 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 . 
