EVOLUTION OF THE MIGRATION RESPONSE: EMIGRATION BY TRIBOLIUM AND THE INFLUENCE OF AGE.

Hypothesis.-Tribolium castaneum is a primary colonist. Adults of this species lead a nomadic existence in which dispersal is intensive and extensive, and highly responsive to density. T. confusum is a secondary colonist. By comparison with T. castaneum, dispersal of T. confusum adults is less intensive, and less responsive to the presence of other individuals. In both species of Tribolium the tendency to disperse persists throughout adulthood. Development of the hypothesis.-Beetles of the Tenebrionid subfamily Ulominae, to which Tribolium belong, are typically associated with rotting wood and the loose bark of trees (Good, 1933). T. castaneum is apparently found in this habitat in forests of the Indian subcontinent (Good, 1933), and Sinha (1968) notes that T. castaneum and T. confusum are occasionally found separately in Canadian forests. Feral populations of Tribolium have been observed in the nests of bees and birds, and in manure (Hinton, 1948; Lefkovitch in Mertz, 1971; Jones, 1967). The beetles, however, are most abundant in warehouses where they infest stored products. Such "domestic" populations of both species show a cosmopolitan distribution. The diets of T. castaneum and T. confusum overlap broadly, and include a bewildering array of plant and animal products (Good, 1933) which need only be sufficiently soft or small in size to be eaten. Immatures and adults of both species are also predaceous and cannibalistic (Park et al., 1965). These activities are modified according to the nutritional quality of their food (Sokoloff and Lerner, 1967). To be sure, comparative nutritional studies (Sokoloff et al., 1966a, 1966b; Sinha, 1966, 1972) have documented species differences in the suitability of specific foods for development and reproduction, but, on the whole, one is impressed with the opportunistic generality in the diets of both species. These and related aspects of the biology of Tribolium are comprehensively reviewed by Sokoloff ( 1974). Tribolium habitats are undoubtedly temporary. Whether a rotting log or a sack of flour, a Tribolium habitat can be expected to support a variable number of generations before it completely deteriorates. It is not unlikely, then, that Tribolium populations should alternate between short periods of rapid population growth and prolonged periods of decline. To this characteristic Mertz (1971) attributes much of the natural selection for long reproductive life (iteroparity) in Tribolium species. If breeding habitats are widely distributed in space, a long life span would give females time to search for them (cf. Cole, 1954). If we make the reasonable assumption that persistent habitats also arise infrequently, then it follows that iteroparity, which may have evolved in response to prolonged population decline within persistent local habitats, will also be adaptive when those habitats finally deteriorate. Tribolium females produce eggs continuously. They may lay them over a wide range of habitats, each with a low probability of successfully supporting their offspring to adulthood. Such temporal and spatial uncertainty in the survival of immatures will also promote the evolution of iteroparity (Murphy, 1968). Adaptive dispersal is an element of