by Anne of anne_rats
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! (article on finding these odorant receptors)
Photo courtesy of R. A. of the Dapper Rat
The message from the olfactory neurons speeds along a pathway to the olfactory bulbs, which are stem-like projections from the forebrain. The olfactory bulb is covered with about 2,000 tiny basketlike structures, each the diameter of a human hair, called glomeruli. The glomeruli are the basic units of olfactory perception. Each glomerulus is tuned to a specific odorant (odor molecule). So, a different pattern of glomeruli is activated when a rat smells different odors, such as bananas, caraway seed, spearmint, and the complex smell of peanut butter! These activation maps also change when the odor concentration increases, and when highly similar odors are presented in sequence, such as a series of aldehydes that only differ in their number of carbon molecules, which demonstrates how rats can discriminate between subtly different odors.
At this page, you can type in a chemical compound and see the corresponding map of activated of glomeruli in a rat's olfactory bulb!
During olfactory learning, this odor-induced pattern of activity in the olfactory bulb is sharpened, such as when a newborn rat learns its mother's unique odor (Brennan & Kevarne 1997)
Rats have a second way to detect odors, called the vomeronasal organ, or VNO. The VNO in mammals is situated in a pouch off the nasal cavity. In rats, 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. This dead-end position means that air can't flow into it, like the olfactory epithelium of the nose (Agosta 1992). 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 (more on the VNO).
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 (Brennan 2001), though it does detect some colatile pheromonal compounds as well (Trinh and Storm 2003, Zufall et al. 2002). Unlike the vast numbers of receptors in the olfactory epithelium, there are only 30-100 kinds of olfactory receptors in the VNO, and only one or a few per cell.
The messages from the vomeronasal organ shoot up a separate pathway to the accessory olfactory bulbs, and from there to the amygdala, then to both the preoptic area and the hypothalamus, areas known to be involved in reproductive behavior (see Meredith, FSU Neuroscience Program, for a summary).
The VNO is critical in chemical communication between animals -- mate attraction, courtship, copulation, aggression, and parental care are all mediated by the VNO (Bradbury and Vehrencamp 1998).
Chemical signals are found in all sorts of secretions, such as urine, feces, and secretions from the skin glands. They are picked up by sniffing or licking an individual, or through odors that have been deposited on the ground or volatilized into the air.
One of the most familiar methods of chemical communication in rats is urine marking. Sexually mature males are the most prolific urine markers, though sexually mature females may show some urine marking as well, especially on the night before they come into heat (Calhoun 1962, p. 151). Urine marking is therefore considered an advertisement of one's presence and a sex attractant -- adult males advertise and females choose their mate from among the advertisers (Doty 1974). Female urine marking may be an advertisement of sexual receptivity.
Chemical secretions contain an enormous amount of information (Agosta 1992). Through odors contained in secretions such as urine, rodents can determine all the following about the animal who produced the odor:
So, urine contains all sorts of highly personal information!
References: Agosta 1992, Brown 1975 & 1977, Giesecke 1997, Mackay-Sim 1980.
Chemical signals may even lead to changes in the recipient, too. Urine contains pheromones that accelerate or decelerate puberty in immature females, influence the timing of the estrus cycle, and pheromones that cause a male to mount a receptive female. Specifically, male odor accelerates female puberty, while female odor delays it and suppresses estrus in sexually mature females. The odor of a strange male may also cause a newly pregnant mouse to reabsorb her litter. (Agosta 1992).
Chemical signaling plays an essential role in animal communication. Such chemical signals are involved in all aspects of courtship and mating, aggression, parental behavior, and foraging:
Courtship and mating behavior
Chemical signals are essential for the proper performance of courtship and mating behavior (Larsson 1971). For example, if males don't have access to information from their nose or vomeronasal organ, they cannot mate. Visual and auditory cues are not enough (Sachs 1997). Sexually experienced males may get by with either the nose or the VNO, but inexperienced males are severely impaired if their VNO is hampered. If the VNO is removed in female mice, male odor no longer accelerates puberty, adult females no longer influence each others' estrus cycles, and foreign males no longer cause miscarriage in recently impregnated females (Meredith, of FSU). Female odor also enhances sperm production in dominant males, but not in subordinates (Koyama 2000). Females require olfactory cues for normal reproductive behavior as well (Aron 1979).
Olfaction is used in aggression, too. Lactating mother rats, who usually display increased aggression toward strange adult rats, do not do so if their sense of smell is impaired (Kolunie & Stern 1995, Ferreira et al. 1987). Male rats usually display some aggression toward each other, and this inter-male aggression is normally increased if they can smell a receptive female. However, if the males' sense of smell is impaired, they become less aggressive toward each other and there is little increase in aggression when they are presented with the odor of a receptive female (Bergvall et al. 1991, Cain 1974).
Odor is critical for newborn animals, as they locate their mother's teats through smell. After birth, mothers spread their amniotic fluid onto their teats, and the infants crawl toward the familiar smell and latch on to the nipples. If the mother is prevented from licking and spreading the secretions on herself, the babies cannot find her teats, but they can find her teats again if the mother's saliva or amniotic fluid is brushed back on (Teicher & Blass, 1977). A few days later, the pups are attracted to the smell of their own saliva. Wash the nipple and they stop suckling, spread infant saliva around and they nurse again (Teicher & Bass, 1976).
Odor is critical for maternal behavior. Impair a mother's sense of smell, and her parental care decreases drastically, leading to the death of many of her pups (Kolunie & Stern 1995, Fleming 1971).
Newborn rats learn about what to eat later in life through odor cues received through the mother's milk. Later in life, when rats forage independently, their food choices are influenced by the scent of foods recently eaten, carried on fur, whiskers, and breath of other rats (Galef 1996).
Keeler (1942) found that albino rats took twice as long to back away from a pungent-smelling piece of garlic as normally pigmented rats (9.87 seconds vs. 4.85 seconds). In another experiment with different rats, Keeler found that pigmented rats backed away from a piece of garlic after 7 seconds, but albino rats never backed away at all. In fact, after 15 seconds three of the albinos in the experiment actually tested the garlic with their teeth to see if it was edible.
Sachs (1996) tested male albino and pigmented rats' response to the remote cues of a female rat in heat. Sachs used a testing chamber that was divided in half by two wire mesh screens separated by a few centimeters, so the male rat on one side could see and smell a female in heat on the other side but could not touch her. Sachs found that 83% of pigmented male rats became aroused, but only 4% of albino rats did.
In conclusion, albino rats appear to have a dulled sense of smell and/or reduced responsiveness to olfactory cues.