This week’s BEACON Researchers at Work blog post is by MSU graduate student Emily Weigel.
What would a fish say if it could talk? How about, “Hey, baby. What’s your sign?” Male threespine sticklebacks court females in a constant game of flashy zig-zag dances and showing off with the hope that a female will respond favorably. Most of the time, to use the baseball analogy, the male strikes out; that is, the female swims hurriedly away, and the male must go sadly back to the nest alone tonight. On occasion, however, the male is able to get to first base, and perhaps even hit a home run. So, what makes a male attractive, or “sexy”? Does a male’s sex appeal depend on where he is or what time of year it is? Do all females agree on what makes a sexy male?
I study the mating behavior of threespine sticklebacks. Threespine sticklebacks (Gasterosteus aculeatus aculeatus) are small fish found throughout the Northern hemisphere. About 10,000 years ago, at the end of the last Ice Age, marine sticklebacks invaded recently formed freshwater lakes and streams. Since then, these fish have changed through time in similar and often predictable ways—that is, they’ve evolved in parallel across several lakes, making them an ideal species to study for evolution in action.
In just a few lakes in Canada, two closely-related stickleback species are found— benthics and limnetics. Benthic and limnetic sticklebacks differ extensively in many ways, including size, shape, color, and feeding habits, yet their genetics reveal that the species pair evolved quite recently from a common ancestor. Although these species live in the same lakes and come into regular contact with one another, they manage not to interbreed. It turns out females are really good at finding males of her own species. Females use species-specific traits to identify the right species males, and to choose the best of those males, the females rely on sexual signals to decide the male’s “sexiness.” Often, the brighter, bigger, or stronger a male is, the sexier he is considered.
Now here’s the problem: in one lake (Enos Lake, British Columbia), a well-established stickleback species pair recently collapsed into a hybrid swarm in less than 20 generations. What gives? Why did the females stop choosing to mate only with males of their own species? What could have caused this? It seems scientists don’t know yet how quickly this type of loss can occur, under what circumstances (environmental change, low mate availability, etc.), and whether trait or preference loss (or a relaxation of associated requirements) occurs first. Let’s try to find out.
One of the big determinants of behavior is often experience, so that seems a practical starting point. Experience can be broadly defined to include social experience (i.e., mate availability, population density, mating history), age or seasonality; variation in these factors can lead to local adaptation, population divergence, and even the evolution of new species. Working with Dr. Robin Tinghitella and the BEACON Sexual Signaling Group, we designed an experiment to test a female’s mate choices throughout the breeding season. Wild-caught fish were uniquely marked and assigned to one of two “experience” treatments that differ in sex ratio (mimicking early and late breeding season conditions; male-biased and female-biased, respectively). We’ve since measured female preferences and male mating behavior every time the females reached reproductive condition. To return to the Baseball analogy, we can assign a score — or a base — to every act the female commits to show she’s interested. So first base might be approaching the male, second following him, third is examining his nest, and, the home run, entering the male’s nest to deposit eggs. We then test a female with first a dull, then a medium, then a bright male and see who along this “sexiness continuum” the female chooses to accept. That way we can tell if a female is relatively choosy or relatively relaxed and whether those preferences change over time.
We expect treatment females who have experience with few males to be less choosy (essentially, to let more males run the bases and score), and this makes logical sense: better to mate with someone than no one at all. On the other hand, treatment females who regularly see many males will be choosier (thus males will strike out or be called out more). Because the tests with each male occur at several time points throughout the season, we also expect females to relax their mating requirements as the season goes on, such that the females maximize the number of offspring they can have. The theory is, if females with relaxed mating requirements sometimes accept males who lack the preferred sexual signal, loss of male signaling traits could occur. Once male signaling traits or the associated preferences are lost, the species pair could interbreed, leading to the collapse and loss of species.
The question of why females accept certain males is something in which I’ve always been interested. Since starting graduate school at MSU in the Fall of 2010, I’ve had the pleasure of working and collaborating on empirical and theoretical experiments using sticklebacks, mathematical modeling, and even the digital organisms in Avida. Through these avenues I’ve been better able to understand how mates are chosen in systems and populations with sexual signals, and the evolutionary consequences of what happens when signals are lost.
So, guys, the females are ready. Batter up!
For more information on Emily’s work, you can contact her at weigelem at msu dot edu.
This post has been submitted to the National Evolutionary Synthesis Center (NESCent) travel award contest to attend ScienceOnline2012.