The costs of predation may exert significant pressure on the mode of communication used by an animal, and many species balance the benefits of communication (e.g. mate attraction) against the potential risk of predation. Four groups of toothed whales have independently evolved narrowband high-frequency (NBHF) echolocation signals. These signals help NBHF species avoid predation through acoustic crypsis by echolocating and communicating at frequencies inaudible to predators such as mammal-eating killer whales. Heaviside's dolphins () are thought to exclusively produce NBHF echolocation clicks with a centroid frequency around 125 kHz and little to no energy below 100 kHz. To test this, we recorded wild Heaviside's dolphins in a sheltered bay in Namibia. We demonstrate that Heaviside's dolphins produce a second type of click with lower frequency and broader bandwidth in a frequency range that is audible to killer whales. These clicks are used in burst-pulses and occasional click series but not foraging buzzes. We evaluate three different hypotheses and conclude that the most likely benefit of these clicks is to decrease transmission directivity and increase conspecific communication range. The expected increase in active space depends on background noise but ranges from 2.5 (Wenz Sea State 6) to 5 times (Wenz Sea State 1) the active space of NBHF signals. This dual click strategy therefore allows these social dolphins to maintain acoustic crypsis during navigation and foraging, and to selectively relax their crypsis to facilitate communication with conspecifics.

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Knowledge of the occurrence and distribution of cetaceans is particularly important for conservation and management, but is still limited within Namibian waters. We collated 3211 cetacean records from the Namibian Exclusive Economic Zone (EEZ) for the period 2008 to 2016 and applied the principle of minimum cross entropy (referred to as MinxEnt) to predict habitat suitability. MinxEnt is a generalised form of maximum entropy modelling allowing incorporation of additional information such as sampling bias. The habitat suitability of nine cetacean species or species' groups (five odontocete species, two mysticete species, two taxonomic groups: pilot whales Globicephalus spp and balaenopterids Balaenopterid spp) were predicted per season, in relation to environmental variables likely to drive cetacean presence: sea surface temperature, chlorophyll a concentration, water depth or distance to shore, seabed slope and habitat complexity. The environmental variable which most frequently influenced species habitat suitability was depth, and it was the main environmental driver of six species or species' groups: dusky Lagenorhynchus obscurus and bottlenose dolphins Tursiops truncatus, pilot, humpback Megaptera novaeangliae and southern right whales Eubalaena australis and the balaenopterid group. Further, Heaviside's dolphin Cephalorhynchus heavisidii habitat was best predicted by distance to shore in all seasons, while common dolphin Delphinus delphis habitat was best predicted by habitat complexity and sperm whale Physeter macrocephalus habitat by chlorophyll a concentration. We identify distinct spatial patterns in species' habitat suitability and provide baseline maps which can be used by managers of wildlife resources.

Arctic marine mammals face a multitude of challenges linked to climate change, including increasing anthropogenic noise from ship traffic. The beluga whale (Delphinapterus leucas), a predominately Arctic endemic cetacean, relies heavily on acoustic communication, with documented overlap between their vocalizations and hearing range and ship noise. Some belugas migrate through areas with the highest levels of ship traffic in the Pacific Arctic and exposure to ship noise is highly probable. Here, we document the responses of nine satellite-tagged Eastern Beaufort Sea belugas to encounters with ships in the Beaufort, Chukchi, and Bering Seas during July-December 2018. We report 177 occasions when ships were within 125 km of tagged belugas and quantified changes in lateral and vertical movements to investigate individual behavioral responses to ship approaches within 50 km (n = 23). Belugas' swim speed was negatively correlated with ship distance, showing possible changes in swim speed up to 79 km away. Changes in lateral and vertical movements, indicating disruption of