Research at Oxford University showed that swarming is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin. This causes the locust to change colour, eat much more, and breed much more easily. The transformation of the locust to the swarming variety is induced by several contacts per minute over a four-hour period. It is estimated that the largest swarms have covered hundreds of square miles and consisted of many billions of locusts. Plagues of locusts appear in both the Bible and the Koran, including one of the Biblical Plagues of Egypt, where locusts ate all the crops of Egypt.