Most professional deer stalkers will tell you how sensitive a deer’s sense of smell is – while stalking, the slightest change in wind direction or air eddy in the forest can scupper your chances for the rest of the day. The problem is compounded by the fact that humans typically have a very poor sense of smell, which makes it difficult to understand how significant smell can be. Sadly, in the same way that studies on the visual capabilities of deer have only been conducted on a few species, studies on cervid olfaction are similarly restrictive.
We can gain an insight to the importance of scent in a deer’s world by looking at the structure of the animal’s brain in conjunction with that of its nasal cavity. Deer have relatively larger olfactory bulbs (the scent-processing parts of the brain) than we do; they also possess a considerably greater surface area of olfactory epithelium than humans. In a study of the olfactory epithelium of the Roe deer published in 1975, German anatomist Albert Kolb found that the average area of olfactory epithelium was 90 sq-cm (14 sq-in). If we compare that to an adult human, which typically has about 10 sq-cm (1.6 sq-in), we can see that a Roe deer’s sense of smell is potentially nine-times more sensitive than ours. Coupled with larger bulbs and increased epithelial area, deer also have a long nasal passage, terminating in a moist rhinarium (nose).
A moist nose helps improve the sense of smell; volatile scent particles stick more easily to wet noses, while the side of the nose being cooled by the prevailing wind helps the animal establish the direction from which the scent has come. Studies in domestic dogs have found that wet mucus on the nose can also help to pre-sort odour molecules hitting the nose, by slowing down their passage into the nasal canal.
Early behavioural studies attest to the ability of deer to find and assess food by smell. In 1934, Joseph Dixon published a paper in the journal California Fish and Game detailing the results of his studies on the food habits and life history of Mule deer (Odocoileus hemionus) in California. Dixon found that his subjects were able to tell good acorns from those with worm infections and those that were hollow by smell alone. Similarly, in a 1977 special report of Arizona Game and Fish Department, Theodore Knipe described how White-tailed deer were able to locate oak leaves and acorns under several inches of snow using cues that could only have been olfactory.
In addition to the main nasal process, deer have another scent-detecting gland, sometimes referred to as their “second nose”, called the vomeronasal organ (VNO). The VNO was first described by Danish anatomist Ludvig Jacobson (as such it is sometimes referred to as the “Jacob's Organ”) in 1813 and, in deer, it takes the form of a diamond-shaped lump of tissue on the roof of the mouth. The nerves run from the VNO, along the nasal septum, to the vomeronasal bulb (sometimes called the 'accessory olfactory bulb'), which contains the same type of sensory cells as the main olfactory bulb.
The VNO is considered to play a role in assessing the sexual readiness of females and perhaps helping to sync the male's reproductive condition to that of the nearby females. It is certainly interesting that the brain connections for the nasal and VNO nerves are apparently different. As University of Georgia deer biologist Karl Miller points out in his “Deer Talk With Their Noses” article, the VNO connects to the part of the brain that controls the reproductive condition of the deer, rather than connecting to the same part as the nasal passage.
Studies on White-tailed deer have shown that, although the VNO is used to sample urine in order to assess a female's impending oestrous, even if the organ is removed, the deer are still able to tell when a female is in season. This is in contrast to many other studies that have shown how a damaged or missing VNO can lead to suppression of reproductive activity – this was first demonstrated in 1953 with male guinea pigs, which failed to mount females when the VNO was impaired. The male deer samples the female's urine using a flehmen response, where he curls his upper lip and lifts his head into contact with the urine stream – the animal may also wrinkle its nose and cease breathing for a moment. Flehmen is frequently observed in rutting deer, but is common among the ungulates and other mammals, including cats. It is theorised that the act of flehmen serves to move fluid-based pheromones (i.e. in the urine or vaginal secretions) from the mouth to the VNO. In his 'Deer Talk' article, Miller notes that “Deer use the VNO exclusively to analyze urine”.