Quantitative measurement of motor activity during the immobility reflex elicited by clamping in developing rats.

The immobility reflex can be elicited in developing rats by clamping the neck. Quantitative measurement of motor activity during such clamp-induced immobility was made by placing the rats in a jiggle cage. These measurements showed that there are two stages in the ontogenic development of this reflex; from day 5 to day 25 the inhibition of motor activity increased with age; from day 30 to day 90 the inhibition of motor activity was substantially reduced. It is postulated that in the first stage immobility could facilitate carrying by the mother whereas the second stage, when immobility is not as profound, could be related to escape from predators. Older rats must be ready to run if other strategies such as freezing and camouflage fail to avoid an attack from the predator.