Cooperative signaling as a potential mechanism for cohesion in a gregarious sawfly larva, Perga affinis

Abstract

During periods of travel or dispersive activities (e.g., foraging), group-living animals face the common challenge of maintaining a cohesive unit. At the basic level, this challenge is no different for vertebrates than it is for arthropods and is solved through communication. Gregarious larvae of the Australian sawfly, Perga affinis, communicate via vibrational signals. The most common signal, tapping, involves striking the substrate with the sclerotized tip of the abdomen. This study investigates the role of tapping as a mechanism of cohesion, specifically in situations between a separated larva and a group. As nomadic foragers that move daily to new feeding locations and readily coalesce with other colonies, the possibility of separation and potential re-aggregation arises regularly. Experiments demonstrated that tapping facilitates cohesion as groups responded to the tapping of lone larvae and did so preferentially over other larval behaviors. Additionally, separated larvae respond to tapping by the group through increased walking activity. It is also possible that they receive directional information from the group's vibratory signals, although visual cues may influence orientation as well. Tapping represents a cooperative signal and, as such, I investigated the level of investment of both parties in the communicative exchange. While individual larvae invested more in the exchange than the group, the exchange is analogous to the Raise-the-Stakes model of cooperation in that groups gradually increased their investment according to the cumulative time spent tapping by the lone larva. The mutual but asymmetrical benefits received through cooperation are discussed and compared with similar situations between parents and offspring. Not all larvae in the group participated equally, suggesting individual differences in signaling propensity or strategy.