Snarling dogs may merely be depressed

Dogs that growl and bite people may not be hostile by nature - merely depressed, according to new research. Scientists who studied badly-behaved pets concluded that they had low levels of the "feel good" hormone serotonin. In humans, low serotonin levels are known to be linked to depression and anxiety.

The researchers hope their findings will lead to new ways of treating canine aggression, which is the most frequent behavior problem seen by vets. In the study, reported in the journal Applied Animal Behavior Science, blood sample were taken from 80 dogs which had been referred to veterinary teaching hospitals after their owners reported that they had behaved aggressively towards humans. Samples from normally-behaved dogs were also taken. Tests showed that the aggressive dogs had significantly lower concentrations of serotonin in their blood – 278 units compared to 387.

The lowest readings of all were from a group of dogs whose bad behavior appeared to be an attempt at self-defense. The aggressive pets also had significantly higher levels of a second blood chemical, the "stress hormone" cortisol – 21 units compared to 10. Just what had caused the dogs to have different levels of the hormones in their blood is not known, but the findings could open the door to news ways of diagnosing and treating aggression.

The researchers, from Zaragoza university in Spain, said the results suggested that antidepressant drugs, which can raise serotonin levels in depressed humans, may be of help in treating aggressive dogs, in conjunction with other techniques to change their behavior. Mark Johnston, spokesman for the British Small Animals Veterinary Association, said, "The ability to identify dogs with lower levels of serotonin may help in identifying those dogs who could benefit from the use of pharmacology."