A two-choice discrimination method to assess olfactory performance in pigtailed macaques, Macaca nemestrina

Abstract

Four pigtailed macaques were trained in a new two-choice olfactory discrimination method. They learned the initial task within 3 months, requiring a maximum of 900 trials. After the method was established, we investigated the olfactory threshold of three monkeys for the odors peanut, iso-amyl acetate, and n-pentanoic acid. The animals detected peanut odor in dilutions as low as 1:10 000. They were able to perceive iso-amyl acetate up to a 30 000-fold dilution (animals F1 and M2), respectively in a 30 Mio-fold dilution (animal M1). The sensitivity for n-pentanoic acid ranged between a dilution of 1:30 000 (F1), 1:100 000 (M2), and 1:300 000 (M1). A comparison with the thresholds of other species demonstrates that the olfactory sensitivity of pigtailed macaques is not necessarily inferior to that of species that are believed to have a very keen sense of smell, such as dogs and rats. The sensitivity for certain odors seems to reflect their biological relevance for the tested species. The fact that the threshold for peanut odor obtained in this study is lower than the one found in a previous study with pigtailed macaques using a multiple olfactory discrimination method indicates that the new two-choice discrimination method is a better candidate for the assessment of olfactory abilities in pigtailed macaques.

Introduction

It is now widely acknowledged that the sense of smell plays an important role in the regulation of a variety of behaviors in prosimians and New World monkeys. Most of the species that have been investigated so far possess specialized scent-producing skin glands and show conspicuous scent-marking behavior [11], [42], [50], [55]. They are able to gain information about species, gender, individuality, social status, and reproductive state contained in conspecific body odors [8], [9], [10], [27], [36], [62]. Basic psychophysical functions, such as olfactory discrimination ability and olfactory sensitivity, have been studied systematically in squirrel monkeys and demonstrated an unexpectedly well-developed sense of smell in this New World monkey species [25], [32], [33], [34], [35], [37], [38], [39]. For example, it could be shown that the olfactory sensitivity of squirrel monkeys for certain odors, such as amyl acetate and cineole, equals or even excels the olfactory sensitivity that has been found in rats or dogs, species that are believed to have a very keen sense of smell [37].

In Old World monkeys, the situation is much less clear. Because Old World monkeys, with a few exceptions, do not possess scent glands and do not show obvious scent-marking [14], [15], [21], [53], it is assumed that the sense of smell is of only minor importance for their behavior [13]. Furthermore, Old World monkeys, opposed to prosimians and New World monkeys, are thought to lack a vomeronasal organ and an accessory olfactory bulb, structures that are known to be frequently involved in processing social information through body odors [12], [29], [41].

Only a few studies have investigated olfactory abilities in Old World monkeys, and the results were either discouraging or contradictory. Whereas Michael and Keverne [43] reported that male rhesus monkeys were able to detect the reproductive state of female conspecifics by means of olfaction, Goldfoot et al. [16] failed to replicate these findings. Demaria and Roeder [5] studied the response to different urine samples in pigtailed and stumptailed macaques, but they did not observe different responses towards urine samples taken from other macaque species compared to neutral stimuli. The few studies, which applied instrumental conditioning paradigms to investigate olfactory abilities in macaques, were concerned with the effect of brain lesions and reported difficulties with the method [4], [49]. It took up to 10 months to train macaques in an olfactory discrimination task. Only one study reported a rapid acquisition of an olfactory discrimination within 4 days by pairing the negative stimulus with an aversive taste stimulus [52]. Smith [60] reported about preliminary investigations of olfactory discrimination and olfactory sensitivity in rhesus monkeys using an olfactory chamber placed over the monkeys' head, but to our knowledge, further results were never published.

These findings, respectively the lack of results, indicate that it is crucial to develop an adequate method to investigate olfactory abilities in Old World monkeys. In an attempt to develop such a method, we trained three pigtailed macaques in an olfactory discrimination task using an instrumental conditioning paradigm. The method is explained in detail elsewhere [23]. The method was first developed for squirrel monkeys, a small New World primate species, and then transferred to pigtailed macaques. It is based on a multiple olfactory discrimination task that simulates olfactory-guided foraging behavior. The method was established in less than 4 months and proved to be a useful instrument for the investigation of olfactory performance in pigtailed macaques. Nevertheless, it also had several shortcomings. For example, the macaques developed a tendency to store the manipulanda (Eppendorf reagent cups equipped with an odorized absorbent paper strip) in their cheek pouches. Further, the monkeys sometimes stopped sniffing at the odorized paper strips, and the method did not allow us to force them to do so. Both disadvantages tended to occur in challenging tasks. Thus, we wanted to optimize the method by further adapting it to the species-specific needs of the pigtailed macaques. Here, we present a new two-choice olfactory discrimination task that allowed us to better control the investigatory behavior of the macaques. To gain more knowledge about the basic olfactory abilities of pigtailed macaques, we tested their olfactory sensitivity for three odors. One of the odors had already been used in the previous method [23], allowing us to compare the two methods by the results of the threshold task.