their fruits defend themselves against some herbivores. eating the neotropical plant "annona muricata" (soursop, "graviola, guanabana") is a causal agent in "atypical parkinsonism". the causative agent, annonacin, is present in the seeds and leaves of many of the annonaceae, though not in any significant quantity in the fruit flesh. it is thought to be responsible for up to 70% of parkinsonian conditions in guadeloupe. exposure is typically through traditional food and natural medicines. it is hard to interpret this. presumably some favoured herbivores can eat it without ill effects. in some plants, seeds contain irritants to encourage early defecation.