Abstract
The effects of different types of reproductive cost on the pattern of life histories [animal] are examined. Reproductive cost may be the generally deleterious effect of present reproduction on future survival or fecundity or both. The circumstances in which reproduction at a given age should be maximal, and the conditions under which reproduction at given age should be zero are discussed. Semelparity and of the optimal age at maturity are considered. A model applicable for homoiotherms and poikilotherms is presented.