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Evolution of the Migration Response: Emigration by Tribolium and the Influence of Age

James R. Ziegler
Evolution
Vol. 30, No. 3 (Sep., 1976), pp. 579-592
DOI: 10.2307/2407581
Stable URL: http://www.jstor.org/stable/2407581
Page Count: 14
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Evolution of the Migration Response: Emigration by Tribolium and the Influence of Age
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Abstract

Literature relating to the ecology of Tribolium castaneum and T. confusum was reviewed in order to draw inferences about the roles of these two species as colonists of temporary habitats. From this information it may be inferred that T. castaneum is a primary colonist, and that T. confusum is a secondary colonist. On the basis of this inference it was hypothesized that T. castaneum would exhibit higher intrinsic levels of emigration than T. confusum. Emigration by both species was expected to persist throughout life. To test this hypothesis beetles were permitted to emigrate from cohorts containing 100 precisely aged individuals. There were seven age classes ranging from 2 to 90 days. The results showed that T. castaneum is indeed much more emigratory than T. confusum. They departed virtually to the last individual in all experimental cohorts. T. confusum tended to emigrate more slowly, and to cease their emigration at low densities. In both species emigration proceeded in two phases: numbers initially declined in a density dependent manner until a certain density was reached. Thereafter, further emigration occurred more slowly and at a constant rate. In cohorts of T. confusum this final rate was virtually zero. In both species initial emigration was relatively low for sexually immature adults. It peaked at or after the onset of reproduction, and thereafter declined with increasing age. Progressively older beetles changed to the slower mode of emigration at increasingly higher densities. The evolution of the emigration response was discussed in relation to habitat conditions. It was suggested that if cues were absent or unreliable stereotyped genetic determinism for the ability to emigrate should be favored. More reliable habitats should favor increased environmental determinism of the emigration response. If cues are perceived early in the life cycle they may permit the evolution of specialized developmental morphs which exhibit contrasting migrant and nonmigrant phenotypes. In less stable habitats in which reliable cues may occur at any time during the life cycle the optimum response may be more generalized. Such a generalized response would involve short-term physiological and behavioral adjustments to prevailing habitat conditions. It would permit a single individual, through the proper timing of emigration, to partition optimally its reproductive effort between the original habitat and the habitat it is likely to colonize. The emigration of Tribolium is largely of this last generalized type.

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