Reproductive effort, offspring size and benefit-cost ratios in the classification of life histories

There have been many attempts to document links between reproductive allocation and factors such as adult body size and demography. This paper suggests that among closely related taxa, two dimensionless numbers, each a benefit-cost ratio summarizing reproductive timing, allocation and demography, are invariants and thus are useful to classify life histories. The two numbers are E/alpha and C . E, where E is average adult life span, a is age-at-first-reproduction and C is average mass (per adult) devoted to reproduction per unit of time, divided by the average adult body mass (m); C is usually called 'reproductive effort'. Since E-1 is the average adult mortality rate, C/E-1 is the reproductive effort (benefit) per unit death (cost). Similarly, E/a is the amount of time for reproduction (E) divided by the time cost to get there (a). Combining these two numbers with the relative size (I) of an offspring (I/m) yields a new classification scheme for life histories; this is contrasted with other classification schemes (e.g. r and K).