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Reproduction in the iguana conolophus subcristatus on fernandina island galapagos ecuador clutch size and migration costs



Reproduction in the iguana conolophus subcristatus on fernandina island galapagos ecuador clutch size and migration costs



American Naturalist 121(6): 757-775



Adult land iguanas (C. subcristatus) on Fernandina Island, Galapagos, live in an unpredictable, predator-free environment. They lack resource competitors. The population structure allows direct comparison of adaptations to varying environments because the entire population can be asssumed to share the same genetic background. Reproductive effort apparently varies as a function of habitat quality. Females from good habitats not only lay larger clutches but also maintain higher proportions of reserves for survival during harsh conditions. Differential selection in varying habitats should favor offspring from females adapted to harsh conditions. Thousands of females use the island's caldera for nesting. They may migrate (aerial) distances exceeding 10 km to the 1495-m high summit and finally climb into the 900-m deep caldera for nesting. Migration costs are accordingly high and are estimated to constitute half the reproductive effort. The relative clutch mass (RCM) is the lowest recorded for iguanines, and is also low compared to a number of smaller lizards. If migration costs are added to the clutch weight, the corresponding value is between the mean and maximum for other lizards. The value is unexpectedly high for a long-lived species controlled by resources (adults). The expenditure per egg is low. This presumably is an adaptation to the extreme predation pressure at and shortly after hatching, when hawks and probably also snakes concentrate at the nest sites. Suitable nesting areas used by other iguanine and land iguana populations are not used by Conolophus on Fernandina. The high investment and associated risks to reach the caldera must be counterbalanced by advantages of nesting there. Three possibilities are discussed. Climatic conditions in the caldera may enable the population to place egg production and mating in the most favorable (warm, rainy) season; nests were found in fumarole-heated soil which could be used to concentrate hatching into a much shorter period than laying (an antipredator mechanism) and sex determination on the basis of incubation temperature.

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