Lateral view of the brain of a typical (A) tetrapod (the mammalian prairie vole, Microtus ochrogaster) and (B) teleost fish (the cichlid Astatotilapia burtoni). Surface features are shown for orientation as lower case letters. Deeper structures of the AVP (tetrapod) or AVT (teleost) circuit associated with territorial behaviour are shown as upper case letters. The black dots represent AVP- (or AVT-) producing neuron cell bodies with axons projecting to the lateral septum. The anatomy of the septal AVT circuit in birds and reptiles is similar to that of the septal AVP circuit in mammals. Teleosts do not possess an anatomical lateral septum, but the ventral region of the ventral telencephalon is thought to be its homologous equivalent. BNST: basal nucleus of the stria terminalis; cer: cerebellum; cor: cerebral cortex; hyp: hypothalamus; LS: lateral septum; MEA: medial amygdala; mid: midbrain; ob: olfactory bulb; ot: optic tectum; POA: preoptic area; sc: spinal cord; tel: telencephalon; VP: ventral pallidum.