Induced immobility in nervous and normal Pointer dogs

Pointer dogs learn complex hunting tasks such as vigorous aim-directed activity in tracking specific birds and sudden rigid immobility "on point" at appropriate distance from the prey. In the presence of a human, dogs of the Arkansas Line of Nervous Pointers show markedly reduced activity. In close quarters, where they cannot flee, they usually react to humans by freezing. These responses do not extinguish. The breed demonstrates a strong capacity to inhibit motion. In our nervous dogs, this trait is greatly exaggerated. In the search for an objective and rapidly modifiable target motor sign to correlate with biochemical and pharmacological events, we discovered that 10 of 10 nervous dogs, in contrast to kennel mates of a normal line, developed tonic immobility (TI) which was induced by manual inversion into a sling and stroking. This is the first systematic demonstration of TI in mature dogs. The TI was accompanied by reduced telemetered heart rate (HR) compared to baseline. Release from the inverted position was accompanied by marked HR increase. In the absence of humans, both before and after the inversion, all nervous dogs showed little or no movement about the testing area. This was in contrast to the nine normal line dogs, which were comparable in age and sex. These normal dogs were very active before and after inversion into the sling. Five of the normals remained in the sling but did not show head and neck immobility and were generally relaxed. Their HRs increased slightly while in the inverted position, thereafter returning to baseline. The four normals which righted themselves soon after the inversion showed reduced HR with the resumed activity. We discuss possible mechanisms and review some of the implications for psychiatry and behavioral biology.