In breeding season, male Cope’s gray treefrogs gather in groups near ponds, creating a dense chorus that attracts females. It has long been assumed by evolutionary theorists that females pick the fittest male specimen based on acoustics.
However, a theory is that females may experience mental overload or struggle with sound density. Researchers needed to figure out how much impact, if any, this has on selection.
Scientists set up controlled testing to evaluate the difference that extra noise makes.
What attracts female Cope’s gray treefrogs to preferred males
Researchers from the University of Tennessee, Knoxville, collaborated with others from the Smithsonian Tropical Research Institute to study Cope’s gray treefrog breeding behavior.
Field crews collected 52 female specimens from breeding sites in the eastern United States. They were housed in a specialized enclosure in a lab, which was designed in a semi-circular shape to isolate acoustic variables.
One of the observable patterns was that females of the species (Hyla chrysoscelis) consistently choose males with longer call durations.
In courtship based on sound, the longer length of the call indicates that the male has stamina and energy; therefore, he is genetically fit for reproduction.
The overlapping sounds of all the males gathered at one pond are acoustic chaos. This led biologists to believe that females have trouble picking ideal mates because individual signals get drowned out in the background noise.
Researchers needed to separate the physical sound masking from female frogs’ mental processing limits. Their test involved tracking how females decided on mates when the call clarity remained consistent as the number of calls increased.
By maintaining a constant call volume, the team could isolate whether the number of male options in a shared space changes female behavior.
An experiment setup that isolates choice overload
The 52 female treefrogs were placed one by one in the middle of the temperature-controlled test arena.
Speakers set at different angles broadcast recordings of male calls at equal volume.
Each test involved playing a single long call with one, three, and then seven shorter calls being added for distraction. Controlled choice environments involved two, four, and eight simultaneous calls.
Another layer was added to the experiment with parallel trials. Speakers played a continuous, overhead, non-directional chorus of background frog calls.
With the use of an automated tracking system, every female frog’s exact path and speed of travel across the arena floor were logged and mapped.
When a female got within four inches of an active speaker, it was recorded as a mating choice.
Movement time and final target accuracy were tracked across all the trials. Frogs that stayed still or failed to select a speaker within 15 minutes were recorded as non-selections.
Mental choice overload lies behind pairing decisions
When the results of the test were returned, it was clear that the level of background chorus noise barely impacted female decisions. However, the sheer number of males calling in competition had an effect on all metrics of mate selection, according to the Smithsonian Institution.
When the females were given two options, they chose the longer call 76 percent of the time.
On average, they took 114 seconds to reach the target speaker. Only 3 percent failed to pick a mate at all.
The average decision time increased to 183 seconds, while 18 percent of the females were marked as non-selectors.
Choice overload operates independently of sensory interference
More choices reduced accuracy, increased decision time, and increased the instances of non-selection even without sensory interference, confirming that cognitive choice overload is independent of acoustic masking.
However, background chorus noise was far from irrelevant, as it actively modified these decision dynamics and buffered certain choice-overload effects.
One of the important insights is the explanation for why, even though females prefer longer calls, males that produce shorter, less energetic calls persist in natural populations.
Dense breeding groups mean exceeding female mental processing limits. Their choice of the ideal mate becomes unreliable. Less suitable suitors get to mate by chance, and choice overload becomes a barrier to evolution.
The full study can be found here: Hemingway C, Rodrigues A, Vélez A … Choice overload constrains mate choice independent of energetic masking. iScience, 2026; 29
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