Social housing of rats: life-span effects on reaction time, exploration, weight, and longevity.

Sprague-Dawley rats were housed throughout their lives either in group cages or singly in standard laboratory cages. Locomotor activity was observed at selected ages and, in addition, operant conditioning procedures were used with an extensively trained subset of the animals to study reaction time and fixed ratio responding. The major effect of prolonged isolation was to induce timidity and inactivity during open field and emergence tests. Responding by isolates was depressed relative to socially-housed animals at 13 and 19 months of age although the higher levels of the social animals progressively declined during tests at 19 and 25 months. By comparison, neither housing condition nor age (7 to 25 months) systematically influenced the well-practiced operant responses (response rates, postreinforcement pauses, reaction times, foreperiod responses). A serendipitous finding was that isolated animals tended to die at earlier ages, an outcome that may be related to the fact that isolates also tended to weigh more throughout the experiment.