Sensory Physiology In Dogs "2" : Olfaction & Taste

Olfaction in dogs

In free-ranging dogs or wolves, the olfactory system is important to detect prey. The dog can usually detect the direction where the scent is coming from. In doing so, the moisture of the nose may be important, since it may allow the dog to identify the wind direction

The olfactory system of the dog is extremely sensitive compared to that of humans. This fact is not only the consequence of the larger area of the olfactory epithelium of the dog compared with humans (5 cm2 in humans and 150 cm2 in the German shepherd) (Brüggemann et al., 1965), but also of the higher density and sensitivity of the sensory cells. For some compounds, the sensitivity of the dog’s olfactory system is millions of times higher than in men (Brüggemann et al., 1965). Hence, it is clear that smells play a much more important role to the dog than to humans: when a male dog meets a female, olfactory cues provide information about her state in the sexual cycle, for example. Each individual dog can probably be identified by its personal olfactory profile. Therefore, a change of the individual odour of a dog can cause problems, since it might not be identified by another dog: change of smell can turn a dog into an ‘alien’ individual. Drugs used for anaesthesia can change the personal odour, a problem well known in cats.

Dogs, mainly males, mark their territories with urine. It contains information about sex and perhaps about state in the ranking order: generally the alpha-male raises the hind limb extremely high, so that the urine is deposited higher than for lower ranking males. The posture during urination demonstrates the position in the ranking order. In neutered female dogs the rising of the hind limb during miction is sometimes observed too.

Dogs also use defecation for marking. Some individuals try to deposit their faeces as high as possible, for example on fences or trunks. The scratching of the ground after defecation is rather an imposing behaviour than a dispersion of faeces or odours. This behaviour is more extensive when another male passes. In this situation, sometimes the scratching dog starts to growl.

Dogs used for hunting or used in the military or police service are trained to find deer, persons, etc., by sniffing. Here, it is necessary to distinguish between primary and secondary tracks: the primary track is the scent of the person; plants and insects for example, which have been destroyed by the footsteps of the searched person or prey, cause the secondary track and deliver additional scents. Thesen et al. (1993) distinguish three phases during tracking the footprints of a person. In the first phase (searching phase), the dog is going straight forward while sniffing close to the ground. When it has found the track, it identifies the direction the person was walking by comparing the intensity of the scent of the first footprints (phase 2, deciding phase). During tracking (tracking phase, phase 3), the pattern of sniffing is similar to that of phase 1. When tracking, the dog sniffs with a frequency of six inhalations per second for about 15 times and then a respiration follows. In phase 2, the respiration follows each period of 35–50 sniffs. 

In dogs searching for drugs the decision phase (phase 2), which is the most strenuous one, might be the only one occurring. After about 20 minutes of work, the dog is usually exhausted. Sniffing leads to an increase of the energy needed for inspiration and expiration, which can be demonstrated by blood samples (Strasser et al., 1993). One can speculate that pack hounds overcome the problem of exhaustion by alternating with each other in tracking. As a speculation, I want to put forward another hypothesis: scent hounds, like bassets, beagles and bloodhounds, track their prey using their noses. The breeds mentioned have very long, hanging ears. Maybe these serve a function in whirling up the scent and guiding it to the nostrils. 

Taste in dogs

 In the dog, taste functions similarly to in humans (Kurihara and Kashiwayanagi, 2000). In dogs, there are five types of receptors too. For the dog, the receptor sensitive to umami is probably very important, for dogs eat meals with high protein content. However, as already stated by Huxley, cited above, taste is a function of olfaction. Taking into account the tremendous sensitivity of the dog’s olfactory system, we can assume that it has a much greater range of taste impressions than a human does. Olfaction seems to be more important in dogs than taste: if a dog smells a preferred substance, this can easily be used to trick it to eat another, nonpreferred substance (Overall, 1997). The main purpose of olfaction and taste presumably is to select the correct food and to avoid harmful or poisoning ones.