Dear Peter,

few year ago Andreas Burkart, Clemens Schlindwein and me have done a comparative study of buzzing frequency in relation to flight frequency (PDF attached) in neotropical bees including one bumblebee. The rule the the buzzing frequency is twice as high as the flight frequency holds only for large bees. the smaller the bees the higher is the buzzing frequency and the more similar are buzzing and flight frequencies.

Klaus

National Public Radio recently did a number of pieces on the sounds animals make. Many of the sounds are outside human hearing. Here is the link:

http://www.npr.org/series/93588962/wild-sounds

Maybe one or more of these pieces could provide a contact of someone who could help. It would require recording sounds at a high enough resolution and digitizing the result.

Sounds like a fun project.

Our analysis of bumble bee buzz frequencies indicate a range of flight buzzes (not sonication buzzes) with fundamental frequencies from ~125-250 hz. So that is in line with the idea that the buzzing frequency is twice as high as the flight frequency.

Check with Steve Buchmann.

Peter, you may also be interested in some recent work on Pedicularis by Corbet and Huang.  They show a positive correlation between wingbeat frequency and sonication buzzes, but the correlation coefficient is below the magic 2:1 ratio.  

Thank you for the link Klaus, quite an interesting paper that I have not found I'm my searches.

I also second David's recommendation of Contacting Steve Buchmann, he has a massive amount of knowledge on this subject and is very approachable! 

Article Buzz pollination in eight bumblebee-pollinated Pedicularis s...

Dear Peter,

I am not an entomologist or anything of the sort, however if you are interested in analysing the frequency content of bee sounds may I suggest the Voicebox Toolkit for Matlab. It has many functions, including fundamental frequency analysis, which are tailored to human voice analysis but can be applied to most any sound (occasionally requiring minor modification). 

http://www.ee.ic.ac.uk/hp/staff/dmb/voicebox/voicebox.html

Thank you, Chris.  Best wishes,  Peter

First, there are many species of bumblebees and the (buzzing)sounds of them are not the same. Even inside single species there are workers, queens and so on of various size and the size is important for frequency range of sounds. And even single animal can make sounds of various frequencies in different behavioral context. There are also differences in spectra of sounds, some sounds are ±pure tones and others have pronounced higher harmonics, sometimes with the same fundamental frequency - therefore a comparison with the musical scale is not that simple. 

Dear Matija,

your statements are very interesting concerning the intrainididual variation of the buzzing frequency. In my opinoin there seem to be two hypotheses:

1. Individual bees actively adjust their buzzung frequency to maximize pollen release or electrostatc charge of pollen grains.

2. The variation in the individual buzzing frequency is caused by mechanical properties of the buzzed flowers, i.e. rather passive.

Klaus

The variation in buzzing frequency could be an allometric function of body size. 

Dear David,

I agree that it is possible that in bumblebees buzzing frequency could be an allometric function of body size. However, this would not answer any quenstion about intra-individual variation of buzzing frequency.

Extending and combining earlier comments: No part of the system is constant. So there could be a behavioral component where the bee tries to optimize the buzz. There could be limits to the bees ability to change the buzz where the limits are set based on some function of body size and wing size. Damage to the wing will influence the buzz, and age might also be a factor. All of this is in addition to small morphological variations in the flowers that will influence how the flower resonates to the buzz of the bee. You could record the vibrations in the plant caused by the buzzing of the bee. It probably doesn't play any role in communication, but it is another way of measuring the system and allows one to look at a plant "response" to the bee's buzzing.

It would also be interesting to know the evolutionary consequences of the variability in the system.

I agree: There could be a behavioral component where the bee tries to optimize the buzz and there could be morphological variations in the flowers that will influence how the flower resonates to the buzz of the bee. Both would explain the variation in buzzing frequency within an individual bee.  What is left open is the question whether the bees are capable to actively modify their buzzing frequency.

This might be worth a look:

1) The Sound of bees, Poels, J (Poels, Jeanpyer), EUROPE-REVUE LITTERAIRE MENSUELLE, Issue:1033, Pages:339-339, Published:MAY 2015, Publisher REVUE EUROPE, 4 RUE MARIE-ROSE, 75014 PARIS, FRANCE, ISSN: 0014-2751

(The article is in French, so I cannot read it.)

I have to follow up on all the ideas.  There are various constraints on buzzing, one being the physical dimensions of the thorax and how the indirect flight muscles can cause vibrations in harmonics with muscular contractions and relaxations. Peter