How spontaneous is spontaneous quantity discrimination in companion dogs?

Although quantity discrimination and the factors affecting it have been widely studied in the domestic dog using a variety of paradigms, little attention has been given to the possible effects of the used paradigm itself. In the present study, we employed a paradigm in which naive companion dogs were repeatedly presented with a free choice between two quantities of food (2vs.4, more difficult comparison, and 1 vs.8, simpler comparison). Dogs did not undergo any previous training and could freely choose to feed from either plate. After the choice was made, the second plate was withdrawn without letting the dog to eat its content. We hypothesized that a preference for the larger quantity of food may emerge as a consequence of experiencing the experimental procedure, and may not be expressed by the dogs on their first choice (hypothesis 1). If so, rewards experienced in the first trial may affect behaviour on subsequent trials (hypothesis 2). Data were analysed using generalized linear mixed models. Both hypothesis 1 (general performance P < 0.0001, i.e., above chance level; first choice P = 0.06, i.e., chance level) and 2 (P = 0.001) were confirmed by the experimental results. The difficulty of the numerical comparison and the neuter status of the dogs had a significant effect on the overall performance, but not on the first choice nor on the likelihood of redirecting after the first trial. The results suggest that domestic dogs are highly sensitive to the results of their experience and adjust their behaviour accordingly, even after one single event. Future studies may help to disentangle the role of the food seen vs. eaten by the dog and the possible role of unintentional human feedback after the first choice.