In social insects, changes in behavior are often accompanied by structural changes in the brain. This neuroplasticity may come with experience (experience-dependent) or age (experience-expectant). Yet, the evolutionary relationship between neuroplasticity and sociality is unclear, because we know little about neuroplasticity in the solitary relatives of social species. We used confocal microscopy to measure brain changes in response to age and experience in a solitary halictid bee (Nomia melanderi). First, we compared the volume of individual brain regions among newly-emerged females, laboratory females deprived of reproductive and foraging experience, and free-flying, nesting females. Experience, but not age, led to significant expansion of the mushroom bodies—higher-order processing centers associated with learning and memory. Next, we investigated how social experience influences neuroplasticity by comparing the brains of females kept in the laboratory either alone or paired with another female. Paired females had significantly larger olfactory regions of the mushroom bodies. Together, these experimental results indicate that experience-dependent neuroplasticity is common to both solitary and social taxa, whereas experience-expectant neuroplasticity may be an adaptation to life in a social colony. Further, neuroplasticity in response to social chemical signals may have facilitated the evolution of sociality.

We conducted this study in Touchet, WA, USA between 27 May–19 June 2016 (Experiment 1) and 29 May–27 June 2018 (Experiment 2). Newly-emerged (NE; < 24-h old) female alkali bees were collected while leaving their natal nests by placing traps over bee beds. For Experiment 1, there were 3 treatment groups: 1) newly-emerged (NE), 2) lab-reared, and 3) reproductive females. NE females were sacrificed immediately (< 24-h old) upon returning to the laboratory, whereas lab-reared females were housed individually in the laboratory for 10 d. Reproductive females (unknown age) were collected from bee beds and sacrificed upon returning to the laboratory. For Experiment 2, NE bees were randomly assigned to one of two treatment groups: 1) solo or 2) paired. Solo females were kept alone for 10 d, and paired females were given a nesting female cage-mate for the same duration. For paired bees, only the brains of focal females were used (i.e., we did not dissect the brains of cage-mates).

The deposited datasets include raw volumetric measurements from separate brain regions. These volumes were obtained using autofluorescence-based confocal microscopy (Zeiss LMS 710, Jena, Germany) on whole-brain mounts to generate image stacks for all samples. Each structure and the whole brain were traced individually on each slice of the confocal stack. Experiment 1 and 2 trace intervals were every 5 µm and 10 µm optical slice, respectively. Volumetric measurements were generated using serial reconstruction.