Olfaction in Rodent Control

A brief survey is presented of current knowledge on olfaction in rodents and the various roles that odors may play in modifying rodent behavior. Some species at least uti lize olfactory cues: to locate food items; to recognize their mother and mates; to mark territory; as an involuntary population density regulator; possibly to recognize predators; as a warning cue against a repellent or toxic substance (poison-bait shyness); and probably, for many other behavioral purposes. The value of using a r t i f i c i a l odors in rodent baits to increase bait acceptance is not yet well documented. The addition of attractive natural odors may increase detection of low-preference foods, but there is l i t t l e evidence that a strange odor can improve palatability for any prolonged period. Much more research is needed before rodent control methodology can f u l l y exploit the olfactory acuity of w i l d rodents. A review is made here of knowledge on the role of smell in the ecology and behavior of w i l d rodents as it may relate to rodent control. Since so l i t t l e is known about the role of smell and taste in the acceptance and rejection of baits by w i l d f i e l d and commensal rodents, it is hoped that this report w i l l stimulate more research in this control area. Every rodentcontrol operator has his own theories about the significance of human odor on rat traps or the addition of a few drops of aniseed o i l in a bait. Host of the evidence, however, is subjective. This paper discusses a number of aspects of olfaction and gustation in rodents which seem to bear directly on the role of odor in rodent control. Also reported are a few of our own preliminary findings. According to Moulton (1967), the a b i l i t y to detect, analyze, and exploit odors appears to reach it s highest degree of development among mammals. Mammals are especially flexible and efficient in deploying their sensory resources and tend to make more use of a l l available cues. Many exploit odors extensively in trail-following, recognition of territory, of young, of mates, and of other social groups, as well as in the detection of food and predators. Furthermore, it is becoming increasingly evident that odors may control certain reproductive functions by acting as pheromones, as discussed later. Since the pairing of eyes and ears help an animal determine the direction of a stimulus, Moulton asks whether the pairing of the external nares, located so close together, serves a comparable function (tropotaxis). It could be, if very sensitive receptors could make simultaneous comparison of odor intensities or time of arrival. The high mobility of rodents and most mammals, however, may make this type of stationary orientation unnecessary. The volatile constituents of food actually in the mouth may also be smelted through intercommunication between olfactory and buccal cavities. According to Moulton (ibid.), Whitten observed that f l u i d from the mouth can travel up a cleft in the upper l i p of the mouse to enter the nasal cavity. The snout of a mole is really a chemotactile sense organ, l i k e the snout of a pig, where both tactile and olfactory information can be extracted simultaneously (Moulton, ibid.). Mammalian chemoreception is associated primarily with taste buds or olfactory epithelium, and there appears to be no question that rodents have a highly effective olfactory modality, though l i t t l e is known about the extent to which w i l d rodents use their olfactory senses to locate food items they may or may not have encountered previously. Research at Davis (Howard and Cole, 1967; Howard, Marsh, and Cole, 1968; Howard, Palmateer, and Marsh, 1969) has substantiated that at least in some w i l d rodents olfaction is high in the sensory hierarchy in richness of cognitive detail, in variety of sensory experience, and in motivational s i g n i f i cance. Even so, research techniques must become much more sophisticated before we w i l l know how to capitalize f u l l y on the sense of smell of rodents in improving current rodent-control methods. L i t t l e is known about the extent to which various species of w i l d rodents use their olfactory senses to locate or avoid various food items; the significance, if any, of smell in affecting the palatability of foods to rodents; and how odor cues facilitate the locomotion, spatial orientation, and other ethological responses of rodents. It is generally known that the act of ingestion, normally a response to hunger, may be reinforced or inhibited by seemi n g l y unrelated types of behavior and previous experiences. Some suspected or recognized factors include maternal instincts, social interand intra-specific stresses, population density and structure, hierarchy and dominance, tameness and fear, imitation and imprinting, general health and d a i l y rhythm of the individual, season, temperature, and weather. Our premise is that odor is what usually i n i t i a l l y motivates a rodent to search for and pick up specific food items to satisfy its appetite, and that each rodent learns to associate i t s gustatory experiences with the odor of the item. Also rodents apparently have sufficient