Titi monkeys live in small family groups built around a bonded adult pair. Tail-twining is the visible sign of the bond: the pair sits pressed together with tails coiled around one another, and the posture is directed selectively at the partner rather than at any nearby animal. Pairs also duet at dawn, both partners contributing to a coordinated call that stakes out the territory.
Unusually among monkeys, males carry the infant most of the time, transferring it to the mother mainly to nurse. Infants placed with a choice show distress at separation from the father rather than the mother, which is a reversal of the usual primate pattern.
Experimental work has found elevated cortisol when a pair is separated and changes in brain activity when a partner is in view, so the bond has a physiological signature and not merely a behavioural one. Whether the mechanisms map closely onto human pair bonding, which is the reason for the research interest, remains an inference from a small number of species.