Olfaction in primates can be considered as a neglected sense due to the limited research done in that field compared to the large scale research performed in the sense of vision. Conclusions from most research conducted before the 21th century led to the nowadays outdated interpretation that primates have a poor sense of smell. This statement is mostly based on the finding that the relative size of olfactory brain structures has decreased during primate evolution. In addition, a high fraction of olfactory receptor genes (OR genes) are believed to have lost function during primate evolution and are described as olfactory receptor pseudogenes (OR pseudogenes). In New World monkeys, 18.4% (± 5.6%) of OR genes are OR pseudogenes. A larger fraction has been found in Old world monkeys (29.3% ± 2.4%) and an even larger proportion of OR pseudogenes is found in the great apes (33.0% ± 0.8%). In humans, 51.8% of OR genes are pseudogenes. One hypothesis believed to partly explain the increased fraction of OR pseudogenes in primates are the possession of full trichromatic colour vision. Rouquier et al (2000) drew the conclusion that the increased proportion of OR pseudogenes during primate evolution would explain a reduced sense of smell. Nevertheless, Cometto-Muñiz and Abraham (2009) reported an identified OR pseudogene in the human genome being activated by a mixture of odorants. Thus, we cannot exclude involvement of the OR pseudogenes in the primates’ ability to detect and discriminate odours. It is well-established that olfactory discrimination, the ability to tell an odour from another, is of relevance for the primates’ foraging behaviour. For instance, spider monkeys are known to use olfaction to inspect food when vision alone is not sufficient to determine the edibility. Further, studies confirmed that primates use olfactory discrimination in natural behaviours other than foraging such as predator recognition and assessing the reproductive state of a female.

Genetic and anatomical studies suggesting primates to have a poor sense of smell are contradicted by behavioural studies reporting primates to have a keen sense of smell for a variety of odorants. Among the countless possible odorants to test most studies so far tested odorants relevant to the primates’ natural behaviour. Subsequent studies demonstrated that spider monkeys, for example, possess a highly developed olfactory sensitivity for structurally related monomolecular substances such as aliphatic esters, alcohols and aldehydes, carboxylic acids, monoterpenes, steroids, thiazoles, thiols and indols, alkylpyrazines, aromatic aldehydes, green odours as well as proteinogenic amino acids.

In the present study, the spider monkeys’ olfactory sensitivity for another group of structurally related substances, aliphatic ketones, was investigated. Aliphatic ketones are monomolecular substances consisting of a carbon chain and a keto-group. In nature, aliphatic ketones are present in human urine and body odour. Aliphatic ketones are also present in a variety of fruits thus indicating a possible behavioural relevance for primates, especially frugivores such as the spider monkey. A previous study found a well-developed olfactory sensitivity for aliphatic ketones in two species of nonhuman primates, the squirrel monkey and the pigtail macaque. In the present study eight aliphatic ketones were used, both homologous ketones (2-butanone to 2-nonanone) and C7-ketones (2-heptanone to 4-heptanone).

      The aims of the present study were three-fold:

Firstly, to determine the olfactory sensitivity in spider monkeys for the eight aliphatic ketones used in this study;

Secondly, to investigate the relevance of carbon chain length and placement of the functional group on the spider monkeys’ olfactory sensitivity; and

Thirdly, to perform across-species comparisons with data from previous studies in order to assess whether the size of olfactory anatomical structures or the number of functional olfactory receptor genes correlate with olfactory sensitivity.