The breeding and non-breeding distributions of male and female marine iguanas, Amblyrhynchus cristatus, and their mating behaviour were analysed. The population size was low because of a long lasting El Nino. The distribution of female-sized iguanas was more clumped during the mating season than before and afterwards. Model iguanas placed outside territories attracted more females during the breeding season than during the non- breeding season. The majority (75%) of large males established clustered territories while the remainder established single territories. The locations of the clusters were not related to physical characteristics of the habitat and were only partially related to non-breeding female densities. Non- territorial 'sneaker' males, similar in size to females, attempted copulations inside territories when territorial males were absent. Moderate sized, non-territorial males remained in the vicinity of territories and attempted to copulate forcefully with female-sized iguanas. This form of harassment was much higher outside territories, but levels did not differ between lek and single territories. Therefore, while harassment may lead to an increase in grouping among female-sized iguanas, it probably does not influence the type of territory visited. Females preferentially mated with the largest territorial males and reproductive success for these males was independent of lek size. The reproductive success of smaller territorial males, however, increased with lek size. Lekking in marine iguanas, therefore, may represent a 'hotshot' phenomenon where small territorial males associate with large males to increase reproductive success. The likely mechanism behind clustering appeared to be a greater propensity for females to mate in leks where stimulation rates were higher.
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