1. Competition for food at high densities during larval development leads to reduced adult weight in the northern temperate dung beetle Aphodius ater.
2. Analysis of female beetles caught in the field showed that numbers of eggs and total egg load per female were correlated positively with beetle size.
3. Female beetles reared at different population densities during larval development in the laboratory were analysed with regard to their lifetime fecundity and reproductive lifespan.
4. High population densities during development had a negative influence on the number of eggs per female and on reproductive lifespan. Lifetime fecundity was correlated positively with female weight.
5. It was concluded that competition during larval development in the first generation of offspring will result in a lower number of offspring in the second generation in Aphodius ater, and thereby reduce parental fitness.