Assessing social signals, such as vocalizations, figures prominently in the outcome of aggressive encounters, including the potential to win a fight or prevent escalation resulting in physical injury. Among vertebrates, the neural circuitry integrating sensation, for example what an individual hears, with modulation of aggressive output, is distributed throughout the forebrain and conserved across vertebrate lineages. Social behavior is thought to be an emergent property of activity across this network, and audition plays a prominent role in modulating aggression in primates and birds. In mammals, although sensory integration within specific forebrain regions is thought to strongly influence aggressive behavior, approaches to examining and understanding this integration remain a challenge, in part because of the large size of mammalian brains and limits on brain-wide optical access to the full network. To overcome this problem, we propose a new model, Danionella dracula, a genetically tractable miniature species of fish that is transparent into adulthood, produces robust sonic and visual (postural) signals solely in aggressive contexts, and a close relative of zebrafish (Danio rerio), which are not sonic. Genetic toolkits developed in zebrafish are readily adapted in D. dracula. Our long- term goal is to use this new model to test if an evolutionarily conserved forebrain network of neuronal populations is activated during aggression, sensitive to acoustic signals, and, using optogenetic approaches, can be manipulated at the cellular level to effect aggressive behavior. To reach this goal, we propose in Aim 1 to map a forebrain aggression network in D. dracula and use cutting-edge methods for brain-wide 2 and 3- photon calcium imaging of auditory-responsive neurons to identify forebrain regions involved specifically in acoustic-related aggression. Aim 2’s goal is to map projections and gain genetic access to a brain region identified in Aim 1 for identifying connectivity and causal testing of a role for this region in aggression and on activity across the aggression network. To this end, we will generate a transgenic line with GCaMP and photoactivatable green fluorescent protein (paGFP), that will allow visualization of projections in functionally characterized neurons, uncovering anatomical pathways between auditory neurons within aggression circuits. Using a promoter characterized in zebrafish Gal4 lines we will create a transgenic D. dracula line expressing an opsin in a population of excitatory neurons in an auditory-sensitive region and determine their connectivity to other candidate brain areas involved in aggression. Completion of these aims will provide the necessary transgenic lines and methods to establish D. dracula as a new vertebrate model for neural circuits of aggression. The small size, transparency, and robust sonic-aggressive behaviors of this species’ adults, and phylogenetic relationship to zebrafish will allow genetic and brai...