Acoustic characteristics of air puff-induced 22-kHz alarm calls in direct recordings

https://doi.org/10.1016/j.neubiorev.2005.04.007Get rights and content

Abstract

Alarm calls were induced in adult Wistar rats by an air puff. Emitted calls were digitized and directly recorded on a computer hard drive. The long-duration 22-kHz calls were emitted almost exclusively in series. Initial calls in the series tended to have the longest durations, higher frequency range, and the highest degree of frequency modulation, as compared to other calls. The frequency modulation always appeared as a downward sweep and seemed to represent a tuning of individual calls to a 3 kHz communicatory band. Regardless of the maximum frequency, rats always reached approximately the same minimum frequency, common to all calls. Thus, the broader was the frequency range of a given call, the longer the call duration. It is postulated, therefore, that rats emit 22-kHz calls at the minimum possible ultrasonic frequency they are able to produce, which is synonymous with peak frequency. It is further postulated that production of alarm calls in series, with long call duration and the invariably low ultrasonic frequency, maximizes successful communication in dangerous situations. Exceptions to this rule were observed immediately following air puffs, suggesting that acoustic parameters of the initial calls may differ from the alarming properties of the remaining 22-kHz calls.

Introduction

Adult and juvenile rats emit 22-kHz calls in a number of aversive situations, including agonistic encounters, social threat, pain, or other stressful behavioral situations (for more details see Sales and Pye, 1974, Miczek et al., 1991, Blanchard et al., 1992, Brudzynski et al., 1993, Fu and Brudzynski, 1994). In order to explain the biological function of 22-kHz calls in the interaction between two individuals, a number of hypotheses have been proposed. The calls have been postulated to signal submission and cause decrease in aggressive behavior of attacking opponents (Lore et al., 1976), to play a role in inhibiting female agonistic behavior during mating (Brown, 1979), or in signalling post-ejaculatory behavioral inhibition and withdrawal (Barfield and Geyer, 1975). Twenty-two kilohertz calls were also postulated to have a general ‘desist-contact’ function and signal an inactive and non-interactional state to the companion animal (Smith, 1979, Van der Poel and Miczek, 1991). In general, 22-kHz calls have been suggested to be emitted as a result of arousal (Bell, 1974) and as a possible expression of emotional state, particularly a state of fear in threatening situations where escape is not possible (Van der Poel and Miczek, 1991, Cuomo et al., 1993).

These 22-kHz vocalizations have also been recorded in a number of situations where rats do not interact directly with each other or are even in isolation (Blanchard et al., 1992, Francis, 1977, Kaltwasser, 1990a, Kaltwasser, 1990b, Brudzynski et al., 1991, Brudzynski and Ociepa, 1992). It has been postulated for this social context that 22-kHz calls occur as an alarm call directed to members of the colony and other conspecifics (Blanchard and Blanchard, 1980, Blanchard et al., 1990a, Blanchard et al., 1990b, Blanchard et al., 1991). The alarm calls would inform the colony about approaching or potential danger (e.g. a predator). The rat-receivers in the colony do not need to be in a close proximity to the caller but should remain within hearing range. This is particularly evident during presentation of a predator (cat) in an open surface of the visible burrow system (Blanchard and Blanchard, 1989). Exposure of a cat to individually kept rats prompted no ultrasonic calls, while a similar presentation of the predator was effective in inducing 22-kHz calls in an established colony, where other familiar conspecifics were present in the vicinity (Blanchard et al., 1991). The communicatory and alarming properties of 22-kHz calls have been well documented by the responses of the receivers of these calls (Sales, 1991, Blanchard et al., 1991, Blanchard et al., 1992, Brudzynski and Chiu, 1995, Brudzynski, 2001, Nobre and Brandão, 2004).

In established rat colonies, ultrasonic communication with 22 kHz calls represents a higher order of defensive organization because the entire colony may benefit from the defensive behavior of one individual (Blanchard et al., 1991, Brudzynski, 2001). This behavior is regarded as a successful social strategy (Edmunds, 1974, Brudzynski, 2001). Many different menacing events (e.g., approach or presence of a predator or any large animal, startling stimuli, etc.) can be communicated to the entire colony with obvious benefits for the colony's survival. For example, it was found that 22-kHz alarm calls emitted by a dominant rat in response to a cat caused all members of the colony to run for cover and remain hidden in the burrows for more than 5 h (average time to enter the surface area), even though the cat was removed hours ago (Blanchard and Blanchard, 1989).

In the present study, the calls were induced by an air puff, which is a startling stimulus with both acoustic and tactile properties (Taylor et al., 1991), and has been reported to induce defensive behavior (Knapp and Pohorecky, 1995). This is a reliable method of inducing 22-kHz calls, which have been suggested to reflect fear and anxiety (Knapp and Pohorecky, 1995, Naito et al., 2003, Sánchez, 2003), as well as general distress (Molewijk et al., 1995, Spencer et al., 2004), and which were, in turn, believed to be a trigger for the alarm response. However, the most recent behavioral–pharmacological study, using Pavlovian conditioning to an electric shock, suggested that anxiety and not fear is the driving force behind the 22-kHz calls emitted between application of conditioning stimuli (Jelen et al., 2003).

One of the purposes of the present study was to re-analyze and characterize acoustic features of 22-kHz alarm calls of adult laboratory rats using direct recordings without frequency division (i.e. without lowering the original sound frequency to the audible range). Previous studies of ultrasonic calls in rats were done with the aid of a bat detector, a device which converts ultrasounds to the audible range of frequencies. The frequency conversion could potentially distort the signal or cause the loss of some of its original acoustic features (e.g. distortion of the maximum frequency, Holland and Brudzynski, 2004). Thus, the present analysis was conducted using direct digitized recordings from the microphone to the computer hard drive. Furthermore, an attempt was made to analyze the calls via a reliable automated acoustic analysis system, which allowed for the inclusion of large numbers of calls to a detailed analysis.

Section snippets

Animals

The study was performed on 10 adult male Wistar rats (Charles River, St Constant, Quebec, Canada). At the time of the experiment, body weight of individual animals was between 260 and 350 g. After arrival, animals were housed in polycarbonate cages in pairs with dust-free wood chip bedding and plastic dark hiding compartments. The cages were kept in a common room with a constant ambient temperature of 22±2 °C and with 12:12 h light-dark cycle. Animals received standard pellet Rodent Lab Diet #

Results

All calls emitted by rats (up to 110 calls per rat) were collected and analyzed. A total of 689 calls were collected from all vocalizing animals with 110 calls per rat, with exception of two animals with approximately 70 calls per rat. These calls were also included in the analysis because their average acoustic values were within similar limits as others. Except one animal, rats did not vocalize after the first air puff. They needed 2–12 successive air puffs to emit their first alarm call.

Discussion

Emission of 22-kHz calls by a single rat without an immediate contact with the home-cage companion or with the colony is regarded as defensive and alarming in nature. The terms ‘defensive’ and ‘alarming’ pertain in this context to the social group, hence, 22-kHz calls can be discussed as true social communicatory signals. The alarming property of the 22-kHz calls might not hold in other behavioral situations, in which two individuals interact directly in a close proximity to one another,

Acknowledgements

The authors express their thanks to Drs Robert and Caroline Blanchard, Departments of Psychology and Neurobiology, University of Hawaii, for their supportive and insightful discussion. The help of Dr Edward Sternin, Department of Physics, Brock University is also appreciated. The study was supported by a grant from the Natural Sciences and Engineering Research Council of Canada to S.M.B.

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