most species of animals synthesise their own vitamin c. it is therefore not a vitamin for them. synthesis is achieved through a sequence of enzyme driven steps, which convert glucose to ascorbic acid. it is carried out either in the kidneys, in reptiles and birds, or the liver, in mammals and perching birds. the loss of an enzyme concerned with ascorbic acid synthesis has occurred quite frequently in evolution and has affected most fish, many birds; some bats, guinea pigs and most but not all primates, including humans. the mutations have not been lethal because ascorbic acid is so prevalent in the surrounding food sources.