Mariana Muñoz-Romo uncaps a clear plastic tube and waves it under my nose. The contents, a bit of dry, yellow paste called a forearm crust, collected from a male bat, smells vaguely like incense.

“Women seem to like the smell and, in general, men hate it,” says Mariana, postdoctoral fellow in Rachel Page’s bat lab at STRI. “People say that crusts smell musky, sweet, fruity or floral.”

In 2017, doctoral student Victoria Flores, working in the same lab, wrote the first paper describing this mysterious, tar-like substance on the forearms of fringed-lipped bats (Trachops cirrhosus). During three years of catching these bats for another experiment, Flores kept noticing that only mature adult males with large testes had crusts on their forearms. Furthermore, the crusts were most common during the last three months of the year, when the bats seemed to be mating. This led Victoria to believe that the crusts may be important for attracting a mate.

“I prefer not to call them crusty males, because it sounds disrespectful,” says Mariana, “but only some of the males seem to develop these crusts, and we want to know why.”

Across the animal kingdom, males put a lot of energy into to impressing females. They flaunt fabulous feathers—like the peacock’s tail; extra fur, like a male lion’s mane, and supersized teeth or tusks, to demonstrate their strength and superiority. And because bats spend most of their active time in the dark, it makes sense that male bats might use scent to attract females.

In addition to helping one bat recognize another, smells are also honest advertisements of a bat’s physical condition. Potential partners can often tell if another bat has recently been eating or is sick, based on its smell.

But when Victoria and Rachel set out to discover if the crusts were important as signals to potential mates, they hit a dead end.

Victoria set up a maze to discover how bats react to their crusty comrades. Bats could choose to crawl down one of two tubes in a Y-shaped maze toward the scent of another bat. But, contrary to what she predicted, females did not move toward the scent of a male with forearm crusts. And males preferred to approach the scent of crust-free males. Victoria published her findings and moved on.

Meanwhile, Mariana, then professor of zoology and researcher/director of the Applied Zoology Lab at the Universidad de Los Andes in Mérida, Venezuela, decided to take up the case. During her PhD in Thomas Kunz’ lab at Boston University, she studied the long-nosed bat (Leptonycteris curasoae), which has a smelly patch of skin on its back. Female bats were attracted to enclosures containing samples of the patch smell.

“There are several different possible explanations for this kind of perfume on the fringe-lipped bats,” she said, “But at this point, we still don’t know what the substance is, where it comes from or what it is for.” However, she adds: “the fact that this odorous trait is only present in adult males displaying enlarged testes, has some chemical compounds identical to those present in dorsal patches, and is more conspicuously developed during the short mating season, is evidence to support a potential mating/attraction function.”

According to her work so far, it is likely that Victoria and Rachel’s idea that the crusts are important for mating is correct.

“The intensity of odors in some animals is related to testosterone, a male sex hormone, so my first experiment was to ask if there was a relationship between the size of the crust and the levels of testosterone in males.” To determine testosterone levels, she took small blood samples, following international veterinary care standards. And she found that, in general, the more testosterone, the larger the crust. 

Next I will ask if females that are ready to mate are more attracted to the males with crusts than females that aren’t reproductive.”

Determining exactly when females are in estrus is tricky. Last year, she examined cells gathered from female bats’ vaginal cavity using tiny pipette tips. The vaginal cells change shape as the females become more fertile, so this is the best way to know if females are ready to mate.

“There is still so much that we don’t understand about bat’s lives,” Mariana said. “We were driving our truck on a road in the middle of the forest and I smelled a really strong odor near one of the drainage pipes below the road. We discovered a whole group of male bats inside that all had crusts and enlarged testes. Why were they there together and what were they doing? Sometimes male birds—manakins, for example—hang out together and dance or sing to attract females. Maybe these males attract more females with a stronger odor if they group together, but we still have no idea.”

Mariana will continue to adjust the conditions and design new experiments based on her previous experiences and recent observations of fringe-lipped bats in the wild, until she solves the mystery of this complex, odorous trait.