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Electronic Supplementary Material
ESM 1: Experiment 1
1. a. Pre-experimental Period: Training Methods
Prior to beginning testing, subjects were trained in the exchange paradigm, by shaping the exchange action in several steps. Step one involved learning to exchange a non-food item for a food item by dropping it into a plastic container. The container was placed directly under the subjects’ head as the non-food item was given to them, a food reward was displayed on offer to them, and the reward was given as soon as the object fell into the container. The container was then positioned between the dog and the experimenter, and the dog received a reward only when the non-food item was dropped into the container. This allowed the subjects to learn how to take an item in their mouth, manipulate it and drop it into the container in order to exchange. A plastic container was used to avoid any past associations the subjects may have with human gestures (such as begging or compliance to a hand signal). A wooden barrier (an extendable wooden baby gate) was then introduced; this was placed in an indoor doorway area to create a barrier between the subject and experimenter. Subjects adjusted to exchanging the object through (or over, for Jet and Faye, due to larger head size restricting movement) the barrier into the container to receive the food item.
The subjects then progressed to exchanging a non-attractive edible item (e.g. a piece of carrot or dry pasta), through the barrier into a plastic container, to receive an attractive food item (not used in any of the experiments). At this stage, the dog was verbally encouraged to exchange and the exchange behaviour was reinforced by receiving a reward in return. All subjects were enthusiastic during training sessions, and exchanges were generally quickly performed. Each training session consisted of 10 trials / exchange attempts; training sessions were short in duration (approx. 5 min), one session per day, and occurred between 2-5 times per week.
The experimenter then checked that for 3 consecutive trials, the dog could exchange the food item which was lowest on each subject’s food preferences in return for a highly attractive food (e.g. pieces of chicken or cheese). Then a training session (of 10 trials) was conducted under test conditions, to assess the dogs’ ability to complete the task with no verbal or any behavioural support from the experimenter. Dogs had to exchange a piece of carrot in return for an attractive food which was not used otherwise in the experiment. The criterion was set at 80% success exchanging; all subjects met this criterion in the first session (minimum 90%). This process was then repeated with the item of lowest preference. When subjects were performing at 80% success exchanging with the least favoured item for a high-value food, they proceeded to testing. Subjects required between 11 and 20 (mean = 15) training sessions in total to reach these criteria.
ESM 2: Experiment 2
Pre-experimental Period: Additional Training Methods
The dogs were initially introduced to a different container, and shown that when the lid was on the container, it was not possible to make an exchange; exchanges were only possible when the lid was removed. If the dog ate, broke or returned the item too soon, the initial item was given back to the dog, the second item discarded, and the trial ended. As during the initial training period, unattractive food items were used in training, and the subjects’ behaviours were shaped, from trying to exchange immediately (early stages) until they watched for lid movement and were able to maintain a 2 s delay with an unattractive food item (e.g. carrot, and progressing to plain biscuit) for a highly attractive food item (e.g. sausage or cheese) in return. Training sessions consisted of 10 trials / attempts and were approximately 5 min in duration, with one training session conducted per day.
During training, verbal encouragement was given to subjects and second chances were given if an attempt was made to return the item prematurely; however, once subjects began successfully delaying returns, the testing procedure was gradually introduced (experimenter made no response, other than to discard the reward item and record the dog’s response on a data sheet). When subjects maintained a 2 s delay and exchanged in 80% of trials in a training session, the subject moved into the preliminary testing phase. Subjects required between 6 and 16 training sessions (mean = 9.5) to reach this criterion.
For preliminary testing, each subject was tested using the food item to be used in the experiment at a 1 s delay, following testing procedure (for 2x the size in return, one 10-trial session); when subjects performed at the criterion of 50% successful exchanges, they were moved into the experimental phase. Two subjects met this criterion in the first testing session (Ellie and Bonnie); the other two subjects (Bailey and Jet) were each returned to the training phase for 5 additional training sessions before meeting this criterion. Subjects required between 6 and 21 training sessions in total to complete the pre-experimental training (mean = 12).
ESM 3: Video Clips of Exchange Tasks
Video 1= Bailey successive exchange task for items increasing in quality
Video 2 = Jet delayed exchange: 20 s delay
Video 3 = Bonnie delayed exchange: 80 s delay (video plays at double speed during delay interval)
Video 4 = Ellie delayed exchange: 320 s delay (video plays at double speed during delay interval)