Olfactory sensitivity for aliphatic alcohols in squirrel monkeys and pigtail macaques

SUMMARY The view that primates are microsmatic animals is based mainly on an interpretation of neuroanatomical features, whereas physiological evidence of a poorly developed sense of smell in this order of mammals is largely lacking. Using a conditioning paradigm, we therefore assessed the olfactory sensitivity of three squirrel monkeys (Saimiri sciureus) and of four pigtail macaques (Macaca nemestrina) for a homologous series of aliphatic alcohols (ethanol to 1-octanol) and isomeric forms of some of these substances. In the majority of cases, the animals of both species significantly discriminated concentrations below 1 part per million from the odourless solvent, and with 1-hexanol individual monkeys even demonstrated thresholds below 10 parts per billion. The results showed (i) that both primate species have a well-developed olfactory sensitivity for aliphatic alcohols, which for the majority of substances matches or even is better than that of species such as the rat, (ii) that both species generally show very similar olfactory detection thresholds for aliphatic alcohols, and (iii) that a significant negative correlation between perceptibility in terms of olfactory detection threshold and carbon chain length of both the aliphatic 1-and 2-alcohols exists in both species. These findings support the idea that across-species comparisons of neuroanatomical features are a poor predictor of olfactory performance and that general labels such as `microsmat' or`macrosmat', which are usually based on allometric comparisons of olfactory brain structures, are inadequate to describe the olfactory capabilities of a species. Further, our findings suggest that olfaction may play an important and hitherto underestimated role in the regulation of behaviour in the species tested.