Bowen JL et al. (2008). Vibration-mediated territoriality in the warty birch caterpillar Drepana bilineata. Physiological Entomology. 33: 238-250.

The authors infer the function of vibrations made in the warty birch caterpillar

Drepana bilineata through observations of staged encounters of individuals and present the first evidence of communication of territories via vibrational signaling in caterpillars. The authors collected Drepana bilineata moths from a wild population in Ontario, Canada from which larvae were reared for experimentation. As part of the preliminary observations, caterpillars of various ages were observed in order to gather information about their general life history, behavior, and communication. For thirteen late-instars, video and audio recordings were made from which total duration of vibrational signals (anal scraping and mandible drumming), signal bout duration and number of signals per bout were measured. In addition, signals from five different instars were examined using laser Doppler vibrometry to visualize their spectral characteristics. Finally, the authors staged 56 encounters between residents and intruders of similar size in a controlled setting containing one twig and one leaf of a certain size. These interactions were recorded and distance between individuals and signaling rates were measured. A winner was dubbed when his component left the leaf; if no winner was determined after 30 minutes then the contest was considered a ‘tie’.

The authors learned that the lifecycle of the caterpillar involves 5 instars all of which live solitarily. They observed that residents began giving vibrational signals when other caterpillars attempted to approach the leaf edge. Also, at the third instar, individuals produce a mat of silk on the leaf which is used as a shelter. The authors found that mandible drumming by an individual produces a broadband sound that lasts about 37.9 milliseconds on average while anal scraping, also a broadband call lasts 125 milliseconds on average. In the staged encounters, residents won more trials (61.5%) than intruders (5.8%) or trials ending in a tie (32.7%). Also, residents signaled more overall than intruders and were the first of the contestants to signal in the majority of trials. As the distance between the two contestants decreased, the rate of signaling increased. Finally, when the intruder came into contact with the residents, the resident often hit the intruder showing that encroachment on a territory can be physically costly. In combination, these results suggest that vibrations are a signal used by caterpillars to portray territorial boundaries.

This study presents a case where despite being equally matched in size (presumed symmetry in fighting ability), the residents won more contests than the intruders. As we learned in class this resident advantage may be caused by several factors, though asymmetry in territory value is the most likely explanation in this case since the leaf is assumingly more valuable to the resident than to the intruder; much time and energy was invested in the construction of the silk shelter on the leaf. The resident did not always win though which is not surprising since, as we learned in class, the existence of an arbitrary convention that the resident always wins in very unlikely again because there is often an asymmetry in value of territory. Also, we discussed in class that owners would benefit from knowing the distance to the intruder so that they don’t waste as much energy signaling when there is little threat. This experiment showed that the rate of signaling (which is likely energetically costly) increases when intruders get closer. This knowledge of distance allows the resident to gauge whether physical altercation will be likely. The residents may be able to infer the distance of intruders as a consequence of the fact that vibrational signals are broadband and thus frequency degradation may be used to detect location.