Narrow Acoustic Field of View Drives Frequency Scaling in Toothed Whale Biosonar.

ABSTRACT

Toothed whales are apex predators varying in size from 40-kg porpoises to 50-ton sperm whales that all forage by emitting high-amplitude ultrasonic clicks and listening for weak returning echoes [1, 2]. The sensory field of view of these echolocating animals depends on the characteristics of the biosonar signals and the morphology of the sound generator, yet it is poorly understood how these biophysical relationships have shaped the evolution of biosonar parameters as toothed whales adapted to different foraging niches. Here we test how biosonar output, frequency, and directivity vary with body size to understand the co-evolution of biosonar signals and sound-generating structures. We show that the radiated power increases twice as steeply with body mass (P ∝ M1.47 ± 0.25) than expected from typical scaling laws of call intensity [3], indicating an evolutionary hyperallometric investment into sound production structures that may be driven by a strong selective pressure for long-range biosonar. We find that biosonar frequency scales inversely with body size (F ∝ M-0.19 ± 0.03), resulting in remarkably stable biosonar beamwidth that is independent of body size. We discuss why the three main hypotheses for inverse frequency scaling in animal communication signals [3-5] do not explain frequency scaling in toothed whale biosonar. We instead propose that a narrow acoustic field of view, analogous to the fovea of many visual predators, is the primary evolutionary driver of biosonar frequency in toothed whales, serving as a spatial filter to reduce clutter levels and facilitate long-range prey detection.