Whale geography: A species-centric approach applied to migration

Abstract

Understanding the biogeography of a species begins by mapping its presence over time and space. The use of home ranges, breeding and feeding areas, migration paths and movement patterns between the two are also inherent to their ecology. However, this is an overly simplified view of life histories. It ignores nuanced and complex exchanges and responses to the environment and between conspecifics. Having previously advocated for a more species-centric approach in a discussion of ‘whale geography’, I look to better understand the driving factors of migrations, and the information streams guiding the movement, which is key to the biogeography of large whale species. First, I consider the processes underlying the navigation capacities of species to complete migration, and how, and over what scales, sensory information contributes to cognitive maps. I specifically draw on examples of large-scale, en masse migrators to then apply this to whales. I focus on the acoustic sense as the principal way whales gain and exchange information, drawing on a case study of grey whale (Eschrichtius robustus) calling behaviour to illustrate my arguments. Their consistent employment of far-propagating calls appears to be tied to travel behaviours and probably aids navigation and social cohesion. The range over which calls are being propagated to conspecifics, or perhaps being echoed back to the individual, underlies the distance over which the cognitive maps are being both formed and employed. I believe understanding these processes edges us closer to understanding species biogeography.

Keywords

Whales soundscape migration acoustics biogeography species-centric

I previously advocated for a species-centred approach in biogeographical studies of animals (Burnham, 2017). I used the example of whales’ use of acoustics to advance the discussion of ‘active space’ as a key feature of the animal’s ecology. Active space, in its most general sense, is the area over which a sensory signal can be detected and decoded by a receiver. Acoustic active space is the extent that auditory information from a signaller can be detected and discriminated as intended by a receiver. It is the range over which a whale can communicate with conspecifics or receive information from its own echo. The concept of active space represents a significant refinement in realized niche, confirming it to be an attribute of the individual, or perhaps population, in relation to its environment (Hutchinson, 1978). Again, I look to the acoustics of whales and active space to gain a greater understanding of their capacity to complete long-distance migrations as part of their life history. I present my interpretation of how acoustics may facilitate navigation and population-wide movement. I speculate as to how calling aids way-finding, group cohesion and habitat assessment. I draw on examples of long-distance, en masse migrations from different taxa to frame the interpretation of data from whale migration to inform my conclusions. Taking a species-centric approach, I examine the type and scale of sensory data available that factors into navigational choices.

Movements of individuals, and collectively as flocks, herds or schools, is a result of ecological, physiological or behavioural mechanisms (Strandburg-Peshkin et al., 2015, 2017). Migration between breeding and feeding or refuge areas is a response to endogenous rhythms or environmental signals, to enhance resource acquisition, reproductive success or the survival of young (Abrahms et al., 2019; Corkeron and Connor, 1999; Wilcove and Wikelski, 2008). It is a widespread phenomenon (Wilcove and Wikelski, 2008) and is most frequently noted in species that rely on shifting or patchy resources (Dingle, 1996). The orientating mechanisms, perceptual cues or memory processes needed to achieve migration are driven by multi-scale sensory information (Table 1; Mueller and Fagan, 2008). Understanding these stimuli may outline both the motivation for, and means to, complete the movement.

Table 1.

Examples of the modality use of species to achieve long distance, en masse migrations, and the scales over which they process information.

