Get off my back: vibrational assessment of homeowner strength

Abstract

Animals may use a variety of sensory modalities to assess ownership and resource-holding potential (RHP). However, few studies have experimentally tested whether animals can assess these key variables through a purely vibrational modality, exclusively involving substrate-borne vibrations. Here we studied social terrestrial hermit crabs ( Coenobita compressus), where competitors assess homeowners by climbing on top of a solid external structure-an architecturally remodelled shell home, inside of which the owner then produces vibrations. In the field, we used a miniature vibratory device, hidden within an empty shell, to experimentally simulate a 'phantom owner', with variable amplitudes of vibration representing different levels of homeowner strength. We found that assessors could use these vibrations to deduce the owner's RHP: for strong vibrations (indicative of a high RHP owner) assessors were least likely to escalate the conflict; for weak vibrations (indicative of a low RHP owner) assessors showed intermediate escalation; and in the absence of vibration (indicative of an extremely weak or absent owner) assessors were most likely to escalate. These results reveal that animals can assess homeowner strength based solely on substrate vibrations, thereby making important decisions about whether to escalate social conflicts over property.

Keywords: property ownership; resource-holding potential; shells; signals and cues; sociality; vibration.

Figure 1.

Experimental design using ‘phantom owner’ to test whether highly social terrestrial hermit crabs (Coenobita compressus) assess homeowner strength via substrate-borne vibrations. (a) Schematic of field set-up on beach, including: (1) camcorder on a tripod; (2) webcam; (3) arena containing shell, inside of which was a miniature vibratory device; (4) microcontroller, lithium polymer battery, and haptic motor controller inside an IP-rated housing and (5) rugged laptop, where the experimenter was positioned. Top inset shows hardware associated with the vibratory device, including: (6) haptic motor controller; (7) FLORA microcontroller; (8) lithium polymer battery and (9) eccentric rotating-mass coin motor (the miniature vibratory device being embedded inside the shell with putty). (b) Picture (video still) from above the experimental arena, with an individual actively assessing the vibrating home—an empty, architecturally remodelled Nerita scabricosta shell, without an actual owner inside. (Online version in colour.)

Figure 2.

Response of assessors to different levels of vibration, denoted by symbols (left-to-right): no vibration, weak vibration and strong vibration. (a) Total assessment time (s) (i.e. time spent touching the shell). (b) Proportion of flip attempts (i.e. number of individuals out of the total sample that attempted to flip over the shell and expose its entrance, which is a measure of conflict escalation). Mean ± s.e. shown (see electronic supplementary material, table S1 for sample sizes).