Cassowaries are large and prehistoric looking birds with large casques (prominent head ridge) that are believed to amplify deep acoustic sounds made by the bird. There are also theories that the bony casques act like a helmet to protect the bird from falling seeds where they forage, or from collisions with trees as they sprint through the forest. Cassowaries are capable of producing a low frequency boom sound that is at the lower limit of human hearing, and is the lowest frequency sound produced by any bird. It is believed that this helps them communicate with each other in dense brush. They are shy birds that will avoid humans at all costs. To see one is a rare sight as they are very capable of disappearing into the forest long before they are discovered. Cassowaries can live 40-50 years in the wild. They are the third tallest and second heaviest bird in the world, after the ostrich and the emu. People in countries where they live are well aware of the hazards of approaching a cassowary and even military troops arriving in New Guinea are warned to show them proper respect. Documented cases of aggression from the cassowary have been used to study their behaviour. Most of these cases involve attacks on humans, and many others on domestic dogs. In the majority of the cases studied, the attacks have come when the birds are being fed or when they expected food. Several attacks were on humans that had been feeding the birds previously and had developed a sense of trust. This perception was unfortunately misguided. Rarer cases had involved attacks in self defense when people were attempting to kill or injure the cassowaries. With the highest recorded numbers of attacks on people and the severity of wounds inflicted, it is no wonder this bird is commonly referred to as the World's most dangerous bird. Like all animals, they should be treated with respect and caution. Feeding wild animals can have immediate and disastrous consequences.