• Lower levels of serotonin have been associated with aggression and antisocial behavior in humans.
• The mean serum concentrations of serotonin were significantly lower in the aggressive dogs in this study.
• No significant difference was seen in lipid levels between the aggressive and nonaggressive dogs in this study.

The most common behavioral complaint of dog-owning clients is aggression, specifically "dominance aggression." Studies in humans and other species have suggested that aggression may be influenced by levels of serotonin and lipids. In this study, serum concentrations of serotonin and lipids (total cholesterol, triglycerides, as well as high- and low-density lipids and very-low-density lipids) were analyzed in 18 normal dogs and 23 aggressive dogs. Aggression was determined by the history and independent assessment of the dogs using a canine overt aggression chart that listed 30 everyday situations involving the dog and owner or the dog and other dogs. Reactions to each situation were scored: growl: 1; lip lift: 2; snap: 3; and bite: 4. Dogs were considered aggressive if they scored more than 5 points. Dogs scoring 0 overall were considered normal. Mean serum concentrations of serotonin were significantly lower in the aggressive dogs (12.0 ng/ml) than in the normal dogs (32.5 ng/ml), but the differences in serum lipids between the 2 groups were not statistically significant. The low serum serotonin concentrations found in this study were similar to those of another study in dogs that measured serotonin and used selective serotonin reuptake inhibitors to treat the dogs.11.

Use of fluoxetine to treat dominance aggression in dogs. Dodman N, Donnelly R, Shuster L, et al. JAVMA 209:1585-1587, 1996.

Relationship between the serum concentrations of serotonin and lipids and aggression in dogs. Cakirogula D, Meral Y, Sancak A, Cifti C. Vet Rec 161:59-61, 2007.