Sniffing frequency of Tupaia belangeri: A measure of central nervous activity (Arousal)

1. The sniffing behavior of tree shrews of both sexes has been studied. A microphone bearing a piece of filter paper impregnated with the odor to be tested was placed beneath an opening in the floor of a passage attached to the home cage. The sniffs were recorded via the microphone on magnetic tape. 2. Habituation of the animals to the test odors was apparent in the number of sniffs made during a given time. (a) During each experimental session the number of sniffs per unit time decreases progressively. In general more than 50% of all sniffs counted during a 30 min session occurred within the first minute, and more than 80% during the first 5 min. The bursts of sniffing were nearly constant in length (cf. Fig. 2), while the pauses between bursts became longer. (b) When a given test odor was presented repeatedly on successive days, the total number of sniffs decreased from one experiment to the next, after a few days reaching a "basal level" which then remained nearly constant (Fig. 1). (c) A new odor in the same category produces an increase in number of sniffs per session, but not to the level associated with the first test of the preceding odor (Fig. 1). In subsequent sessions the animal again habituates to the stimulus. 3. The basal level of sniffs per session in response to a great variety of odors is the same for a given animal, but individuals differ in their basal levels (Table 1). Changes in excitatory state of an individual produce no change in number of sniffs per session. 4. "Sniffing frequency" - the frequency of sniffs within a burst - however, is less variable than the total number of sniffs over a longer period. (a) Within the individual sniffing bursts, the inter-sniff interval is nearly constant (SD for 113 bursts 4.3±0.2%; cf. Fig. 2). (b) Throughout a given experimental session, sniffing frequency, within the many bursts, was about constant (SD for 425 experiments: <3%). (c) When experiments are repeated under identical conditions, the sniffing frequencies measured in each are approximately the same.