Advait Jukar & Laura Bradley
Behavioral mechanism: morphological and physiological factors that allow the behavior to take place.
The question of how migratory animals find their way has been under investigation for centuries (Kirschvink 1985).
Initiation of migration
A change in the photoperiod in the feeding grounds has been seen to be a probable cause for the initiation of migration (Craig et al 2003). It has also been suggested that humpbacks in different hormonal states react differently to photoperiods, possibly resulting in the variation of migration timing. Thus, sex, size and reproductive state can determine the timing of migration. Photoperiodic information is translated via neuroendocrine pathways into hormonal signals that regulate gonadal activity and, in all seasonally-breeding mammals studied thus far, it is the action of photoperiod on the pineal gland that stimulates these physiological changes (Craig et al 2003). The pineal gland of the humpback whale appears to have a typical mammalian structure, suggesting its function is analogous to that in other mammals. The availability of food at higher latitudes and lack thereof in the wintering grounds serves as an additional possible cue for migratory timing (Craig et al 2003).
A number of hypotheses have been proposed for the sensory mechanisms of migration, such as the use of a sun compass, skylight polarization, infra sound and magnetism. Many of these cues are not available to migrating mysticetes (Kirschvink 1985).
Humpback whales migrate great distances from their summer feeding grounds to their winter breeding grounds. The exact mechanism is still unknown, but a possible explanation is the use of magnetoreception, or a magnetic sense. The existence of magnetite particles in the brains of dolphins and anterior dura matter of humpback whales may explain magnetoreception (Walker et al 1992; Kenny et al 2001).
With a magnetoreception system, a humpback whale would be able to follow the magnetic maxima and minima to maintain track of relative longitude using the marine magnetic lineations. Marine magnetic lineations (Figure 7) in the major ocean basins are aligned in a North-South fashion (Walker et al 1992), resulting from the geometry of the spreading ridges which formed after the break-up of Pangea during the Mesozoic era (Kirschvink 1985). An animal could use these lineations by counting or following minima to keep track of relative longitude during long migrations if it were sensitive enough to the magnetic field, while the smooth North-South variation of magnetic inclination would provide unambiguous latitudinal position. Depending on the age, depth and latitude of the sea floor, the magnitude of these anomalies can range from a few hundred to several thousand nanotesla, which are well within the theoretical limits for magnetite-based magnetoreception as well as the sensitivity range inferred for this system in homing pigeons and honey bees (Kirschvink 1985).
It has been observed that local minima are preferred by humpback whales because the high gradients are more prone to a positive susceptibility bias from seamounts and oceanic fracture zones. (Kirschvink 1985). Fin whales have been shown to migrate in regions of low geomagnetic field gradients in the specific north-south fashion that is characteristic of mysticete whales. These findings are consistent with earlier analyses that showed that strandings occur most often in areas where magnetic minima intersect the coast (Walker et al 1992). Magnetoreception is also believed to be a potential mechanism of right whale migration (Kenny et al 2001).