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.
In both rodents and humans90,91, individuals with a higher tendency to attribute incentive salience to classically conditioned cues predicting rewards (sign trackers) are more vulnerable to addiction than goal trackers, who focus primarily on the (location of the) reward itself88,92, see the meta-analysis by93. Tendency to sign-track vs. goal-track is associated with the risk of addiction and is also related to variations in the dopaminergic system and stress physiology88.
While it was not explicitly measured in the current study, in dogs, a tendency to sign track might be advantageous in a training context – i.e., maintaining motivation would be easier in dogs that are not only sensitive to rewards but also attribute value to the cues predicting these rewards, even if not always followed by a primary reinforcer. Sensitivity to reward – and propensity to attribute incentive salience to reward-predictive cues – would thus be highly relevant traits in relation to trainability and might be selected for especially in working dog breeds.
Several publications state the importance of ‘obsessive’ play motivation for working dog success42,94,95,96,97,98,99. Dogs with extreme toy motivation are believed to show better focus, reduced distractibility and greater trainability97,99. However, if such motivation becomes addictive-like, it needs to be questioned whether the well-being of these dogs may be compromised. If dogs prioritise toy interactions over other essential aspects of their daily lives this may have maladaptive effects, as is the case in humans with behavioural addictions100,101. Certainly, adverse health consequences may arise from repetitive ball chasing, like straining ligaments and joints56. Moreover, welfare would be compromised when dogs experience a high level of frustration when access to their reward is prevented (cf102.
Anecdotally, when play motivation becomes excessive, irritability, high arousal levels, and frustration may negatively affect dogs’ trainability and work103. Indeed, as stated by Mathews96, the high ‘drive’ of search dogs often makes them unsuitable as family pets, which is also supported by owner reports that pet dogs with extreme motivation for toys are often problematic to control102.
Thus, it needs to be questioned when play becomes maladaptive. Do high-AB dogs still ‘like’ to play, or have they progressed to primarily ‘wanting’ and the compulsive need to continue104?. Similar to the escalating engagement seen in human behavioural addictions105, some dogs would repeatedly spin, jump, focus or bark towards the unavailable toy on the shelf for the duration of the subtest. Two dogs even managed to destroy the box enclosing their favourite toy. These behaviours might be likened to the repetitive actions observed in individuals with behavioural addictions41. Nonetheless, such behaviours may also occur in other behavioural phenotypes such as canine compulsive disorder or autism spectrum-like behaviours106. Further research is needed to elucidate commonalities and differences between such phenotypes in dogs.
Behavioural addictions in humans often involve emotional dependency on specific activities107. Whether dogs similarly seek comfort, stimulation, or stress relief through persistent engagement with the toy could not be determined in the context of the behaviour test. In the questionnaire, “Is attached to the favourite toy” was the only question not significantly associated with the AB-T score. Thus, further research is required to determine whether dogs develop an emotional dependency on their toys (as described anecdotally).
To better understand the origin and possible functional underpinnings of excessive toy-directed behaviour in dogs, future research should examine whether similar patterns of excessive object play occur in non-domesticated canids. While data are limited, recent studies have shown that both hand-reared and wild wolf pups engage in object play108. For instance, wolf pups have been observed developing a preference for toys and spending increasing amounts of time with them over time108. Hand-reared wolf pups will even retrieve objects to humans109. In the wild, wolves have also been seen interacting with human-made objects110. These findings suggest that object play is not unique to dogs but rather could represent a broader trait shared by canids. Comparative studies are needed to assess how common and functionally relevant such behaviours are in wolves, which would help clarify the biological basis of the addictive-like behaviours observed in some dogs.
Being the first of its kind, this study has its limitations. As no gold standard exists, the study is exploratory, and our categorisation of dogs into high and low addictive-like behaviour groups, determined by a data range split, was somewhat arbitrary. Nonetheless, the assignment of high- and low-AB categories corresponded well to the first author’s personal assessment of addictive-like tendencies in the participant dogs.
In interpreting the questionnaire results, it is important to acknowledge the potential biases associated with using owner-reported questionnaires. Owners may unintentionally project their perceptions or expectations onto their dogs’ behaviours, potentially leading to discrepancies between reported and observed behaviours in behavioural test. This is particularly relevant in cases where owners have multiple dogs, as they are likely to compare their pets to one another, influencing their assessment, such as by underestimating or overestimating certain behaviours. For instance, an owner with a highly active dog may rate their less active dog as overly calm. Integrating owner reports and objective testing allows for a more comprehensive and accurate canine behaviour evaluation.