Limited vocal compensation for elevated ambient noise in bearded seals: implications for an industrializing Arctic Ocean

Abstract

Vocalizing animals have several strategies to compensate for elevated ambient noise. These behaviours evolved under historical conditions, but compensation limits are quickly being reached in the Anthropocene. Acoustic communication is essential to male bearded seals that vocalize for courtship and defending territories. As Arctic sea ice declines, industrial activities and associated anthropogenic noise are likely to increase. Documenting how seals respond to noise and identifying naturally occurring behavioural thresholds would indicate either their resilience or vulnerability to changing soundscapes. We investigated whether male bearded seals modified call amplitudes in response to changing ambient noise levels. Vocalizing seals increased their call amplitudes until ambient noise levels reached an observable threshold, above which call source levels stopped increasing. The presence of a threshold indicates limited noise compensation for seals, which still renders them vulnerable to acoustic masking of vocal signals. This behavioural threshold and response to noise is critical for developing management plans for an industrializing Arctic.

Vocal plasticity in harbour seal pups

“…The underwater vocalizations of harbour seals in [ 46 ] had a very high SNR (50–70 dB), perhaps also contributing to the lack of observed Lombard effect. However, bearded seals did increase their call amplitudes under higher ambient noise with similarly high SNR [ 45 ]. Garnier & Henrich [ 59 ] reported how the Lombard effect on human speakers helped maintain a +12.5 dB SNR under noise conditions on average, where the SNR would otherwise be negative without any vocal intensity modification.…”

“…Drawing on this evidence, the observed lack of general amplitude shift in our experiment may be due to the high SNR, caused by relatively low playback noise levels. Another explanation could be that seal pups always vocalize close to their physical limits, and thus cannot adjust to different noise conditions (see also [ 45 , 46 , 67 ]).…”

“…Within marine mammals, previous studies targeting amplitude shifts in the presence of background noise have mainly focused on cetaceans, among them bottlenose dolphins and humpback whales [ 20 – 22 , 25 , 44 ]. Within phocid pinnipeds, bearded seals have been shown to increase the amplitude of their underwater calls in higher ambient noise conditions [ 45 ]. To our knowledge, the only studies in harbour seals targeted adult male underwater vocalizations, finding little to no adjustment to noise [ 46 ].…”

Assessment of the impact of anthropogenic airborne noise on the behaviour of Cape fur seals during the breeding season in Namibia

“…Vocal adjustment i.e. an increase in vocalisation amplitude in response to an elevation in the ambient noise level (Lombard effect) has been reported in bearded seals (Fournet et al, 2021). Finally, seals have been shown to experience temporary or permanent hearing threshold shifts in response to noise (harbour seal, Kastak et al, 2008;Kastelein et al, 2013;Reichmuth et al, 2019).…”

Sperm whales exhibit variation in echolocation tactics with depth and sea state but not naval sonar exposures

“…For example, a calling animal may be able to compensate for increased noise by increasing the call amplitude (Lombard effect; Lane & Tranel, 1971), or shift call frequency or temporal pattern relative to the masker (Hotchkin & Parks, 2013). Lombard effects have been reported in social calls of several species of marine mammals, ranging from partial ≪1 dB to full 1 dB increase in source level for each 1 dB increase in ambient and/or anthropogenic noise, including pinnipeds (Fournet et al, 2021) and cetaceans, including killer whales ( Orcinus orca ; Holt et al, 2009), humpback whales ( Megaptera novaeangliae ; Dunlop et al, 2014; Guazzo et al, 2020), right whales ( Eubalaena glacialis ; Parks et al, 2011), and bowhead whales ( Eubalaena mysticetus ; Thode et al, 2020). Echolocating dolphins and porpoises are known to adjust their click source level adaptively depending on the prey distance and environment (Au & Benoit‐Bird, 2003; Jensen et al, 2009; Li et al, 2006).…”