Rana temporaria is an “explosive breeding” frog, meaning that they reproduce in a brief window of the year, and thousands of frogs gather in one area at the same time. In these species, the males often significantly outnumber the females and harass and coerce them into amplexus, which is the technical term used for mating in amphibians. Research prior to Dittrich's study suggested that female frogs in explosive breeding species are passive during the mating process, but she found the exact opposite.

In fact, Dittrich wasn’t initially studying the behavior of the females at all. The experiment she ran was to determine whether male frogs had a size preference when selecting mates — that is, did they prefer a larger- or smaller-sized mate? In the experiment, one male was placed in a box filled with 5 cm (2 inches) of water and two females of different sizes, and their behavior was recorded via webcam for an hour.

“But they didn’t show any size preference,” Dittrich said.

Instead, Dittrich noticed three behaviors that the female frogs repeatedly used when they wanted to avoid mating: physically rotating away, mimicking a mating call and tonic immobility. Rotating away from the male was the most common strategy — since amplexus must occur with the male behind and on top of the female, the females would flip onto their back to push the male frogs underwater. Dittrich recorded that behavior in 83% of amplexed females, and it was almost always the first attempted strategy.

If rotation was unsuccessful, 48% of the amplexed females began mimicking mating calls, as if to communicate, “I’m a male frog, not a female frog.” The tonic immobility was a last resort — 33% of amplexed female frogs stiffened their limbs, with “arms and legs outstretched from the body after being amplexed by a male.”

Interestingly, smaller frogs were more likely to go into tonic immobility than larger frogs. Dittrich theorized this might be because tonic immobility is not a conscious condition, but a response to high levels of stress. Smaller females could be younger, less experienced and more prone to high levels of stress than larger females, and thus more likely to show tonic immobility.

https://www.snopes.com/news/2023/11/18/frog-playing-dead-mating/