Attracting a mate is an expensive endeavour – there are dances to learn, the right outfits to wear and an entire array of vocalizations to perfect. Because males are usually the sex that does the searching, most scientists have focused on the costs that they bear. But in a few species conditions can arise that cause females to invest in finding a mate. In these situations, females have to balance several costs: the potential cost of not producing any offspring if she fails to find a mate, the cost of mating itself and the cost of signalling to attract a mate.

To best balance these costs, theory predicts that females should increase their signalling effort the longer that they spend without a mate, as the potential cost of failing to find a mate increases. Leigh Simmons, a researcher from the University of Western Australia, decided to test this hypothesis using the solitary ground-nesting Dawson's bee. As males only search for females immediately after the females emerge as adults, females who get left out during the first pass must find ways later to attract the attention of males by using a particular blend of chemicals found on their cuticles that function as a pheromone perfume. Would females that had failed to mate the first time round invest more in the chemical composition of their cuticle chemicals to attract a male on the rebound?

First, Simmons nabbed 50 female bees right as they emerged from their burrows. He froze one set immediately, then isolated another set in a sort of bee nunnery away from males for a day and then froze them. Finally, he unleashed the remaining females to a group of male bees, allowing some to mate and perform their post-copulatory courtship behaviour, while others only mated (to isolate the effects of insemination on cuticle chemistry); and a final group was allowed to mate and nest after. This gave him five groups across which to compare the composition of their cuticle covering: one that was young and unmated, another that was older and unmated, one that had mated but not had post-copulatory courtship, one that had mated and had post-copulatory courtship, and one that was allowed to have all the normal mating behaviours and then nest afterwards. Simmons then used hexane to dissolve the chemicals from the cuticle of each bee before using gas chromatography to identify and quantify each chemical.

He found a total of 21 compounds that dissolved out of the female bees’ cuticles, and the composition of the cuticle chemicals extracted from the female bees that had been unmated for a day had altered significantly. Meanwhile, the cuticle components of the bees that had mated (either with or without post-copulatory courtship) were more like those of the freshly emerged bees, and the chemical composition of the nesting bees’ cuticles differed from that of all of the other groups. So, the unmated female bees had altered the composition of their cuticle chemicals, while mated female bees did not.

Not a lot is known about the costs of pheromone signalling or the mechanisms by which females can change their signalling, but in desperate times, it seems that at least female Dawson's bees break out their best perfume to find that lucky guy.

Sexual signalling by females: do unmated females increase their signalling effort?
Biol. Lett.