Modality	Use	Example species	Scale^a
Geomagnetic reception	Magnetic signatures, field lines	Amphibians	Micro to meso
		Bats	Micro to meso
		Birds	Meso to large
		Baleen whales	Meso to large
		Crustaceans	Meso to large
		Fish	Meso to macro
		Insects, bees	Meso to large
		Odontocete whales	Meso to large
		Sea turtles	Meso to macro
		Small mammals	Micro to meso
Vision	Leading lines, habitat/geometry features	Amphibians	Micro to fine
		Birds	Meso to large
		Baleen whales	Micro to fine
		Insects	Fine to micro
		Primates	Fine to meso
		Ungulates	Fine to meso
Celestial features		Amphibians	Fine to meso
		Birds	Fine to meso
		Sea turtles	Fine to meso
Conspecific signalling (direct)		Bees	Micro to fine
		Fish	Micro to fine
		Primates	Fine to meso
		Ungulates	Fine to meso
Conspecific signalling (indirectly through cues)		Primates	Micro to fine
		Small mammals	Fine to meso
Conspecific visual contact (including following)		Baleen whales	Micro to large
		Birds	Micro to fine
		Primates	Micro to fine
Olfactory/chemoreception	Locational unique signatures	Amphibians	Micro to meso
		Baleen whales	Fine to large
		Birds	Meso to large
		Fish	Fine to meso
		Insects	Micro to fine
		Ungulates	Fine to meso
	Conspecific (scent trails)	Insects	Micro to fine
		Ungulates	Micro to fine
Somatosensory	External stimuli	Baleen whales	Meso to large
		Crustaceans	Fine to meso
		Fish	Fine to meso
		Insects	Fine to meso
		Ondotocete whales	Meso to large
	Conspecific contact	Baleen whales	Micro to fine
		Crustaceans	Fine to meso
Audition	Low-frequency echoes	Baleen whales	Fine to large
		Birds	Meso to macro
	High-frequency echoes	Bats	Micro to meso
		Odontocete whales	Micro to meso
	Conspecific calls	Amphibians	Micro to meso
		Baleen whales	Meso to large
		Birds	Micro to meso
		Elephants	Meso to macro
		Large cats	Fine to meso
		Primates	Micro to fine
		Wolves	Fine to meso

Baleen whales are the case study and so are highlighted in bold.

^a Micro = 0–100 m; Fine =100 m–5 km; Meso = 5–100 km; Large = 100–500 km; Macro = 500 to >1000 km.

Three principal mechanisms of long-range navigation have been described: landmark navigation or ‘piloting’, vector navigation and ‘homing’ using cognitive spatial maps. Piloting is a process of following a sequence of reference points, presented on different scales, which are reinforced by indicator cues, between familiar areas (Brown and Gass, 1993). The use of ‘leading lines’ and large-scale geometry, such as shorelines or mountain ranges, seen in birds (Boinski and Garber, 2000) and primates (Gouteux et al., 2001; Mackinnon, 1974), informs scene-related memory (Rolls and Wirth, 2018). A migrator using vector navigation follows a directional bearing for a given time or distance and may use landmarks to confirm orientation. The use of spatial maps incorporates both landmark and vector mechanisms with information from sensory and environmental cues to confirm direction and course. Integrating inputs from large to fine scales, ‘homing’ allows navigation to an endpoint outside the sensory field of the starting point, despite displacement or obscured cues. Understanding how sensory information adds to the construction of internal maps will aid our understanding of both navigation and site selection by migrators. Here, I will discuss the senses in turn, drawing on example species to illustrate their use in navigation (see Table 1 for summary). This will form the basis of my discussions on the mechanisms that underlie migration, applied to the whale case study.

The geomagnetic sense is the most widely applied physical cue to animal navigation, where the location and angle of intersection of field lines and the intensity of signals form a bi-coordinate map (Lohmann et al., 2007; Wiltschko and Wiltschko, 1996, 2005). Locations have distinct magnetic signatures based on the magnetic inclination, which varies by latitude as the field lines curve around the Earth, and magnetic intensity (Lohmann et al., 2004, 2007, 2012; Putman et al., 2013, 2014a, b). Signatures may be imprinted when an animal is young to aid long-distance travel and natal homing (Lohmann et al., 2008a; Lohmann and Lohmann, 2019). Birds, particularly homing pigeons, insects, small mammals, crustaceans, fish and sea turtles are thought to use geomagnetic cues (Table 1; Bingman and Cheng, 2005; Lohmann and Lohmann, 2019; Lohmann et al., 1999, 2007; Wiltschko and Wiltschko, 2005). These signals are enhanced by barometric pressure (e.g. for bees, Aidley, 1981), wind (e.g. birds and locusts, Baker, 1978), or water currents and tidal rhythm (e.g. fish and horseshoe crabs, Aidley, 1982; Dingle, 1996).

Visual cues are taken from landscape features, as mentioned for ‘piloting’, or perhaps orientating to the sun, moon or celestial markers. This is prevalent in birds, tied to their internal circadian rhythms (Table 1; Aidley, 1981; Akesson and Hedenstrom, 2007; Wiltschko and Wiltschko, 2003) and sea turtles (e.g. Avens and Lohmann, 2003; Lohmann et al., 2008a, b; Muheim et al., 2006).

