Samuel S. Snow, research fellow at IAST, and his co-author Richard O. Plum (Yale University) published their latest paper in the journal Evolution, on June 1st 2023. Samuel Snow tells us about it!
We explore a new theoretical evolutionary possibility for systems where males and females are in conflict over mate choice. We show that females may “remodel” males’ capacity to sexually coerce in order to expand and promote their own freedom of mate choice by evolving new mate preferences for “autonomy-enhancing” traits.
- Your field of study is evolutionary biology, how would you define it?
Evolutionary biology aims to understand how species evolve over time. It allows us to improve our understanding of how the diversity of life on earth (including we humans) came to exist and how it could have gone differently. What we learn from those facts provides context for understanding how we and other organisms function gives us a vision on how things could unfold in the future.
- What is your paper about?
We try to understand the evolution of the interactions between individuals of different sexes and what this means for the evolution of social behaviors, their morphologies (the forms of their bodies) and how they choose mates.
For that purpose, built a new mathematical model to help us gain insight into how females can actively “remodel” males’ ability to sexually coerce by evolving mate preferences for new male traits. We were inspired by the evolution of bowerbirds, a group of bird species from Australasia.
Based on observations of different species of bowerbirds, we can infer that over the course of millions of years, bowerbirds’ behavior has changed. Females used to be coerced by males into choosing them as mates. However, today, the males of many bowerbirds species build very peculiar structures out of sticks: these “bowers” are special courtship display arenas that seemingly allow the female to be courted while being protected, giving them the opportunity to escape easily if they feel threatened.
We hypothesized that females may have evolved preferences for visiting males that build bowers specifically because they help resist coercive mating and allow females to choose mates freely.
A female can evaluate the structure of the bower from far away, so even though the structure interferes with a male’s own ability to coerce, he has no choice but to build one if he wants a female to visit him! Over evolutionary time, females with preferences for bowers would be more successful because they are better able to mate the males with courtship displays they prefer, and males that build bowers would be more successful because more females would prefer them.
Check out this video of Bowerbird courtship behavior.
Using our mathematical model, we show that it is indeed possible for females to remodel the way males court females in order to enhance their own sexual freedom of choice. We then explore the various conditions under which it is possible, or not.
For example, we found that females can only remodel males if the “bower” is protective enough to provide an advantage to the females that visit, but not too protective (at least initially) so that the males lose too many mating opportunities. However, once the process gets going, it likely becomes easier and easier for females to make further gains.
We were also surprised to find that the protective male trait acts as a “public good” because it benefits all females, no matter their own preferences. So any female can have autonomy and take advantage of the evolution of bowers even if they did not necessarily want it in the first place. This can cause cycles in the evolution of the bowers.
- What can we learn from this?
This is not only about birds. Yes, our model helps us understand bowerbirds and their evolution in a new way, but the remodeling process could really happen in any species. The “bowers” that the males build are just one example of an “autonomy-enhancing” trait. Other autonomy-enhancing traits could be things like obviously smaller body size that makes it harder to do violence, or reduced teeth. In fact, we do observe that human males are less violent and much smaller relative to females than chimpanzees, our closest relatives. Our model suggests it is plausible that this kind of remodeling process could have occurred in our own lineage as well.
- How did you study this? Which tools or raw material did you use?
We used abstract theoretical modeling, inspired by natural history and inferences from comparative approaches made possible by advances in genetics. We spend a lot of time observing wild birds and their behavior in order to understand their diversity and its origins.
- Why these results are relevant to the public eye?
Our theoretical modeling approach is based on the details of birds but the facts we learn about the evolutionary process are generalizable to all sexually-reproducing organisms, including, potentially, us humans. Beyond understanding how remodeling of male coercion may have played out in our own evolutionary past, our new results demonstrate the importance of allowing for more complexity in behavior to understand evolutionary dynamics that have very important consequences. Usually when researchers model sexual behavior they only allow the simulated females to have very simple preferences. In our case, we imagine that they can simultaneously but independently have preferences for different aspects of the males (the mating display and the autonomy-enhancing trait).
We show that what might have been happening in bowerbirds might have happened in humans as well. So on the one hand, it reminds us that humans are fundamentally animals and can evolve via the same processes. On the other hand, we only attain our results because we recognize that animals like bowerbirds have very complex social lives; they are more like us than you might think!