The idea that the size of animal populations may be regulated by factors intrinsic to them, such as behaviour, has a long history in ecology. Although this idea is now rejected as a general mechanism, it may apply in some species where females are damaged during courtship attempts, such as the Mediterranean tortoise Testudo hermanni. The sex ratios (adult males : adult females) of most dense Greek populations were more extreme (over 1.5:1) than could be accounted for by the earlier age at maturity of males, and some ranged to over 6:1. The sex ratio was correlated with the population density of adult males and the frequency of courtship attempts on females, and negatively correlated with the longevity of females. A high population density of males limits the density of adult females. This intrinsic regulation of population size is a consequence, rather than a goal, of a particular type of courtship behaviour (persistent thrusts by the male using a tail with a sharp terminal spur) in T. hermanni. Existing population densities at many Greek sites are probably unnaturally high, owing to the reduction of natural predators.