Irradiated by LabRat
It helps to know that gibbons* are extraordinarily strong for their size (that makes sense when you contemplate how they get around), like most primates prone to go for the face, eyes, hands, and testicles when attacking, and as we see here, bright enough to be bloody-minded.
*Because I am an insufferable pedant: It’s an Agile Gibbon, not a monkey. They’re apes. They don’t have alpha males- gibbon groups don’t have individual leadership. They are, however, VERY territorial, and it is the males that do the actual tearing and biting for the most part when groups meet.