Can a spider “sing”? If so, who might be listening?

Alexander L. Sweger – swegeral@mail.uc.edu
George W. Uetz – uetzgw@ucmail.uc.edu
University of Cincinnati
Department of Biological Sciences
2600 Clifton Ave, Cincinnati OH 45221

Popular version of paper 4pAB3 presented at the 2014 169th ASA Meeting in Pittsburgh, PA.

While we are familiar with a wide variety of animals that use sound to communicate- birds, frogs, crickets, etc.- there are thousands of animal species that use vibration as their primary means of communication. Since sound and vibration are physically very similar, the two are inextricable connected, but biologically they are still somewhat separate modes of communication. Within the field of bioacoustics, we are beginning to fully realize how prevalent vibration is as a mode of animal communication, and how interconnected vibration and sound are for many species.

Wolf spiders are one group that heavily utilizes vibration as a means of communication, and they have very sensitive structures for “listening” to vibrations. However, despite the numerous vibrations that are involved in spider communication, they are not known for creating audible sounds. While a lot of species that use vibration will simultaneously use airborne sound, spiders do not possess structures for hearing sound, and it is generally assumed that they do not use acoustic communication in conjunction with vibration.

The “purring” wolf spider (Gladicosa gulosa) may be a unique exception to this assumption. Males create vibrations when they communicate with potential mates in a manner very similar to other wolf spider species, but unlike other wolf spider species, they also create airborne sounds during this communication (Figure 1). Both the vibrations and the sounds produced by this species are of higher amplitude than other wolf spider species, both larger and smaller, meaning this phenomenon is independent of species size. While other acoustically communicating species like crickets and katydids have evolved structures for producing sound, these spiders are vibrating structures in their environment (dead leaves) to create sound. Since we know spiders do not possess typical “ears” for hearing these sounds, we are interested in finding out if females or other males are able to use these sounds in communication. If they do, then this species could be used as an unusual model for the evolution of acoustic communication.

Figure 1: An image of a male “purring” wolf spider, Gladicosa gulosa, and the spectrogram of his accompanied vibration. Listen to a recording of the vibration here:

and the accompanying sound here:

Our work has shown that the leaves themselves are vital to the use of acoustic communication in this species. Males can only produce the sounds when they are on a surface that vibrates (like a leaf) and females will only respond to the sounds when they are on a similar surface. When we remove the vibration and only provide the acoustic signal, females still show a significant response and males do not, suggesting that the sounds produced by males may play a part in communicating specifically with females.

So, the next question is- how are females responding to the airborne sound without ears? Despite the relatively low volume of the sounds produced, they can still create a vibration in a very thin surface like a leaf. This creates a complex method of communication- a male makes a vibration in a leaf that creates a sound, which then travels to another leaf and creates a new vibration, which a female can then hear. While relatively “primitive” compared to the highly-evolved acoustic communication in birds, frogs, insects, and other species, this unique usage of the environment may create opportunities for studying the evolution of sound as a mode of animal communication.