How do ants use scraping and knocking vibrations to warn their nestmates?

Details

Researchers observed defence signaling and behavior in Dolichoderinae ants from the Malay Peninsula in the field and brought the ants to the laboratory for measurements. They discussed the results in comparison with corresponding behavior in the European ants Camponotus ligniperda (Latreille 1802) and C. herculeanus (Linnaeus 1758). Six of the seven tropical species investigated produced vibration signals; the exception was Dolichoderus tuberifer Emery 1887.

The vibrations were produced in two ways: knocking and scraping. A high-speed camera recording 500 pictures/s was used to examine signal production. Five species—Dolichoderus thoracicus (Smith 1860), D. burmanicus Bingham 1903, D. sulcaticeps (Mayr 1870), and two undescribed species—scraped the ground with their mandibles while standing. These signals had a distinct pattern of three clearly distinguishable sequences and generally repeated 3–5 times, although they could occur up to 15 times. Each signal element lasted 0.04–0.14 s, depending on temperature. Intervals between elements lasted 0.22–0.36 s, and frequencies ranged from 1–19.5 kHz.

The sixth signaling species, Dolichoderus patens (Mayr 1870), instead used its gaster to produce signals. It struck the ground with its gaster and could move slowly while producing vibrations. Its signal lasted 0.012–0.019 s, with breaks of 0.058–0.162 s. The signal repeated up to 30 times, and even more often in a few cases, with detected frequencies of 0.5–22 kHz. In general, only workers appeared to generate vibration signals; reproductive individuals were never observed doing so.

Behavioral patterns were examined in D. thoracicus and D. patens. D. thoracicus workers responded to perceived vibrations differently depending on their level of excitement. At low excitement, they froze motionless. When more excited, they also produced signals or increased their running speed manyfold and showed a lower threshold for aggression toward visible moving objects. When objects were obscured, threatening behavior—spreading the mandibles—was observed. This behavior occurred outside the nest.

In contrast, D. patens workers neither moved vigorously nor reacted aggressively, but instead moved slowly toward the nest. At low excitement, they waved their gasters; when excessively stimulated, they knocked them against the ground. On perceiving these signals, reproductive individuals of both species quickly moved into the nest.