While ecological and conservation consequences of combining animals of varied genetic backgrounds have been widely discussed the demonstration of effects that stem from lineage mixing remains elusive. Since management agencies relocate populations or supplement them with individuals regularly, the opportunity for either inbreeding or outbreeding depression may be high; still, any putative effects will go unnoticed without detailed knowledge of life-history and behaviour. Here, we report potential consequences of lineage mixing in a restored population of North American Bos bison studied for five years. In 1984 two allopatric lineages became sympatric in Badlands National Park, South Dakota, they differed in both founding population size and the number of demographic bottlenecks experienced since 1907. Measures of reproductive variance in both sexes were employed to estimate effective population size based on 261 copulations and the survivorship of calves between 1985 and 1989. We assumed that the reproductive variance and mortality documented in this study are representative of the bison's recent past and based on this assumption we calculated N-e separately for each generation for which the lineages were allopatric. Four potential correlates of fitness were studied in the new sympatric population: (1) female fecundity; (2) juvenile survival; (3) growth rates, and (4) female age at puberty. Of these, neither female fecundity nov juvenile survival was associated with lineage but growth rates were more rapid and ages at puberty were lower for F-1 purebred (inbred) juveniles than for F-1 hybrid (outbred) juveniles. Possible consequences of this variation in the F-1 generation include (1) higher winter mortality in the slower growing line as well as (2) decreased lifetime production of young; both are life-history parameters that could be interpreted as long-term selection against outbreeding. However, these data by themselves do not constitute support for an outbreeding depression hypothesis. The failure of males from one lineage to mate at all prevented the possible combinations of the F-1 generation needed for the appropriate statistical contrasts. Nevertheless, these interpretations (1) substantiate a level of variation in life-history parameters stemming from lineage mixing; and (2) suggest that advice regarding prudent conservation strategies must be sought concerning the generic histories of individuals and populations to be selected for re-introduction. This applies to both wild and captive populations. The prevalence of outbreeding and inbreeding tolerances within populations of managed and protected species needs verifiable documentation.
