This study represents the beginning of the exploration of addictive-like behaviour in domestic dogs. Convergent behavioural measures support the existence of an addictive-like behavioural phenotype in 33 of the 105 tested highly play-motivated dogs. Note that we specifically sought dogs exhibiting extreme behaviour; thus, this proportion is not a reflection of the general population. Perhaps not surprising, working breeds – many of which are known to have been artificially selected for high toy or predatory motivation74,75,76 – were overrepresented in the sample.
As predicted, dogs classified as high-AB dogs based on the detailed AB-T score (Addictive-like Behaviour Test score) scored significantly higher than low-AB dogs on the individual criteria craving, salience, and lack of self-control in the behaviour test. Contrary to the prediction, mood modification (when given access to a toy) did not differ between high and low-AB dogs. In retrospect, however, this lack of difference between the two groups strengthens our argument that we were measuring a phenotype beyond mere enjoyment of play. Still, despite the significant differences between high- and low-AB dogs in the other investigated addiction criteria, Salience, Craving and Loss of Self Control, there was generally high variation between individuals.
In line with the predictions, high-AB dogs showed higher durations of focusing on and trying to access an inaccessible toy than low-AB dogs, often prioritising attempting to access the toy over eating or interacting with the owner. Thus, there was general agreement between the three alternative methods of coding the data (detailed behaviour score, addiction criteria, and quantitative coding), indicating internal consistency.
The external validity of the behaviour test was demonstrated by significant associations of the AB-T score with 18 out of 19 questions from the addictive-like behaviour questionnaire filled in by the dogs’ owners, intended to measure addictive-like behaviour in everyday life. Nonetheless, although significant, the effect sizes were relatively low, indicating that no single question would have predictive value for assessing a tendency for addictive-like behaviour in dogs.
In studies using animal models of substance addiction, one way to differentiate an addiction from drug use that occurs due to lack of choice is to present the subject with a choice between the addictive substance and other highly desirable stimuli. If an individual continues to take the drug at the expense of these other options (such as consumption of a food reward), this points to the possibility of addictive-like behaviour77,78. Consistent with this, high-AB dogs showed a loss of interest in other relevant stimuli, focusing on the inaccessible toy and foregoing the opportunity to consume food or to engage with their owner. The latter is also reminiscent of behavioural addictions in humans, leading to a decline in social interactions79.
The intense toy-seeking and loss of interest in other stimuli, despite the availability of food or social interaction – considered as indicators for salience and persistence – might resemble “hyperfocus,” a trait associated with ADHD and autism in humans80,81. However, unlike typical hyperfocus, which often emerges in the absence of competing stimuli, dogs in our study were presented with alternative salient rewards (e.g., the toy was placed on a shelf while the owner actively invited the dog to engage in social play; in another subtest, food was available in a puzzle toy while the preferred toy was inaccessible in a closed container), and they still showed a preference for the inaccessible toy. Like dogs with ADHD, dogs in the current study with high AB-T scores in general exhibited high impulsivity (labelled as “loss of self-control”), and some individuals displayed heightened activity (which could be interpreted as the hyperactivity component of ADHD64,65 in particular during the cool-down period. Thus, further research is needed to explore commonalities and differences between addictive-like behaviour and ADHD-like behaviour in dogs. While dogs with a high tendency for addictive-like behaviour might exhibit many characteristics of dogs with ADHD, the converse is not necessarily true – dogs might show ADHD-like behaviour without displaying any hyperfixation on toys.
Another characteristic of addicted individuals is that they are willing to pursue their addiction even if it has adverse consequences82. In the current study, “adversity” was elicited by the owner and the experimenter leaving the room in order to assess the effect of social isolation on the behavioural addiction criteria. Isolation in an unfamiliar place is well-established as a stressful experience for dogs83,84,85,86. However, this subtest was not a good measure of addictive-like behaviour: Time spent interacting with the toy while the dog was alone did not differ significantly between high-AB and low-AB dogs. For welfare reasons, we decided against exposing the dogs to more severe stressors; however, it cannot be ruled out that this subtest was not “aversive” enough. The dog was left alone for only 30 seconds, and the subtest took place in the middle of the test when the dogs were already habituated to the test room. It is also possible that individual differences in subjects’ separation distress, independent of play motivation, affected the results. Additionally, there was no clear contingency between interacting with the toy and the ‘adverse’ outcome (owner leaving). Future studies could potentially enhance the design by providing the dog with an explicit choice, such as by placing the toy in a separate room, away from the owner and the experimenter. This could help determine whether the dog is willing to risk being alone in an unusual or new environment when it normally prefers the safety of being near its owner. Such a design would better reflect the conflict between competing motivations (social security vs. reward seeking) and could offer a more valid test of the criterion of persistence under adversity.
Still, the importance of continued efforts to engage in the behaviour despite adverse consequences was demonstrated in the questionnaire, where one of the highest associations with the AB-T score was found with the question, “My dog will continue to play with a ball/toy despite adverse consequences”. This suggests that some dogs may fulfil the criterion of continuing the addictive-like behaviour despite adverse consequences in real life, even if this could not be demonstrated in the behaviour test.
A critical factor in addiction is the propensity to attribute incentive salience to classically conditioned cues predicting rewards87,88. In humans, cues associated with addictive behaviours, such as specific locations or objects, can induce craving and drug administration88,89. In dogs, a toy such as a ball could represent such as a conditioned cue. It may achieve its value, for example, by the experience of chasing and catching. For many domestic dogs, balls or other toys possess incentive salience, according to the three criteria by Robinson and Berridge49: they (1) “elicit approach” (i.e. they become “wanted” and act as “motivational magnets”); (2) “they can energise ongoing actions by eliciting cue-triggered wanting”; (3) “they can act as reinforcers in their own right, reinforcing the acquisition of a new instrumental response (measurable by conditioned reinforcement)” (cf49, p. 3139].).
The perceived value of the toy was demonstrated in our study by many dogs having difficulty relinquishing the toy. It can be speculated that balls become ‘motivational magnets’ by being associated with species-typical predatory behaviour (cf48. The high salience of the toy was especially apparent in subtests where dogs were foregoing available alternatives such as freely available food or social play with the owner, at the expense of trying to regain their inaccessible toy.
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