Function discrimination between odorants

Individual Discrimination. Discrimination between odors of individual males was assessed using a habituation-discrimination paradigm in which females were first exposed to male flank gland scent or urine, two odors with multiple behavioral functions (Johnston, 1990), on four consecutive 3-min trials separated by 15-min intervals (Johnston et al., 1993 Johnston, 1993). On the fifth (test) trial the animal was exposed to a novel male s odor as well as the now-familiar male s odor. These odors were presented on glass plates (7.7 x 17.8 cm) that were placed on the floor of the female s home cage and removed after each trial. One half of the plate contained one male s urine (100 pi of thawed urine) or flank scent (rubbed directly on plate) and the other side was left clean (trials 1-4) or contained the other male s odor (test trial). The amount of time the female spent investigating both sides of the plate was recorded with stopwatches. 

In complex odor mixtures, the ratios of odorant concentrations may be important features that animals use to discriminate. This has been shown to be especially true of pheromone blends, where the ratio of the concentration of each pheromone compound in the blend may affect several behaviors, from anemotaxis to contact with the source of emission. Variability of emission of the ratios of odorants in pheromone blends can occur within individuals and across populations or between species. The strength of the effects of variation on behavior appears to be dependent upon the species involved, however. In non-pheromonal odors, the ratios of odor compounds in scent may also be useful for discriminating among odor-emitting objects. Differences attributed to scent as a function of the ratios of odorant compounds may be more subtle than perceptual differences that occur from the subtraction or addition of different odorant compounds to an odor mixture. As these differences may be hard to detect, the extent to which animals use differences in the ratios of compounds may be governed by the cost of making a mistake between scents with different ratios. 

The comparisons of psychophysical tests in different species [29] are in direct conflict with the implications of the decline of functional olfactory receptor genes from rodents through the primate series to humans. It appears that we have a lot more to learn about the relation between counting receptor genes and testing for detection and discrimination of odors. This implies that there is not a one-to-one relation between genes and behavior. 

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