Body size, demography and foraging in a socially plastic sweat bee: a common garden experiment

Abstract

Phenotypic plasticity may evolve when conditions vary temporally or spatially on a small enough scale. Plasticity is thought to play a central role in the early stages of evolutionary transitions, including major transitions such as those between non-sociality and sociality. The sweat bee Halictus rubicundus is of special interest in this respect, because it is socially plastic in the British Isles: Nests are social or non-social depending on the environment. However, sociality comprises a complex suite of interrelated traits. To further investigate social plasticity in H. rubicundus, we measured traits that are potentially integral to social phenotype at a northern site, where nests are nonsocial, and a southern site where nests can be social. We found that foundresses at non-social sites were smaller, produced offspring of a size more similar to themselves, initiated nesting later, and took longer to produce their first female offspring. They began provisioning earlier in the day, finished earlier, and collected more pollen loads. Common garden experiments suggested that these differences represent mainly plasticity, as expected for traits involved in the overall plastic social phenotype, with only limited evidence for fixed genetic differences in foraging. Conditions during overwintering did not have major effects on a foundress' subsequent behaviour.

Limited social plasticity in the socially polymorphic sweat bee Lasioglossum calceatum

“…Yanega 1997; Field et al 2010, 2012; Kapheim et al 2012, 2015a; Kocher et al 2013; Rehan and Toth 2015), and investigating these mechanisms requires taxa that straddle this transition (Field et al 2010; Rehan and Toth 2015). Socially polymorphic sweat bees (Hymenoptera: Halictidae) are ideal models for this purpose, because different populations of the same species exhibit either eusocial or solitary behaviour (Soucy and Danforth 2002; Chapuisat 2010).…”

“…Field transplants are critical to addressing this question and yet are rarely performed (Yanega 1997; Field et al 2012). Reciprocal transplants of the socially polymorphic sweat bee Halictus rubicundus Christ between social and solitary populations in the UK revealed that social phenotype is plastic with respect to the environment (Field et al 2010, 2012).…”

“…Reciprocal transplants of the socially polymorphic sweat bee Halictus rubicundus Christ between social and solitary populations in the UK revealed that social phenotype is plastic with respect to the environment (Field et al 2010, 2012). Evidence suggests that B1 females become workers only when emerging sufficiently early in the season (Field et al 2010; see also Hirata and Higashi 2008), and that nest foundresses may be able to adjust the size of B1 offspring depending on anticipated social phenotype (Field et al 2012, but see Field et al 2010). Conversely, significant mitochondrial differentiation exists between eusocial and solitary populations of North American H. rubicundus , suggesting that social phenotype might have a fixed genetic component (Soucy and Danforth 2002).…”

“…A laboratory common garden experiment also suggested that social phenotype may have a fixed genetic component in Lasioglossum albipes Fabricius (Plateaux-Quénu et al 2000). However, fixed genetic differences between social phenotypes have never been demonstrated experimentally in a natural field setting, which is critical to fully account for unmeasured environmental variables (Yanega 1997; Field et al 2012). …”

“…Critically, since offspring are mass provisioned, B1 body size may largely reflect investment decisions by foundresses at the time of provisioning (Plateaux-Quénu 1983; Richards and Packer 1994). If social phenotype is plastic, foundresses transplanted to the south from northern solitary populations might respond by producing small B1 offspring that in turn remain at their nest as workers, or perhaps initiate their own nests instead of entering hibernation (Field et al 2010, 2012). Instead, we find that most transplanted foundresses and their offspring show no evidence of plasticity, indicating that inter-population differences in social phenotype predominantly reflect genetic differentiation.…”