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A General Theory of Clutch Size

Martin L. Cody
Evolution
Vol. 20, No. 2 (Jun., 1966), pp. 174-184
DOI: 10.2307/2406571
Stable URL: http://www.jstor.org/stable/2406571
Page Count: 11
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A General Theory of Clutch Size
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Abstract

The principle of allocation of time and energy is used to formulate a general theory to account for clutch size in birds. "Advantages" are figured as the axes of a three-dimensional graph, and phenotypes allocating their energy in particular ways as points in space forming a surface and enclosed solid. In the temperate zones most energy is used to increase the reproductive rate r. In the tropics the carrying capacity of the habitat is more important, resulting in a smaller clutch size. Different phenotypes will be more fit in different environments, as optimum allocation of energy differs. Previous theories of clutch size are discussed, and incorporated into this general theory. Increase of clutch size with latitude is analyzed, and accounted for by the theory. Predictions are made that all stable environments, the tropics, islands, coasts, will favor reduced clutches. Examples are quoted in which instability of conditions results in increased clutch size. The situation of predation-free species is examined, and it is predicted that the clutch size of such species will remain relatively unchanged with latitude changes. These predictions seem to be verified in all cases where data are available to test them.

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