Module 7: Migration & Navigation

Virtually all baleen whales (mysticetes) undertake annual round-trip migrations between high-latitude summer feeding grounds (where productivity is high) and low-latitude winter breeding grounds (where water is warm enough for neonatal thermoregulation). The energetic paradox is obvious: whales leave food-rich waters precisely when food is most abundant, travel thousands of km to breed in food-poor tropical seas, and return the next summer. The reproductive benefit of warm water for calves is thought to outweigh the energetic cost of migration plus forgone feeding time.

1.1 Odontocetes: Less Migratory

Toothed whales generally migrate less than baleen whales. Sperm whales follow squid distributions globally but are not strictly migratory. Orca populations occupy relatively fixed territories; some make seasonal movements tracking prey (Chinook salmon runs for southern residents) but most stay within a ~500 km range. Bottlenose dolphins exhibit coastal resident populations along with more wide-ranging offshore populations.

Using the cost of transport (COT) derived in Module 1, the energy required to swim mass \(M\) over distance \(d\) is:

For a 30-ton gray whale (M = 30,000 kg) with COT ≈ 0.7 J/(kg m) over 22,000 km =\(2.2\times 10^7\,\text{m}\):

This is a reasonable fraction of the gray whale's pre-migration blubber depot; it is why whales must feed intensively during the summer season to lay down enough fat for migration plus reproduction. For lactating females, energetic demands can be supra-physiological: producing ~200 L/day of 40%-fat milk while not feeding for months requires the mother to catabolize her blubber.

2.1 Why Migrate at All?

The evolutionary puzzle of mysticete migration is not yet fully resolved. Hypotheses include:

• Predator avoidance: tropical breeding waters have fewer killer whales; calves are vulnerable
• Thermoregulatory benefit for calves: newborn whales have low surface area: volume and limited blubber; warm water reduces heat loss
• Ectoparasite reduction: warm water may disfavor some whale lice and barnacles
• Sexual selection / display: breeding grounds serve as lekking arenas for humpback song

Pitman et al. (2019) argue compellingly that predator avoidance (orca predation on neonates) is the dominant driver, based on observed orca attacks at mid-latitudes during the migration.

3.1 Geomagnetic Sensing

The Earth's magnetic field has two geographically-varying components usable for navigation: the inclination angle\(\,I(\theta)\,\) (the angle between the field vector and the local horizontal) and the intensity\(\,B(\theta, \phi)\,\). For a dipole approximation:

A cetacean could in principle determine its latitude by measuring either quantity. Indirect evidence for magnetic sensing in cetaceans includes (i) the presence of magnetite crystals (Fe₃O₄) in whale brain tissue (Bauer et al. 1985, Kirschvink 1990), (ii) live-stranding events that correlate with geomagnetic anomalies and low-inclination coastlines (Klinowska 1985), and (iii) statistical correlations between stranding rates and solar-storm-disturbed magnetic fields (Vanselow et al. 2018, Granger et al. 2020). Direct behavioral demonstration remains elusive — a major open problem in marine mammal navigation.

3.2 Acoustic Landmarks

Seafloor bathymetry, coastline geometry, and ambient acoustic features can serve as landmarks. A sperm whale diving to 1 km can listen for the echoes from seamounts, shelf breaks, and continental margins. The SOFAR channel (Module 4) may also carry identifiable acoustic “signatures” from specific geographic features — distant surf, specific shipping lanes, or shelf-break reverberation — that an experienced whale could use as waypoints.

3.3 Memory and Social Learning

In social species like gray whales and humpbacks, calves follow their mothers on their first migration, learning the route through observational learning. Subsequent migrations draw on this imprinted knowledge. This explains fidelity to specific breeding and feeding grounds across generations: migratory tradition is culturally transmitted alongside genetics.

3.4 Celestial Cues

Lateral-line and eye-based detection of polarized light is documented in many fish but not confirmed in cetaceans. However, given the enormous eyes of most cetaceans (sperm whale: ~15 cm; blue whale: larger than a grapefruit), visual cues from the sun position, moon, and possibly stars cannot be ruled out. Some behavioral evidence from orca movements suggests sun-position orientation.

Mass strandings — live beaching of two or more whales at one time — have been documented for millennia. Over the past fifty years, systematic analysis has identified several distinct mass-stranding syndromes:

4.1 Single-Species Social Strandings

Pilot whales and sperm whales, with their tight matrilineal bonds, sometimes strand as whole pods. One individual (often injured or disoriented) becomes stranded; others follow due to social cohesion. Long-finned pilot whale mass strandings at Farewell Spit, New Zealand, involve hundreds of animals at a time.

4.2 Solar-Storm Correlated Strandings

Statistical analyses (Vanselow & Ricklefs 2005; Granger et al. 2020) show that sperm whale strandings on the North Sea coast correlate with geomagnetic disturbances — perhaps because the distorted field during solar storms supplies anomalous navigational cues. In K-index > 4 conditions, stranding frequency rises substantially. This is strong (if indirect) evidence that these animals use the geomagnetic field for navigation and get confused when it is disturbed.

4.3 Sonar-Induced Beaked Whale Strandings

Discussed in Module 2, beaked whale mass strandings tightly correlated with military mid-frequency sonar exercises exhibit gas-bubble lesions consistent with DCS. The navigational component of these events may involve panic-driven atypical surfacing, compounding the acoustic trauma.

4.4 Disease and Biotoxin-Induced Strandings

Harmful algal blooms, morbillivirus outbreaks, and biotoxin exposure (domoic acid, saxitoxin) can cause neurological disruption and consequent disorientation. These are often diagnosed post-mortem; distinguishing from other causes requires systematic necropsy programs.

Modern cetacean tracking draws on several complementary technologies, each with distinct tradeoffs in resolution, longevity, and invasiveness:

• Satellite tags: transmit GPS coordinates when the dorsal fin breaks the surface. Current generation (e.g., Argos, Iridium) provide positions accurate to ~100–1000 m. Tag lifetimes vary from weeks to ~1 year
• Acoustic tags (DTAGs): record on-board sound, depth, orientation (via 3-axis accelerometer and magnetometer) at high temporal resolution. Used for fine-scale dive and vocalization studies
• Passive acoustic monitoring: networks of hydrophones detect whale calls and estimate range/bearing via multiple-array triangulation
• Photo-identification: natural markings (fluke patterns in humpbacks, saddle patches in orcas) allow recognition of thousands of individuals over decades
• Biologging with cameras: on-animal video reveals fine-scale prey interactions and conspecific behavior

5.1 Straight-Line Migration

Horton et al. (2011) analyzed satellite track data from migrating humpback whales and found that the paths are straighter than predicted by random-walk or correlated-random-walk models, even when simulated using all known environmental gradients. The straightness suggests the whales navigate by reference to some absolute external cue — consistent with geomagnetic or celestial compass use — rather than purely by following environmental gradients.