Cheal, M. L., Foley, K., Kastenbaum, R. 1986. Brief periods of environmental enrichment facilitates adolescent development of gerbils. Physiology and Behavior 36, 1047-1051.

The enrichment experience [large outdoor environment] for one hour a month [only!] .. facilitated adolescent development. Area of the ventral gland increased more rapidly in gerbils given environmental enrichment.