Seeing with sound: Navigation in the tusked whale of the Arctic
Dubbed “the unicorn of the sea”, the Narwhal Monodon monoceros inhabits Arctic waters off the coasts of Russia, Norway, Canada, and Greenland that are seasonally covered by pack-ice. Here, Narwhals utilise leads and polynyas (open breaks within pack ice) to breathe (Figure 1). Measuring between 4-6.1m in body length, with a spiraled tusk that has been documented to exceed 2.7m long, the Narwhal’s total length is comparable to the that of the length of a bus. Mottled in colouration, Narwhals actually change colour as they age. Calves are born blue-grey in colour and become increasingly paler following maturation. Narwhals are related to Orcas and Beluga whales and travel in groups of ~15 individuals; however groups exceeding several hundreds of members have been observed. Unlike most whale species that migrate over vast distances, Narwhals demonstrate varying degrees of site fidelity and predicted migratory patterns – with large numbers often overwintering under sea ice in Baffin Bay-Davis Strait.
The Narwhal is an odonocete, or toothed whale. However it differs from other whales as it has no teeth in it’s mouth, and instead possess two overgrown or vestigial canines. In males, prominent growth in one (and in rare circumstances two) teeth is demonstrated; here forming a sword-like, spiraled tusk that grows through the whale’s upper lip. The function behind such elaborate dentistry has, until recently, remained unknown to science – various theories suggest that the Narwhal’s tusk, alike with the antlers of Red deer stags, is used in display and courtship behaviours; alternatively the tusk may be used to widen polynyas/gaps in the sea ice. Somewhat more eccentric theories have proposed that the Narwhal’s tusk is used to “spear” prey items. Indeed the tusk of the Narwhal has long evoked interested, both in the scientific community and in popular culture, but it’s not just the animal’s snaggletooth that makes it remarkable. According to a recent study, the Narwhal uses sound to navigate in it’s environment – effectively enabling the tusked whale of the Arctic to see with sound.
“Echolocation: to find my prey and to find my way”
Like any other air-breathing mammal, the Narwhal has to surface in order to breathe – around every 4-6 minutes on average. Narwhals use echolocation to navigate beneath the pack ice in order to find breaks of open water. Additionally, echolocation is also used to locate prey items whilst diving and swimming in an environment almost devoid of light. Echolocation (in its simplest terms) is the use of sound waves and echoes in space, a practice which enables an organism to determine the location of objects within the environment. Cetaceans produce a series of high and low frequency sounds transmitted through their forehead (melon) which produce a series of sonic waves. When the sonic waves come into contact with an object, a further series of echoes “bounce back” to the cetacean – generally, the greater the distance the object is from the animal, the greater the delay of the return of the echo. Within the jaw, are cavities consisting of fatty tissues which channel reflected sounds towards the cetacean’s ears and brain where they are then interpreted. Once the cetacean receives the reflected sound, it forms a mental image of the object’s location size, and the speed it is travelling if the object isn’t stationary, i.e. a prey item. Multimodal integration of several organs within the cetacean are responsible for this sensory output and relay.
Dolphins, for example, have no vocal chords, yet are capable of producing a wide range of clicks and whistles all of different frequencies. The process of sound production is driven by the intake and recycling of air. The following video demonstrates the major components of the biological machinery responsible for echolocation, here examined using a 3D computer animation.
The larynx is the driver of sound production in cetaceans. The organ is surrounded by muscles which control the flow of air through twin passageways in the skull, each sealed by a retractable plug. When the larynx is pushed upwards, the two plugs open and air is pushed vertically towards the phonic lip complexes – this is where sound is generated, with the phonic lips either working in unison or alone. Pressurised air from below causes the phonic lips to vibrate rapidly, producing a series of high and low frequency sounds. The sounds produced are what are used in echolocation, enabling the organism to navigate its environment using sound. Video Credit: LIVING WATERS – Dolphin Echolocation/Illustra Media/YouTube (channel).
How do Narwhals use echolocation?
Narwhals inhabit a complex environment where sound travels almost 5x faster than what it does in the air. Hearing and the processing of sounds therefore hold important biological implications in respect to communication, reproduction, feeding behaviours,and navigation. Knowing that Narwhals use echolocation, a study revealed that they are capable of producing sounds at rates of almost 1,000 clicks per second. The clicks (produced in the phonic lip complexes) enable the Narwhal to scan their environment; creating a large acoustic picture with a higher resolution than any other animal is capable of producing, with the exception of perhaps the Beluga whale.
