14.7–14.10: A life history is like a species summary.

Closely related to goats and sheep, the aoudad is native to North Africa.

Some animals reproduce with a “big bang.” Antechinus is an Australian mouse-sized marsupial, and the males are classic big-bang reproducers (FIGURE 14-13). At one year of age, they enter a two- to three-week period of intense mating activity, copulating for as long as 12 hours at a time. Shortly after this, the males undergo rapid physical deterioration—they lose weight, much of their fur falls out, their resistance to parasites falls—and then they die. (Although occasionally a female will live for two or three years, most die following the weaning of their first litter.)

Figure 14.13: Reproductive strategies.

Other animals are a bit—but only a bit—more restrained in their reproduction. The house mouse reaches maturity in about one month and produces litters of 6–10 offspring nearly every month, sometimes generating more than a hundred offspring in its first year of life.

And some animals could not be farther from big-bang reproducers. The little brown bat is also mouse-sized, yet does not reach maturity until one year of age and typically produces only a single offspring each year. It can, however, live more than 33 years in its natural habitat.

Why all the variation? And is one strategy better than the others, evolutionarily? One of the most important recurring themes in biology is that evolution nearly always finds more than one way to solve a problem. As the marsupial mouse, house mouse, and little brown bat illustrate, there are many possible responses to the challenge of when to reproduce, how often to reproduce, and how much to reproduce in any given episode. The answers to these questions make up an organism’s life history—the species’ vital statistics, including age at first reproduction, probabilities of survival and reproduction at each age, litter size and frequency, and longevity. Life histories tell us as much about a species as possible in a small amount of information.

Life histories vary from the big-bang reproductive strategy in Antechinus, in which the male’s reproductive investment—all of the material and energetic contribution that an individual will make to its offspring—is made in a single episode, to strategies, such as that of humans, in which organisms have repeated episodes of reproduction. Plants also have life histories, and, like animals, they have a range of strategies. Annuals, such as corn, wheat, rice, peas, marigolds, and cauliflower, usually reproduce once and then die. Perennials, such as apples, raspberries, grapes, ginger, and garlic, on the other hand, reproduce repeatedly, often living for many years.

Returning to the question posed above—which life history strategy is best?—we need to consider two questions.

1. What is the cost of the reproductive investment during any reproductive episode? Producing offspring is risky in several ways. The act of mating can be risky (by increasing the likelihood of an individual’s being eaten by a predator, for example). And the wear and tear of reproduction on an individual’s body takes its toll. Thus, the number of offspring an organism produces can only go so high before it becomes detrimental. An organism might produce four offspring in an episode, for example, without significantly increasing its risk of dying from the effort. But if, by chance, it produces five or six, this may take such a toll that the individual is unlikely to live to have another litter. Thus, for many organisms, a smaller litter is favored by natural selection because it enables the individual to have additional litters in the future, maximizing its lifetime reproductive success.

2. What is an individual’s likelihood of surviving to have future reproductive episodes? If predation rates or other sources of mortality are very high, an individual might not be alive in one month. If so, it makes less sense to defer reproduction: why save for a future unlikely to come?

Taken together, the answers to these questions help us understand the variety of life history strategies we see in nature.

Rodents are toward the fast extreme of reproductive strategy. They experience very high mortality rates in the wild, and natural selection has consequently favored early and heavy reproductive investment. Rodents provide relatively little parental care but produce many litters, each with many offspring. Organisms with this type of life history tend to have relatively poor competitive ability but a high rate of population growth; each individual’s likelihood of surviving is low, but so many are produced that the odds of two offspring surviving are high.

At the slow extreme, humans have such a low probability of dying in any given year that they can defer reproduction, or at least reproduce in small amounts—once every few years—without much risk, enabling them to have just one offspring at a time but to invest significant parental effort in maximizing its likelihood of surviving. Organisms with this type of life history generally have evolved in such a way that their competitive ability—the likelihood of an individual surviving—is high, although their rate of population growth is low. That is, not many offspring are produced, but those that are have a high likelihood of surviving (FIGURE 14-14).

Figure 14.14: With such a low probability of dying in any given year, humans can defer reproduction.