The Life of the Smooth Newt (Triturus vulgaris) after Metamorphosis

Abstract

After metamorphosis, smooth or common newts (Triturus vulgaris vulgaris [Linn]) disperse on land, where they live as juveniles before returning to the water as breeding adults. The population age—structure was obtained by dissecting the standard length frequency distribution, a procedure which is justified in the text. Juveniles have an uncomplicated age—structure, the number of individuals in successively later age—classes being reduced by mortality and by recruitment to the adult population. Among adult newts, the most frequent age—class is nearly central, indicating the existence of extensive variation in the age at maturity. Growth continues throughout life, the rate of growth decreasing with age. The annual survival rate of adult newts as estimated by a census method, and from the age—structure, was found to be ~ 50%, being rather greater in ♀ ♀ than in ♂ ♂. Less reliable data indicate the annual survival rate of juveniles was ~ 80%. Maturity is attained between the ages of 3 and 7 yr, most individuals reproducing for the first time at 6 or 7 yr of age. The number of eggs laid increases with age from ~ 100 at age 3 to ~ 400 at age 12; there is also an effect of ♀ age on oocyte size, which may influence subsequent larval survival. Smooth newts breed annually. Reproduction diverts surplus energy from somatic growth, and thereby decreases potential future fecundity in animals whose fecundity is proportional to body size. Individuals maturing at different ages, therefore, have different schedules of fecundity, which were estimated through the use of microbomb calorimetry. Population size was found to be lognormally distributed, with a mean of ~ 70. The sex ratio of juvenile newts was near equality, but a majority of ♀ ♀ comprised most adult populations. This imbalance is due to greater mortality of adult ♂ ♂, which results in an age—specific trend in the sex ratio. There are 2 breeding migrations: 1 in autumn and 1 in spring. It is likely that newts migrate during autumn when about to reproduce for the first time, and thereafter migrate during spring. Terrestrial newts appear to move little; there is some evidence that colonization of newly dug ponds is achieved by the infrequent emigration of juveniles. During its life, a smooth newt occupies a succession of ecological niches. Moreover, different individuals may occupy different niches, or may occupy the same niches for different periods of time. It is speculated that in large newt populations the complex life history is able to trap genetic variation and to dampen fluctuations in population size. However, it is suggested that these group attributes have arisen largely as the result of selection between individuals. Breeding only once (rather than repeatedly) will reduce fitness because the additional fecundity necessary to balance the loss of later reproduction cannot be attained. Early maturity appears to be favored in ♂ ♂ and late maturity in ♂ ♂ these opposed selection pressures may contribute to the observed age variation at maturity. Finally, the sex ratio does not vary with population density, and is therefore thought to be controlled by natural selection.