Acoustic crypsis in southern right whale mother–calf pairs: infrequent, low-output calls to avoid predation?

Abstract

Southern right whales (<i>Eubalaena australis</i>) invest substantial amounts of energy in their calves, while facing the risk of having them predated upon by eavesdropping killer whales (<i>Orcinus orca</i>). We tested the hypothesis that southern right whale mother-calf pairs employ acoustic crypsis to reduce acoustic detectability by such predators. Specifically, we deployed multi-sensor DTAGs on nine lactating whales for a total of 62.9 h in a Western Australian breeding ground, and used a SoundTrap to estimate the concomitant acoustic background noise. Vocalisations were recorded at low rates of <10 calls h^-1 (1 call per dive) and at low received levels between 123±8 and 134±10 dB re. 1 µPa RMS depending on call type. We conclude that such acoustic crypsis in southern right whales and other baleen whales decreases the risk of alerting potential predators and hence jeopardizing a substantial energetic investment by the mother.