Lifetime reproductive effort.

In a 1966 American Naturalist article, G. C. Williams initiated the study of reproductive effort (RE) with the prediction that longer-lived organisms ought to expend less in reproduction per unit of time. We can multiply RE, often measured in fractions of adult body mass committed to reproduction per unit time, by the average adult life span to get lifetime reproductive effort (LRE). Williams's hypothesis (across species, RE decreases as life span increases) can then be refined to read "LRE will be approximately constant for similar organisms." Here we show that LRE is a key component of fitness in nongrowing populations, and thus its value is central to understanding life-history evolution. We then develop metabolic life-history theory to predict that LRE ought to be approximately 1.4 across organisms despite extreme differences in production and growth rates. We estimate LRE for mammals and lizards that differ in growth and production by five- to tenfold. The distributions are approximately normal with means of 1.43 and 1.41 for lizards and mammals, respectively (95% confidence intervals: 1.3-1.5 and 1.2-1.6). Ultimately, therefore, a female can only produce a mass of offspring approximately equal to 1.4 times her own body mass during the course of her life.