Exploitation of an ant chemical alarm signal by the zodariid spider Habronestes bradleyi Walckenaer

Intraspecific signals are vulnerable to exploitation by predators that are not the targets of the signal. This cost has been documented for several acoustic, visual and chemical signals, but not for chemical alarm pheromones. We reveal a novel form of exploitation of an ant alarm pheromone by the cursorial spider Habronestes bradleyi (Zodariidae), a specialist predator of the highly territorial and aggressive meat ant Iridomyrmex purpureus. We demonstrate experimentally that H. bradleyi locates I. purpureus workers engaged in territorial interactions with conspecifics, by using the alarm pheromone of the ants as a cue. The spiders are attracted to an airborne cue, identified as the alarm pheromone 6-methyl-5-hepten-2-one, which is produced by injured or alarmed I. purpureus workers but not by inactive I. purpureus, or injured workers of other sympatric ant species. These data demonstrate a novel cost of producing alarm pheromones.