Recently, Professor Zhang Fang's research team at the School of Life Sciences, Anhui Normal University, for the first time, confirmed the presence of blinking visual signals in frogs, indicating that this type of signal communication is not limited to just a few primates but may also exist in other vertebrates. The related research findings were recently published in Contemporary Biology.
The correct caption for the image of the concave-eared frog (male above, female below). Image provided by the Zhang Lab.
Male concave-eared frogs. Image provided by the Zhang Fang research group.
Blinking is a behavior unique to animals with active eyelids. Generally, blinking is divided into physiological blinking and blinking as a social signal.
"Physiological blinking is a reflexive response, such as blinking to remove foreign objects from the eyes. The other type is using blinking as a form of communication signal, which current research suggests is limited to humans and some primates," Zhang Fang explained in an interview with Chinese Science Bulletin.
In the evolutionary history of animals, frogs' ancestors were the first group to transition to land life, and almost all extant frogs have eyelid structures capable of blinking.
In studies of sexual selection, there has been extensive research both domestically and internationally on what types of males female frogs choose. However, the process of close-range courtship interaction between male and female frogs is still not clear.
The concave-eared frog is a unique species in China and is the first frog species confirmed to use ultrasonic communication. It is currently distributed in a few streams in provinces such as Anhui, Zhejiang, Jiangsu, Jiangxi, and Fujian.
Zhang Fang noted that during preliminary field observations, they noticed frequent blinking behavior in concave-eared frogs prior to mating. They speculated that blinking behavior might play an important role in female choice in concave-eared frogs.
To investigate this, the Zhang Fang research group conducted controlled mate pairing experiments, video playback experiments, and field observation experiments to confirm this hypothesis. That is, female concave-eared frogs send blinking signals to the "preferred" male frogs, and only male frogs that receive the blinking signal can successfully mate with the female frogs; otherwise, they will be kicked away by the female frogs.
Specifically, the controlled mate pairing experiment involved placing two male concave-eared frogs with one female concave-eared frog and observing how blinking behavior affected mating success.
In late March to early April each year, concave-eared frogs enter their breeding season. At this time, Zhang Fang and his team would go to Huangshan to find concave-eared frogs and record the blinking behavior of female concave-eared frogs. After technical processing, they would play the videos to male concave-eared frogs. This is the video playback experiment.
Zhang Fang explained that they placed two screens of the same model in front of male concave-eared frogs, with one screen showing a blinking female frog and the other showing a non-blinking female frog (the same female frog was presented on both screens), and then observed how male concave-eared frogs made their choice.
During the early video playback experiments, the students overlooked the fact that concave-eared frogs, like other frogs, are "myopic." During the experiment, they found that male concave-eared frogs did not react to blinking or non-blinking female frogs. Later, they solved this problem by shortening the distance between the screen and the male concave-eared frogs.
The first two experiments were conducted under controlled conditions. In a natural environment, do concave-eared frogs produce the same response? This was also a challenge in this study, to record the complete natural mating process of concave-eared frogs in the wild.
"This requires a great deal of patience. We often track with infrared cameras and film for several hours without any results," Zhang Fang said.
Regarding the research findings, several reviewers from Contemporary Biology highly praised them, stating, "I believe this is an interesting article that emphasizes the fact that animals can communicate with each other through subtle behaviors, which from our human perspective, might seem trivial." "Blinking behavior in non-human animals is rarely studied, so work in this area is valuable."
Zhang Fang said, "The research findings represent a significant breakthrough in frog communication behavior and have important implications for understanding the origin and evolution of animal visual signals."
Zhang Fang speculated that pressures from sexual selection and habitat environment might have jointly driven female concave-eared frogs to evolve blinking as a social signal. This is also the next research direction for the Zhang Fang research group.
Related paper information: https://doi.org/10.1016/j.cub.2024.01.023
