Female red mason bees, Osmia bicornis, from the U.K. don’t usually mate with males of the same species from Germany. It’s in the way he vibrates. Males living in different regions vibrate their thorax differently. But with the help of magnets, German males can mimic the moves of their British counterparts and successfully copulate with British females. The findings, published in Current Biology this week, suggest for the first time that bees use vibrations in addition to chemical signals (like sex pheromones) to help recognize their mates.
During courtship, a male red mason bee embraces a female while sitting on her back and vibrating his thorax, rubbing himself against her, and passing his antennae over hers and his forelegs over her eyes. After all this, she can either accept him for copulation or reject him and throw him off her back. You can watch a video of a male trying to copulate (unsuccessfully) with a female here. Researchers thought that the length of uninterrupted vibrations was a sign of male strength and fitness, but this hasn’t really been tested. Perhaps vibrations carry information about their identity. There are two main subspecies in Europe – O. bicornis rufa and O. bicornis cornigera – and the only way to tell them apart just by looking is the color of the hair on the tip of their abdomen (one is red, the other is black).
University of Ulm’s Taina Conrad and Manfred Ayasse devised a test to see if male vibrations signal where they come from. They glued a small magnet onto the thorax of a British or German male using resin. After he initiated his pre-copulatory embrace, the pair was placed onto an inductor connected to a frequency generator. The team then transferred the signal of a male from the opposite subspecies though the electromagnetic field onto the magnet. Males bearing the magnetic strap-on vibrated accordingly.
British females became receptive to German males, German females became receptive to British males. It seems females prefer males from their own region, the team found, and vibrations are the main signals used to make this choice. "We were really surprised to find that bees use vibrational signals not only as a sign for fitness but also for information on where a male comes from," Conrad says in a statement. "This is complex information, and we did not expect this to be encoded in this signal."
That females show a strong preference for males from their same region demonstrates divergence between the two subspecies. Female choice is thought to be one of the main types of sexual selection, Conrad explained to IFLScience, and examples range from bird feathers to fish colors to frog calls. Observing speciation is difficult because of the long timeframe, but we might be looking at the first steps of a separation.
Image in the text: T. Conrad & M. Ayasse, Current Biology 2015