High Source Levels and Small Active Space of High- Pitched Song in Bowhead Whales (Balaena mysticetus) Outi M. Tervo 1 *, Mads F. Christoffersen 1 , Malene Simon 2 , Lee A. Miller 3 , Frants H. Jensen 4 , Susan E. Parks 5 , Peter T. Madsen 6 1 Arctic Station, University of Copenhagen, Qeqertarsuaq, Greenland, 2 Greenland Climate Research Centre, Greenland Institute of Natural Resources, Nuuk, Greenland, 3 Institute of Biology, University of Southern Denmark, Odense M, Denmark, 4 Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, United States of America, 5 Department of Biology, Syracuse University, Syracuse, New York, United States of America, 6 Zoophysiology, Department of Bioscience, Aarhus University, Aarhus C, Denmark 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 18562 dB rms re 1 mPa @ 1 m and a high mean centroid frequency of 444648 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. Citation: Tervo OM, Christoffersen MF, Simon M, Miller LA, Jensen FH, et al. (2012) High Source Levels and Small Active Space of High-Pitched Song in Bowhead Whales (Balaena mysticetus). PLoS ONE 7(12): e52072. doi:10.1371/journal.pone.0052072 Editor: Alexandre Roulin, University of Lausanne, Switzerland Received July 4, 2012; Accepted November 15, 2012; Published December 26, 2012 Copyright: ß 2012 Tervo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was funded by the Oticon Foundation (grant # 08-3469 to Arctic Station, OT). OT and MC were additionally funded by AP Møller og Hustru Chastine Mc-Kinney Møllers Fond til almene Formaal, MS by a PhD scholarship from the Oticon Foundation, FHJ by a Danish Council for Independent Research, Natural Sciences post-doctoral grant, SEP by a grant from the U.S. Office of Naval Research, and PTM by frame grants from the Danish Natural Science Research Council. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: outiter@gmail.com Introduction Whales rely on sound as the primary modality for communi- cation, orientation and finding food [1]. Sound moves through water with high speed and, for lower frequencies, with little attenuation, which favours long-range social signaling [2]. The acoustic properties of a communication signal such as source level, directionality, frequency, bandwidth and duration, will greatly influence the type of information that can be communicated. 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 [4–6]. The active space has important implications for the evolution and function of acoustically mediated behaviour. 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]. Some animals have very small active spaces such as whispering moths that can only hear each other over a few centimetres [8]. Baleen whales, on the other hand, produce powerful signals at low frequencies [9], providing the basis for long range communication [10]. Blue whales (Balaenoptera musculus) and fin whales (B. physalus) produce simple narrowband songs [9] with the lowest frequencies and highest energy contents of any animal. Their songs and calls have dominant frequencies that range from 15 to 29 Hz[11–14] with mean source levels around 186 to 189 dB re 1 mPa (root- mean-square, rms) @ 1 m [12–14]. The combination of high source levels and low sound frequencies, where little sound energy is lost due to absorption, results in active spaces of hundreds to thousands of km for blue and fin whales under natural ambient noise conditions [10,14,15]. But what defines the frequency of animal vocalizations? Fletcher (2004) [16] and Gillooly and Ophir (2010) [17] have presented convincing evidence for an inverse relationship between animal size and the peak frequency for sound production. Larger animals in general produce lower frequency signals at higher sound pressures than do smaller animals [16–17]. Hence, large animals will generally have a larger active space than small animals for the same power output. Large balaenopterid whales such as fin and blue whales fit such scaling predictions by being the largest marine PLOS ONE | www.plosone.org 1 December 2012 | Volume 7 | Issue 12 | e52072