Olfactory cues reinforce spatial features with unique chemical signatures that extend up to hundreds of kilometres, reflecting mountain ridges, river outlets, coastlines and oceanic features, such as seamounts and the continental shelf break (Benevenuti et al., 1994; Nevitt and Bonadonna, 2005; Wallraff, 2004). Olfactory-guided navigation may be achieved through the recognition and association of odours with specific locations (e.g. Papi’s mosaic model, Ioalè et al., 1990) or by the differentiation of odour intensities and dissipation gradients over a landscape (Wallraff’s gradient model, Wallraff 1974). Between conspecifics, the use of scent trails has been documented for wildebeest for a leader to guide followers (Table 1; Dingle, 1996; Yahner, 2012) and insects (Baker, 1978; Boinski and Garber, 2000; Kennedy, 1983; Wilson, 1965).

Acoustic information may be garnered through passive listening or actively sought (see Table 1). Navigation by passive infrasonic cues has been found in fish, guided by the noise of surf and water movement (Farina, 2014; Sand and Karlsen, 1986). Birds use noise from breaking waves and wind off mountain ranges for large-scale cues (Aidley, 1981; Hagstrum, 2000). Echolocation in bats is an example of active acoustics. The return signals of ultrasonic calls can give detailed information to form an acoustically derived mental map aiding prey capture, navigation, orientation and habitat assessment (Knörnschild et al., 2012).

The examination of sensory information reception and perception describes how individuals may use each modality to aid navigation. Migration, however, is more typically undertaken as a group. Information on migratory routes may be transmitted culturally, from mother to young (Baker, 1978), inherited (e.g. monarch butterflies, Brower, 1996), or conveyed between individuals (Table 1), such as the honey bee ‘waggle dance’ (Riley et al., 2005; von Frisch, 1967) or ‘voting behaviours’ in buffalo herds during pre-migration milling (Prins, 1996). Moving as a group shares the knowledge of both route and destination (Boinski and Garber, 2000; Scott et al., 2014), as well as increasing navigation accuracy (Baker, 1978; Hamilton, 1967; Mueller et al., 2013; Pettit et al., 2013).

Mechanisms to maintain cohesion between individuals during migration have been noted in several species (see Table 1). Spiny lobsters demonstrate ‘queue’ behaviour while maintaining physical contact (Dingle, 1996). Some primates travel with the rule ‘keep at least one individual in sight’, with calls used if vision is obscured (e.g. the ‘bark’ call of baboons, Ingmanson, 1996). Also ‘trail markers’ may be employed. Bonobos (Pan paniscus) deliberately break and disturb foliage to mark the travel direction (Ingmanson, 1996), whereas wildebeest communicate direction and suitability of habitat through penal gland secretions (Baker, 1978). Vocalizations may be used to initiate and maintain travel or convey directional information. Calls like ‘rallying’ cries, described for primates, wolves, hyenas, coyotes and lions (Gautier and Gautier-Hion, 1977) or ‘let’s go’ rumbles heard from elephants (Leighty et al., 2008; Poole et al., 1988) initiate migration. Calls signalling to ‘travel in that direction’ or ‘continue travelling’ come later (e.g. howler monkeys, Carpenter, 1934). In birds, contact-calling underpins social functions and maintains cohesion or movement coordination within groups (Marler, 2004). It is present in many taxa and, during migration, can transmit distance information between signallers by the modification of call structure (e.g. ring-tailed lemurs, Oda, 1996; Japanese macaques, Sugiura, 2007 and rate (e.g. squirrel monkeys, Boinski, 1991; ring-tailed lemurs, Oda, 1996; chacma baboons, Rendall et al., 2000).

The examples from other taxa now serve as a backdrop to discuss the sensory modalities that inform migratory processes in cetaceans. The migrations of baleen whales represent some of the longest mammalian movements and are often repeated annually between productive, seasonal foraging habitats and breeding areas. Cues to navigation may be geomagnetic (Allen, 2013; Kremers et al., 2014), come via auditory, visual, olfactory and chemosensory routes (Bastian, 1967; Kenney et al., 2001; Torres, 2017) or use topographical features as landmarks (Barlow et al., 2009; Becker et al., 2012; Dalla Rosa et al., 2012; Forney et al., 2012; Friedlaender et al., 2006; Garrigue et al., 2015; Gregr and Trites, 2001; Keiper et al., 2005; Kirschvink et al., 1986; Yen et al., 2004). I will discuss how each modality may add to cognitive spatial maps to afford whales such accuracy in these large-scale movements.

