Body temperature and grooming in the Mongolian gerbil.

The Mongolian gerbil, Meriones unguiculatus, like many other rodents, releases a complex mixture of pigments and lipids from the Harderian gland during a self-groom. The material exits from the external nares of the nose, is mixed with saliva, and spread widely over the pelage. Cold temperatures, especially, are effective in initiating grooming. A self-groom is associated with an increase in body temperature (compensatory thermogenesis in the cold). In addition to acting as a chemosignal, the Harderian material serves two major homeostatic functions: (1) the lipids on the pelage act to insulate the animal against cold and wetness, and (2) the lipids and pigments darken the pelage and increase radiant absorption. Body temperature is thus maintained at a higher level than would otherwise be the case. The amount of Harderian material found on the pelage varies with Harderianectomy, sandbathing, and ambient temperatures. Animals prevented from sandbathing accumulate excess lipids on the pelage, and cold temperatures facilitate the acquisition of lipids on the pelage. Under hot temperatures the grooming of Harderian substances is repressed and the frequency of sandbathing is increased. Thus pelage lipids are reduced in two ways. The amount of Harderian material released during an autogroom is inversely related to the ambient temperature, whereas the amount of saliva used for evaporative cooling is positively related to ambient temperature. The net effect is that pelage lipids are increased and maintained during cold conditions, and are reduced during hot conditions. In hot environments the gerbil switches from the spread of Harderian material for insulation to the spread of saliva for evaporative cooling. The gerbil optimizes its body temperature by varying the frequency of grooming and sandbathing, and by altering the amount of Harderian material and saliva released. Other species living in arid environments may use similar mechanisms to stabilize body temperature. Self-grooming is a critical behavior for meeting thermal needs, and is complexly integrated with related processes.