Offspring Size and Number

If we look across all organisms, we find that some species produce only one or a few large offspring per reproductive bout (e.g., most birds and mammals), others produce 10s or 100s of intermediatesize offspring (e.g., most plants and insects), and yet others produce many 1000s of offspring (e.g., some marine invertebrates). How can we account for such broad variation? In this chapter, we review many of the environmental and demographic variables that influence the evolution of offspring size and number. In the first section, we discuss how the trade-off between offspring size and number is an important determinant of offspring size. An individual’s resources can be allocated to three basic functions— growth, somatic maintenance, or reproduction. Resources directed toward reproduction can in turn be used to produce either many small offspring or a few large offspring. Thus, for a fixed amount of resources available for reproduction, it necessarily follows that there is a trade-off between the number and size of offspring during a given bout of reproduction. Trade-offs between offspring size and number during a single reproductive bout are a primary determinant of offspring size for most semelparous organisms, which reproduce once in their lifetime (e.g., salmon and century plants). For iteroparous organisms, however, lifetime reproduction is divided into many discrete bouts, with intervening periods of no reproduction. Evolutionary explanations for the number and size of offspring in these organisms must also consider how reproductive effort during any one period affects future survival and reproduction. The second part of our chapter considers the evolution of offspring number among long-lived, iteroparous organisms, especially vertebrates. We focus on the clutch sizes of birds that produce altricial (nidicolous) young. Because each nestling requires much parental care, we expect strong selection toward producing the most appropriate number of offspring for a given environment. The trade-off between current and future reproduction can also affect semelparous animals if offspring must be distributed among scattered resources. Many insects, for example, lay eggs on small, discrete hosts, and their sedentary offspring often cannot move between hosts. A female that places too many eggs on a host faces the same diminishing returns as a bird that produces more nestlings than it can provision.