New paper published in Evolution! Our study uses quantitative theoretical modeling to explore a new evolutionary possibility for systems involving sexual conflict over mate choice. Rather than evolving resistance traits that directly interfere with male coercive strategies, potentially leading to a co-evolutionary arms race, we show that females may evolve additional mate preferences that “remodel” male coercive capacity in order to expand and promote their own freedom of mate choice, or sexual autonomy.
A ket aspect of this work is that it demonstrates the importance of allowing female reproductive traits and behaviors greater complexity and biological realism when we build theoretical models to guide our intuition about the evolution and diversity of mating interactions.
Read on for more:
Sexual conflict is often characterized by the evolution of female resistance that directly interfaces with the male coercive strategy. A now-famous example is duck genital biology, wherein the vaginal tract of female ducks (shown on the left) is a convoluted anti-corkscrew, co-evolved with the male morphology (here on the right), seemingly allowing females to block unwanted fertilizations and retain the benefits of mating with their social partners in the face of violent coercive attempts by other males.
As you can see, these dynamics can potentially lead to a costly co-evolutionary arms-race, where the evolution of female resistance selects for more effective coercion, and so on.
In our paper, we explore a novel alternative to this arms race scenario, which we are calling “remodeling.”
Suppose females, faced with male coercion, could evolve an additional preference for a new male trait that enhances female freedom of mate choice?
Let me show you a heuristic example from nature where we think this might be going on:
This is the courtship display arena of the Tooth-billed Bowerbird (Scenopoeetes dentirostris) from Australia. A female that arrives at one of these leaf piles has never been observed longer than 3.8 seconds before being accosted by the male. Not so easy to evaluate a mating display in that time.
This, on the other hand, is a Satin Bowerbird (Ptilonorhynchus violaceus) courtship arena. It’s not a nest, but a structure built by the male called a bower. When the female comes to visit, she sits right in the middle.
The male displays out front and she will often spend several minutes evaluating his display. If the male wants to attempt mating, he has to go around to the back of the bower, giving the female an opportunity to escape out the front. Fascinatingly, it appears that the bower structure functions specifically to protect females from coercive mating.
We suggest that, instead of evolving direct resistance to unwanted male advances and touching off an arms race, females can actively remodel males, that is, change the males themselves, by evolving mate preferences for male traits such as the protective bowers that enhance freedom of mate choice – even though the bower is likely costly to males and hinders their coercive mating opportunities.
To explore this, we built a theoretical, proof-of-concept model where females can evolve to first choose what kind of bower to visit in order to then evaluate a male’s display
We show that, under certain sets of conditions, when male bowers are protective to females, bowers, and the female preferences for them, can invade and persist in the population in spite of the costs due to their utility in enhancing females’ sexual autonomy. Indeed, you can see in that in this model simulation, the attractive male display trait in the population, shown as the warmness of the color, concurrently increases because females are getting what they want more often.
Obviously, a striking aspect of this is the oscillations.
As bowers come into the population, fewer matings are coerced, and females with a preference for bowers get an advantage by mating more often with attractive males, as shown by the orange line. However, the protective bowers are out there in the population now, and can act as a “public good.” Females without a preference for protective bowers (the blue line) can freeload, reaping the benefits of increased freedom of choice without paying any of the costs of specifically seeking out bowers, and so they become favored briefly until bowers decrease again. Eventually it converges on an equilibrium where overall males rarely coerce and females have substantially increased ability to choose mates.
Although the bowerbirds are a useful example, the “remodeling” process could gain purchase in any scenario where preferences can act on multiple scales allowing females to avoid direct coercion. For example, in the paper, we speculate that such a process could represent an alternative (or complementary) underlying mechanism for hominid “self-domestication,” wherein females forming friendship coalitions or choosing to associate with males with outward traits uncorrelated with aggression could have led to modern levels of reduced coercion and infanticide, and reduced sexual dimorphism in traits such as body size and canine teeth.
- Duck genitals: Dr. Patricia Brennan
- Tooth-billed Bowerbird court: Macaulay Library ML278241221
- Satin Bowerbird Bower: Dr. Hazel Parry
- Satin Bowerbird Courtship video: Carlo Ferraro (ferrarofilms.website)
- Chimpanzee: Ronan Donovan