1.Introduction: Definitions, Background -- 2. Competition For Limiting Resources, Hamilton’s Rule And Chesson’s R* -- 3. Flexible And Derived Varieties of Mammalian Social Organization: Promiscuity In Aggregations May Have Served As A Recent “Toolkit” Giving Rise To “Sexual Segregation”, Polygynous Social Structures, Monogamy, Polyandry And Leks Abstract -- 4. Multimale-Multifemale Groups And “Nested” Architectures: Collaboration Among Mammalian Males -- 5. Higher “Grades” Of Sociality In Class Mammalia: Primitive Eusociality -- 6. Ecological Models As Working Paradigms For “Unpacking” Positive And Negative Interactions Among Social Mammals -- 7. Mechanisms Underlying The Behavioral Ecology Of Group Formation -- 8. The Evolution Of Mammalian Sociality By Sexual Selection -- 9. Proximate Causation: Functional Traits And The Ubiquity Of Signaler To Receiver Interactions: From Biochemical To Whole Organism Levels Of Mammalian Social Organization -- 10. Synopsis.

This brief discusses factors associated with group formation, group maintenance, group population structure, and other events and processes (e.g., physiology, behavior) related to mammalian social evolution. Within- and between-lineages, features of prehistoric and extant social mammals, patterns and linkages are discussed as components of a possible social “tool-kit”.  "Top-down” (predators to nutrients), as well as “bottom-up” (nutrients to predators) effects are assessed.  The present synthesis also emphasizes outcomes of Hebbian (synaptic) decisions on Malthusian parameters (growth rates of populations) and their consequences for (shifting) mean fitnesses of populations.  Ecology and evolution (EcoEvo) are connected via the organism’s “norms of reaction” (genotype x environment interactions; life-history tradeoffs of reproduction, survival, and growth) exposed to selection, with the success of genotypes influenced by intensities of selection as well as neutral (e.g. mutation rates) and stochastic effects.  At every turn, life history trajectories are assumed to arise from “decisions” made by types responding to competition for limiting resources constrained by Hamilton’s rule (inclusive fitness operations).