Changes in Life History within an Individual's Lifetime

A central goal of life history theory is to understand the selective factors that generate the diversity of reproductive patterns observed in nature. Within lifetime changes in reproductive investment will determine an organism's fitness; however, this area of life history theory has received less attention than comparisons among population that characterize life history traits as a single population mean. Reproductive allocation can be affected by multiple cues; the integration of these cues across an organism's lifetime generates the diversity in life history strategies observed in nature. Life history studies should examine the interacting effects of multiple cues on life history strategies to generate better predictions and generalizations of age-related changes in reproductive investment. An individual's life history strategy is inherently multivariate consisting of a coordinated suite of life history traits that, when combined across the organism's lifetime, determines its fitness. Life history strategies can therefore be described as a trajectory through multivariate space defined by life history traits. Here I describe life history trajectory analysis, a multivariate analytical approach for quantifying and comparing phenotypic change in life history strategies; this methodology is adapted from an analytical framework originally described for studies of morphological evolution. Life history trajectories have attributes (magnitude, direction, and shape) that can be quantified and statistically compared among taxa to determine if life history patterns are predictable. Using the life history trajectory analysis, I demonstrate the effect of prior experience on reproductive allocation in the burying beetle Nicrophorus orbicollis. The effect of prior experience resulted in a terminal investment or accentuated response to age-based cues, or resulted in a conservative investment strategy or reproductive restraint. In the livebearing fish Gambusia affinis, females adjust the level of reproductive investment to current reproduction based on age- or environment-based cues. Age-0 females decreased the level of reproductive investment to current reproduction in late summer prior to the onset of fall and winter months. Old females, on the other hand, increased the level of reproductive investment as the summer progressed. The reproductive restraint and terminal investment patterns exhibited by age-0 and age-1 females, respectively, were consistent with the predictions from the cost of reproduction hypothesis. These studies demonstrate how the life history trajectory analysis provides an analytical tool to test predictions of life history theory. Additionally, I provide evidence that organisms use multiple cues to determine the level of reproductive investment and that the strength of the effect of each cue will depend on the age of an individual.