A two-choice discrimination method to assess olfactory performance in pigtailed macaques, Macaca nemestrina Fabienne Hu Èbener*, Matthias Laska Institut fu Èr Medizinische Psychologie, Ludwig-Maximilians-Universita Èt, Goethestraûe 31, D-80336 Munich, Germany Received 19 April 2000; received in revised form 16 October 2000; accepted 3 November 2000 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. D 2001 Elsevier Science Inc. All rights reserved. Keywords: Pigtailed macaques; Primate olfaction; Olfactory threshold; Behavioral testing 1. 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 conspic- uous scent-marking behavior [11,42,50,55]. They are able to gain information about species, gender, individuality, social status, and reproductive state contained in conspe- cific body odors [8 ± 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±35,37±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-mark- ing [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 discoura- ging 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 maca- ques, but they did not observe different responses towards urine samples taken from other macaque species compared to neutral stimuli. The few studies, which applied instru- * Corresponding author. Tel.: +49895996-650; fax: +49895996-615. E-mail address: fabienn@imp.med.uni-muenchen.de (F. Hu Èbener). Physiology & Behavior 72 (2001) 511 ± 519 0031-9384/01/$ ± see front matter D 2001 Elsevier Science Inc. All rights reserved. PII:S0031-9384(00)00447-9