Stereotypies resulting from a deviation in the ontogenetic development of gerbils.

As a reaction to intensive housing conditions, Mongolian gerbils (Meriones unguiculatus) regularly develop stereotypic digging in early ontogeny. To determine how the development of stereotypic digging deviates from the development of digging in a natural setting, the development of digging in laboratory cages and an artifical burrow were compared. In the burrow treatment, young gerbils were raised inside the burrow and left it for the first time at a mean age of 21 days. They showed a strong tendency to retreat into the burrow and spent 91.6% of the time in it. Their digging consisted of short, non-stereotypic digging bouts. In contrast, digging of the young gerbils in the laboratory cages became increasingly longer and consisted of long-lasting, stereotypic digging bouts along with short, non-stereotypic bouts. This development deviated from the development in the burrow significantly from day 24. Stereotypic digging was interpreted as an intensification of digging due to a lack of retreating structures in the laboratory cages. Stereotypic digging was considered as an abnormal behavior because it failed to regulate retreating behavior.