Smell as a signal among non-human primates. (Answer in 150 words)

- Smell as a signal among non-human primates. (Answer in 150 words)
Introduction
Chemical signalling is the oldest primate communication channel, and its importance across the order tracks the reduction of the olfactory apparatus.

Body
1. Anatomy and behaviour: specialised glands — brachial and antebrachial in the ring-tailed lemur, sternal in marmosets, anogenital and suprapubic in many strepsirhines — with urine-washing, substrate rubbing and faecal deposition. Reception is by the olfactory epithelium and the vomeronasal organ, accessed through the flehmen grimace.
2. What the signals convey: territory and pathway marking, group, individual and matriline identity, dominance rank — the lemur's "stink fights" waft brachial secretion from the tail — and female reproductive state, which advertises ovulation in nocturnal species lacking visual swellings.
3. Distribution and trend: greatest in strepsirhines and nocturnal forms, retained in New World monkeys, much reduced in catarrhines, which lose a functional vomeronasal organ. Macrosmatic gives way to microsmatic as diurnality, trichromacy and visual signalling advance.

Conclusion
Odour remains a cheap, persistent, night-working channel. Its decline in apes reflects visual specialisation, not any failure of the medium.