Horses sniff dung for several reasons - for communication, social behaviour, and environmental awareness. 
Horse dung (c) Avrezn Dreamtime.com
Dung contains chemical cues that can convey information about the individual who deposited it, such as their identity, sex, and reproductive status. By sniffing dung, horses can recognize and gather information about other horses in their vicinity. It may also help horses understand social structures and hierarchies within a group, allowing them to recognize dominant individuals and avoid potential conflicts.
Horses may form "stud-piles" where they deposit dung in specific locations to mark their presence. This behaviour can communicate territorial boundaries or indicate that a particular area is frequently used by certain individuals or groups
Research by Audrey EM Guyonnet, and Ian Q. Whishaw, at the Canadian Centre of Behavioural Neuroscience, University of Lethbridge, Alberta looked at how horses use location , odour, and memory related to their encounters with dung.
In their study, which is reported in Behavioural Processes, they assessed the response of horses when they were led, at varying time intervals, to objects chosen by the experimenter or to dung deposits, which they were allowed to sniff.
Twenty-two horses were included in the study. which was carried out in a total of six riding arenas—two indoor and four outdoor. –
The research team took video recordings of the horses encountering the objects and the dung Frame-by-frame video analysis assessed several factors: the way the horses approached the objects or dung deposits, the duration of sniffing, nostril use, ear position, and blinking associated with dung investigation.
The researchers found that horses consistently approached and sniffed dung deposits for a longer time compared to non-dung objects.
While they were sniffing, horses moved their heads across the dung deposits. They showed no specific nostril or ear preference when investigating the target and they tended to blink as they disengaged from sniffing.
Horses showed a reduced likelihood of approaching and shorter sniff durations when revisiting dung deposits encountered earlier the same day, regardless of the location. The researchers suggest that this indicates a strong short-term memory for dung and its location.
However, horses showed poor memory for dung visited on the previous day, suggesting that their memory for dung is good on the same day but significantly diminishes by the next day. This pattern reflects adaptive forgetting, allowing horses to focus on current environmental cues. (“Adaptive forgetting” refers to the brain's mechanism of intentionally forgetting certain information to optimise memory function and cognitive efficiency. It helps individuals prioritise relevant and important information while discarding outdated, irrelevant, or redundant data. This process is thought to enhance learning, decision-making, and overall cognitive performance.)
The researchers suggest that this phenomenon of adaptive forgetting, where memory for dung visited the previous day fades, may be beneficial for horses. It optimizes risk assessment by preventing unnecessary interruptions in foraging caused by conspecifics and ensures that their attention remains focused on current and potentially more relevant environmental cues.
For more details, see:
Audrey EM Guyonnet, Ian Q. Whishaw,
Adaptive forgetting of place/object memory for dung in the domestic horse (Equus ferus caballus): Memory for a day.
Behavioural Processes,
Volume 217,
2024,
105025,
ISSN 0376-6357,
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