Period length in cyclic animal populations

Although cyclic animal dynamics have long been a focus of scientific interest, the variable cycle lengths are poorly understood. Based on a review of the literature, we suggest that period length in animals showing multiannual cycles is related to the life span of their food plant rather than to any trait of the animal, such as mass or fecundity. We envisage that this pattern is brought about by a prolonged mobilization of induced defenses in longer lived plants, which can better afford periods of low reproductive output than can short-lived plants. On the basis of this hypothesis we expect animals with similar diets to show similar cycle lengths, irrespective of taxonomy and size. A path analysis, on the vertebrate herbivores, shows that 92% of the variation in cycle length is explained by food-plant longevity and that body mass adds little to this.