In this study, I describe the increase in territorial behaviour of siamangs kept in two adjacent cages
after the formation of a new pair. The formation of a new pair resulted in a significant increase of a
form of inter-group agonistic behaviour (“arm-pulling”), which occurred exclusively among animals of
the same sex. The increase in conflicts cannot be fully explained by the simple increase in the number
of these territorial animals, but appears to be directly influenced by the formation of a new pair.