Novel approach to automatically classify rat social behavior using a video tracking system

•Current animal models, if involving social behavior at all, are limited to very short and simple measurements.•Social behavior is mostly manually annotated, despite available automated observation technology.•We developed a method that combines velocity of movement with inter-individual distance of rat pairs.•Our methods result in different behavioral classes that are naturally present in juvenile rats.•Our approach allows automated and objective measurement of social rat behavior. In the past, studies in behavioral neuroscience and drug development have relied on simple and quick readout parameters of animal behavior to assess treatment efficacy or to understand underlying brain mechanisms. The predominant use of classical behavioral tests has been repeatedly criticized during the last decades because of their poor reproducibility, poor translational value and the limited explanatory power in functional terms. We present a new method to monitor social behavior of rats using automated video tracking. The velocity of moving and the distance between two rats were plotted in frequency distributions. In addition, behavior was manually annotated and related to the automatically obtained parameters for a validated interpretation. Inter-individual distance in combination with velocity of movement provided specific behavioral classes, such as moving with high velocity when “in contact” or “in proximity”. Human observations showed that these classes coincide with following (chasing) behavior. In addition, when animals are “in contact”, but at low velocity, behaviors such as allogrooming and social investigation were observed. Also, low dose treatment with morphine and short isolation increased the time animals spent in contact or in proximity at high velocity. Current methods that involve the investigation of social rat behavior are mostly limited to short and relatively simple manual observations. A new and automated method for analyzing social behavior in a social interaction test is presented here and shows to be sensitive to drug treatment and housing conditions known to influence social behavior in rats.