Adaptive forgetting of place/object memory for dung in the domestic horse (Equus ferus caballus): Memory for a day.

The domestic horse (Equus ferus caballus) makes dung deposits to form "stud-piles" and compulsively examines dung droppings, suggesting that dung contains species-relevant information. The present study investigates horses' use of location (place), odor (object) and memory for dung sniff encounters. Horses were video recorded in 2 indoor and 4 outdoor riding arenas as they were taken at different time intervals to experimenter-determined objects or dung deposits that they could sniff. Frame-by-frame video analysis measured approaches, sniff duration, nostril use, ear position and blinking associated with dung investigation. Horses approached and sniffed dung-deposits for longer duration than non-dung object in all test locations. They made head movements across the extent of dung-deposits when sniffing, showed no nostril or ear preference directed to the target, and blinked as they disengage from sniffing. Reduced approach probability and sniff duration showed that they displayed good place/object memory for dung previously visited at similar and different locations on the same day but poor memory for dung visited on a previous day. Adaptive forgetting of object/place memory for dung after a previous day's dung visit may optimizes risk assessment, including the possibility of premature interruption of foraging by conspecifics.