Show or Tell? Testing Peacock Spiders’ Multi-Modal Mating Display
By Adrienne Antonsen
Peacock spiders are most well known for the prismatic colors males display during courtship and their endearing little dances. But the show doesn’t stop there. Beyond the dazzling visual spectacle, peacock spider males also sing through vibration while they woo. Ever since this incredible display was first filmed and brought to light by Australian biologist Jürgen Otto, Ph.D., about a decade ago, peacock spiders have captured researchers’ attention.
Recently, a group sought to independently investigate the two modes of the male peacock spider’s courtship ritual: the visual display of their colored fans and the vibratory signals they send through the substrate. The results of this study, by Madeline Gerard and Damian Elias, Ph.D., of the University of California, Berkeley, and Michael Kasumovic, Ph.D., of the University of New South Wales in Australia, were published in October 2018 in the journal Behavioral Ecology.
Multi-modal signaling is notoriously difficult to analyze in terms of the relative contribution of each signal type. First, the signals must be interpreted from the organisms’ point of view. While certain signals may seem particularly noteworthy to human observers, they may be perceived completely differently by the study animal. Wavelengths beyond the human visual range may be at play, or sounds, chemicals, or electrical signals we are initially blind to could be involved as well. Then, to determine how these signals interact, experiments must be designed such that behaviors can be analyzed independently from one another even when performed simultaneously.
To accomplish this with the peacock spider Maratus volans, researchers used a 2×2 factorial design by varying both the light and vibrational environment. The spiders were observed under either full-spectrum or long-wavelength reduced light, and they were placed on substrates that either allowed or blocked vibrations from being transmitted. A virgin female was placed in an arena with a male, and the scientists recorded the time males spent displaying their fans and vibrating and the mating success rate.
The researchers previously had found that Maratus volans males had increased mating success when they spent more time both visually and vibrationally signaling to females, but the visual displays had twice the magnitude of an effect. So, they predicted mating success in the recent study would be more negatively impacted under reduced light conditions than reduced vibrations. This prediction turned out to be correct, with the highest mating success (66 percent) happening under full-spectrum light and unblocked vibrational conditions. Under reduced long-wavelength light, whether vibration was restricted or not, male peacock spider mating success rate was nearly halved. When full-spectrum light was paired with non-transmitted vibrational signaling, success rate fell in an intermediate range (49 percent).
The findings, then, show that female peacock spiders are more responsive to male visual displays than vibrational communication. So, why vibrate at all? The researchers noted males increased vibrations when females weren’t looking at them, and this often spurred females to redirect their attention back to the display at hand. Thus, male vibrational signals often serve as a wingman of sorts to the visual display, bringing female focus back to their colored fans.
The scientists took their study one step further by adding another treatment, watching the peacock spiders under reduced long-wavelength light while increasing brightness. What they found surprised them: Copulation success rose back up to the rates observed under full-spectrum light. This indicates that it’s the pattern contrast, not the color, of Maratus volans males that females are responding to.
With more than 50 species of Maratus peacock spiders discovered in the past decade, each with its own unique coloration and behavior, it remains to be seen whether these same patterns of relative signal effect hold true across the genus. Life-or-death stakes are at play when males court their ladies, and unimpressive suitors often become a tasty meal instead of a mate. That’s why having multiple strategies, including both song and dance, is of utmost importance when these enchanting arachnids take center stage.
Adrienne Antonsen is a graduate student in entomology at North Dakota State University. Email: firstname.lastname@example.org