Odor detection thresholds in tree swallows and cedar waxwings
1991; Oxford University Press; Volume: 108; Issue: 1 Linguagem: Inglês
ISSN
1938-4254
Autores Tópico(s)Insect Pheromone Research and Control
ResumoMost Procellariiformes, cathartid vultures, pigeons, and kiwis use their sense of smell for orientation and foraging (Stager 1964, Wenzel 1968, Grubb 1974, Papi 1986, Waldvogel 1989, Lequette et al. 1989). Otherwise, little is known concerning the importance of olfaction to birds, although anatomical evidence suggests that many species should possess an acute sense of smell (Bang and Cobb 1968, Wenzel 1973, Bang and Wenzel 1986). Recently we began to collect behavioral and physiological data on the olfactory ability of passerines (Clark and Mason 1987, 1989; Mason et al. 1989; Clark and Smeraski 1990). Passerines are of special interest because the relative size of their olfactory bulb is among the smallest reported for birds and, conventionally, olfactory prowess is assumed to correlate positively with relative bulb size (Bang and Cobb 1968). The present study is designed to assess the sensitivity of passerines to a standard odorant. While by no means a complete assay of olfactory function, the results of experiments such as this one permit interspecific comparisons of sensitivity, and they serve as a relative index of olfactory ability. Such evaluations are prerequisite to speculations concerning whether or not a species uses olfaction in the wild. Five adult Tree Swallows (Tachycineta bicolor) were trapped at nest boxes in May 1988 at Tinicum National Wildlife Refuge, Philadelphia, Pennsylvania, and transported to the Monell Center. In August 1989, five adult Cedar Waxwings (Bombycilla cedrorum) were obtained from USDA/APHIS personnel in Gainesville, Florida, and transported to the Monell Center in Philadelphia. In the laboratory, the birds were housed individually in cages in a room with a constant ambient temperature of 23?C, and a constant 14L: 10D cycle (0700-2100). Water, food (crickets, mealworms, and grapes for the Tree Swallows; banana mash, blueberries, and apples for the waxwings), and medicated shell grit were available ad libitum. The diet was supplemented daily by vitamin solution injected into the fruit. Four birds of each species remained in good health and maintained weight throughout the experiments. I used cyclohexanone (CH) [C6H10O, mw 98.14, bp 155.60C, d204 0.9478] as the standard odorant. This substance was selected because its physicochemical properties and binding to receptor proteins were the subject of previous studies in our laboratory (Mason et al. 1984, 1987). Although CH is without any known biological significance to either species, vertebrate olfactory receptors are sensitive to a wide range of reagents that are not encountered naturally (Fazzalari 1978). This suggests that the functional design of such receptors is for the perception of volatile chemicals per se. Besides, the search for biologically relevant odorants for passerines seems premature in the absence of data that imply that olfactory sensitivity exists. Tree Swallows and Cedar Waxwings were evaluated for their ability to detect odor using a cardiac conditioning paradigm (Michelsen 1959, Walker et al. 1986). Cardiac conditioning is a technique to train animals to associate electric shock with a strong odor cue. The cardiac acceleration that occurred in response to odor presentation, but prior to shock, was interpreted as a conditioned response. Details on how subjects were prepared, signal processing, and the odor delivery system are published elsewhere (Clark and Mason 1989). Briefly, birds were restrained and placed within a darkened sound-attenuating chamber with their nares placed at the exit port of a dilution olfactometer. Heart rate was monitored with a Type II ECG lead configuration (Sturkie 1965) via a high impedance probe, amplifier, and oscilloscope. The frequency of heart beats was counted by processing the 'R' component of the amplified ECG signal to a TTL pulse via a Schmitt trigger circuit, and recording the timed pulses via custom software to a computer. Birds were trained to attend to the reinforced stimulus (S+) by presenting 10 s of CH at 5% vapor saturation (% VS) and following it immediately with electric shock (2.5 VDC for 2 s) applied across the legs of the subject through the recording electrodes. The ECG signal was lost at this point, so records were kept only during the 10 s before and 10 s during stimulus delivery. This concentration was selected because previous evidence suggested that 5% VS represents a strong olfactory cue, but not so strong as to elicit trigeminal responding (Walker et al. 1979). To control for possible attentiveness to cues associated with operation of the olfactometer, birds were presented with 10 s of a nonreinforced control (SO; i.e. humidified, filtered air, matched to the vapor saturation of the S+). The S+ and So were presented in paired trials with the sequence within pairs randomly selected. The intertrial intervals between all stimulus presentations were randomly selected (60-300 s) via the computerautomated dilution olfactometer. A cardiac acceleration of at least 10% of the prestimulus heart rate in response to stimuli was interpreted as positive. To avoid fatigue, birds were never given more than 100 trials per day. The minimum number of trials given any bird was 30. If the training criterion was not met (3:1 S+:So positive response over 20 consecutive trials), the bird was given a day's rest, and training was re-
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