Even when cloned animals are born apparently healthy there are doubts about how quickly they will age.

Results from a number of studies on the lifespan of clones are beginning to build up. Much of the work so far has been done on mice because they have relatively short life spans which makes studying them easier – you get results faster – than using cows which may live for twenty years!

In a controlled study almost all of a batch of cloned mice produced by the National Institute of Infectious Diseases in Tokyo died earlier than their naturally bred cousins. The mice all appeared the same when they were young, so perhaps some effects of cloning aren't apparent in early life.

Twelve male clones were compared with control groups of 7 naturally conceived males and 6 "test-tube" conceived mice. Just over 2 years later, 10 of the cloned mice had died compared to 3 of the non-cloned mice. They had died from a number of causes including pneumonia, liver disease/cancer and poor immune systems.

In an experiment with cloned mice at the University of Hawaii, 1 in 3 clones born looking normal became massively overweight within a few weeks. There is a gathering amount of evidence that many clones contain some aspects of their body systems which don’t work well in the long term.