Dolphins have to “shout” to be heard in a noisy environment
Dolphins rely heavily on sound production and reception to navigate, communicate, locate food and maintain an awareness of threats. So it is surprising that the energetic costs in response to man made noise have rarely been studied in marine mammals so far in the literature.
We all know that ocean noise from shipping and other marine traffic is disruptive to echolocating cetaceans causing individuals stress, disrupting their navigation, feeding and ability to detect predators over expansive areas, but what are the energetic implications?
A recent study, published in the Journal for Experimental Biology, investigated energetic costs related to an increase in vocal effort. Oxygen usage was recorded in captive dolphins at rest and during two minute vocal periods. The Dolphins were trained to produce two different call types on command: a quiet low amplitude call and a much higher amplitude call that was ten decibels louder.
The authors found that the louder the dolphins vocalised the more oxygen they used with individuals using 80 percent more oxygen when producing the higher amplitude call compared to when resting. To compensate for increases in environmental noise such as from ship noise, traffic, seismic exploration and construction activity, dolphins not only may produce louder vocalisations in order to be heard over the background noise, but they may also increase the duration and repetitiveness of calls.
This acoustical alteration affects the animals’ health because it is energetically expensive. This increased metabolic cost could accumulate when in areas of chronic noise exposure, meaning individuals may struggle to replenish the energy lost. On top of this, their food sources may possibly be limited so it may be even harder to compensate for the extra oxygen usage. This could be particularly detrimental to young dolphins and nursing females who are already struggling to eat enough to maintain the high energy levels required.
Since marine mammals depend on a naturally quiet ocean to survive, we must make every effort to protect the areas that are already quiet so we can compare the behaviour of animals in these quiet zones to those living in noisier areas. Only then will we have a better idea of the impact human generated noise is having on acoustically sensitive marine organisms.
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