It is caused a bizarre quirk of the RTW engine that means that after the casulaties have been healed as long as there is at least one soldier for every animal you don't lose any elephants, for example a unit of 6 elephants has 18 men in it (3 for each animal), if all the unit was killed on the battle feild but 6 soldiers were healed after the battle you would still have all the elephants (but no men in the towers on their backs). Its a completely conter-intuitive system having elephants being more replacable that humans but thats how it works.