The Mating Game
By Marc Airhart. Illustrations by Jenna Luecke.
Across the animal kingdom, males and females of the same species are often locked in a battle of the sexes. The instigator is evolution itself. It drives them to develop weapons, tactical tricks and defensive maneuvers that aid in an animal’s fight to pass its genes on to a new generation.
Scientists in The University of Texas at Austin’s Department of Integrative Biology, home to the nation’s sixth-ranked program for ecology and evolutionary biology, say this process of sexual conflict can lead to complex behaviors and traits. As males and females engage in a tug of war over control that’s never completely won or lost, more than mating happens. Animals may grow smarter, more beautiful and perhaps even better at parenting.
Livebearers, a group of small freshwater fish that includes guppies and swordtails, make for good research subjects for evolutionary biologists like Molly Cummings. In these fish, females get pregnant and gestate just like mammals, meaning, as with humans, their choice of a mate matters.
“When females make those mate choice decisions, it comes with a long-term investment,” said Cummings, a professor of integrative biology. “They’re out of the reproductive pool for a month while they’re pregnant.”
Since females are choosy about when and with whom they mate, males who want to keep their genes in the pool have to outwit each other and even their potential mates. Swordtail males, for example, have evolved different mating strategies. Ones who focus on courtship are big, have vibrant colors and dance to woo prospective mates. Ones that are coercive are small and try to force themselves on females by sneaking up from behind.
In experiments that introduced each type of male, Cummings discovered that the female swordtails exposed to courting males – those good-looking, bring-them-home-to-mom suitors who (bonus) like to dance – expressed genes associated with active learning pathways in their brains. In contrast, being around shrimpy, coercive brutes for 30 minutes turned those same learning pathways off.
“This is super provocative because it suggests that your mating system can influence your general cognition,” Cummings said. “Does hanging out with courting males make the females smarter?”
To confirm that result and better pin down exactly how the female brain responds to the mating strategies of males, Cummings and her students, including undergraduates in the Freshman Research Initiative, are orchestrating a series of remarkable long-term experiments with female fish. Some females grow up around all types of male mating behaviors. Others meet only courting males or only coercive ones. Still others have no males around at all. Then the team sees how the female fish exposed to these different environments perform on various tests of cognitive abilities and behavioral traits. (For a fish, that means measuring traits associated with sociability, anxiety, exploration and even simple math.)
Cummings said it’s possible that having to choose between subtly different courting males could lead female fish to become better learners.
“Females aren’t just slaves to their sensory systems,” Cummings said. “They actually engage their brains.”
Music to Her Ears
When it comes to reproductive traits, life often imitates an old country-western duet where the guy sings, “I’m going to wear the pants,” and the gal fires back, “But I’ll tell you which pair to wear.” It’s the female brain that tends to dictate what is attractive.
In an effort to understand why males do things that can be dangerous to them – like display flashy colors and sing noisy tunes – Mike Ryan, professor of integrative biology, studies an inch-long native of Central America called the túngara frog. Lonely males crouching along the brims of puddles call out in the night, hoping to attract females, while also hoping to avoid attention from nearby frog-eating bats. Males start their calls with a low-pitched whine and then sometimes tack on a series of higher pitched sounds called chucks. The females are attracted to calls with more chucks, but the bats are attracted to the sound, too.
Ryan and his colleagues discovered that female túngara frogs have two hearing organs – one that best hears low-pitched whines and the other that best hears high-pitched chucks. The team found females evolved these organs before the males started making the sounds.
“Thus, females are the biological puppeteers, making the males sing exactly what their brains desire,” Ryan wrote in his 2018 book A Taste for the Beautiful.
That male traits are driven to evolve by the way the female brain processes information – in other words, by what females find beautiful – is a big idea in evolutionary biology. It’s why peacocks produce dazzling tail feather fans and why male nightingales sing their intricate and endlessly creative songs.
Ryan has gone on to uncover some surprising quirks of the female frog’s brain. For example, when he and his graduate student Amanda Lea played for female túngara frogs the calls of two males, females would reliably choose the one with the most attractive calls (i.e., chocked full of chucks and with just the right pitches and timing). But then if a third, much-less-attractive call is added to the mix, about a third of the time, females would switch to the one with the second-most-attractive calls. This “decoy effect” works in humans, too – it’s exploited by retailers to get us to buy more expensive cars and by online advertisers to get us to click on ads.
Ultimately, the female brain, which evolved to do many other things besides finding mates, has its own limitations and quirks, and it conjures up an aesthetic that males strive to match.
“I argue that beauty only exists because it pleases the eyes, ears or noses of the beholder,” Ryan wrote. “More generally, that beauty is in the brain of the beholder.”
Never Gonna Give You Up
Males and females in a species sometimes have different priorities for pairing up and childrearing. Cute rodents called prairie voles form strong, lifelong bonds and raise generations of pups together. Steven Phelps, a professor of integrative biology, began studying the brains of male prairie voles to see if he could determine what made them monogamous. He was surprised to find that, within the same species, some males are more faithful and some are less faithful, and that the difference comes down to a subtle genetic variation in their brains.
Phelps discovered that both the monogamous and the philandering males continued to flourish in the wild. But why would monogamy evolve in the first place if males generally benefit from having more partners to help them spread their genes? Wouldn’t the less faithful trait be favored?
A vole’s surroundings may factor into the equation, Phelps hypothesizes. When female mates are few and far between, monogamy really is the best strategy. Watching after one’s young helps a male ensure his offspring survive and receive resources, while being protected from predators and rival female voles, who are known to commit infanticide.
“If their young need two parents to care for them in order to survive, and a male mates with multiple females and doesn’t stick around to care for his young, every offspring dies,” said Phelps. “So it doesn’t really get him anything.”
As population densities get higher, though, animals tend to become less faithful partners. Phelps and a graduate student are looking to see how that plays out in the male vole’s brain.
“We’re looking to see if there might be variation in selection on this brain mechanism depending on whether the population is high density or low density,” said Phelps. “What we think is that the philandering males do really well at high densities and that the exclusive males do really well at low densities.”
From female fish wising up to males’ tricks, to male frogs dutifully playing females’ preferred tunes, to voles putting aside differences for the sake of their offspring, how males and females behave in pairs results from an evolutionary arms race playing out within species. As this war of the sexes continues, nature’s battlefield displays an exquisite variety of colors, sounds, smells and behaviors as pleasing to members of the opposite sex as they are endlessly interesting to us.
Follow more news from UT evolutionary biologists, find stunning images from our biodiversity collections and watch a peregrine falcon who lives atop the UT Tower on a live “Falcon Cam” at biodiversity.utexas.edu. More from UT Austin scientists and their work that has researchers rethinking evolution can be found in this cover article in The New York Times Magazine.