Sensory Physiology In Dogs : Hearing & Vision

Hearing in dogs

Dogs have an excellent auditory system. They can locate the source of sound far better than man: dogs can distinguish two different sources of sound when the angle between the connecting lines (head-source) is as small as 1° 26′. In man the corresponding value is 4° 18′ (Buddenbrock, 1952). This ability is very important in predatory behaviour performed in complex terrains (e.g. wood with scrub). The direction can be detected by motions of the earlap and by using the difference of phase of the sound waves arriving at the left and right ear. The dog is able to hear ultrasound (frequencies higher than 20 kHz) up to 60 kHz (Overall, 1997). What is the benefit of this ability? In accordance with Peters and Wozencraft (1989), one can assume that puppies communicate in distress situations with the mother by means of ultrasound. We know from physics that the damping of an oscillation correlates negatively with its frequency. Therefore, ultrasound is used when the signal should not be heard at greater distances to avoid possible predators detecting it. In predatory behaviour, it could play a role in finding prey, since some rodents use ultrasound in communication too. Fox (1965) distinguishes between the following types of canine vocalizations: 
1. Infantile sounds like crying, whimpering and whining. 
2. Howling. The interpretations of this type of sound are controversial. In wolves, two functions are discussed: to get in contact with other members of the pack and to mark the territory by sound. In some dogs, howling is elicited when the owner is absent. 
3. Aggressive growling. 
4. Submissive whining. 
5. Territorial defensive barking. From the acoustical signal, the receiver dog can presumably estimate the size of the vocalizing dog. Dogs range in body mass from chihuahua to Saint Bernard, a 100-fold difference. In addition, the vocal tract length differs tremendously and, because of that, also the acoustic signal (Riede and Fitch, 1999).

Vision in dogs

Unlike humans, the dog has no fovea centralis but an area centralis. As in the fovea centralis, that is where the highest density of photoreceptors is found. The effect of this is that in this area the resolution of optical signals is higher than in the rest of the retina. The importance of vision depends to some extent on the breed of the dog: it is less important in breeds such as bassets, beagles and bloodhounds, which track prey using their noses (‘scent hounds’). It is of more importance in dogs hunting in free areas like Afghans and greyhounds, which have good sight (‘sight hounds’). However, in general, visual acuity is higher in humans than in dogs. The colour vision of the dog has been a matter of discussion for years. As already mentioned, dogs (like many other mammals) have only two types of cones. Some therefore believe that dogs are unable to distinguish different colours in the same way as humans do. However, in an experiment using a discrimination task, it was clearly shown that dogs can differentiate colours. The dogs had to identify the designated correct colour out of magenta, cyan and yellow. Finding the correct colour was rewarded by food, and the dogs could clearly distinguish all three colours (Antolini-Messina, 1996). In another experiment also using a discrimination task, it was found that the ability to distinguish different scales of grey is poorer than that of humans (Pretterer et al., 2004), but that the dogs could readily discriminate between red, green, blue and two types of turquoise and all scales of grey. None of these animals was able to distinguish between a third type of turquoise (wavelength: 480 nm) from the corresponding grey shade. Therefore 480 nm is the so-called neutralpoint for the dog (a colour with this wavelength looks like grey) (Pretterer, 2000). The behavioural aspect of vision in the dog is that it plays a major role in finding prey. Predatory behaviour presumably makes use of a cascade of stepwise sensory information. By olfaction, the dog finds the direction where possible prey could be found. With the help of vision and hearing it finds the prey and the instinctive killing behaviour is then released. Visual signals also play an important role in intra- and interspecific communication: the posture of the whole body, the mimic of the face, ears, mouth, muzzle, the fur (flat or raised), the tail held upwards or downwards or wagging: the ears are erect, there is a gentle slope in the hip, a relaxed tail and no piloerection can be seen. When a dog’s fear level increases, the head is lowered, the ears are pulled back and become limper. The lips become looser and the eyebrows are arched, the tail is lowered or between the hind limbs. When aggression increases the ears are raised, the fur piloerected, the teeth shown, nostrils and pupils are widened, the tail is upright or wagging, and the hind limbs stretched out . The submissive dog, on the other hand, makes itself look smaller and in extreme situations, it rolls on its back, flexes its limbs and feet, exposes its belly and tucks in its tail. In some breeds with great deviations from the appearance of the wolf, problems in sending visual signals can occur. For example, dogs with hanging ears or short snout may have problems sending appropriate signals and sometimes may be misunderstood by other dogs. Another example is the chow chow, which normally has stretched hind limbs, which is misinterpreted by some dogs as demonstrative behaviour.