Plants respond to environmental cues to alter their morphologies, physiologies, and defenses.  Plants use volatiles emitted or released following damage by herbivores as a source of information.  For both species for which we have measured fitness consequences of induction triggered by exposure to volatiles from damaged neighbors, estimates of plant fitness increased as a result of plant-plant signaling.  If these results are general and volatile cues make neighbors more resistant and more fit, why would plants emit these cues?  Herbivores and parasitic plants exploit these same cues to locate hosts, making them even more costly for the plant that emits them.  

Results/Conclusions

1) Volatiles provide direct defenses and also attract predators of the herbivores.  2) Volatiles are often required for within-plant communication and have advantages as within-plant signals compared to vascular signals.  3) Volatile communication may be more effective between kin, although evidence for kin selection is lacking.  4) Volatiles emitted following damage have other fitness related consequences.  For example, MeJA emitted following damage is a powerful inhibitor of seed germination.  These alternative explanations are not mutually exclusive and several may provide sufficient benefits for plants to overcome the fitness costs of emitting active cues.