How does biotremology identify with bioacoustics?

Question

I am wondering how much biotremology (vibrational communication) differs from bioacoutics (e.g. sister disciplice or sub-branch). From what I understood (Biotremology (2016), What is Biotremology? (2019)), biotremologists insists on that biotremology should be a sister discipline of bioacoustics and not a sub-branch mainly for the 2 following reasons:

• the type of mechanical waves they study. This paper opposes biotremology to bioacoustics because of the type of waves they study (to discuss on definition of acoustics, see this thread):

[Bioacoustics studies] Acoustic waves: purely longitudinal waves in a homogeneous medium (gas, liquid, or solid). [Biotremology studies] Surface-borne waves: waves (Rayleigh or bending) at a boundary between two distinct media — particles oscillate perpendicular to the plane of wave/energy travel.

• The high number of different medium where the wave propagates before being detected by an organism. This book foreword says:

This means that the effects of the communication environment on the efficacy and evolution of signals are potentially much greater for vibrations compared to sound.

However, even if bioacoustics may have originally been the study of animal sounds that humans can hear (i.e. 20-20k Hz air-born sounds detected by pressure sensors), bioacoustics has extended to sound frequencies not audible by human (e.g. ultrasounds echolocation), to other medium (e.g. underwater communication) and to sensors sensitive to physical quantities distinct from pressure (e.g. dipter’s antenna particle-velocity sensor).

In this respect and from the eyes of somebody not studying vibrational communication, biotremology could be seen as "just" another sub-branch of bioacoustics, i.e. the study of communication through the transversal component of surface-borne waves in substrates.

1. Are the 2 differences that I listed above the main ones that justifies the distinction or did I miss something?

2. How much the distinction is spread in the community of researchers studying vibrational acoustics communication? Is there groups of this community who still refer themselves as bioacousticians or is this split largely established?

Answer 1

Ok, this is a tricky one but I'll have a go!

1. The first difference you list (wave direction perpendicular, and non-compressional waves in boundaries) is what makes the distinction to my understanding. Bioacoustic studies do include things that we as humans cannot hear, but these are still pressure waves (which are not included in biotrem studies).

Regarding animals which have particle velocity sensors and use them in air or water (without detecting pressure)- here we get into a sort of grey area between disciplines which is fun to think about.

Can we include those animals in biotremology? are they communicating using substrate-borne vibrations? Well, we can form a strong argument in either direction, since it really depends on how strict we are with our biotremological definitions of "boundary/surface", "substrate", "particle motion", "vibrational", and how particle motion in air and water (being a single media) fit with those. (if you actually want the full arguments either way, feel free to message me, it's long!)

Currently, those types of examples are in bioacoustics as a default, because that discipline has been defined for a lot longer. basically my point here is, we don't have all the answers yet, some areas are still tricky- we're talking about a discipline only defined around 2014- however, what we DO know is that many many animals simply are not "acoustic" as they cannot detect pressure waves. They have distinct, specialised receptors for sub-borne, non-compressional, waves.

2. Most biotremologists are also bioacousticians, and flit between the two, but it really depends on study animal. For example if I am recording:

e.g. 1--- the call of a frog- it can be measured in the air as sound and in the ground as vibration. which is the relevant stimulus to the animal? that's a tricky question to answer! so, we have to measure both stimuli, and it falls in both disciplines.

e.g. 2--- the courtship song of an insect may vibrate the surface of a leaf, but does not produce sound. This case is more in a biotremologists remit.

The final aspect to note is that it's generally accepted now that vibrational communication was the evolutionary precursor to acoustic communication, as found in insects. Hence sister discipline, not sub-discipline. Yet unlike bioacoustics, we did not have the technology to detect sub-borne waves until relatively recently, and most of these vibrations are not detectable by humans, we simply did not know that animals were using vibrations in substrates! Hence the lag in research and the new-ness of the discipline.