Are there differences in impulsivity between sexes in domestic dogs?

Impulsivity is a complex personality trait, which, broadly, relates to a subject tendency to perform choices or actions driven by strong drives and with a lack of proper inhibitory control. Different facets of impulsivity have been shown to differ between sexes in humans and in other animal species [1,2]. However, sex differences in impulsivity have not been described in domestic dogs. The aim of the present study was to address this aspect. Thirty-two pet dogs were enrolled for this experiment (16 males and 16 females) and underwent two assessment procedures: 1) The proximity task, a newly devised procedure for the assessment of impulsive choices, in which dogs are initially trained to learn on which side (right/left) they can find the largest of two amounts of food, which are presented in bowls placed at the same distance (approx. 2.5 m) from the dog. Dogs choices for either of the two bowls are then assessed in a test phase, where the smaller food amount is placed systematically closer to the dog. 2) The cylinder task, a validated procedure that assesses impulsive actions. In this procedure, dogs are first trained to approach the side of an opaque container to get a piece of food. In the following test phase, the front of the container is transparent, and dog’s attempts to get the visible food from the front part of the container (failed trials), across 15 consecutive presentations, are indicative of the inability to inhibit an impulsive response upon presentation of an appetitive stimulus. In the proximity task, dogs’ choices were significantly affected by the distance of the smaller amount of food (P<0.001, Friedman). Dogs chose the smaller amount on only 20% of presentations (median) when the two bowls were at the same distance; choices for the bowl with less food proportionally increased as the latter was placed closer to the dog, reaching 100% (median) when the bowl was at 80 cm from the dog. These results conform to the idea that the dogs’ performance in the test is indicative of dogs’ inability to resist to an immediate, but smaller, gratification (impulsive choice) and providing a first indication for the validity of the procedure. In the cylinder task, dogs attempted to reach the food from the front of the box on 4 of 15 trials (median; min=1, max=10). There was no correlation between the number of choices for the smaller amount of food (averaged across all distances) in the proximity task and the number failed trials in the cylinder task (Spearman’s rho = 0.72, P=0.64), suggesting that the two procedures measure different and unrelated facets of impulsivity. There were no differences between sexes in either the number of choices for the smaller food amount in the proximity task (P=0.76, Mann-Whitney), or in the number of failed trials in the cylinder task (P=0.086), thereby not supporting the existence of sex-related differences in these measures of impulsivity in dogs. However, the large variability and the small p-values found in the comparison of failed trials in the cylinder task warrant the possibility that differences between sexes in impulsive actions may arise if a larger sample was to be tested.