 Studies of free-ranging African Basenjis and New Guinea Singing Dogs indicate that their behavioral and ecological traits were the result of environmental selection pressures or selective breeding choices and not the result of artificial selection imposed by humans.Dog pups show unrestrained fighting with their siblings from 2 weeks of age, with injury avoided only due to their undeveloped jaw muscles.This fighting gives way to play-chasing with the development of running skills at 4–5 weeks.