Em http://www.nature.com/articles/s41467-016-0012-yChristoph Gruter, Francisca H. I. D. Segers, Cristiano Menezes, Ayrton Vollet-Neto, Tiago Falcon,Lucas von Zuben, Marcia M. G. Bitondi, Fabio S. Nascimento & Eduardo A. B. AlmeidaNature Communications 8, Article number: 4(2017)doi:10.1038/s41467-016-0012-yDownload CitationBehavioural ecologyEntomologyEvolutionEvolutionary ecologyReceived:31 August 2016Accepted:16 November 2016Published online:23 February 2017AbstractThe differentiation of workers into morphological castes represents an important evolutionary innovation that is thought to improve division of labor in insect societies. Given the potential benefits of task-related worker differentiation, it is puzzling that physical worker castes, such as soldiers, are extremely rare in social bees and absent in wasps. Following the recent discovery of soldiers in a stingless bee, we studied the occurrence of worker differentiation in 28 stingless bee species from Brazil and found that several species have specialized soldiers for colony defence. Our results reveal that worker differentiation evolved repeatedly during the last ~ 25 million years and coincided with the emergence of parasitic robber bees, a major threat to many stingless bee species. Furthermore, our data suggest that these robbers are a driving force behind the evolution of worker differentiation as targets of robber bees are four times more likely to have nest guards of increased size than non-targets. These findings reveal unexpected diversity in the social organization of stingless bees.IntroductionDivision of labor among cells, organs, or individuals is a fundamental feature of complex biological systems1,2,3. In social insects, division of labor among workers is widespread and the most advanced forms of division of labor are found in species with morphologically distinct worker phenotypes4,5,6. In many ant and termite species, for example, colony defence is performed by a soldier caste (or sub-caste)4,5,6. Having workers with morphological adaptations for specific tasks such as foraging or defence is likely to improve colony functioning and performance because workers are more efficient at performing these tasks7,8,9. For example, more specialized ant soldiers (or majors) are more effective at nest defense10, whereas minors are better at brood care11. Given the benefits of task-related worker differentiation, it is puzzling that physical worker castes are extremely rare in social bees9 and absent in wasps6. It has been argued that developmental constraints12,13, individual-level selection5,13, the presence of a powerful sting5 or the fact that colonies with winged workers can more easily avoid aggressive interactions5 might prevent the evolution of physical castes in bees and wasps.Division of labor is mainly based on temporal castes in both groups of highly eusocial bees, the honey bees (Apini) and the stingless bees (Meliponini)5,6,14,15: workers first perform nursing duties inside the nest before moving on to general nest maintenance duties and, finally, they perform the outside tasks of guarding and foraging. However, the recent discovery of soldiers in the Neotropical stingless bee Tetragonisca angustula9 suggests that more complex caste systems might exist in this relatively understudied tribe. In T. angustula, colonies are defended by a small16 but dedicated group17 of entrance guards that are both larger (~ 30%) and of different shape than their nestmates9. Having larger soldiers is beneficial for colonies because body size is directly linked to the fighting ability of T. angustula guards9.Given this discovery