ABSTRACT: The European bison Bison bonasus is the largest extant terrestrial mammal on the European continent; however, the species went extinct in the wild in 1919. Restoration started in 1929 in Poland’s Białowiez˙a Primeval Forest using captive individuals sourced from zoological gardens and breeding centres. Of the 7 founders, 2 individuals contributed 85% to the genetic make-up of the lowland line of the species. The Białowiez˙a bison population numbered 820 in 2008, but very low genetic diversity and a high level of management have raised questions as to whether it still conforms to evolutionary predictions. We tested whether the sex ratio of European bison calves conformed to the Trivers-Willard hypothesis at the population level, i.e. whether it became increasingly female-biased as bison condition deteriorated following increased population density. We found that increased population density and reduced female body mass led to increasing female-biased calf sex ratios, whereas mast years (abundant food resources) corresponded to male-biased sex ratios. Despite the high degree of inbreeding and management, European bison are still responding as expected to variations in female body condition; however, the precautionary principle cautions managers of small populations that artificial selection can alter the evolutionary strategy of wildlife even though we did not detect this in theBiałowiez˙a bison population.