Studies that compared neural AVP, AVT, or V1a in males of different species. Comparisons are interpreted in the context of the inverted-U function that characterizes resource defense theory. Social organization is shown on the x-axis as a proxy for population density. The mating system that corresponds to each type of social organization is shown above the x-axis. For each inter-species study discussed in the text, each species is placed on the x-axis according to its species-typical social organization and mating system, and the levels of AVP, AVT, V1a in males for the two (or more) species compared are plotted on the y-axis relative to each other (without units). (A) In mammals, males of species with larger territories also exhibit greater septal circuit AVP and V1a. (B) In estrildid finches, mating system is dissociated from social organization, indicating that ecological factors other than population density or resource density shape mating system in these species. Studies all used monogamous species and AVT and V1a are not associated with territoriality or mating system. (C) In fishes, males of species with larger territories also have higher levels of neural AVT and V1a. 1V1a density in lateral septum [65, 135, 136], 2AVP-expressing axons in lateral septum , 3V1a density in lateral septum , 4V1a density in lateral septum , 5AVT neurons in BNST, baseline Fos levels in AVT neurons and social induction of Fos in AVT neurons, receptor binding density in lateral septum [61, 147], 6Density of axons in ventral part of ventral telencephalon and size of cells in the gigantocellular POA that expressed AVT [95, 149], 7V1a2 mRNA in the magnocellular and gigantocellular regions of the POA .