Repeated evolution of soldier sub-castes suggests parasitism drives social complexity in stingless bees

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.

Cryptic castes, social context and colony defence in a social bee, Tetragonula carbonaria

“…A larger body and greater head width may benefit guards, who are most likely to engage intruders in physical contests, where a large body size is likely to be useful. Our results are consistent with a recent study documenting worker differentiation in ten other stingless bee species (Grüter et al., ), where size and colour dimorphism is associated with previously unknown guard castes. Age polyethism (Wilson, ) is not precluded as a driver of task allocation in T. carbonaria : younger workers may remain in the nest, engaging in in‐nest duties, subsequently adopting either foraging or guarding tasks depending upon their sensilla density.…”

Colony-level non-associative plasticity of alarm responses in the stingless honey bee, Tetragonisca angustula

“…2014; von Zuben et al 2016; Grüter et al 2017b). In the context of a non-nestmate or heterospecific scout discovering the nest entrance, the goals of a colony should be as follows.…”

“…The nests of T. angustula have a single tube-like defense structure with a single nest entrance/exit, patrolled by morphologically distinct guards (Grüter et al 2012; Hammel et al 2016; Grüter et al 2017a; Grüter et al 2017b). These guards perform two types of guarding behaviors, standing and hovering.…”

Maternal investment in a bee species with facultative nest guarding and males heavier than females

“…Therefore, it is possible to measure the maternal investment by weighing offspring (Tepedino & Torchio, 1982;Bosch & Vicens, 2002) or by using another proxy parameter that is correlated with weight. The most commonly used parameter of size in aculeate Hymenoptera is the head width (Stark, 1992;Strohm & Linsenmair, 1997b;Roulston & Cane, 2000;Bosch & Vicens, 2002;Grüter et al, 2017;Dew et al, 2018). Body mass and head width are also correlated in Ceratina bees (Rehan & Richards, 2010a;.…”