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For which animal is the sense of smell so dominating a sense of perception that about two-thirds of its brain is dedicated to it?
The answer is the Great White Shark, (Carcharadon carcharias). It can smell a drop of blood in water 2 km away.
However, it would be wrong to say for sure that the great white shark has the absolutely best sense of smelling of all animals. It might be so, but it is impossible to compare because the sense of smell depends on olfactory receptors of different sorts. One sort detects only one sort of odour, while other sorts detect others. That means that the sense of smelling can be targeted to certain odours. The shark's ability to smell blood might very well be what it needs; perhaps it doesn't feel other smells as good at all. What we can assume is that there is no other animal for which the sense of smelling is so dominating, at the cost of many other abilities, and probably at the cost of other senses.
Generally, of course the sense of smell is better for those with many receptors, which are those with the most developed olfactory epithelium, which is found back in the nasal cavity. The number of genes involved in detecting scents is also different for different species. It is justified to assume that the sense of smelling is better when the number of scent-detecting genes is higher. So what can we say about animals?
Bears may very well have the best sense of smell of all animals. They can detect a dead animal 40 km away. Everyone moving in a nature inhabited by bears must be aware of that. Bears cannot see well, but if the wind moves in the right direction (from you towards the bear) it surely knows you are there long before you are aware of its presence. Chances are that it avoids you and you will never know how close to it you were.
Dogs are well-known for having a good sense of smell, and amongst dogs the bloodhound has the strongest sense of smell of all. It's far beyond any human ability, but still not impressing if we compare to a bear.
So, wonder all cat lovers, how about cats, do they smell better or at least as well as dogs?
No, they are not able to smell as well as dogs, they have fewer scent receptors, but their ability are clearly superior to the human sense of smell. Some research, however, indicates that cats are better than dogs at discerning different smells. Different species have different needs, and their senses of perception are adapted to that.
Elephants also have a great sense of smell. The African elephant has the highest number of genes dedicated to detecting smells. Many of its odour-detecting receptors are found in the trunk.
On the whole, the sense of smell is well developed among mammals, although there are exceptions. Humans, for instance, are mammals with a very weak sense of smell. However, there is reason to believe our pre-primate ancestors had a more acute sense of smelling than we have, but that its capacity fell while vision evolved to a dominant sense of perception.
Apart from sharks, other non-mammal animals with a strong sense of smell are snakes and moths.
As for birds, there has been an endless debate whether or not they have any sense of smell, or, given some obvious examples, whether all birds have it.
Vultures and kiwis are examples of birds that obviously have a strong sense of smell, but as for birds in general, more studies are required to settle the question.
How about insects? Well, they have a sense of smell although it works differently. They detect chemicals in the air mainly via their antennae. On the whole, smell is very important in most insects' life; they navigate with it, find food with it, find a mate with it, and they communicate by chemical signals detected as odours.
Spiders also have a sense of smell and they pick up the odours with sensors on their legs.
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Images. In order of appearance:
Great white shark. Photo: skeeze/Pixabay. CC0/Public Domain.
Bear. Photo: Daniel Eledut/Unsplash. CC0/Public Domain.
Elephant. Photo: Will Shirley/Unsplash. CC0/Public Domain. The image has been cropped and digitally enhanced.
Vulture. Photo: Free-photos/Pixabay. CC0/Public Domain. The image has been digitally enhanced.