Everyone knows that some animals, at least cats and dogs, keep their noses wet and cool. But why did they evolve it? What evolutionary advantages does it provide? Smell-sensing cells need cooling for precision, may be? Or the liquid dissolves smellful gases into itself, simplifying analysis this way? Or may be it just simplifies cleaning the nose from the previous sniffing act?
Why did it evolve ?
Mammals with rhinaria tend to have more acute olfaction, and the loss of the rhinarium in the haplorrhine primates is related to their decreased reliance on olfaction, being associated with other derived characteristics such as a reduced number of turbinates.
The rhinarium is very useful to animals with good sense of smell because it acts as a wind direction detector. The cold receptors in the skin respond to the place where evaporation is the highest. Thus the detection of a particular smell is associated with the direction it came from. The rhinarium is adapted for different purposes in different mammals according to ecological niche.
In aquatic mammals, development of lobes next to the nostrils allow them to be closed for diving. In mammals that root, the rhinarium often develops into a hard pad, with the nostrils off to the side. In the elephants it has become a tactile organ. In the walrus, it is covered with stiff bristles to protect it during foraging for shellfish. In many animals the form and purpose of the rhinarium remains to be elucidated.
Essentially it is the case of having a better sense of smell provided by the extended surface area, which has evolved to fit whatever the organism in question needs. I'll update this when I find this in a few more papers.
I took this from the wiki: http://en.wikipedia.org/wiki/Rhinarium
How does it work ?
A model of the canine nasal airway, developed from a 3-D reconstruction of high-resolution magnetic resonance imaging (MRI) scans, was used to study the aerodynamics of canine olfaction. Simulation results reveal that a unique olfactory airflow pattern exists within the canine nasal cavity during sniffing that is critical for efficient olfaction.
Calculations show that this novel olfactory airflow pattern provides a crucial residence time for odorant absorption in the sensory region and promotes spatiotemporal fractionation of odorant mixtures along the olfactory epithelium.
The canine nose functions similar to a chromatograph that imposes odorant-specific deposition patterns upon the thin mucus layer covering the nasal cavity....The results show that deposition patterns are primarily influenced by the intricate olfactory flowfield and the odorant solubility in the mucus layer.
Highly-soluble odorants are quickly absorbed near the entrance to the olfactory region, and thus do not reach the periphery with significant concentrations. In contrast, insoluble odorants are deposited more evenly and may even exit the olfactory region without being completely absorbed. Predicted odorant deposition patterns correspond with the anatomical organization of olfactory receptors known to occur in keen-scented (macrosmatic) mammals, providing a mechanism that helps explain the excellent olfactory acuity of the dog.
In short the mucus on a dog's nose 'pre-sorts' smells by trapping the passage of some odour particles more than others along the nose, with different types of molecules being picked up at different "depths" in the nasal cavity.
You can read about it here:
Craven, Brent, Gary Settles, and Eric Paterson. "The Aerodynamics and Transport Phenomena of Canine Olfaction." APS Division of Fluid Dynamics Meeting Abstracts. Vol. 1. 2008.
Lawson, Michael, et al. "Odorant Transport and Deposition in the Canine Nasal Airways." APS Division of Fluid Dynamics Meeting Abstracts. Vol. 1. 2009.
Craven, Brent A. A fundamental study of the anatomy, aerodynamics, and transport phenomena of canine olfaction. ProQuest, 2008.