More to butterfly mating than meets the eye: Female butterflies choose mates based on the scent of male pheromones.
How do butterflies choose their mate? Research has largely focused on male visual attraction towards female colour patterns, but a recent study finds there are more signals at play here. Kathy Darragh from the University of Cambridge shares more on the research conducted for the paper, “Male sex pheromone components in Heliconius butterflies released by the androconia affect female choice“. The study found that as well as males choosing mates based on colour pattern, females are involved in the mating decision using chemical cues of male pheromones.
Animals can communicate with each other using a variety of different types of signals. Some might be very obvious to us as humans, such as the songs of birds or their colourful plumage. Others, such as electrical communication in fish, or chemical signaling, may not be easily detectable to us, and have consequently received less attention. I am a third year PhD student at the University of Cambridge and one area of animal behavior of particular interest to me is how animals choose their mates. This is an important process as it will determine who produces offspring for the next generation, which can lead to evolutionary change. To unravel this, we can begin by identifying which signals animals are using to make these decisions.
A fundamental sensory system in animals is olfaction, or the sense of smell. Smell is very important for some groups in particular, such as moths which can detect tiny amounts of chemicals in the air, allowing them to find mates over long distances. In contrast, the closely-related butterflies are thought to mainly rely on vision to choose their mates. This is possible as they fly during the day and many have colourful wing patterns which could be used in mate choice.
We do know that in some butterflies chemicals produced by males, known as pheromones, are involved in interactions between the male and female during courtship, and are important for female mating decisions. We chose to investigate if this was also true in Heliconius butterflies. These are a diverse group of Neotropical butterflies, and research on Heliconius has contributed greatly to our understanding of evolutionary processes such as adaptation and speciation. In this group, research on mate preferences has largely focused on male visual attraction towards females, which is greater towards females sharing their own colour pattern, but less attention has been given to other signals involved in mate choice.
In our recent publication with PeerJ we combined morphological, chemical and behavioural data to investigate the role of male sex pheromones in the butterfly Heliconius melpomene. Achieving this required input from specialists in different fields, resulting in a collaborative project between chemists and biologists at the University of Cambridge, the Smithsonian Tropical Research Institute (STRI) in Panama and Technische Universität Braunschweig in Germany.
The project began in Cambridge, where Sohini Vanjari used scanning electron microscopy work in Cambridge to identify specialized scales which are only found on the wings of male butterflies. These scales have brush-like structures, which we think are involved in pheromone release by increasing the surface area. Together with Florian Mann, she identified multiple male-specific chemical compounds which are found in the same region as the specialized scales and not detected on female wings.
We then expanded the project to STRI in Panama. This long field-season lasted most of the first year of my PhD studies, where I spent my days in the insectaries in a small town called Gamboa. Here we are able to rear captive stocks of Heliconius melpomene in insectaries, using both butterflies and host plants found in the local area. This involves a lot of gardening and plant care, which in the humidity of the tropics means we are all very sweaty, all the time!
I wanted to determine the behavioural role of these compounds and test if they are important for female mate choice. The method I decided to try was to block the region of the wing where these specialized scales and pheromones are found with nail varnish, which has previously been successful in other butterflies. This initially proved challenging but with practice and perseverance, we refined the procedure. I gave female H. melpomene a choice between two males, only one of which was “pheromone-blocked”, and found that females only mated with the males which still had their pheromone regions exposed. These male-specific chemicals localized to specialized wing scales are therefore important for female acceptance during courtship and therefore play a role in mate choice in Heliconius melpomene.
I think that our findings contribute to a general trend in the field, which is moving away from a simplified view of mate choice. In Heliconius, we now know that as well as males choosing mates based on colour pattern, females are involved in the mating decision using chemical cues. Whether females also use visual cues or males also use chemical cues has yet to be determined. We can think about mate choice as a complex decision where in many cases there are numerous inputs of information, and both individuals can be involved in the decision; how individuals use multiple types of information to make decisions about the best mate to choose is an open question.
Once we understand which signals are being used, we can begin to ask which are the most important, what information are they conveying and how the decision is made. Mate choice and signal evolution within a species can also help us to understand why different species are reproductively isolated. If different species have different pheromones which are an important part of mate choice this could prevent hybridization. It will be interesting to see if this is important for reproductive isolation in Heliconius.
I would like to thank everyone involved in the project, and in particular Sohini Vanjari and Florian Mann who were key to the initiation and early development of this project.