Buzzing boundaries: tiny caterpillars vibrate to defend leaf tip territories

Territorial displays include some of the most elaborate behaviours in the animal kingdom. In this study, we investigated the territorial behaviour and vibratory signalling of neonate warty birch caterpillars (Falcaria bilineata; Lepidoptera: Drepanidae), which reside solitarily on birch leaves and defend the leaf tip. Upon hatching, these tiny caterpillars - no larger than 2 mm - seek out and establish a small solitary territory (∼1 cm wide) at the leaf tip, where they lay silk mats, feed and advertise their presence by producing multicomponent vibratory signals - buzz scrapes and drums. When a conspecific neonate (intruder) is introduced to a leaf occupied by a resident, the resident increases its signalling rate up to four times that when undisturbed, and even more - up to 14 times - if the intruder enters the territory. Intruders rarely manage to take over the resident's defended space, with most confrontations (71%) ending in the resident maintaining control. Residents signal significantly more than intruders at all stages of the contest. If physical contact occurs, residents flee by dropping from the leaf by a silk thread. This results in territorial contests that involve no physical aggression, relying entirely on vibratory communication. These vibratory displays most likely function to establish spacing between conspecifics on a tree branch, but these complex signals may also function to exclude other members of the vibratory community by mimicking something dangerous, such as a jumping spider.