Theory of Natural Selection. Clarendon. Oxford. Garay. J.. 2009. Cooperation in defence against a predator. J. Theoret. BioL 257. 45-51. Haldane. J.B. S., 1932. The Causes of Natural Selection. Longmans Green & Co.. London. Hamilton. W.D.. 1971. Geometry for the selfish herd. J.Theoret. Biol. 31.295-