Back breeding is an approach to livestock breeding and part of a family of de-extinction practices. In back-breeding programs, specimens, for example of certain domestic cattle breeds, are selected for breeding based on traits they are thought to share with an extinct ancestor species. This approach differs from conventional livestock breeding in that it tends to use other breeds and selects for a wider range of traits other than, say, meat or dairy yield. Sometimes, back-breeding projects deliberately avoid traits (such as aggressiveness) even when these are thought to have been present in the extinct species. Depending on when the species went extinct, it can be difficult to trace back its characteristics. Back breeding initiatives tend to rely on a combination of excavations and historical descriptions or depictions (cave drawings or paintings) of the species concerned. Sometimes these projects are based on idealised understandings of the species and do not consider the variety of traits that may have been present among different individuals.