Vibration sensitivity and a computational theory for prey-localizing behavior in sand scorpions

Vibration Sensitivity and a Computational Theory for Prey-Localizing Behavior in Sand Scorpions1

SYNOPSIS. As burrowing, nocturnal predators of small arthropods, sand scorpions have evolved exquisite sensitivity to vibrational information that comes to them through the substrate they live on, dry sand. Over distances of a few decimeters, sand conducts low velocity (~50 m/sec) surface (Rayleigh) waves of sufficient amplitude and bandwidth (200

VIBRATIONAL COMMUNICATION AND PREY LOCALIZING BEHAVIOR OF SAND SCORPIONS

Beyond direct contact, vibrational communication implies the existence of a medium capable of conducting mechanical waves between a message sender and a message receiver. Judging from the steady growth of literature on vibration sensitivity in psammophilous animals (Salmon and Horsch, 1972; Brownell, 1977, 1984; Aicher and Tautz, 1990; Henschel, 1997; Brownell and Polis, 2001; Mason and Narins, 2001), sand appears be a particularly favorable substrate for conduction of mechanical information. In this paper we describe the prey localizing behavior of the Mojave

Desert sand scorpion, Paruroctonus mesaensis, and some of the physical properties of loose sand that make it a good conductor of surface waves. The exquisite sensitivity of Paruroctonus to vibrational information is certainly not limited to signals emanating from prey alone, however, and likely mediates intraspecific vibrational communication in the form of "juddering" movements (sudden strong lurches of the body forward), tail thumping and tail wiping behaviors commonly observed in psammophilic scorpions (Tallarovic et al., 2000; Brownell and Polis, 2001). In this way an evolutionary connection may be drawn between passive detection of vibrational information for purposes of orientation to food and the more evolved process of active signal generation for purposes of communication between conspecifics.

Sand scorpions make excellent models for analysis of surface vibrational information, as much for the physical simplicity of the substrate they inhabit as for the elegant simplicity of the predatory behavior they display. In many respects the scor-pion's orientation to prey disturbing the flat, featureless surface of a sand dune is the same sensory problem that orb spiders face in orienting to prey caught up in a web, or that water striders encounter in locating prey struggling at the surface of a pond. With these similarities in mind, we expect the study of predatory behavior in sand scorpions to transcend vibrational media and address the more general question of how animals make sense of mechanical information propagating through space. For this we present a computational model conceived for the specific circumstances and behaviors of sand scorpions but simple enough to apply as a general theory for perception of vibrational information conducted along free surfaces.

Vibration-source localizing behavior quantified

Vibration-source localizing behavior quantified

Paruroctonus mesaensis is an ambush predator of insects and other scorpions, always hunting at night from a motionless rest position outside its burrow on the sand surface. When prospective prey approach within 20 cm. the sand scorpion first assumes an alert posture with its body slightly elevated from the substrate and the pedipalps (organs of prey capture) extended and open forward. Subsequent disturbances of the sand by the prey evoke a sequence of quick rotations with forward movements toward the target until the pedipalps can grasp and immobilize the prey for stinging. Scorpions are naturally fluorescent under portable blacklights (Fig. 1) so that images of this predatory orientation response (POR) can be captured on film at night and subsequently measured for accuracy in the laboratory. The two components of the PORthe angle of turning and the distance moved toward the prey-were both shown to be highly accurate within 10 cm distance (Brownell and Farley, 1979a), confirming the impressions from field observations that prey within this radius are nearly always captured in a single orientation response.

A second orientation behavior of Paruroctonus, the defensive orientation response (DOR). has more utility for experimental analysis of vibration sensing behavior because it is easily and repeatably evoked in the laboratory when animals are agitated into defensive posture. The DOR is characterized by the same accurate rotation of the body to align the pedipalps toward a vibrational disturbance but there is no forward movement. Figure 2 shows an example of the accuracy of DOR for stimuli presented at 8-10 cm distance. With this angular component of vibration source localization assayable under controlled conditions, we have a means of demonstrating that the signals used by the scorpion to determine stimulus location are mechanical waves propagating along the surface of sand (Brownell, 1977, 1984).

Wave propagation in sand

When considering the prospects for vibrational communication between animals, our attention is drawn immediately to the solid substrates through which the signal must pass. From a physical perspective, most natural substrates are massive, rigid and highly inelastic to conduction of mechanical waves when compared to air and water. Unlike air and water which, by comparison, are excellent conductors of sound, light and volatile chemical substances, solid media are difficult to move and absorbent to mechanical waves. But dry sand, like that found in desert dunes is a surprisingly good conductor of the wave type that carries most of the energy away from surface disturbances-the Rayleigh wave.