Narwhals produce a series of high directionality clicks achieved in high intensities in the forwards direction. Such sounds reduce the effects of clutter echoes from the surface or pack ice movement. This creates the effect of narrow beam sonar; the successive production of clicks enables the Narwhal to scan it’s surroundings in a complex acoustic environment almost devoid of light. The echolocation beam in Narwhals is asymmetrical in the vertical plane. Analysis of both the animal’s composite beam and single click beam have inferred that there is a wider shape below the beam axis than above it. This narrow beam further reduces the effects of echoes derived from the surface or pack ice. Alike with bats, Narwhals aim their calls in various directions, a strategy used to gather different signals from their surroundings. This directional scanning behaviour both increases the sampling field and also directs the narrow sonar beam. Such behaviour increases the sampling area in which to search and locate prey, whilst also enhancing the location of singular prey items in the final stages of approach. Directional beam scanning may also be a strategy that enables the Narwhal to locate breaks in the sea ice (Figure 2).
Significance of the Narwhal’s tusk – functional adaptation or elaborate ornament?
As previously explained, the echolocation beam emitted by Narwhals is asymmetrical – the beam is narrower above the beam axis than the below it. However it is unwise to presume that such asymmetry in the Narwhal is a result of its tusk. Such asymmetry is instead an evolutionary advantage deriving from the functional anatomy of the Narwhal’s head.
Both males and tusk-less females appear to possess similar capabilities to echolocate. The tusk, therefore, doesn’t appear to have any functional significance in navigation and echolocation. If there was indeed a survival advantage in receiving and sending acoustic signals, surely female Narwhals would also possess the same feature. In male Narwhals, the tusk may have an evolutionary purpose likewise to that of a peacock’s gaudy plumage or a lion’s mane – here used in sexual display. Larger males are (generally) considered to possess a larger tusk; a larger tusk would determine a higher social rank than that of smaller males, which would subsequently grant access to more females. Male Narwhals have often been observed to “cross swords”, almost resembling the carefully executed movements in fencing, in Arctic bays and fjords (Figure 3).
A sensory function?
One concensus has suggested that the tusk of Narwhals is in fact a sensory organ. Unlike most mammalian teeth, the Narwhal’s tusk is not coated with enamel. Cementum channels, which are also present in human teeth, allow seawater to enter the tusk. Here, seawater travels via a network of tubules to the base of the tusk. The aggregation of tubules at the base facilitates nervous activity. Sea water excites nerve endings here, sending both chemical and temperate signals to the animal’s brain. Male narwhals are therefore considered to be highly attuned to changes in temperature regimes and the chemical composition of seawater – an evolutionary adaptation to locate both prey items and mates.
Anthropogenic change: sound in the ocean
Narwhals demonstrate specialization in a narrow habitat niche within a limited geographic range. The Narwhal is subsequently a sensitive Arctic mammal in respect to climate change and anthropogenic impact changes. Climate change is an unavoidable dilemma which in the long term may create an ice-free Arctic; ultimately affecting the behaviour of Narwhals. However a much more demanding issue in regards to Narwhal behaviour and distribution is the increasing rate of shipping activities in and around the species range. Narwhals demonstrate high site fidelity to Baffin bay, an area located off of Western Greenland, where over 80% of the world’s total population overwinter and feed. The summering site for the majority of the population (i.e. Lancaster sound) is now subject to year-round shipping activities resulting from intensive sea ice retreat. Both locations essentially essentially create an “acoustic bowl”; increasing anthropogenic use of the Arctic increase the clutter echoes experienced by Narwhals. Such noise pollution masks sound orientation and communication. Narwhals have also been observed to “freeze” in response to visually sighting ice-breaking vessels. The Narwhal’s site fidelity and apparent lack of behavioural plasticity (change) may prevent them from leaving sites experiencing high levels of anthropogenic noise pollution (Figure 4.)
For Narwhals, listening is as important as seeing. Increasing noise pollution may alter their behaviour, potentially driving them away from areas vital to the population survival.