Résumé
Although most life history traits in birds and mammals show an allometric relationship with body weight (Brody 1945; Lack 1968; Peters 1983; Calder 1984), such studies have failed to show a clear relationship for the components of fecundity: litter size and number of litters per year. By using a functional definition of the fecundity as the product of the number of litters per year with litter size, however we find an allometric relationship with allometric exponents of-0.15 in birds and-0.22 in mammals. the observed value of the allometric exponent for each order is discussed with regard to the theoretical value expected for variables dependent on time according to Lindstedt and Calder (Lindstedt and Calder 1981; Lindstedt et al. 1986). This has direct implications for investigation of demographic strategies.