Pygmy blue whale calls have been getting deeper for decades, and marine biologists have yet to identify a cause for the change. It’s not just the pygmy blue whale, however; researchers around the world have observed that whales of all species have been deepening or changing their cries in recent years. According to a publication in the Journal of the Acoustical Society of America, the rate at which the voices of some blue whales are changing is faster than has previously been detected:
The peak frequency of Sri Lankan pygmy blue whale calls decreased from approximately 107 Hz to 100 Hz over a decade corresponding to a 0.55 Hz/year rate of decrease. To date, this is the largest rate of decrease observed for any blue whale call.
The 7 Hz decrease in average pitch is similar to the amount of change in human male voices during puberty. One theory behind the decrease in whale song pitch is that the whales are getting bigger due to less frequent whaling worldwide; since whales are living longer, this thinking goes, they are able to grow larger and therefore have a deeper resonance to their vocalizations. However, not all aspects of the whales’ calls are getting louder, indicating that this change is more specific.
Another popular theory argues that there is an increasing amount of noise pollution in the world’s oceans, causing whales to have to adjust their vocalizations in order to be heard above the din. Deep sea mining, marine traffic, and naval exercises all contribute to the amount of acoustic pollution throughout the world’s oceans. Several whale beaching incidents in recent years have already been attributed to marine noise pollution.
The lead author of the pygmy blue whale study, Jennifer L. Miksis-Olds of the the University of New Hampshire in Durham, said that the change in whale voices could imply a new phase of whale evolution:
We’re seeing an evolution of some type. We just don’t know what aspect of the environment they’re adapting to.
Whale communities have been previously observed to change their vocalizations in response to other groups’ songs, suggesting that whale songs might have a “cultural” component to them. It has already been observed that some whales’ evolution is driven by cultural factors, so the deepening of whale vocalizations could be due to an unknown change in whale cultural behaviors.