Geomagnetic and somatosensory perception of ocean conditions probably dominates navigation in oceanic dolphins (Torres, 2017). Their use has been hypothesized for cetaceans on a range of scales (Bauer et al., 1985; Kremers et al., 2016a, 2016b; Wartzok and Ketten, 1999). The migratory pathways of both fin (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) have been correlated to magnetic fields; however, the route taken by the whales was more direct than if they had strictly used geomagnetic inclination and intensity measures (Allen, 2013; Walker et al., 1992). Somatosensory vibrissae and follicles function as pressure sensors in baleen whales, detecting water density, temperature, stratification, turbulence, currents or frontal location, and wave movement (Baumgartner and Mate, 2003; Bost et al., 2009; Kenney et al., 2001; Lohmann et al., 2008a, b; Norris, 1967; Tynan, 1998). These may identify locations by processing information on regional discontinuities in water properties or larger scale oceanic patterns (Kenney et al., 2001; Torres, 2017).

Visual cues inform spatial awareness on a small scale (1–100 m, Table 1; Torres, 2017), with acuity, environmental conditions and water turbidity as limiting factors. The ability of large whales to detect and resolve stimuli is difficult to discern. Lateral eye location precludes binocular vision, and, probably, the received image is not in colour, but with the highest sensitivity in the blue–green spectrum (Jacobs, 1993).

Baleen whales have retained the olfactory function, whereas it has been lost in toothed whales (Berta et al., 2014; Thewissen et al., 2011). Mysticetes may therefore be able to detect chemicals that indicate prey productivity or salinity levels and orientate themselves based on olfactory gradients (Hagelin et al., 2012; Nevitt, 2000, 2008; Nevitt and Bonadonna, 2005). Location may also be discerned via the innervation of small fungiform papillae associated with taste buds through gustation. Although ephemeral and spatially restricted (10–100 km, Table 1; Torres, 2017), inputs such as salinity and temperature and their gradients may also distinguish locales (Torres et al., 2008; Tynan et al., 2005).

Audition is the primary sense for cetaceans as determined by innervation (Ketten, 1997). Information from multiple scales contributes to localization and orientation data to form an acoustically derived cognitive map (Table 1). Passive listening to the soundscape, particularly the processing of low to infrasonic frequencies, can inform a listener on wide spatial scales (10–100 km; Torres, 2017). Surf noise and breaking waves, for example, provide information to whales over great distances (Allen, 2013; Wilson et al., 1985). The species complement of an area may add to this, forming a unique biophonic sonic marker (Allen, 2013; McWilliam and Hawkins, 2013).

The acoustic sense may also be actively engaged in echolocation, as previously described for bats. For cetaceans, the definition of echolocation must be considered broadly, whereby an individual uses return signals from its own calls to locate itself in space. The use of long-range infrasonic calls has been noted for blue whales (Balaenoptera musculus) during migration and bowhead whales (Balaena mysticetus) in under-ice travel (Clark and Ellison, 2004; Ellison et al., 1987; Norris, 1966, 1969; Oleson et al., 2007a, b; Payne and Webb, 1971; Thompson et al., 1979). The long wavelengths of these calls should follow an omnidirectional propagation path, but call projection and perception processes are not fully understood (Clark, 2004; Clark and Ellison, 1988). Contact-calling between individuals is carried out by long, patterned, frequency-modulated calls in humpback, blue, fin and right whales (Eubalaena glacialis), used at high repetition rates during migration (Allen, 2013; Clark, 1982; Clark and Ellison, 1988; McDonald et al., 1995; Oleson et al., 2007a, b; Širović et al. 2013; Wiley and Richards, 1978). In addition, they may function in ‘herd ranging’ and group cohesion over extensive space (Clark and Ellison, 2004; Norris et al., 1999; Payne and Webb, 1971). The calls may be directed to conspecifics as orientating cues or used by unintended receivers or ‘eavesdropping whales’ (Allen, 2013).

I use the case of the use of acoustics by grey whales (Eschrichtius robustus) during migration to further a discussion of the formation of sensory-derived cognitive maps in baleen whales. The timing and migration route between calving and breeding lagoons in Mexico and summer feeding grounds in the Bering and Chukchi seas for grey whales are well documented. However, the mechanisms they use to achieve this is not well understood.

