The sound structure of elephants is very much advanced. Over the course of the past few decades, there have been many studies completed on the infrasound hearing capabilities of elephants. The leading authority on this topic in the world would be Katy Payne who recently published the book Silent Thunder. Interestingly, in the book Elephants by Eltringham a passage on an elephant´s hearing is described in full in the following passage: "The elephant is usually considered to have an acute sense of hearing, although firm data on the matter are hard to find. The large pinnae are not necessarily indicative of good hearing because their primary functions lie in temperature control and in social signalling. Elephants do not appear to communicate over long distances so perhaps there is little need for particularly good hearing, but it is difficult to be certain whether and elephant has not heard a distant sound or is simply ignoring it." (P.22 Eltringham) Luckily, it was discovered that this was totally wrong and that elephants do communicate over huge distances. Thanks to Katy Payne, the general public is no longer told about elephant coordination between groups coined elephant ESP. Also, they are now spared the descriptions about elephants which mark their hearing as poor; her discovery of infrasonic communication has opened the door to numerous implications about the social world of elephants. It has opened the door allowing those who care to better understand the elephant. Joyce Poole has also worked in extending our understanding of the depth of this communication and its implications. The infrasound level is a range of sound waves which humans are incapable of perceiving with their own ears. However with the help of certain devices, much can be learned about about the elephant and their modes of communication. Generally, "elephants hear and respond to each other's loud calls from distances as great as four kilometres away"(Payne P.121). This under normal conditions would mean elephants can communicate and hear a message within a fifty+ square kilometres range. With David Larom and Michael Garstang´s work with long range calls and atmospheric conditions it was found that elephants have the capability of 9.8 kilometres away and within a 100 square kilometre range. At evenings in Africa the air temperature within 300 metres of the ground becomes inverted causing low-frequency sound to be reflected back to the ground instead of normally dissipating into the sky. Basically, around dusk elephants can communicate with each other over a much greater distance. Katy Payne´s research has found that elephants do most of their calling in the late afternoon. This is a time when sound transmission is good but is not perfect. An easy explanation for not calling a lot during the night is obvious to anyone who has spent the night out in the African bush alone; the lions and many other predators are awake and hunting in the middle of the night and it would be foolish to draw unneeded attention to yourself. Thus, for its effectiveness wages against survival, elephants have found an ideal balance in maximizing this tool that they are so lucky to possess. This structure allows for distress signal or other communication to be sent out and received by other elephants. This infrasound ability can be noticed by the sudden reaction of elephants to lift there head from drinking, bathing or eating, and act in a peculiar fashion. This may indicate that they have heard something that has warned them about some danger or could just be a female sending out estrus calls. Infrasound take social communication to new levels within the natural animal kingdom. The evolutionary function for such communication is fairly obvious. Firstly, the fact that females are only really receptive to males just a few days every three to four years there is a small window within a big land to allow for the biggest bull to find a receptive mate. Given they can communicate over long distances, elephants gain a great evolutionary advantage in being able to communicate over great distances. The general trend applies across species including our own: the stronger the bull is that mates with the female, then the more likely the children are going to survive. Second, the ability to tighten the bond between groups allows for stronger social relationships, and reduces wasted energy in trying to keep together when moving over vast distances.