Flashy Courtship Traits Evolve Twice as Fast as Combat Weapons

The team — Mohammadali Dashtbali and Jonathan M. Henshaw of the University of Freiburg, Allen, and colleagues — built on the 1981 model to track three inherited traits: male ornament size, male weapon size and female preference strength. The simulation assumed an effective population of 1,000 individuals and incorporated genetic drift, the random shifts in gene frequencies that occur in any finite population.

The model's logic is straightforward. Weapons always pay off in combat, regardless of female tastes, so natural selection anchors them near a stable optimum. Ornaments only help a male if females currently favor them — and female preferences themselves wander over time, creating a moving target that keeps ornaments churning.

The simulations confirmed it. Ornaments fluctuated widely whenever low preference costs allowed drift to move female tastes. Freezing preferences artificially slowed ornaments down to weapon pace. Drifting preferences alone accelerated ornament evolution even without strong genetic coupling. The model did flag exceptions: weapons won the race when strong preferences carried heavy survival costs, or when weapons were much cheaper to grow and maintain.

What did the real-world data show?

The researchers compiled published trait measurements and time-calibrated evolutionary family trees for 531 species across 10 datasets, each ranging from 17 to 103 species. To control for broad biological differences, they matched each weapon-bearing group to its closest available ornament-bearing relative, producing five paired comparisons:

• Leg spine length in leaf-footed bugs vs. eye-stalk span in stalk-eyed flies
• Bite force in anole lizards vs. throat-bone length supporting display fans in agamid lizards
• Leg spur length vs. neck-feather color in landfowl such as pheasants and grouse
• Tusk length in hoofed mammals vs. vocal pitch in carnivorans
• Canine tooth length vs. vocal pitch in primates

After adjusting traits for body size, ornaments evolved faster than weapons in four of the five pairs under both evolutionary models tested. Insects were the lone exception. Combining all pairs under the better-supported stabilizing model — the Ornstein-Uhlenbeck model, which pairs random fluctuations with a pull toward an optimal size — gave the twofold overall gap.

"We observed large differences in the rate of trait evolution in different groups of animals, only some of which was explained by whether the trait was a weapon or ornament," Allen noted. Rates ranged from 0.02 to 0.19 across datasets.

How solid is the evidence?

Early findings, several caveats. The twofold advantage emerged only under the stabilizing model and only after statistically removing body size — a regression-based method the authors acknowledge can introduce bias. Under the alternative random-walk model, or without body-size adjustment, the overall difference wasn't statistically significant. Female preferences also couldn't be measured directly, and two comparisons span vast evolutionary time — the lizard families split roughly 146 million years ago, the insect lineages about 351 million years ago.

The model itself simplifies reality, tracking one ornament and one weapon in a well-mixed population with no representation of combat mechanics. Traits with dual functions, like fiddler crab claws, were excluded entirely.

"More broadly, we think the study illustrates how combining theoretical and comparative approaches can reveal mechanisms that would be difficult to identify from either approach alone," Allen said. "More generally it shows how evolutionary models can help us understand the diversity of animal life in the past and present, and allow us to predict what might happen in the future."

The bottom line: flashy display traits appear to change faster than fighting hardware, and wandering mate preferences are the likely engine — though the size of the gap depends on how you measure it.