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Rat-natomy: Sense of Smell
A Nose like any other?
Air
enters the rat's nostrils and flows past a patch of skin rich with smell
receptors called the olfactory epithelium. Here are olfactory neurons,
which are tipped with little hair-like cilia that project into a thin
bath of mucus at the cell surface. Odor particles in the air, called odorants,
bind to special receptors on the cilia of the olfactory neurons, and their
binding triggers a neural response that shoots up to the brain.
Incredibly, there are between 500 and 1,000 types of olfactory receptors,
coded for by between 500 and 1,000 genes! That is a staggering number of genes,
about 1% of the rat's DNA. That
means that in rats, one in out of every 100 genes is involved in the detection
of odors. This jaw-dropping number of genes involved in olfaction gives
an idea of how important the sense of smell is to a rat!
More than just a Nose!
Rats have a second way to detect odors, called the vomeronasal organ, or VNO. The VNO is located in
a cigar-shaped passage in the floor of the nasal cavity, right next to the
septum, with a narrow opening just inside the nostril. When rats sniff and lick,
molecules from the environment stick to the moist nose and dissolve, and are
then transported to the VNO suspended in mucus. The VNO dilates and constricts
to pump the odor-bearing liquid inside rapidly.
The vomeronasal organ primarily detects pheromones, chemical
signals transmitted between members of the same species. It specializes in
nonvolatile chemicals found in the urine and other secretions; mate attraction,
courtship, copulation, aggression, and parental care are all mediated by the VNO.
Thanks to the VNO and it’s ability to detect chemicals beyond those one
might smell, rats are able to use chemical signals for communication. These signals are found in all sorts of things, such as
urine, feces, and secretions from the skin glands. These chemical signals are
picked up when the rat sniffs or licks an individual, or through odors that have
been deposited on the ground or in the air.
“Single
female PEW looking for sexy, dominant male...”
It’s the VNO’s fault that our ratties leave piddle puddles behind! For a rat, urine marking is the equivalent of a personal ad
-- adult males advertise and females choooose their mate from among the
advertisers. Female urine marking may be an advertisement of sexual receptivity.
Rats can learn a lot about the advertisers from their “ads,”
including: