Olfactory sensitivity for aliphatic esters in squirrel monkeys and pigtail macaques

Using a conditioning paradigm, the olfactory sensitivity of four squirrel monkeys and three pigtail macaques for a homologous series of aliphatic esters (ethyl acetate to n-octyl acetate) and isomeric forms of some of these substances were investigated. With only few exceptions, all animals of both species significantly discriminated concentrations below 1 ppm from the odorless solvent, and in several cases, individual monkeys even demonstrated thresholds below 1 ppb. The results showed (a) both primate species to have an unexpectedly high olfactory sensitivity for aliphatic esters, which for the majority of substances matches or even is better than that of species such as the rat, the mouse, or the dog; (b) squirrel monkeys to generally perform better than pigtail macaques in detecting aliphatic esters; and (c) a significant negative correlation between perceptibility in terms of olfactory detection thresholds and carbon chain length of the n-aliphatic esters in both species tested. These findings support the assumptions that between-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 a species' olfactory capabilities.