Fig 1.
Spectrogram of a typical flight buzz by a Bombus balteatus queen.
A spectrogram represents the energy of the audio signal within time-frequency bins, where the magnitude of each bin corresponds to the energy within a frequency range during a narrow time frame. The lowest band (approximately 175 Hz in this example) corresponds to the wing beat frequency. The sound is a harmonic series with energy at integer multiples of the wing beat frequency (known as the 1st harmonic or fundamental frequency).
Fig 2.
Characteristic frequency of flight buzzes for workers and queens of two alpine bumble bee species.
Buzz frequency was negatively related to wing length (indicated by the red line) (y = 259.2–7.57x), an estimate of body size that is not biased by pollen and nectar loads of the bees.
Fig 3.
Characteristic frequency is related to bumble bee tongue length.
(A) Variation in characteristic frequency for two alpine bumble bees, Bombus balteatus and B. sylvicola is explained by tongue length (y = 238.2–7.96x), indicating that acoustic signals reflect functional trait diversity. (B) Literature reports of tongue length for workers and queens of 17 bumble bee species also correlate with characteristic frequency (y = 5.48 + e-0.376x). Tongue length measurements correspond to the weighted means of published accounts for each species and caste combination (S1 Table).
Fig 4.
The number of buzzes recorded is correlated with the number of bees counted during visual observations.
Fig 5.
Acoustic surveys predict pollination services in two alpine flowering forbs.
When flowers of Trifolium dasyphyllum (circles) and T. parryi (triangles) are left open to bumble bee visitors, buzz abundance (A) predicts the average number of seeds per plant in each of three 0.01 km2 plots. When bumble bee pollinators are excluded, buzz abundance fails to predict seed set (B), illustrating that acoustic surveys document pollination services by bumble bees. Gray shading around the line represents the standard error of the mean [47].