The effects of brief daily separation, also known as "handling," during the first 2 weeks of life on play behavior and fearfulness toward a predatory odor were assessed in juvenile rats. Handled rats were more playful than nonhandled control rats, and while handling had no effect on the direct response of these rats toward a predatory odor, handled rats did not exhibit a conditioned suppression of play when tested later in the same context where they had been exposed to the predatory odor. Handled rats were still wary of the environment in that they continued to show a heightened level of risk assessment behavior. These data suggest that early postnatal experiences may play a significant role in determining how an animal deals with predatory threats later in life.
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Siviy, Stephen M. "Effects of Neonatal Handling on Play Behavior and Fear Towards a Predator Odor in Juvenile Rats." Journal of Comparative Psychology. 2008 Feb;122(1):1-8. http://dx.doi.org/10.1037/0735-7036.122.1.1