In the present study, the spider monkeys needed between 530 and 1102 trials to reach the learning criterion in the food-rewarded two-choice olfactory discrimination paradigm. An earlier study reported that spider monkeys tested with the same method needed between 330 and 360 trials to reach the learning criterion of 24 correct decisions in 30 trials. The numbers obtained in the present study are thus about 1.6-3.2 times higher than the numbers reported in the previous study. Possible reasons for this difference in learning speed include the previous experience of the experimenter, the odor stimuli used (different odorants were used in the previous and in the present study), a slightly different experimental set-up and the set of individuals that took part in the experiments (that is, the animals' personality; the "least shy" individuals proceeded faster than the more reserved individuals).

Pigtail macaques, the other primate species tested in a very similar food-rewarded two-choice olfactory discrimination paradigm as used in the present study, needed 480-900 trials to reach the learning criterion. Pigtail macaques tested in a multiple-choice olfactory discrimination paradigm reached the learning criterion after 140-280 trials, and squirrel monkeys tested with a very similar multiple-choice method reached the learning criterion between 75 and 125 trials. (Please note that the experimental set-ups of a two-choice versus multiple choice discrimination paradigms are different, and the results are thus not directly comparable. In the case of squirrel monkeys, for example, 75-125 trials equal about 450-750 stimulus contacts.)

To compare with non-primate mammals tested on a similar two-choice discrimination paradigm as used with the spider monkeys in the present study, Asian elephants reached the learning criterion after only 60 trials and South African fur seals needed 240-440 trials to reach the learning criterion. Other species tested on somehow comparable odor discrimination tasks (i.e. in tasks that use operant conditioning) include mice, rats and dogs. These species needed less than 200 trials, 40-320 trials, and 60-120 trials, respectively, to reach at least 80 % correct decisions. Even though only rough between-species comparisons of acquiring the initial olfactory discrimination can be done as the experimental set-ups as well as the learning criteria used vary between studies, primates seem to learn odor cues on average slower than the other mammalian species tested.

In the present study, the number of trials needed to reach the learning criterion during the intramodal transfer tasks varied between 30 and 510 trials while the spider monkeys tested in the previous study reached the learning criterion in intramodal transfer tasks straight during the first 30 trials. These different results between the two studies, again (as demonstrated by the results of initial odor discrimination), suggest that the circumstances during the previous study and the current study differed, interindividual variation in olfactory learning speed is considerably large in spider monkeys and that the nature of odor stimuli used may affect the learning performance.

Pigtail macaques and squirrel monkeys needed between 20 and 100 trials, South African fur seals 40-120 trials, Asian elephants 60-300 trials and mice usually less than 200 trials to reach the learning criterion in intramodal transfer tasks.  As the number of trials that the spider monkeys needed to reach the learning criterion during the intramodal transfer tasks ranged from 30 to 510 trials, the results of the other species tested fall into this range. Therefore, the spider monkeys’ learning speed in intramodal transfer tasks is comparable to that of pigtail macaques, squirrel monkeys, South African fur seals, African elephants and mice. Primates might thus be less prepared to use olfactory cues in the initial solving of a problem compared to non-primate mammals, but the efficiency of their olfactory system in discriminating between odors is not necessarily inferior. In visual discrimination tasks primates outperform for example rats, suggesting that primates are more prepared to use visual, rather than olfactory cues.

At the group level, the spider monkeys in the present study showed a significant negative correlation between the number of trials needed to reach the learning criterion and the number of transfer tasks completed. Based on this finding spider monkeys are able to form an olfactory learning set, similar to visual and spatial learning sets that primates have been reported to be able to acquire. The finding that the spider monkeys learned to discriminate between several odor pairs and even acquired a learning set, questions the traditional view that primates would have a poor sense of smell.

It is wothwhile to mention that despite the acqusition of an olfactory learning set, the number of trials that the animals needed to reach the learning criterion did not drop all the time systematically as the animals completed more transfer tasks. Instead, some odor pairs seemed more difficult for the animals to learn than others. Possible reasons for the inconstant performance of the animals accross different transfer tasks include 1) the biological relevance of the odorants used for spider monkeys, 2) different odor perception between human subjects and spider monkeys (even though human subjects perceived all the odorants used qualitatively different from each other, this may not be true for spider monkeys) and 3) the previous experience of the spider monkeys (some animals may have encountered some of the odorants used in their earlier lives). It is, however, challenging to determine the reason for inconstant performance of the animals across tasks because each individual had different odor pairs that were difficult to learn, because of the low number of individuals that took part in the experiments and because there is little knowledge about the natural occurrence of the odorants used in the present study. Thus, it is difficult to determine which of the used odorants have behavioral relevance for spider monkeys.

The fact that all animals reached the learning criterion in all memory tasks straight during the first 30 trials (which is faster than they reached the criterion during the initial learning) suggests that their good performance was indeed due to memory rather than due to a quick relearning. Neto even scored 100% correct decisions in the 5-week memory test and thus did not make a single mistake. The spider monkeys’ performance in long-term odor memory tasks is comparable with that of other mammalian species such as Asian elephants, dogs, mice, pigtail macaques, South African fur seals and squirrel monkeys, who also remembered the reward value of previously learnt odors after several weeks or even months. Similar long-term olfactory memory capacities across different mammalian species suggest that long-term odor memory is an important cognitive ability not only for non-primate mammals, but also for primates.

The present study shows that spider monkeys (Ateles geoffroyi) can be trained on a food-rewarded two-choice olfactory discrimination paradigm and that their learning speed in acquiring the initial odor discrimination is comparable to that of other primate species tested with similar methods. Spider monkeys acquire the initial olfactory discrimination slower than non-primate mammals tested on comparable tasks, but their learning speed in intramodal transfer tasks, at least with certain odorants, is comparable to that of non-primate species tested. As a group, the spider monkeys even showed acquisition of an olfactory learning set, that is, the animals needed fewer and fewer trials in consecutive tasks to reach the learning criterion. This suggests that primates may be more prepared to use other than olfactory cues in the initial solving of a problem but that once they learn the concept, their learning speed with novel odor discrimination problems is not generally slower than that of non-primate mammals. The nature of the odor stimuli used seemed to affect the learning performance as with certain odorants a surprisingly high number of trials was needed to reach the learning criterion. Finally, the spider monkeys showed no signs of forgetting the reward values of previously learnt odors even after a 6-week retention interval. Thus, the long-term odor memory of spider monkeys is comparable to that of other mammals such as dog, mouse and rat.