All animals evaluate the salience of external stimuli and integrate them with internal physiological information into adaptive behavior. Natural and sexual selection impinge on these processes, yet our understanding of behavioral decision-making mechanisms and their evolution is still very limited. Insights from mammals indicate that two neural circuits are of crucial importance in this context: the social behavior network and the mesolimbic reward system. Here we review evidence from neurochemical, tract-tracing, developmental, and functional lesion/stimulation studies that delineates homology relationships for most of the nodes of these two circuits across the five major vertebrate lineages: mammals, birds, reptiles, amphibians, and teleost fish. We provide for the first time a comprehensive comparative analysis of the two neural circuits and conclude that they were already present in early vertebrates. We also propose that these circuits form a larger social decision-making (SDM) network that regulates adaptive behavior. Our synthesis thus provides an important foundation for understanding the evolution of the neural mechanisms underlying reward processing and behavioral regulation. J. Comp. Neurol. 519:3599-3639, 2011. INDEXING TERMS: social behavior; comparative neuroanatomy; amphibian; reptile; bird; teleost; reward system; social behavior network; limbic system; neural circuitsThroughout their lives, all animals constantly face situations that provide either challenges (e.g., aggression, predation) or opportunities (e.g., reproduction, foraging, habitat selection) (for a detailed review, see O'Connell and Hofmann, 2011). In all cases, environmental cues are processed by sensory systems into a meaningful biological signal while internal physiological cues (e.g., condition, maturity) and prior experience are integrated at the same time. This process usually results in behavioral actions that are adaptive, i.e., beneficial to the animal. To accomplish this, an animal's nervous system must evaluate the salience of a stimulus and elicit a context-appropriate behavioral response. Despite tremendous progress in understanding the ecology and evolution of social behavior (Lorenz, 1952;Tinbergen, 1963;Lehrman, 1965;von Frisch, 1967;Krebs and Davies, 1993;Stephens, 2008), it is less understood where in the brain these decisions (e.g., about mate choice or territory defense) are made and how these brain circuits have arisen over the course of vertebrate evolution.Recent research has begun to decipher the neural basis of social decision-making. In mammals in particular, the neural circuits that evaluate stimulus salience and/or regulate social behavior have been uncovered to some degree: the mesolimbic reward system and social behavior network (Fig. 1). It is becoming increasingly clear that the reward system (including but not limited to the midbrain dopaminergic system) is the neural circuit where the salience of an external stimulus is evaluated (Deco and Rolls, 2005;Wickens et al., 2007), as appetitive beh...

Parental care, including feeding and protection of young, is essential for the survival as well as mental and physical well-being of the offspring. A large variety of parental behaviors has been described across species and sexes, raising fascinating questions about how animals identify the young and how brain circuits drive and modulate parental displays in males and females. Recent studies have begun to uncover a striking antagonistic interplay between brain systems underlying parental care and infant-directed aggression in both males and females, as well as a large range of intrinsic and environmentally driven neural modulation and plasticity. Improved understanding of the neural control of parental interactions in animals should provide novel insights into the complex issue of human parental care in both health and disease.

Complex phenotypes typically have a correspondingly multifaceted genetic component. However, the genotype-phenotype association between chemical defense and resistance is often simple: genetic changes in the binding site of a toxin alter how it affects its target. Some toxic organisms, such as poison frogs (Anura: Dendrobatidae), have defensive alkaloids that disrupt the function of ion channels, proteins that are crucial for nerve and muscle activity. Using protein-docking models, we predict that three major classes of poison frog alkaloids (histrionicotoxins, pumiliotoxins, and batrachotoxins) bind to similar sites in the highly conserved inner pore of the muscle voltage-gated sodium channel, Nav1.4. We predict that poison frogs are somewhat resistant to these compounds because they have six types of amino acid replacements in the Nav1.4 inner pore that are absent in all other frogs except for a distantly related alkaloid-defended frog from Madagascar, Mantella aurantiaca. Protein-docking models and comparative phylogenetics support the role of these replacements in alkaloid resistance. Taking into account the four independent origins of chemical defense in Dendrobatidae, phylogenetic patterns of the amino acid replacements suggest that 1) alkaloid resistance in Nav1.4 evolved independently at least seven times in these frogs, 2) variation in resistance-conferring replacements is likely a result of differences in alkaloid exposure across species, and 3) functional constraint shapes the evolution of the Nav1.4 inner pore. Our study is the first to demonstrate the genetic basis of autoresistance in frogs with alkaloid defenses.