Animals

The data were collected from four adult female spider monkeys (Ateles geoffroyi) (Nanny, Piolina, Kelly and Frida) all maintained at Universidad Veracruzana’s field station ”Pipiapan”, near Catemaco, Veracruz, Mexico. The animals were all kept in outdoor enclosures surrounded by their natural environment, tropical forest. All enclosures contained branches and ropes to stimulate some of the spider monkeys’ natural behaviours i.e. climbing and swinging between branches.  The age of the spider monkeys ranged from 8 to 14 years (Nanny and Piolina 14 years, Kelly 10 years and Frida 8 years).Three of the animals (Nanny, Piolina and Kelly) were familiar with the experimental procedure as they have participated in similar studies for several years. Frida started out as a training monkey but was able to participate in the study after some time of practicing allowing me to use her results with two of the odorants. Two of the monkeys (Nanny and Piolina) shared an enclosure. One monkey (Kelly) shared an enclosure with her two offspring and the fourth monkey (Frida) had an enclosure for herself. Nanny, Piolina and Kelly’s enclosures were connected to huts in which the testing took place. Test cages connected these enclosures to the huts. A test cage was also attached to one of the sides of Frida’s enclosure. The tests were performed in the mornings and afterward the spider monkeys were given their daily food ration, consisting of cultivated fruits and vegetables, at 12.00-13.00 pm. The experiments reported here comply with the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publication no. 86-23, revised 1985) and also with current Swedish and Mexican laws.

Odour stimuli

A set of eight odorants was used: 2-butanone (CAS# 78-93-3), 2-pentanone (CAS# 107-87-9), 2-hexanone (CAS# 591-78-6), 2-heptanone (CAS# 110-43-0), 2-octanone (CAS# 111-13-7), 2-nonanone (CAS# 821-55-6), 3-heptanone (CAS# 106-35-4), and 4-heptanone (CAS# 123-19-3). All substances were of the highest available purity (≥96 %) and were obtained from Sigma-Aldrich (St. Louis, MO, USA). A near-odourless solvent, diethyl phthalate (CAS# 84-66-2), was used for diluting the odorants. Gas phase concentrations for the headspace above the diluted odorants were calculated using published vapour pressure data and corresponding formulae. Fig. 1 shows the molecular structure of the odorants.

Method

A food-rewarded two-choice instrumental conditioning paradigm was used for testing the spider monkeys. The test apparatus consisted of a 50 cm long and 6 cm wide metal bar with two cube-shaped boxes with a side length of 5.5 cm attached to it at a distance of 22 cm from each other. Each container was equipped with a tightly closing hinged metallic lid, hanging 2 cm down the front of the container. A metal clip was attached on top of each lid holding an absorbent paper strip. One end of the paper strip was impregnated with 20 μl of an odorant used as a rewarded stimulus (S+) or 20 μl of the near-odourless solvent used as an unrewarded stimulus (S-). The spider monkeys voluntarily entered the test cage connected to their enclosures and they were free to return to the enclosure at any time during the test. The front side of the test cage consisted of a stainless steel mesh with a width of 1 cm and had two openings of 5×5 cm allowing the animal to reach through the mesh to retrieve the food reward. When the test apparatus was presented to the animals the impregnated paper strips reached approximately 3 cm into the cage allowing the animals to thoroughly examine them by only using their sense of smell. The box with the rewarded stimulus (S+) contained a Kellogg's Honey Loop® as a food reward. The box with the unrewarded stimulus (S-) was empty. When presented with both stimuli the spider monkeys were allowed to sniff both options as much as they liked until clearly indicating which box they wanted to be opened. When a clear indication was made the box was opened by the experimenter. The spider monkeys were not allowed to change their minds after indicating at one of the boxes nor indicating at both boxes or indicating before sniffing both options. The apparatus was removed immediately after each decision and was prepared for the next trial out of sight from the spider monkeys.

Each spider monkey performed three sessions, 10 trials in each, per day and dilution step. In each session the right and the left box was baited equally often in a pseudo-randomized order but no box was baited more often than three times in a row. Testing started at a 100-fold dilution of a given odorant. This starting dilution was tested on two subsequent days, resulting in six sessions and 60 trials per animal, in order to get the spider monkeys familiarized with the new odorant. Every time the spider monkeys performed above chance level when combining the results from the three sessions performed per dilution step (except from the six sessions for the starting dilution), they were presented with a 10-fold increased dilution (i.e. lower concentration). This continued until they failed to discriminate the odorant from the solvent above chance level. Consequently, after failing to detect an odorant the spider monkeys were presented with an intermediate dilution step in order to determine a more precise threshold value. Testing was performed in the mornings before the spider monkeys received their daily food ration.