ReviewUltrasonic vocalizations emitted during dyadic interactions in female mice: A possible index of sociability?
Introduction
Ultrasonic vocalizations (UVs) emitted by adult mice for long time have been considered as a component of the sexual behavioral repertoire of males [42]. Sewell [43] was the first to detect UVs also during social interaction in pairs of female mice, especially when they were engaged in olfactory investigation of the other. Subsequently, sporadic studies reporting ultrasonic calls in female mice [15], [16], [45] were published until Maggio and Whitney [29] demonstrated that female mice of different genotypes emitted a large number of UVs during encounters with other females, at rates comparable to those of the males. Despite this convincing study, the opinion persisted that mouse UVs were a chiefly male trait [52].
In the last few years, we have started a line of research aimed at understanding the occurrence and function of UVs in female mice. In particular, using the resident/intruder experimental paradigm, we have demonstrated that the resident female emitted a great number of ultrasonic calls when a same sex partner was introduced in the cage [33]. The ultrasounds recorded were emitted by the resident animal, as demonstrated by several experiments performed with either anesthetized resident or intruder [12], [29]. As recently confirmed in BALB/c mice, ultrasonic vocalizations recorded during female–female encounters ranged from 50 to 70 kHz [23] and are emitted during the first minutes of social interaction. This is in line with data previously reported in female [29], as well as in male mice [42] showing that the majority of vocalizations can be detected in concomitance with high levels of social investigation. Olfactory investigation is very important for the individual identification, allowing animals to gain information on the features (i.e. sex, rank and reproductive state) of a conspecific [7].
We have investigated the emission of UVs during social recognition. Individual recognition of conspecifics, is a crucial prerequisite for a wide range of social behaviors such as the development of bonds and the establishment of hierarchies that limit aggressive interactions and allow group living [24]. According to its original design, the social recognition/social discrimination test was based on the natural propensity of a mouse to investigate another mouse introduced into its home cage [14], [17]. If a different, novel, conspecific was presented, the initial level of social investigation could be reinstated in the resident mouse (dishabituation). We explored the possibility that the observed decrease of ultrasonic vocalizations detected during the re-exposure to the same intruder could have been the consequence of the reduction in social interest of the resident towards the already familiar animal. Thus, we hypothesized that this reduction in UVs could be used as a measure of social recognition [12]. Indeed, when re-exposed to the same partner, there was a decline in the number of UVs emitted by resident mice when 15, 30, or 60 min passed elapsed between the two social sessions. After 24 h, this effect disappeared. In contrast, when the resident was exposed to a novel female partner, the number of UVs remained unchanged. Interestingly, female mice treated with the classic cholinergic antagonist scopolamine when re-exposed to the familiar conspecific after 30 min from the first social exposure did not show the expected decrease in UVs. Scopolamine seemed specifically disrupting the social memory process and not the UVs emission per se. Indeed, at the same dose this drug did not affect the UVs of female mice exposed to an unfamiliar partner. We thus have provided behavioral and pharmacological evidences that ultrasonic calls could be used as an index of social memory in female mice. Moreover, we proved that this parameter was more sensitive than the measures of social investigation or social contacts as we did not find any significant decrease in these parameters. This test has been recently used in a neonatal neurotoxicological study in outbred CD-1 female mice allowing to detect treatment-induced differences in baseline levels of UVs, but not in the recognition capability, since all females showed the expected decrease in UVs when encountering the familiar partner [51].
In mice and rats, olfactory cues coming from conspecifics are also useful to acquire information about the food they have eaten [20], [21], [47]. This kind of phenomenon provides an experimental context suitable to study the ultrasonic vocalisation in parallel to the olfactory investigation. Interestingly, we found that the resident/observer female emitted a great number of ultrasonic calls during the first minute of social interaction, when the intruder had previously fed on a palatable food [33]. By contrast, fewer UVs were associated with partners fed on unpalatable food. These calls were associated with resident's intense olfactory investigation of the partner.
Data reviewed above strongly suggest that ultrasonic vocalizations are an important component of the female mice behavioural repertoire. Indeed, ultrasonic emission is a consistent and robust phenomenon during social interaction in females of this species that can be also “exploited” as an index of social interest in these animals. As already hypothesized for male calls during sexual interactions [38], these vocalizations seem to have an affiliative function facilitating proximity. This proximity is a pre-requisite for the acquisition of relevant information about the intruder by the resident female.
To better characterize this aspect of the female behavioral repertoire neglected so far, in the first part of the present paper we will report a series of experiment showing variations of UVs in female mice according to estrous status, reproductive condition and age. As for aging, rodent models have been used successfully to study the behavioral and neurophysiological changes associated with cognitive changes [22], but the majority of studies have focused on males. However, as males tend to perform better than females in spatial performance tests in general [5], it is important to set up behavioural tests not involving spatial navigation in order to investigate aging cognitive decline in female mice. Being the social recognition test based on UVs specifically designed for females [12], in the second part of this paper, we will expand our previous report on UVs and memory, testing the hypothesis that ultrasonic vocalizations can be used as a useful parameter to investigate age associated deficits in memory in female mice.
Section snippets
Subjects
Outbred albino NMRI mice were used in this study. They were purchased from Harlan, Italy, at about 45 days of age (weight 23–25 g) and housed in group of 4 in Plexiglas cages (40 cm × 23 cm × 15 cm) with sawdust on the floor and water and food available ad libitum. They were maintained on a 12 h light–dark cycle (light on at 7:00) at least for 2 weeks before animal testing. Room temperature was kept at 21 ± 1 °C.
Estrus cycle determination
At the end of the 3 min social interaction session, 40 resident females were checked for the
UVs during social interaction test and resident female estrous cycles
The correlation analysis indicated a significant positive relation between the number of UVs and the time spent by the resident in social investigation (r = 0.35, n = 39, p < 0.05). A negative correlation was found between UVs and cage exploration by the resident animal (r = −0.34, n = 39, p < 0.05) with no other significant correlations detected. The time spent in social investigation of the partner and in cage exploration were affected by the estrous phase (social investigation: F3,35 = 4.05, p = 0.01; cage
Discussion
In the present study we confirmed that during female–female mice social encounters the resident animal utters a considerable number of 70 kHz calls. Moreover, the number of calls seems to be modulated by the estrous phase of the emitter as non-sexually receptive females emit more UVs than receptive ones. The number of calls uttered during interaction with an unknown female partner significantly decreases with pregnancy and ageing. In addition we showed that ultrasounds could be used to assess
Acknowledgment
The financial support of Telethon, Italy (Grant No. GGP05220) is gratefully acknowledged.
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