Relative life expectancy (i.e. the average life expectancy of a species expressed as a percentage of the maximum longevity ever reported for this species) may describe husbandry success in captive populations. By correlating the relative life expectancy with biological characteristics and husbandry factors for different species, reasons for variations in relative life expectancy can be detected. We analysed data for 166,901 ruminants of 78 species and demonstrated the presence of such a correlation between relative life expectancy and percentage grass in the species’ natural diet (not necessarily the diet fed in zoos). This suggests that species adapted to grass (so-called grazers, such as bison and wildebeest) can be managed more easily when compared to species that feed on leaves and twigs (so-called browsers, such as giraffe and moose). Another finding of our analysis is a true success story of zoo animal management: the relative life expectancy was higher in species that were managed by an international studbook than in species not managed this way. This highlights the positive effect of intensive studbook management on the overall husbandry success of the respective species. Translating these results into husbandry recommendations, our approach can help to improve zoo animal husbandry.