Fig 1.

Optimal body size must be placed in the size range for which the production rate divided by mortality rate, P(w)/m(w), where w is body mass, is concave downwards. P(w)/m(w) is expressed in energy units because production is measured in J/day and mortality is measured in 1/day. Because life expectancy is equal to 1/m(w), this expression measures the expected amount of energy allocated to offspring for an animal maturing at size w. If the adult mass is in the range for which the ratio is concave upward, then strong directional selection is expected to increase the body mass rapidly to the point when the shape of this function becomes concave down and then slowly to a size that maximizes fitness. For size‐independent mortality, m(w) can be removed from the vertical axis legend. See Appendix S1 and Kozłowski (2006) for more details and for explanation of the body size optimization condition: if an increase in body size by 1 J increases the expected offspring production (taking into account mortality) by more than 1 J, then growth is adaptive; otherwise, the use of this energy for reproduction becomes adaptive.