Body Size Variation in a Social Sweat Bee, Halictus ligatus (Halictidae, Apoidea), across Urban Environments.

Abstract

Simple SummaryMany animal species that dwell in cities have altered aspects of their behavior, morphology, and physiology in order to survive in human-dominated environments. One way in which animals can adapt to survive in novel habitats is by shifting their body size. Body size is an important and flexible trait for insects because the ability to vary body size is linked to better survival and reproduction. In this study, we quantified body size variation in a species of sweat bee and compared the variation between bees residing in three different urban cities. Though studies have assessed urban bee body size previously, this is the first to compare bees from different cities. Similar to the human experience, no two cities are alike for bees. Therefore, we predicted that bees would show differences in the spread of body size in order to adapt to each unique city. We found that bees in three different environments all showed high variation in body size, but that the variation differed depending on location. This study is one of the first multi-city studies, and this is a trend we hope continues as urban research advances.High morphological variation is often associated with species longevity, and it is hypothesized that urban-dwelling species may require more plasticity in functional traits such as body size in order to maximize fitness in heterogeneous environments. There has been published research regarding the functional trait diversity of urban bee pollinators. However, no two cities are identical, so the implementation of multi-city studies is vital. Therefore, we compared body size variation in female Halicus ligatus sweat bees from May–October 2016 from three distinct Midwestern United States cities: Chicago, Detroit, and Saint Louis. Additionally, to elucidate potentially influential environmental factors, we assessed the relationship between temperature and measured body size. We collected bees in community gardens and urban farms and measured their head width and intertegular distance as a proxy for overall body size. We utilized an ANCOVA to determine whether body size variation differed significantly across the three surveyed cities. Results indicated that H. ligatus females in Chicago, Detroit, and Saint Louis had significantly different body size ranges. These findings highlight the importance of intraspecific body size variation and support our prediction that bees from different urban environments will have distinct ranges in body size due to local ecological factors affecting their populations. Additionally, we found a significant influence of temperature, though this is probably not the only important ecological characteristic impacting bee body size. Therefore, we also provided a list of predictions for the future study of specific variables that are likely to impact functional trait diversity in urban bees.