Holt, M.M., Noren, D.P., Dunkin, R.C., and Williams, T.M. (2015). Vocal performance affects metabolic rate in dolphins: implications for animals communicating in noisy environments. J. Exp. Biol. 218, 1647-1654. doi: 10.1242/jeb.122424
The ocean out loud
The ocean is a surprisingly noisy place. There’s the crashing of waves, the rumble of underwater volcanoes, earthquakes, and deep-sea hydrothermal vents. Near ports and settlements, there’s also the groaning of large ships as they lumber through the water, the hum of motors of smaller watercraft, the chatter and splashing of beachgoers. Oil exploration, fishing expeditions, and military sonar also add to the din.
Aside from geological and human influences, the billions of animals that call the ocean home are also quite noisy. Toadfish are known for grunting and humming seductively to attract mates. Male black drum, also looking for their future Misses, can make a low-pitched throbbing sound so loud that it’s been misattributed to engineering faults in nearby homes (De La Pena, 2008, New York Times). Herring can expel air from their swim bladder out their anal pore (mischievously termed a “Fast Repetitive Tick,” or FRT for short). Seahorses clank together parts of their skull. And, of course, there’s the clicks, squawks, and whistles of whales, dolphins, and porpoises.
Dolphins depend on sound
Sound production is essential for cetaceans like bottlenose dolphins (Tursiops truncatus), which rely on it to communicate with their pod, forage for meals, and navigate the vast ocean (via echolocation, a process analogous to sonar). However, making such a racket can come at a substantial cost – not only in terms of the biological machinery required (e.g. contracting muscles, memorizing sound combinations, etc.), but also the increased risk of being detected by predators or competitors.
While we have a reasonable understanding of how echolocation in marine mammals works on a physiological and biomechanical level, much less is known about how vocalization plays into their overall energy budget. Do chatty dolphins have to invest extra energy into their aural physiological systems? How might dolphins cope with changes in environmental noise levels, particularly in areas where background noise has seen a substantial, recent increase?
Researchers from NOAA and the University of California, Santa Cruz used flow-through respirometry to measure the oxygen consumption rate of two dolphins trained to vocalize on cue. The animals would swim into a metabolic hood installed at the water surface, and breathe into a sealed air chamber. The oxygen content of the air being pumped into the hood was compared with the oxygen levels inside the hood while the dolphin was breathing. The difference between the two oxygen contents allowed the researchers to calculate the oxygen consumption attributable to the dolphin alone (Figure 1). Since the vast majority of energy expenditure by aerobic organisms (which includes all mammals) is oxygen-dependent, tracking oxygen use over time is a reasonably accurate method of estimating total metabolic rate.
Making more noise requires more energy
During vocalization, the oxygen consumption rates of the two dolphins increased by 20% and 50%, demonstrating that vocalization costs some energy. This reported cost is similar to what has been reported in birds and bats, suggesting that the energetic demands of sound production may be consistent across a wide range of species (though further study is needed in this area). As the amount of sound made (vocal effort) increased, such as with louder or more frequent calling, so did oxygen consumption rate, likely due to greater energetic demands in the vocal muscles. The type of sound also seemed to be important – whistles require much higher pressure in the bony nasal passage, and probably use more energy than clicks and squawks (Figure 2). However, more animals would need to be tested to confirm this hypothesis.
Overall, the increase in oxygen consumption with increased vocal effort was modest. Using this data set, the authors calculated that the cost of doubling whistling frequency was only about 7 kJ of caloric content (adult dolphins consume over 36,000 kJ daily, and adult humans less than 9000 kJ). This is especially low when considering that dolphins do not always increase the volume of their vocalizations in line with background noise.
Why does this matter?
Greater human activity and influence in the oceans means dolphins and other vocal organisms will need to adjust to higher levels of background noise. While the increases in oxygen consumption rate during vocal modification (e.g. making louder, more frequent, or different noises) reported here are very modest (<0.02% of daily expenditure), they can gradually build up in vulnerable groups, such as resident cetacean populations near coastal cities or shipping hubs. Young, sick or nursing dolphins would also be at risk, as their metabolic demands are already elevated due to high rates of growth, immune activity, and milk production, respectively. This study provided an interesting look at how dolphins may use vocal modification to cope with increasingly noise-polluted environments, and the potential energetic costs of this behaviour.