1. This study investigates the evolution of life-history traits in parasitic and free-living nematodes. A database on 35 species was assembled for values on: body size of female nematodes; egg production; life expectancies of both adult and free-stages; the length of maturation time for free-living nematodes and prepatent period for parasites (time needed for an infective stage to reach maturity in its definitive host). Comparative methods were used to account for the effects of phylogeny. 2. The well-known allometries of life-history traits with body size were found, such as the allometry between size and total fecundity. Daily fecundity and prepatent period of parasitic nematodes are both correlated with body size, suggesting that delaying maturity increases fecundity. 3. Phylogeny affects relationships between investigated life-history traits, i.e. daily reproductive output (b) and female life expectancy (1/M). 4. Prepatent period (a) in parasitic nematodes is equivalent to maturation time (a) of free-living nematodes. 5. A search was carried out for life-history invariants (Charnov 1993). There was no correlation between prepatent period (a) and daily reproductive output (b) and hence no invariant ab. This could be explained by constrained life styles of reproduction among Nematoda. Indeed, invariant aM (or aM, where M is the mortality rate of adult stage) were found for both parasites of vertebrate ((aM) = 0.23) and free-living, plant and insect nematodes ((aM) = 0.05). 6. A causal chain of evolution of life-history traits of parasite nematodes is proposed and discussed. It is suggested that adult mortality is the main factor driving the evolution of life-history traits.
