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Busy Bees: An Up-Close Look at One Bee Species’ Scramble to Mate

In a large aggregation, hundreds or thousands of male Diadasia rinconis bees patrol the area where females nest, and they compete fiercely for the chance to mate with females that emerge, forming “mating balls,” in which multiple males grapple with each other and attempt to secure a female. (Video originally published supplementary to Russell et al 2018, Journal of Insect Science.)

Common to the Sonoran Desert in the Southwestern United States, the bee species Diadasia rinconis is deemed a “solitary bee,” though you’d be forgiven if you thought that’s a bit of a misnomer.

The term “solitary” in bees contrasts such species with their eusocial, hive-dwelling cousins. Solitary bees don’t follow a caste system or coordinate work the way honey bees do, for instance, but you can often still find a lot of solitary bees all in one place.

For instance, behold the D. rinconis mating “scramble,” in the video at the top of this post. In a large aggregation, hundreds or thousands of male Diadasia rinconis bees patrol the area where females nest, and they compete fiercely for the chance to mate with females that emerge, forming “mating balls,” in which multiple males grapple with each other and attempt to secure a female.

Avery Russell, Ph.D., is a postdoctoral research fellow at the University of Pittsburgh, and he studied bees such as Diadasia in his graduate work at the University of Arizona (UA). He has witnessed the D. rinconis mating scramble first hand. “It was stunning,” he says. “Walking among thousands of bees that were flying just centimeters above the soil and going about their business was truly impressive.”

Russell and colleagues at UA trekked out to the desert in May 2016 and 2017 with high-speed, high-definition cameras and audio recording tools in hand to study D. rinconis mating. Beyond just observing the scrambles, they wanted to get an up-close view of the bees mating to learn more about their courtship and the selective pressures those behaviors produce. Findings from that work are reported in a new study published in August in the open-access Journal of Insect Science.

“Knowing more about where, when, and how solitary bees mate can help us to protect their natural environments,” Russell says. “Similarly, there’s substantial interest in using native bees for agriculture: without knowing their mating habits, it’d be hard to rear any sizeable number. Finally, there’s relatively little known about the biology of solitary bees in general and more studies are necessary to understand what drives the evolution and success of their behavior.”

By capturing footage at such high speed and resolution, the researchers were able to analyze the relationship between male bee size, occurrence and duration of mating, and their patterns of motions during mating.

Researchers at the University of Arizona filmed mating pairs of Diadasia rinconis with high-speed, high-definition cameras in the course of a study in 2016 and 2017, allowing observation and analysis of the bees’ mating behavior in a level of detail never previously achieved. They noted consistent patterns in the sequence of pulsing sounds and movements such as antenna raising, wing flicking, leg stroking, and rocking side to side. (Video originally published supplementary to Russell et al 2018, Journal of Insect Science.)

Overall, they team found an apparent advantage for larger males over smaller ones in mating success, as males found in mating pairs were larger on average than randomly selected males patrolling the aggregation. However, they weren’t able to compare the size of males that entered into mating balls with those that emerged paired with the females, so it’s unclear whether size is an advantage in the mating ball or if females tend to prefer larger males. The researchers also found that larger males spent less time in each mating instance than smaller males did, which they suggest could represent a trade-off: whereas large males are more likely to successfully mate again with other females, smaller males are less likely to do so and may compensate by investing more time in the mating chances they do get.

The detailed analysis provides many new insights into the mating behavior of D. rinconis bees, but “lots of questions remain,” Russell says. “We would like to go back to this system in the future. For instance, it would be wonderful to determine whether females also mate outside of the mating aggregation—and the size of the males acquiring those matings—and whether cryptic female choice occurs—can they reject male sperm after mating?”

Russell also notes that the prospects for researching such ephemeral behavior are brightening with advancing camera technology. “High-definition, high-speed cameras have made it possible to record and precisely characterize behavior in unprecedented detail,” he says. “Insects are small and many important insect behaviors—buzz pollination, courtship, flight, et cetera—happen extremely quickly. Without such technology we would miss most of the behavior.”

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