Dogs that growl and bite people might not be naturally hostile, but are merely depressed, says a new research.

In the study, researchers took blood sample 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 behaviour appeared to be an attempt at self-defence.
The aggressive pets also had significantly higher levels of a second blood chemical, the "stress hormone" cortisol - 21 units compared to 10.
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But the findings could open the door to new ways of diagnosing and treating aggression.
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"The ability to identify dogs with lower levels of serotonin may help in identifying those dogs who could benefit from the use of pharmacology," the Telegraph quoted Mark Johnston, spokesman for the British Small Animals Veterinary Association, as saying.
The study is published in the journal Applied Animal Behaviour Science.
Source-ANI