Is love in the eye of the beholder? Perhaps among humans it is, but not so in other mammals. In the case of some bats, love is detected by nose. In the bat family Emballonuridae at least one of its 51 species (and likely more) uses the sense of smell to find the mate with the greatest genetic diversity.

The bat family, also called sac-winged bats, have bag-shaped glands in each wing that are open to the air. During courtship, males wave their wings in front of the females for a few seconds, dispersing the scent that comes out of these bags. “Males are continually trying to announce themselves by mixing an odorous cocktail with chemical components in their wing bags, making sure the molecules reach the female noses by fluttering the wings in front of them,” Pablo Santos, the lead author of the study, from the Leibniz Institute for Zoo and Wildlife Research, wrote in an e-mail. “Acoustic communication—the ‘social calls,’ not the echolocation used by these animals—is very important in this species, just like in the rest of the bats. But we found that chemical substances that are detected by the sense of smell are also important, at least when females are in the process of choosing a male to mate with.” The findings were published in Scientific Reports.

Previous research had shown these bats do not choose a mate via color displays or elaborate mating dances, the way many other animals do. Rather, Santos and his team focused on odor. About 30 years ago it was discovered males of species such as S. bilineata (striped bat) create their own “perfume” by mixing urine, saliva and penile secretions, which they place in their wing bags. But the reason for this complex behavior remained uncertain.

Santos and his team observed how these bats spend up to an hour a day cleaning and stuffing their wing bags. With the bat body’s warmth, the substance begins to ferment. The team discovered each animal emits an intense odor that broadcasts the chemical composition of what is known as their major histocompatibility complex, or MHC, genotype. The MHC is a family of genes found in all vertebrates that encodes vital proteins in the immune defense system. Such genes play an important role in mate selection—not just in bats, but also in mice and possibly even humans.

After analyzing the genes of about a thousand bats, the researchers studied the exact composition of the MHC genes as well as the molecular structure of three families of olfactory receptor genes: TAAR2, TAAR3 and TAAR8. Because each of these receptors can occur in two variants within one individual, females can have from three to six different receptors on their nose mucosa. The greater the variety of receptors, the more sensitive the female's sense of smell. Females with a higher variety of TAAR receptors are better able to find an optimal partner that complements their genetic disposition. Thus, the descendants of these females have advantages over others.

“Although it was already known that these male bats use their sexy mixture of urine and other compounds to attract females, his study supports the fact that this is determined at the genetic level, and the female can, to some extent, ‘measure the quality of the male’ before mating,” says biologist Bernal Rodriguez of the University of Costa Rica, who has investigated the life of bats but is not affiliated with this study. “The MHC complex is becoming very fashionable to investigate,” he says. “MHC I has been more studied because it handles the sensory and chemical functioning part of some senses. MHC II is the immediate response of the immune system to a virus. The combination of both determines important decisions, such as in humans, where it is known that people use olfaction to select partners,” he adds.

According to Rodriguez, next in his own research is fine tuning: determining which molecules differ from male to male, which makes some males irresistible for some females. “We also need to know: Where do these molecules come from? Do they come directly from the males or are they products of a bacterial fermentation in the bags of the bat wings? In other words, do males smell so intrinsically or is their odor a product of their individual microbiome? I think this is a very interesting question.”