Collective signals in treehopper broods provide predator localization cues to the defending mother

Many animal groups respond collectively to environmental cues. To understand the function of collective behaviour, an important first step is to establish how it varies in response to differences in the relevant cues. Because collective action arises from individual decisions, this in turn requires characterizing the behaviour of individual group members. In the treehopper Umbonia crassicornis, offspring live in cylindrical aggregations along a host plant stem. In the presence of a predator, offspring produce synchronized vibrational signals that elicit maternal defence. Predators typically attack the ends of the aggregation; because the mother must approach the predator to defend her offspring, we hypothesized that variation in collective signals reveals not only the predator's presence, but also its location. The defending mother also produces signals, and we hypothesized that maternal cues influence the signalling behaviour of the brood. We simulated predation at either end of the aggregation. In the presence of the mother, (1) offspring closer to the predator were more likely to signal and (2) more offspring contributed to the group signals when the predator attacked the aggregation at the far end from the mother. Differences in group signals, emerging from position-dependent differences in signalling probability of individual offspring, could be used by the defending mother to determine the end of the aggregation being attacked. A loss of these signalling patterns in the absence of the mother reveals that maternal cues are necessary for the establishment of a reliable relationship between collective signalling and predator location.