One of the first things that caught my attention when I began studying Neotropical bats was their distinctive aroma. I often could identify bat species by scent alone, and in some cases, could even tell if it was male or female.
Scents play critical roles in the lives of mammals. Even for us as humans, scent holds enormous power: aromas evoke vivid memories of the past; through them we relive emotionally charged experiences, and retrieve critical pieces of information. The communicative power of scent is immensely useful to bats, but still not well studied. Since many bat species live much of their lives in darkness, visual signaling is largely inaccessible. For bats, scent can reveal an individual’s identity, aid in the recognition of kin, attract mates, repel rival intruders, and more. We are constantly discovering new components to the communicative power of bat odors. These discoveries never cease to amaze and help us better understand the wonders of bats’ nocturnal world.
One striking case of odor production in bats is that of the male fringe-lipped bat, also sometimes referred to as the frog-eating bat (Trachops cirrhosus). Adult males produce a golden substance that they smear on their forearms with their tongues1 . As this substance dries, it forms what we refer to as a ‘forearm crust’, superficially similar in texture and appearance to amber. The origin of this crust remains mysterious. Behavioral observations suggest that it is a combination of saliva, and other bodily secretions still to be confirmed. The scent of the crust is highly distinctive, and has been described as ‘musky’ and ‘floral’. Even from several meters away in the dense rainforest understory, the human nose can detect the presence of a reproductive adult male fringe-lipped bat by the telltale aroma of his forearm crust.
The fringe-lipped bat’s forearm crust was first described by Victoria Flores and Rachel Page in 2017. Neither females nor young males developed crusts, suggesting that they were related to courtship and reproduction. Victoria’s team later determined that some chemicals in the crust2 were similar to those of reproductive male bats of other species that also displayed conspicuous odors3, and that fringe-lipped bats’ social groups usually included a male with developed crusts4.
I was fascinated by these observations, and in collaboration with Rachel and Victoria, I started a new investigation. My central question was whether fringe-lipped bat crust size was related to testosterone concentration in males. This relationship has been found across species for many male sexual traits.
We found that this was indeed the case in fringe-lipped bats. Just as high testosterone concentrations correspond with majestic plumage or huge antlers in other animals, the size of fringe-lipped bat forearm crusts was positively correlated to testosterone levels. Males with enlarged crusts had high testosterone concentrations5 . Also, as we quantified male crust size and female estrus state across seasons, we found that when the largest male crusts were observed, females were fertile!5
But the highlight of our study was a completely unexpected observation. We captured a group of fringe-lipped bats at the beginning of the mating season after an intense odor led us to their roost. I will never forget that moment. In addition to measuring the size of at least one male’s crust, I was planning to sample females in the group to determine their reproductive condition but… Surprise–There was not a single female! They were all adult males! And even more surprising – all but one had enlarged crusts.
This group of encrusted males explained the intensity of the scent we detected from the road. Their unusual group composition, coupled with the strength of the odor, led me to think of a possible explanation. Perhaps these males were grouping together to create a much more intense scent signal than each individual could produce by itself. “The strength is in the joining,” I thought in that moment. I am now wondering if male fringe-lipped bats come together to form “chemical leks”: congregations of reproductive males that – like groups of loudly chorusing male frogs – are able to attract females to a specific location, where females then select a male to mate with. We are at the very beginning of studying this fascinating behavior. Future investigations will reveal if these grouping males are indeed coming together to waft their collective odors to more effectively attract potential mates.
Bat scents are as fascinating and diverse as bats themselves6. Our understanding of bats’ scent communication is continuously growing, thanks to the multidisciplinary collaboration of biologists and chemists. We are only at the beginning of an exciting, largely unexplored, and scented journey!
https://www.youtube.com/watch?v=ZKK8lbx9DSI
Dr. Mariana Muñoz-Romo is a research fellow at the Smithsonian Tropical Research Institute, Panama, and a former professor from the Universidad de Los Andes in Mérida, Venezuela. Find her on twitter at @munozromo.
References
- Flores V, Page RA. 2017. Novel odorous crust on the forearm of reproductive male fringe-lipped bats (Trachops cirrhosus). Journal of Mammalogy. 98: 1568-1577. doi:10.1093/jmammal/gyx137.
- Flores V, Mateo JM, Page RA. 2019. The role of male forearm crust odor in fringe-lipped bats (Trachops cirrhosus). Behaviour. 156: 1-24. doi: 10.1163/1568539X-00003573.