Embryo growth rates in birds and mammals

Summary 1. Embryo mass growth curves of 36 species of bird and 18 species of mammal were fitted by sigmoidal Gompertz functions, in which k (days −1 ) describes the rate at which the embryo approaches an asymptotic mass A (g). 2. The parameters of the Gompertz function were uncorrelated with parameters of power functions fitted to the same growth data, indicating that the two models describe different aspects of growth. 3. Asymptotes of the Gompertz functions for embryonic growth averaged 2·5 times neonate size, but were well below adult mass. Thus, the pre‐ and postnatal phases of growth are distinct and have different target sizes. 4. Embryo growth rate ( k ) decreases as the −1/4 power of neonate size; values for mammals were 63% of those of birds, on average. 5. Embryo growth rate can be predicted solely by the length of the incubation or gestation period, regardless of the size of the neonate, with the same relationship for birds and mammals. 6. Postnatal growth rate ( k G ) scales linearly with embryonic growth rate ( k ), but is nearly five‐ times more rapid, on average, in birds than in mammals. k G and k have similar magnitude in birds, but k G is relatively much slower than k in mammals. 7. Rate of actuarial senescence ω (year −1 ), measured by the increase in mortality rate with age, is positively related to the rate of embryo growth in both birds and mammals, but is higher in the latter. Moreover, rate of ageing in birds is uniquely related to embryo growth while that in mammals is uniquely related to postnatal growth rate. Thus, development and ageing appear to be differently connected in birds and mammals, although the basis for these relationships is not known.