Described as vocally limited during migration (Crane and Lashkari, 1996; Dahlheim, 1987; Rasmussen and Head, 1965), they are not the obvious candidate for a discussion of audition-informed navigation. However, recent innovative studies using long-term passive acoustic monitoring techniques have shown grey whale vocal behaviours during migration to be much greater than previously recognized. The most frequently used calls are low-frequency, moan-like calls, which create a far-reaching acoustic active space (Figure 1; Burnham et al., 2018; Guazzo et al., 2017; Rannankari et al., 2018). Recordings taken at a mid-migration location off the west coast of Vancouver Island (see Burnham et al., 2018) found these moan calls dominated both northward (>80% of calls recorded) and southward travel (>98% of calls recorded, see Figure 1, adapted from Burnham et al., 2018). These calls have not been described in recordings made of grey whales in foraging or breeding areas (Dahlheim, 1987; Moore and Ljundbald, 1984) and so their function may be distinctly tied to travel behaviours and sending and receiving information over wider spaces. I speculate that they have a role in navigation, where propagation pathways and return signals of both moan calls (class 3, Dahlheim 1987) and moan-like tones in the lowest frequencies (class 3a ‘low moans’, Burnham et al., 2018) provide both location and orientation. These calls may also function as an adhesive mechanism between more distant conspecifics, especially if used in contact call patterns.

Figure 1.

Proportional use of each call type by grey whales on their migrations. A comparison between northward and southward migration acoustics use is shown.

A comparison of call presence and physical presence, taken from geographically disparate locations and lagged appropriately to account for travel time, found that the patterns in calling match those of the progression of individuals migrating (Burnham and Duffus, 2020). Peaks of observed whale passage from a shore station matched that of the acoustic peak in calling, particularly when comparing the low-frequency moan calls. Both visual and acoustic data also demonstrated the bi-modality known in the grey whales’ northward migration, whereby the cow–calf pairs leave the breeding lagoons later than the main body of the population to migrate to foraging locales (see Figure 2, adapted from Burnham and Duffus, 2020). Calls noted in the grey whale repertoire as being more inter-group and socially derived (class 1 knocks, class 2 upsweeps; see Burnham et al., 2018; Dahlheim, 1987) are higher frequency and pitch-modulated. These call types are much more prevalent in feeding and breeding locales, but are still heard on migration (Figure 1), perhaps for more short-range conspecific cohesion. They only supplement the use of moan calls during migration and are perhaps indicative of communication between individuals in close proximity. Comparison of the number of calls per hour in comparable stages of northward and southward migration found that, on average, there were twice as many calls made as the whales travelled to feeding locales than to breeding lagoons (Burnham et al., 2018). I hypothesize that this, as well as the greater use of inter-conspecific call types (∼20% of calls), suggests greater information sharing. This may include travel direction, where calling is analogous to the use of ‘trail markers’ by other taxa. The use of low-frequency signals may unite the migrating whales into a ‘range herd’ connected through ‘large vocally mediated communication networks’ (Payne and Webb, 1971), similar to those described for elephants, whereby low-pitch ‘rumbles’ and infrasonic vibrations connect the herd over tens of kilometres (Langbauer et al., 1991).

Figure 2.

Number of low-frequency moan calls heard compared with visual sightings data of northward-migrating grey whales. Data taken from Burnham and Duffus (2020).

Comprehending the potential capacity that baleen whale acoustical mechanisms provide for orientation and group cohesion is challenging. The extent over which cetaceans can send and receive information, their active space (Burnham, 2017; Clark et al., 2009; Tyack and Clark, 2000), is not yet defined for many species. The calculation of estimated detection ranges of acoustic recording devices is analogous to an active space calculation, whereby the recorder represents a conspecific receiver of the signal from a calling whale. From the recordings described here, most of the recording time (80%) was estimated to have been receiving calls from a range of about 3 km, or small scale. However, the full extent of the detection range (10% of the recording time) was estimated to surpass 6 km, representing meso-scale information exchange (see Burnham et al., 2018). These calculations are based on several parameters, such as the ambient noise levels and the water properties that define sound propagation, but also a lot of assumptions, such as the hearing threshold of whales and the properties of the signal (frequency and amplitude especially, Burnham et al., 2018), which may change based on the individual signaller, or in response to the context of their calling. Much is still to be learned about the use of vocal signalling and acoustic perception by whales and a greater appreciation of the distances that these allow for the exchange of information.

