Rodent ultrasonic vocalizations: behavioural significance

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Rats make many sounds beyond our hearing range. These ultrasonic vocalizations (USVs) appear to be used for communication with other rats, and we have discerned more than a dozen types of USVs. Different kinds of vocalizations predominate in different situations, and certain specific USVs may even reveal when a rat is experiencing positive or negative emotion. Traditionally, it has been thought that rodent's emotions are largely unknowable, primarily because rodents, unlike humans, cannot self-report. However, USVs seem to offer exactly this possibility, making them potentially unique among behavioural measures. Our MAIN GOAL is therefore to better understand what these various kinds of ultrasonic vocalizations signify, and how rats use them to communicate with other rats. So far, we have categorized USVs based on their acoustic properties, but we cannot be sure that rats would categorize these calls the same way as us. Therefore, we will identify which types of calls rats find attractive or aversive (Project 1). Next, we will investigate whether rats can discriminate between different call types when these calls are arranged to signal the availability of reward (Project 2). We will also explore whether some USVs help rats to avoid aggression or to establish social hierarchies (Projects 3 and 4). In particular, we hypothesize that submissive animals use a particular kind of USV ("flat" call) to signal appeasement/submission. To test this, we will study rats in an aggressive context or when competing for food. In Project 5, we will extend our research to mice. Despite a burgeoning literature, there has been very little research into whether any specific call types reflect the mice's emotional state. Therefore, guided by our recent studies of rat USVs, we will investigate whether the administration of rewarding or aversive drugs promotes particular types of call. Devocalization studies can help to establish how and when rodent ultrasonic calls are used for communication. However, this approach currently requires laryngeal nerve section, which is irreversible and causes undesirable side-effects. Project 6 therefore seeks to develop methods to transiently devocalize animals using chemogenetic and optogenetic approaches. Conceptually, this research is part of my larger aim to understand how different aspects of "reward" (positive emotions, positive reinforcement, etc) interrelate. In the longer term, our research could lead to several practical benefits. For example, if ultrasonic vocalizations do indeed reveal different emotional states, they could provide a useful and novel approach for studies linking brain activity to emotions such as pleasure and distress. It may also be possible to use specific types of USVs in order to screen for novel drug therapies for depression and chronic pain, for example. Lastly, this basic research may one day lead to more effective and humane methods for controlling rodent populations that ravage food supplies.