Red Shift, Blue Shift: Investigating Doppler Shifts, Blubber Thickness, and Migration as Explanations of Seasonal Variation in the Tonality of Antarctic Blue Whale Song
Figure 8
Relationship between blubber and tonal frequency.
Seasonal changes around the base frequency measured in this study correlate with seasonal changes in blubber thickness measured by Mackintosh and Wheeler (1929) [23], particularly for males less than 19 m. (A) Time series of intra-annual variation in frequency ratio and blubber thickness. Gray line (right vertical axis) represents the monthly change in frequency ratio (equation 6) measured for all recording sites, while black solid and dashed lines are a summary blubber thickness measurements (left vertical axis) digitised from Mackintosh et al., (1929) [23]. (B) Relationship between blubber thickness and intra-annual measurements of peak-frequency. Dots represent whales greater than 23 m in length, while open circles represent whales less than 19 m in length again with blubber thickness digitised from Mackintosh et al., (1929). Dashed line shows the least-squares fit at all locations to males less than 19 m weighted by the inverse variance of the monthly frequency ratio (intercept −1.55; slope = 4.77; R2 = 0.92; p = 0.004). Males greater than 23 m did not have a significant relationship, so no trend line is shown (intercept = −0.23; slope = 0.69; R2 = 0.28; p = 0.59).