How Vampire Bats Evolved To Live On Blood Alone
New research has discovered some of the genetic changes that allowed vampire bats to live on a diet of pure blood. One of the bats' most important evolutionary traits is the ability to manipulate an anticoagulant protein in their blood and saliva. In humans similar proteins protect against heart attack by breaking up blood clots and clearing vessels.
Really, this has been extensivly documented. Might I suggest starting with the graphic novel Batman: Year One
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