities or just one. Acoustic signals produced by arthropods often demonstrate a wide variety of sound production methods. Such acoustic versatility results from the ability to produce sounds on the majority of the exoskeleton surface, not just dedicated body parts such as vocal folds.
In addition to visual and chemical cues, male wolf spiders also produce different patterns of acoustic/vibratory signals to court females. This study documents how 30 wolf spiders (Schizocosa stridulans) performed seismic vibrations using a laser doppler vibration meter and high-speed videography. Young wild-caught male specimens were tested on a stretched piece of nylon while being exposed to a virgin female spider. Data were shown as spectrograms demonstrating the dominant frequencies and spectrograms showing the intensity of the vibrations, accompanied by anatomical drawings that demonstrate the body motion that generated the vibrations. Courtship-related vibrations were categorized as either rev signals or an idle signals.
A rev signal was comprised of high-frequency vibrations created by flexing the distal joint of the pedipalps in a pulsing, stridulation type of movement. The average frequency was approximately 1000 Hz, but could be as high as 2400 Hz. At the same time, vibrations were also generated by a burst of two or three dorsal-to-ventral shakes of the abdomen with a trilling or tremulation movement, which resulted in a dominant frequency of about 33 Hz. These palpal and abdominal vibrations overlapped in time for a duration of a quarter to half a second.
An idle signal inVolved the spider first adopting a particular stance and then drumming on the substrate a percussive 4 to 11 leg taps, which were loud enough to be heard by human ear. These taps were followed by a high-frequency stridulation of the distal joint of the pedipalps. No abdominal movement was observed during this type of display.
A strength of this study was in augmenting the observations of the natural, intact specimens with experimental manipulations in which the spiders could not move pedipalps or abdomens to show that these movements were sufficient to generate the seismic signals. Finally, the authors closely examined the tibio-cymbial joint of the palps using scanning electron microscopy. Here they found a specialized scraper protrusion on the dorsal cymbium. That male wolf spiders can create complex signals indicates that females must be able to discriminate differences in signal quality or content to guide social interaction. For example, as with some scorpions species, female wolf spiders may be able to locate competing male suitors through the traveling waves of vibration in the ground. Vibrations of low versus high frequency travel through the substrate at different rates. Therefore, the peak of the waves arrive at the female at slightly different times and the magnitude of the disparity in the low frequency and high frequency peaks' arrival times provides position information about where the signalling male is.
Whether these patterns of vibration yield redundant or multiple types of information is still unknown. Many factors likely control the production of these seismic courtship displays, including the presence of other males and the distance between the signalling male and the intended female recipient. An interesting discussion section focuses on how environmental selective pressures can increase the development of sound-yielding structures on the body and may bias multi-component signals to be delivered in parallel or serially. These aspects of the eVolutionary study of animal behavior have yet to be explored. This carefully conducted study is recommended reading for anyone interested in animal communication.