Instrumentation & Measurement Magazine 24-7 - 23

Fig. 2. (a) Example of waveform and (b) spectrogram of fanning in bumble bee colony. Fanning creates a sustained broadband sound with equivalent fundamental
and dominant frequencies. Little acoustical energy is produced above the second harmonic.
muscles (i.e., 'shivering' [7]). Bumble bees can warm their thoraxes
up to 20 °C warmer than ambient temperature and use
this mechanism to incubate brood and maintain nest temperature
[8]. The amplitude from the sound generated by a
shivering bumble bee positively correlates with metabolic heat
flux, suggesting that acoustic monitoring serves as a proxy for
heat production in bumble bee colonies [9].
The degree to which these sounds are communicative to
nestmates is unknown; nonetheless, they are contextually distinguishable
and informative. AMT offers a low-cost, passive
solution to monitoring important colony activity such as the
behavioral tradeoffs between thermoregulation and brood
maintenance or foraging activity. Scientists may learn how behavioral
tradeoffs induced by extreme weather events affect
colony survival, and farmers learn when to intervene if colonies
are under thermal stress.
Foraging
Bumble bees gather pollen and nectar from flowers to feed
the developing brood of their colony. As they come and go
from the colony and interact with nestmates within the colony,
they create distinguishable sounds associated with foraging
that can be used to track behavior and inform management
decisions.
The efficiency with which social bees can relay information
about foraging resources has implications for the survival
of the colony. Honeybees and some tropical bees perform vibratory
'dances' in which they encode messages that inform
nestmates of direction, distance and height of resources. While
bumble bee communication of floral resources is regarded as
rudimentary and thought to occur primarily through pheromones,
several reports describe hurried 'buzz runs' to recruit
October 2021
other workers to forage for novel nectar resources: after arrival
to the colony, foragers move rapidly about the colony, bumping
into and climbing over other bees, depositing nectar into
honeypots, and fanning their wings audibly in rapid, pulselike
buzzes [10], [11] (Fig. 3).
Other reports of foraging related sounds include the 'humming'
or 'buzzing' sounds between the queen and workers
and the larvae while feeding [12], [13] and 'leaving sounds'
[5], [11]. There is also note of 'honking' in workers and 'barking'
in queens, but these are not described quantitatively, and
their biological significance is unknown [5]. Upon return to
the nest, B. impatiens foragers use bursts of vibrations at ~600
Hz to prompt other workers to begin foraging [14]. Playback
experiments of these vibro-acoustics prompted more colony
recruitment events than a similar playback experiment
with white noise, suggesting that bumble bees may use vibroacoustics
as communicative signals. However, the extent to
which bumble bees use other vibro-acoustics for communication
of foraging resources is still poorly understood.
More recently, Heise et al. automated detection [15] and
classification of arrival and departure buzzes [16] in audio
from microphones housed within bumble bee colonies. Arrival
buzzes are characterized by a gradual crescendo and an
abrupt offset (Fig. 3) as the bumble bee approaches and lands
at the colony entrance. Departure buzzes are characterized
by an abrupt onset and a brief decrescendo as the bumble bee
initiates flight and departs from the colony entrance. Heise
et al. developed a model that differentiates the buzz types by
computing smooth amplitude envelopes and calculating the
distance from signal start and stop to the maximum peak amplitude
in the segment [15]. Buzzes are classified as arrivals
when distance from signal start to peak amplitude is greater
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