Abstract
The 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.
Highlights
The differentiation of workers into morphological castes represents an important evolutionary innovation that is thought to improve division of labor in insect societies
Our results show that worker differentiation is common in Neotropical stingless bees and that the evolution of nest-entrance guards of increased body size is linked to the risk of being attacked by parasitic robber bees
Our results show that task-related worker differentiation is relatively common in stingless bees, with 10 of the 28 tested species having entrance guards that are significantly larger than the foragers of the same colonies
Summary
The differentiation of workers into morphological castes represents an important evolutionary innovation that is thought to improve division of labor in insect societies. Having larger soldiers is beneficial for colonies because body size is directly linked to the fighting ability of T. angustula guards[9] Given this discovery in a common Neotropical species, we tested if task-related worker differentiation is more widespread in stingless bees, the largest group of eusocial bees (>500 described species[18]). To this end, we compared the morphology of nest guards and foragers of 28 species from different areas in Brazil. Our results show that worker differentiation is common in Neotropical stingless bees and that the evolution of nest-entrance guards of increased body size is linked to the risk of being attacked by parasitic robber bees
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