Practice question

Maximum intrinsic growth rate generally declines with increasing body mass because large organisms tend to have slower life histories. Metabolic and developmental scaling produce later maturity, longer gestation or development, longer generation intervals, and fewer offspring per unit time. Even when large species have high adult survival, delayed and infrequent reproduction lowers the rate at which descendants accumulate. Small organisms commonly mature quickly and reproduce repeatedly, yielding high rmax under favorable conditions. Across broad taxonomic comparisons, this relationship is often approximated by an allometric power law in which rmax scales negatively with body mass. It is a tendency rather than an invariant rule: temperature, phylogeny, reproductive mode, and ecological strategy create substantial variation around the trend. Body size itself does not directly enter the exponential equation; it influences the age-specific survival and fecundity schedules from which r is derived. Because reproduction occurring early contributes more strongly to population increase than the same reproduction delayed, long generation time is especially important in explaining why elephants have lower rmax than mice or microorganisms.