Scientists face numerous difficulties while working with wildlife in the field, not the least of which is getting access to their subjects. On land, the Great Apes can take a decade to habituate to human presence. Some larger simians, like the bearded sakis of Guiana that Chris Shaffer studies, need to be followed by teams of people to acquire full days of behavioural observation.
Imagine then, the people who really, really, badly want to observe a humpback whale.
These baleen whales migrate long distances, congregating at feeding grounds only to swim across half the globe to breed. So how do you monitor a whale that you just can’t keep up with?
You listen to it sing.
Humpback whales (Megaptera novaeangliae) produce songs that consist of a string of melodious and ethereal sounds that can last up to 30 minutes. We know each string is a ‘song’ because they are able to repeat the melody over and over again with high precision.
Listen to a humpback whale sing.
When a whale sings, it sings its own song, although a few general patterns of song have been identified. They put Pavarotti to shame with their incredible staying power, singing sometimes for several hours at a stretch.
Scientists have come up with an ingenious way to follow the whales around, just by listening to them sing. Acoustic tracking involves either actively emitting a sound to track an object via echolocation, or passively recording sounds produced by the subjects themselves.
In this case, scientists from the Northeast Fisheries Science Center in Woods Hole, Massachusetts, visited the Stellwagen Bank National Marine Sanctuary (SBNMS) in the Gulf of Maine, within the Atlantic Ocean. There, they deployed several listening devices, known as MARUs (Marine Autonomous Recording Devices) on the floor of the ocean in a fixed array. Interestingly, the system was developed by Cornell’s bioacoustics program that typically works with birds, but the same principles apply. After the devices were installed, they were time-synchronised, and the researchers then used passive acoustic tracking to listen for whalesong for a period of three years.
Each device heard whalesong from whales that came near them, and the relative differences in when a song was heard by each device allowed scientists to pinpoint the location of the singing whale.
They recorded tracks that lasted from 30 minutes to 8 full hours. They were able to track these individuals for distances as long as 20 km. Most songs were emitted by actively swimming whales, and in one case, a pair of acoustically interacting whales were also observed.
Although humpback whales are identifiable by unique markings on their bodies, while they are underwater, they are impossible to track. This study provides a method to accurately follow singing whales without having to capture and attach tracking devices to them. Although the sex and life histories of each whale remain unknown, the distinctive songs can be used to tell individuals apart.
Their future could include monitoring of cetacean densities for conservation efforts.
The humpback whale can be expected to be extraordinarily sensitive to sound. It is no surprise then that during the study, several factors were found to disrupt singing. An Ocean Acoustic Waveguide Remote Sensing (OAWRS) experiment was being conducted roughly 200 km from the whales, in which low-frequency pulses were emitted in order to image fish shoals. During this time, the whales abruptly desisted from singing, and this silence could not be explained by anything other than the experiment being conducted 200 km away from the animals.
Their remarkable sensitivity to sound makes the increased noise pollution in the ocean, from sonar experiments to ship traffic, a threat to their ability to communicate and survive. The Monterey Institute’s infographic illustrates this point beautifully.
The National Oceanic and Atmospheric Administration (NOAA) delivered a final report of a detailed analysis of underwater sound in 2012, and they created a map of the relative contributions of various human activities to sound, as well as a geographical map of biologically important areas for cetaceans.
While the ocean is vast, and much of it remains untouched by man, our actions on the surface and under the water have consequences far beyond our imaginations. The whales are always listening!
Stanistreet J.E., Risch D., Van Parijs S.M. & Waterman J.M. (2013). Passive Acoustic Tracking of Singing Humpback Whales (Megaptera novaeangliae) on a Northwest Atlantic Feeding Ground, PLoS ONE, 8 (4) e61263. DOI: 10.1371/journal.pone.0061263.t001
Risch D., Corkeron P.J., Ellison W.T., Van Parijs S.M. & Browman H. (2012). Changes in Humpback Whale Song Occurrence in Response to an Acoustic Source 200 km Away, PLoS ONE, 7 (1) e29741. DOI: 10.1371/journal.pone.0029741.s005
Clapham, P. J. (2000). The Humpback Whale: Seasonal feeding and breeding in a baleen whale. In Cetacean Societies: Field studies of dolphins and whales (Eds.) Janet Mann,Richard C. Connor, Peter L. Tyack and Hal Whitehead. University of Chicago Press, Chicago. pp. 173-196.
Fun Facts and Figures: The Humpback Whale
Megaptera novaengliae, the humpback whale, is a mysticete or baleen whale of the Order Cetacea (family Balaenopteridae). There is only one species in this genus.
- They have very long flippers
- They are moderately large whales, the largest being 17.4 m long
- The females are 1-1.5 m longer than the males
- They gestate for 11-12 months
- They wean their calves at 10-12 months of age
- They wait roughly 2.4 y between births (interbirth interval)(range 1-5 years)
- They first give birth at 5-8 years of age
- The breeding season in the Northern hemisphere is from December to April, and in the southern hemisphere, from June to October.
- Their maximum lifespan could be greater than 48 years, and it used to be much greater but whaling has decimated the larger and older individuals for their blubber.
It prefers nearshore habitats and congregates at traditional breeding grounds. Individuals can be identified by their markings.
They go to high latitudes in the north in the summer, where the water is cold and food is abundant. In the winter, they return to the warmer tropical waters to calve and breed, during which time they do not appear to feed at all (!)
Whales in the two hemispheres feed at alternate times of the year.
While other odontocetes pair up for years, it is not known if the humpback whale does so, or whether they are just congregating over a shared food resource.