Fitness measures in selection analyses: sensitivity to the overall number of offspring produced in a lifetime

Age at first (α) and last (ω) breeding are important life-history traits; however, the direction and strength of selection detected on traits may vary depending on the fitness measure used. We provide the first estimates of lifetime breeding success (LBS) and λind (the population growth rate of an individual) of European badgers Meles meles, by genotyping 915 individuals, sampled over 18 years, for 22 microsatellites. Males are slightly larger than females, and the opportunity for selection was slightly greater for males, as predicted. λind and LBS both performed well in predicting the number of grand-offspring, and both detected selection for a late ω, until the age of eight. Differential selection (S′α) for an early α, however, was only detected using LBS, not with λind. In declining populations (λind < 1) selection favours reproduction later in life, whereas early reproduction is selected in increasing populations (λind > 1). As 41% of badgers were assigned only one offspring (λind < 1), whereas 40% were assigned more than two (λind > 1), this cancelled out S′α measured by λind.