Both active and passive acoustics afford baleen whales auditory spatial cues, which may be far-reaching. The use of vision, skin and vibrissae innervation, chemoreception, magnetoreception, somatosensory perception and gustation enhances locational awareness on micro (<100 m) to macro (>1,000 km) scales (Table 1; Torres, 2017). This allows high-resolution spatial information to be distinguished, and perhaps acoustic inputs to be calibrated through other fine-scale tools, affording the accuracy and site fidelity often seen in migrators (e.g. Allen, 2013; Baker et al., 2013; Calambokidis et al., 2010; Fossette et al., 2014; Hoelzel, 1998; Horton et al., 2011).

The extent to which memory and culture inform migration in whale species remains unknown. For many baleen whale species, calves will travel to the feeding grounds with their mother while still nursing. The route, behaviours and feeding locales in subsequent years are highly informed by this first migration in grey whales (Calambokidis et al., 2010; Rodrıguez de la Gala-Hernandez et al., 2008). It may also be that the use of acoustics by other whales on this highly conserved route forms a ‘highway of sound’, which may inform or reinforce memory maps for yearlings and young whales in future migrations.

In suggesting that grey whales are highly acoustic during migration, it becomes necessary to consider the implications of acoustic disturbance in their success in migration. In the previous discussion of ‘whale geography’ (Burnham, 2017), I described how anthropogenic inputs to the soundscapes (anthrophonies) alter and foreshorten the active space of an individual. Grey whales are exposed to a high level of human-derived noise due to their coastal migration route along the west coast of North America, where their travel path follows and intersects ‘marine roads’ formed by shipping lanes (Pirotta et al., 2019). Modification in call structure, calling regimes and the cessation of calling by grey whales have all been noted as a response to human-derived noise (Dahlheim and Castellote, 2016), as have startle responses and avoidance behaviours (Malme et al., 1983, 1984, 1986, 1988, 1989). The traditional biogeographical means of documenting these behavioural changes would be to simply map the new migration path. However, drawing on the perspective of biogeography of MacArthur (1972) suggests a study of factors of the environment, as well as species morphology and evolutionary biology, energetics and dynamics within and between individuals of a population is implicit to our understanding. In a species-centred approach, we must consider the implications of the additional energy required for alterations of behaviour, the physiological draw of the potential stress and the reduced extent of active space. Also, the variation in each whale – for example, due to age, health and prior experience of the noise – may alter its perception and response to disturbance.

To take a MacArthur-inspired perspective is to understand that the characterization of the soundscape is an integral facet of the biogeography of whales. The evidence from the case study suggests that, during migration, the acoustic sense is fundamental to the formation of cognitive spatial maps, with active space defining its extent. This is a dynamic, constantly updating, information stream from the soundscape. In addition, other sensory inputs may become layered or operate hierarchically on multiple scales as individuals move through phases of migration to their destination (see Table 1). Much is still left to learn about the scale and use of sensory modalities and how whales integrate information sources to successfully navigate between remote locations, as is how the acoustic connection between ‘lone’ individuals forms a networked ‘herd’ and how migratory decisions are made, either as collective individuals (sensory-derived), group consensus or through leader–follower dynamics (Strandburg-Peshkin et al., 2015, 2017).

Here, my discussion presumes that the acoustic sense is the foundation for movement and that active space describes the extent to which information is being received and interpreted for an individual to know its current and target locations. This approach is rooted in the morphological evidence of high investment in the auditory sense (Ketten, 1997). Taking this perspective suggests that other sensory data are secondary. It also suggests that changes in oceanic regimes and soundscapes could have a significant impact on whale populations. The additions of anthropogenic noise to the oceans is a growing concern due to its ability to disrupt behaviours and mask communications for whales (Clark et al., 2009; Holt et al., 2009; Nowacek et al., 2007). The pervasive nature of these disruptions is also beginning to be fully appreciated, which includes acknowledging the extent to which these noise inputs can propagate from the source. By taking the ‘whale geography’ perspective, the potential to disrupt individuals, populations and species becomes apparent. Yet, we still have not determined how changes in prey or soundscape, or oceanic regime may alter the initiation or completion of migration. Borrowing insights from other taxa to understand the importance, energetics, economics and mechanisms of migration – as well as integrating population biology, evolution, ecology, physiology and behaviour – will help us understand the complex life history of cetaceans and the processes that drive their biogeography.

Footnotes

Acknowledgements

D. Duffus entertained many thoughtful discussions on this topic and was helpful in finalizing the manuscript.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

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