The vampire bats
The vampire bat (Desmodus rotundus) has become very specialised indeed. Its front teeth are modified into two triangular razors. It settles gently on a sleeping mammal, a cow or even a human being. Its saliva contains an anti coagulant, so that the blood, when it appears, will continue to ooze for some time before a clot forms. The vampire then squats beside the wound lapping the blood. They fly by sonar and it is said that the reason that dogs, whose hearing is also tuned to very high frequencies, are so seldom attacked by them is that they can hear the vampire bats coming.