Bumblebees Are Larger in Cities, Study Finds

Living in cities has an impact on bumblebee size. Bees are larger in urban areas and, because of their increased heft, they’re more productive than their rural relatives, according to new research.

City life has advantages and disadvantages for bumblebees. Gardens, yards, and parks offer lots of potential food sources. However, cities are warmer than rural areas and bumblebee habitats are broken up by long stretches of concrete and building, causing fragmented environments.

A team of biologists from Germany’s Martin Luther University Halle-Wittenberg (MLU) and the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig were curious about how urban development affects bumblebee evolution. They collected more than 1,800 bumblebees from nine German cities and their corresponding rural surroundings. All the urban locations were botanical gardens and parks full of flowering plants. The rural sites had a buffer of at least 6.2 miles (10 kilometers) from the urban sites, had a low density of roads, and were filled with semi‐natural vegetation.

The biologists focused on three species abundant in the area and widespread in Europe: the red-tailed bumblebee (Bombus lapidarius), the common carder bee (Bombus pascuorum), and the buff-tailed bumblebee (Bombus terrestris).

At each site, the researchers placed potted red clover plants — a bumblebee favorite. They left the plants in each location for five days as a reference for pollination.

At the end of each period, the researchers used a hand net to collect as many bumblebees as they could from each species. They measured the body size of each bee they caught and also counted the average number of seeds produced per each red clover plant in each location.

Their findings, published in the journal Evolutionary Applications, showed that bumblebees from urban areas were larger than their rural counterparts by about 4%. The results were similar for all three species.

The difference in body size may be due to the fact that the bumblebees’ habitats in urban areas are becoming increasingly fragmented.

“Cities are obviously fragmented environments. Parks and gardens, the places in cities where bees can find food resources and nesting possibilities, are usually small and isolated and movement across them is potentially very difficult,” lead researcher Panagiotis Theodorou tells Treehugger. “Nevertheless, bumblebees are common in cities, which they seem to prefer to the even more unnatural modern agricultural landscape.”

Why Size Matters

Bumblebees come in many different sizes. Earlier research found that bigger bees can fly longer distances when foraging for food.

“Being big should therefore be an advantage in the fragmented cityscape, if it enables bees to more easily move from one fragment of vegetation to another,” Theodorou said. “Therefore we reasoned that, if fragmentation is actually imposing a challenge on bumblebees, they should respond to that challenge by being larger.”

Larger bumblebees have better vision, bigger brains, and better memory, says study co-author biologist Antonella Soro. They can travel farther and are less likely to be attacked by predators. They are also better pollinators because they can pollinate more flowers.

“Workers of a bumblebee colony, despite being highly related, display as much as a tenfold difference in body size,” Soro tells Treehugger. “We speculate that a fragmented habitat such as the urban one ‘browses’ through this variability and phenotypically selects the size of bees that better matches that habitat. Habitat matching is thought to be particularly relevant for mobile organisms (and bumblebees are very mobile), which, by moving through the landscape, can find the environmental conditions that best match their phenotype.”

The study’s findings point to how habitat fragmentation could indirectly impact pollination. The researchers say more studies are necessary to further understand how bees respond to urbanization and how that research can be used in urban planning.