Anurans, specifically, have been proven to have an auditory system that is very well developed. Also, they have been shown to use phonotaxis, which is basically movement in response to sound stimuli. Most research done before has shown that they use these two methods when they are in a sexually receptive, or reproductive state, however, this article is interesting because it looks at how and if Hyla cinerea use acoustic signals picked up from acoustic stimuli when not reproducing, or basically sexually inactive. Based on the article, there is a gap in the understanding of how anurans behave because of the lack of information on what they do during the non-reproductive season, which was a motivator for these investigators to do the research. Most of what is known about acoustic and vocal amphibian communication is tied to reproduction because the breeding season is when the communication occurs the most.
According to the article, the researchers claimed that anurans are exposed to acoustic cues within their environment that belong to members of their same species, which could indicate to them that there are potential locations that would be good quality for them to settle. The question was if the frogs (in this case, Hyla cinerea) responded to these acoustic cues even when they are not reproductively active. To test this, according to the article, the investigators used 38 female Green Treefrogs who had no visible eggs or large follicles, and subjected them to four auditory exposures: playing back a call from their same species members, playing back a call from members of a different species (Barking TreeFrog; lives in same habitat and overlaps with Green Treefrog), playing back a white noise (control; a nonstop signal), and plain silence.
Single frog calls were used instead of group frog calls, since the interest of the project was in responses to calls not related to mating in the frogs. The frogs tested were each placed in a dark box with the speaker outside the box, and the speaker was repositioned each round. Frog behavioral activity was measured in terms of whether or not they approached the wall of the arena, the frog's position to the speaker, and how long it took for the frog to touch the wall. After statistical analyses were performed, it was found that the female frogs did not approach the wall, or move to the speaker in response to the acoustic stimuli they were experimentally exposed to. Initially, the investigators had hypothesized that Hyla cinerea movements are influenced by acoustically obtained social information from the environment, but based on these results, their hypothesis was not supported. In every one of the experiments, they found that the frog subjects were randomly positioning themselves to the speaker; there was no trend.
From the beginning of the experiment, investigators had thought that since Hyla cinerea have such a greatly developed auditory system for anurans, and also have been known to use phonotaxis, that they may also use their ability to hear and acoustically communicate in various other behavioral contexts. However, the experiment results showed no movement by the female Green Treefrogs to signals seemingly being emitted from other frogs, and also, their speed and positioning did not differ from the speed and positioning of other animals randomly moving around them. The studies that have been done before that were able to document significant frog behavior, according to the article, have used reproductively active frogs during their breeding season instead of reproductively inactive frogs outside of their breeding season. Based on the insignificant results in this experiment, the investigators concluded that Hyla cinerea's responses to social and sexual cues in their environment is limited only to times where it makes the most sense ecologically (finding a mate and/or a good place to breed), and that in frogs and toads, their reproductive hormone system impacts their vocal communication system. The article concluded that in anurans, the reproductive state the frog is in can affect its auditory system's receptivity to social or sexual acoustic signals, and ultimately, that non reproductive frogs may have a less receptive auditory system.
This article was really neat in that it investigated acoustic communication in anurans, which was a topic discussed in class. When we discussed anuran acoustic communication in class, we talked about how the frogs were affected by noise pollution, and how they recognized calls from same and other species. This article was similar to what we talked about in class in that it investigated how frogs respond to calls from same and other species, except this one is more about when they are not in reproductive mode. This article adds some important information to our understanding of amphibians, and what we know about them because by reading this article, one can see how important reproductive activity is for the frogs' auditory system to be of most use. One can also see how dependent the events of mating and reproduction in frogs are on their receptivity to acoustic stimuli, as a result of the release of reproductive hormones when they are reproductively active. This work is interesting because it suggests that outside of reproduction time, these frogs may not really need to use acoustic communication as much, which is quite different from other animals, and even us.
Most other animals, and humans, use communication and auditory information on a daily basis to help with non reproductive aspects of life, such as finding food and shelter, but it seems from this article that the frogs only really need it for mating and reproduction. Ultimately, this work is valuable for raising more questions/interesting topics for further research, such as: if frogs are not as receptive to acoustic signals when not reproductively active, what is it that allows them to continue being successful in finding food and shelter? It is also valuable because it helps us learn more about how the acoustic systems of frogs and the stimuli they are exposed to can affect (or, in this case, not affect) their behavior, which scientists are still trying to understand even more and more about. It's also cool to know these things about this particular species (Hyla cinerea) because they are a local species that live in our wetland areas here in SC, and in other parts of the southeastern US. Finally, it would be valuable for people studying how noise pollution can affect anuran acoustic receptivity and communication because it suggests that they can do that best when they are reproductively active. This means that it is important for noise pollution to be absent or minimal during times when these animals are reproductively active so that they can find a mate and reproduce successfully. Hyla cinerea are beautiful amphibians, and although they did not respond as expected to acoustic stimuli in this article, we can still learn from just that, and anything learned is valuable to the growing knowledge on these cool animals.
Author's names: Gerlinde Hobel and Ashley Christie
Journal: Zoological Studies
Link: http://www.frogbehaviour.org/wp-content/uploads/Hoebel-Christie-2016.pdf
Citation: Hobel, G., & Christie, A. (2016). Do Green Treefrogs Use Social Information to Orient Outside the Breeding Season? Zoological Studies, 55(17), 1-8. doi:10.6620/ZS.2016.55-17
3 comments:
The question these researchers asked is super interesting and even more interesting is what they found! In their methods did they just play calls from male individuals or was it a randomized mix of female and male individuals? If the study were reversed and used males as the focus, I wonder if they would respond to individual calls as a territorial response? I wonder if there are any correlations or overall generalizations that suggest that females vocalize less than male frogs. However, it makes sense that calls would be mostly used for reproduction as reproduction (and its success) is a direct indication of fitness.
Did they include, as a control, a similar test performed during the breeding season?
I agree that it would be interesting to test male responses in a similar experiment. (Like in most frog species, only the males vocalize.)
Dr. Welch: According to the article, the time of the year that this research was performed was not during the breeding season of this species. The frogs studied were not active reproductively at the time. I would assume that they did not include a test similar to that except in the breeding season since they were only interested in conspecific (outside of breeding season) calls made by the frogs.
Michelle: I agree! The frogs studied in this paper were female green tree frogs which were sexually mature. They were caught in the wild and kept captive at the university for about 8 months before the experiment was done. In the experiment, the frogs were exposed to 4 designed treatments: silence, conspecific call playback, heterospecific call playback, and white noise playback noises. It didn't specify if the playback calls were those of male or female frogs, but it did say that since they were interested in non-breeding season conspecific calls, they chose single frog calls to expose the frogs to instead of exposing them to a chorus of calls (typical during breeding season). You have some interesting points; those would be interesting areas for further investigation!
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