(A) Click train of an echolocation event with a search, approach and buzz phases. Arrow marks start of the approach phase, where the first echo is received by the tag. (B) Expanded version of one of the clicks in A, showing the emitted click and a returning prey echo. (C) Envelope of the cross-correlation of the waveform displayed in B. Δt marks the delay between the sounds and thereby the two-way-travel time (TWT).

Bimodal sound production in Mesoplodon. Apparent output (AO) plotted against the corresponding interclick interval (ICI) for 5000 clicks from sequences randomly picked for both of the tagged animals. Regular clicks have ICIs between 200 and 500 ms with an emphasis around 400 ms. Buzz clicks have apparent outputs 15-20 dB lower and ICIs between 5 and 20 ms. Note the few intermediate data points between the two clusters of regular and buzz clicks, emphasizing the rapid transition between the regular click mode and the buzz mode of the sound generator.

(A) Clicks and echoes displayed in an echogram from a 250 s time span during the acoustically active part of a Mesoplodon dive. Time is on the x-axis, and the y-axis gives the time elapsed from the emitted click to the returning echoes expressed as target range by using the two-way-travel time at a sound speed of 1485 m s^-1 akin to the upside-down display of an echosounder. The dense line at time 0 on the y-axis is the emitted clicks that trigger the beginning of the time window. Sound intensity is indicated by the color, so that yellow is strong and blue is weak. Note the many trains of incoming echoes throughout the sequence, and the bottom echoes emerging between 48 and 95 s. (B) Expanded version of the first 25 s shown in A, showing that the whale at times passes through clouds of echo sources without engaging in capture attempts. (C) Expanded version of 16 s shown in A, showing a click train with echoes from an approaching prey target that is terminated by a buzz during capture. The target echoes disappear right at the buzz and reappear around 224 s during the buzz. Note that the ICIs of the buzz are so short that the clicks are displayed repeatedly like harmonics within a time span of 26 ms corresponding to a two-way-travel path of 20 m.

(A) 3-D rendering of a pseudotrack computed for a foraging event at 675 m depth. The black line marks the swimming track of the whale moving right to left through a 20 s period around a prey capture event. Each dot on the line marks a click, and it is seen how the whale switches from regular click mode to buzz mode (bordered by circles in 2-D projections) with a high repetition rate. Coloration of the dots signifies the roll of the animal. Dark blue is dorsal side up and red is ventral side up. Note how the animal rolls up-side down during the prey interception. Dashed lines in x-y, x-z and y-z planes show the projected 2-D swim track. The closing speed is 1 m s^-1. (B) Plot of time before impact (time 0) and target range estimated from a constant swimming speed (blue line) relative to the target and measured from the echo delay (red line and dots). The good fit supports the conjecture that the impact sound was caused by interception of the ensonified target and that movements of the whale were associated with capture of that given prey.

(A) Depicts the distribution of echoes and buzzes per minute as a function of depth (bin width of 25 m) during two dives of the favorably tagged whale. Note that the occurrence of buzzes (red bars) does not overlap with the depths at which the highest numbers of echoes are encountered. (B) Time distribution as function of depth (bin width of 25 m) during foraging dives. Note that the whale spends the most time between 675 and 725 m, where the main part of the buzzes are produced as shown in A.

Echo parameters, first example. (A) Received echo energy flux density plotted against target range. (B) Apparent output on a relative dB scale plotted against target range. (C) Inter-click interval (ICI) of 50 clicks prior to and during the buzz. Black line gives the two-way-travel time (TWT) for the clicks with detectable echoes, indicated by open circles.

Echo parameters, second example. (A) Received echo energy flux density plotted against target range. (B) Apparent output on a relative dB scale plotted against target range. (C) Inter-click interval (ICI) of 50 clicks prior to and during the buzz. Black line gives the two-way-travel time (TWT) for the clicks with detectable echoes (open circles).

(A) Echo energy flux density (black dots) and apparent output (AO) of the approach phase shown in Fig. 7 plotted against log₁₀ to the target range. α gives the slope on a dB scale of the regression line fitted to the data. (B) Similar plot, but of the approach phase shown in Fig. 8.

Pooled ICI data from all approach phases (N=11) plotted against target range. If the data points at ranges of more than 10 m coming from a single approach are omitted, the slope will change to 6.2 instead of 7.1. Red line marks the two-way-travel time (TWT) as a function of target range.