Donkeys most often show their teeth by curling back their upper lip and raising their nose into the air, a behavior called the flehmen response. It looks strange, even comical, but it serves a real biological purpose: the donkey is drawing scent molecules toward a specialized organ in the roof of its mouth that analyzes chemical signals from other animals. That said, teeth-baring in donkeys can also signal pain, aggression, or dental problems, so context matters.
The Flehmen Response: Smelling With Their Mouth
The most common reason a donkey curls its lip and exposes its teeth is to activate a scent-detection system that goes far beyond ordinary sniffing. Donkeys, like horses and many other mammals, have a structure called the vomeronasal organ located along the lower part of the nasal septum. In hoofed animals, this organ connects to both the nasal cavity and the oral cavity through a small duct in the palate. When a donkey lifts its head, opens its mouth slightly, and peels back its lip, it’s funneling airborne chemicals directly to this organ.
The vomeronasal organ is specifically tuned to detect pheromones, which are chemical signals released by other animals of the same species. Once those molecules reach the organ’s sensory cells, signals travel along a dedicated nerve pathway to a part of the brain’s olfactory system, then on to the hypothalamus and amygdala. These brain regions regulate reproductive behavior, defensive responses, and hormone release. So what looks like a goofy grin is actually the donkey running a sophisticated chemical analysis.
Why Males Do It More Often
Male donkeys (jacks) perform the flehmen response far more frequently than females, and the trigger is almost always reproductive. The event most commonly linked to the behavior is urination by a female. A jack will approach the spot where a jenny has urinated, sniff closely, and then curl his lip to draw the scent upward. He’s checking for pheromones that indicate whether she is in estrus and receptive to mating. Studies of Pêga donkeys recorded males performing the flehmen response roughly seven to eight times per observation session during breeding encounters.
Females do occasionally show the same behavior, though less frequently. Foals also flehmen, often after investigating urine from other animals. In young donkeys it appears to be partly exploratory, a way of learning the chemical landscape of their social group. New or unusual smells, including food, can sometimes trigger a mild version of the response in donkeys of any age or sex.
Aggression and Social Signaling
Not every teeth display is a flehmen response. Donkeys also bare their teeth as a warning. An aggressive donkey will pull its lips back to expose the front teeth, sometimes snapping or lunging. The body language is noticeably different from flehmen: the head stays level or drops rather than tilting upward, the ears pin flat against the head, and the overall posture is tense and directed at another animal or person. Jacks in particular may show teeth when guarding territory, competing with other males, or feeling cornered.
Donkeys also use subtle mouth movements in relaxed social interactions. A submissive younger donkey may “clap” its teeth together in the presence of a dominant animal, a gesture that signals it’s not a threat. This looks quite different from either aggression or flehmen, with the jaw moving loosely rather than the lip curling.
Pain and Discomfort
A donkey that repeatedly exposes its teeth, holds tension in its muzzle, or seems to grimace without an obvious scent trigger may be in pain. Researchers developing a pain assessment tool for donkeys (a “grimace scale”) identified muzzle tension as one of several key facial indicators of discomfort, alongside tightening around the eyes, flattened ears, and a glazed expression. A donkey showing several of these signs together is likely experiencing physical distress rather than responding to a scent.
Dental disease is one possible source of that discomfort. Common dental problems in donkeys include diastemata (gaps between teeth that trap food and cause infection), displaced teeth, and wave mouth, where the grinding surfaces wear unevenly. These conditions cause oral pain and can lead to weight loss and even colic. A donkey with dental problems may hold its mouth oddly, drop food while chewing, drool excessively, or resist having its head handled. Because donkeys tend to hide pain more stoically than horses, any persistent change in mouth behavior is worth investigating.
How to Tell What You’re Seeing
The quickest way to interpret a donkey’s teeth display is to look at the rest of its body. A donkey performing the flehmen response lifts its head high, stretches its neck, and stands relatively still. It typically happens right after the animal has sniffed something on the ground. The mood is calm, even curious. An aggressive display comes with pinned ears, stiff legs, and forward movement toward a target. A pain response tends to be more subtle and persistent: the muzzle stays tight, the eyes look dull or half-closed, and the donkey may stand apart from the group.
If you’re around donkeys regularly, you’ll quickly learn to recognize the flehmen response as the most common version. It usually lasts only a few seconds, the donkey lowers its head, and life goes on. It’s one of the more entertaining behaviors to watch, and it tells you the donkey’s chemical world is far richer than anything we can perceive.