High Source Levels and Small Active Space of High-Pitched Song in Bowhead Whales (Balaena mysticetus)

Outi M Tervo,Susan E Parks,Malene Simon,Mads F Christoffersen,Peter T Madsen,Lee A Miller,Frants H Jensen,Alexandre Roulin

doi:10.1371/journal.pone.0052072

Abstract

The low-frequency, powerful vocalizations of blue and fin whales may potentially be detected by conspecifics across entire ocean basins. In contrast, humpback and bowhead whales produce equally powerful, but more complex broadband vocalizations composed of higher frequencies that suffer from higher attenuation. Here we evaluate the active space of high frequency song notes of bowhead whales (Balaena mysticetus) in Western Greenland using measurements of song source levels and ambient noise. Four independent, GPS-synchronized hydrophones were deployed through holes in the ice to localize vocalizing bowhead whales, estimate source levels and measure ambient noise. The song had a mean apparent source level of 185±2 dB rms re 1 µPa @ 1 m and a high mean centroid frequency of 444±48 Hz. Using measured ambient noise levels in the area and Arctic sound spreading models, the estimated active space of these song notes is between 40 and 130 km, an order of magnitude smaller than the estimated active space of low frequency blue and fin whale songs produced at similar source levels and for similar noise conditions. We propose that bowhead whales spatially compensate for their smaller communication range through mating aggregations that co-evolved with broadband song to form a complex and dynamic acoustically mediated sexual display.

Highlights

Whales rely on sound as the primary modality for communication, orientation and finding food [1]
The fundamental frequency of these notes ranged from 104614 Hz (Fmin) to 13566102 Hz (Fmax), and was generally comparable to the song notes that could not be localized in terms of duration, centroid frequency and spectral parameters (Table 1)
Blue and fin whales produce low frequency songs more than 4 octaves lower than the centroid frequency of the high frequency song notes of humpback and bowhead whales (Fig. 1A) raising the question of what are the active spaces for these high frequency singers? In an attempt to answer that question for bowhead whales, we have measured the source levels and spectral characteristics of bowhead whale spring song to address implications of high frequency singing for the acoustic and social behaviour of this large Arctic balaenid

Summary

Introduction

Whales rely on sound as the primary modality for communication, orientation and finding food [1]. Environmental sound propagation properties and ambient noise levels in addition to source parameters will define the range over which acoustic information can be relayed [3]. The active space of an acoustic signal is defined as the maximum range from the vocalizing animal where the sound level allows a conspecific to detect and decode the signal [4,5,6]. To estimate the active space of a particular communication signal it is necessary to know the source level (defined as the sound level 1 m from the vocalising animal on the acoustic axis [7]), the frequency bandwidth, the sound attenuation of the signal through the habitat, the ambient noise and the hearing capabilities of the listener [5,6]

Methods

Results

Discussion

Conclusion