 Prior to widespread knowledge of the differences between the populations (sometimes called races) from different islands and volcanoes, captive collections in zoos were indiscriminately mixed.Fertile offspring resulted from pairings of animals from different races.However, captive crosses between tortoises from different races have lower fertility and higher mortality than those between tortoises of the same race, and captives in mixed herds normally direct courtship only toward members of the same race.