New research suggests the evolution of attractive traits in males may be limited by the perception of that extravagance by females.
UT, Louisiana State University Health Sciences Center and the Smithsonian Tropical Research Institute have shown in a collaborative study on túngara frogs that as male mating calls become more elaborate, it becomes increasingly difficult for females to notice greater complexity.
Hamilton Farris, a professor at LSU Health Sciences Center, said there is an important distinction between the magnitude of stimuli, such as the complexity of male mating calls or other sexual characteristics, and the way these are perceived by females.
“Since decisions are based on not the actual stimuli but the perception of those stimuli, we went and looked at what the perceptual rules were. In other words, the limits of discrimination and detection in these frogs when they’re doing this mate choice behavior,” Farris said.
The male túngara frogs have a complex call, which is the reason Farris said they were targeted for this study. The call consists of a long, vowel-like “whine,” followed by a varying number of short, percussive “chucks.”
The study was conducted at a Smithsonian Tropical Research Institute field station on the Panama Canal called Gamboa. Female frogs were placed in a dark room with two speakers playing two distinctly different calls repeatedly until the female frog chose one by approaching and touching the speaker. Researchers found female frogs chose the speaker with two chucks instead of one, but would not always choose the speaker with, for example, five chucks instead of four. They conducted the test using between zero and seven chucks.
The proportional basis of this decision follows the scientific phenomenon, known as Weber’s Law of perceiving stimuli at different magnitudes. This law maintains, for example, that while it is fairly simple to tell the difference between holding a one-pound weight and a two-pound weight, it is harder to discern which is heavier between a 100-pound weight and a 101-pound weight even though the absolute difference is identical.
“If it were tail length, you would expect that every generation tails would just keep getting longer on the males because females are choosing the male with the longest tail,” Farris said. “But what Weber’s law does, is that it means that as the signal gets greater in magnitude, whatever it is, the ability to tell the difference between signals in two different males gets harder and harder because the ratio has to be at a certain level. It’s not just the difference.”
A predator of these túngara frogs, the fringe-lipped bat, was given the same test and produced the same results. Karin Akre, a lecturer at UT, said it is traditionally believed predation limits the elaboration of sexual characteristics, but because there wasn’t a measurable increase in risk by adding to these calls, the true limiting factor is the lack of benefit in female response.
“In the evolution of elaborate male mating characteristics, like long songs and tails and bright colors, all the flashy things that males have and do to impress females, there’s been this question of why would it ever end?” Akre said. “What our studies show is that females cognitive abilities can also limit the evolution of these traits because at some point females just can’t tell the difference between two traits once they’re really big.”
Rachel Page, a staff scientist at Smithsonian Tropical Research Institute said they believe these findings reflect a universal quality in animals other than túngara frogs.
“The fact that we show this in frog and bats, and we see the same pattern in humans and in other animals leads us to think this is a shared perceptual mechanism — we all perceive stimuli based on stimulus ratio, not absolute values,” Page said.