Social behaviour of multiple females sharing a nest has been documented for two bee families, seven tribes and genera including over 2000 species – 8% of all nonparasitic bees. Early in social bee evolution, a single female produced a female that became her nest helper. Brood protection, larger reproductive output, or female longevity benefited the coexisting females. Many social bees live in tiny colonies and demonstrate elasticity in behaviour and reproductive options. Larger colonies ultimately evolved permanent castes – females unable to forage, build or defend a nest – and sterile helpers, and also evolved foraging coordination and honey storage. The colonies have stringent nesting requirements and advanced defensive behaviour, including stinging, biting and chemical defense, but also protected nest sites, timidity and crypsis. Advanced sociality was reached by Meliponini in Cretaceous times, and Eocene by Bombini and Apini. Other Apidae had some social behaviour since mid Cretaceous, as did Halictidae since Miocene, and both repeatedly lost it. Scarce resources and short flowering seasons force many kinds of social bees to enter diapause or disperse. In tropical forests, where highly social bees predominate, colonies of Apis are migratory and meliponines display aggressive foraging, regulated nesting or infrequent reproduction.

Key Concepts:

• Social bees must share a nest, but usually the totipotency of one or more females is, at least temporarily, compromised. Efficiency and fitness are realised in terms of individuals among solitary bees and among colonies for social species.
  
• Bee social behaviour evolved by the force of haplodiploid sex determination and kin selection in small groups and culminated with the presence of sterile females or those totally dependant on others for food and nesting.
  
• Colonies of bees are a bold experiment and many fail, but when some enjoy higher fitness than individual nesting females, they persist and at different levels of sociality. If colonies are too costly to maintain natural selection maintains solitary nesting females alone.
  
• Large colonies and uniform sociality are almost exclusively tropical, whereas most social bees within temperate climates have a colony, initiated by a single female or small group, which lasts for a relatively short time and then disbands.
  
• At the pinnacle of bee social evolution, there is honey, the most efficient energy storage medium among bees. Coordinated foraging by worker communication and prodigious food collection ability allow colonies to produce and store honey and to survive during poor foraging conditions.
  
• Advanced social bees are not close relatives. Although the oldest advanced social bee group is 100 million years (My) old, others range between 50 and 20 My in age – they are the meliponines, the honey bees, the bumble bees, the allodapines, the halictines and some neotropical euglossines or orchid bees.
  
• Social parasites are a by‐product or tradeoff of social bee evolution and such bees exist in all major groups.
  
• To defend their honey and brood, in perennial nests, advanced social bees have developed potent sting venom and alarm recruitment, or alternative chemical deterrents such as formic acid and intense biting behaviour; they also nest in well protected and highly limited nesting sites and reproduce infrequently.
  
• Permanence of sociality may result from relative absence of severe annual climate fluctuation.
  
• Social bees utilise many floral and other resources and are active for extended periods; they may serve to indicate environmental health.