LIFE-HISTORY CONSEQUENCES OF BIOENERGETIC AND BIOMECHANICAL CONSTRAINTS ON MIGRATION

In this paper I test the hypothesis that bioenergetic and biomechanical constraints to migration play a pivotal role in shaping the life history characteristics of migrants. Firstly, I examine bioenergetic constraints on the ability to migrate by active transport and how they shape the life histories of insects and fish, and, secondly, the consequences of biomechanical constraints to the migration by passive transport in insects and spiders. In both insects and fish the mass-specific energetic costs of active transport (flight and swimming, respectively) decrease with body size, and hence selection should favor large size in migrant species. Because their habitats are ephemeral, migrant insects must grow rapidly. In fish, migrant species are able to exploit resources unavailable to more sedentary species and hence should also show an enhanced rate of growth. These predictions are supported by comparisons within populations, between populations, and among species in both groups. In contrast to the above, biomechanical factors limit the upper size at which insects and spiders can migrate by passive transport. Theory predicts that ballooning will be most likely in spiders considerably less than 6 mm in length. Therefore, species that migrate as adults are predicted to be smaller than those that do not. This prediction is supported by a comparison of migratory and non-migratory spider species from the United Kingdom. The average length of species that migrate as adults, and of migrating young of spiders too large to balloon as adults, is about 2 mm. Further, within this geographic species assemblage, the size distribution of adult spiders is markedly peaked in the 2 mm region, suggesting that biomechanical constraints on ballooning may have a major influence on the evolution of body size in spiders.