What acoustic parameters can I reference if I suspect speciation?

Question

I work with a remote population of mist frogs in the Amazon, and I have set up multiple recorders at 6 locations that we have surveyed to capture their vocal repertoire. In total, we have surveyed 57 frogs in the different areas. These frogs are very area specific, as they require a precise humidity to survive. Our survey data suggests they do not regularly move more than 500 meters away from their home region. They are also very territorial. They produce constant frequency tonal calls during mating season.

One of my recorders is separated by a few kilometers, as well as a 400 m elevation change. In this area we surveyed 2-3 frogs. I have noticed some difference in the calls I am recording between this and other populations. While the fundamental frequency of the tonal call is typically 1500 Hz, the group higher in elevation has a fundamental frequency of 2000 Hz. The group higher in elevation has shorter calls, and shorter pauses in between calls.

I am curious if this supports speciation? Because this group is so far away we consider it isolated. If so, are there other parameters that I should consider? I do not have a permit to take genetic material at this time.

Answer 1

In general, one can not suggest speciation without genetic confirmation. As interesting as your findings are there are many things that may be contributing to the difference that you are measuring.

Due to the location change, there may be factors in your subgroup's environment, e.g. predators, humidity quality, animal behavior, etc. that are causing the frogs to change their vocalizations. You would need to test for these confounding conditions before you would able to say that this group has a different repertoire.

Your sample size is also very small. Far too small to suggest that you are documenting something unique, and not just natural variation in their vocalizations. Also, are you sure that they can not travel between locations? If not, how did your isolated group get to where it is? Can you guarantee that they do not interact and breed with the other mist frogs?

However, what you do have is a convincing data that warrants further investigation. These difference would be worth highlighting, with caveats, in your study, and will be helpful in your argument to collect genetic material.

Answer 2

I vaguely recall something like a corelation between body size and call frequency in frogs, and a correlation between altitude and body size in most animals. Differences in call parameters could also be due to changes in body size...

Since the function of frog calls is mate attraction, maybe you could try to playback the calls of population B to frogs of population A and vice-versa. This might give you a preliminary indication : if frogs of one population don't respond to the calls recorded in the other population, it could suggest that there might be pre-zygotic barrier. In this case, maybe it would make sense to carry on and look for more evidence of speciation.

Answer 3

Another question to ask is whether temperature affects call frequency for these frogs, like it does in some fish species (e.g. toadfish). Could the altitude difference affect temperature and hence call frequency? Were they recorded at the same time of year? The differences in call duration and time between calls is also interesting. More data would be nice to see if this finding can be generalized.