r-strategists and K-strategists differ in number of offspring and parental care.

While the terms “external” and “internal” fertilization focus our attention on where fertilization takes place, the two modes of reproduction are associated with a host of other behavioral and reproductive differences. One key difference is the number of offspring produced.

External fertilization is generally associated with the release of large numbers of gametes and the production of large numbers of offspring, each of which has a low probability of survival in part because the offspring typically receive little, if any, parental care. External fertilization, in other words, is a game of numbers. For internal fertilization, however, there is much more control over fertilization and subsequent development of the embryo because both fertilization and embryonic development take place inside the female. As a result, animals that use internal fertilization typically produce far fewer offspring and invest considerable time and energy into raising those offspring.

These two strategies represent two ends of a continuum of reproductive strategies that was first described by the American ecologists Robert H. MacArthur and E. O. Wilson in 1967. Organisms that produce large numbers of offspring without a lot of parental investment are r-strategists, and those that produce few offspring but put in a lot of parental investment are K-strategists (Fig. 42.8; Chapter 46). Fish are r-strategists; humans are K-strategists.

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FIG. 42.8 r-strategists and K-strategists. r-strategists, like fish, produce many offspring with low probability of survival, whereas K-strategists, like primates, produce few offspring and put a lot of care into raising their young.

According to this model, the environment places selective pressures on organisms to drive them in one or the other direction. In general, r-strategists evolve in unstable, changing, and unpredictable environments. In such an environment, there is an advantage to reproducing quickly and producing many offspring when conditions are favorable. By contrast, K-strategists often evolve in stable, unchanging, and predictable environments, where there tends to be more crowding and larger populations. Here, there is intense competition for limited resources, so traits such as increased parental care and few offspring are favored.

These two strategies represent the extremes of a spectrum, and there are notable exceptions. An octopus, for example, can lay up to 150,000 eggs, typical of r-strategists, but the female guards and takes care of them for a month or more, typical of K-strategists. However, the two strategies highlight different reproductive patterns across animals and suggest important links between number of offspring, parental care, and the environment.