Lockley began his book
Animal Navigation with the words: Many mechanisms of
spatial cognition have been proposed for animal navigation: there is evidence for a number of them. Investigators have often been forced to discard the simplest hypotheses - for example, some animals can navigate on a dark and cloudy night, when neither landmarks nor celestial cues like Sun, Moon, or stars are visible. The major mechanisms known or hypothesized are described in turn below.
Remembered landmarks Animals including mammals, birds and insects such as bees and wasps (
Ammophila and
Sphex), are capable of learning landmarks in their environment, and of using these in navigation.
Orientation by the Sun '', uses the Sun and its
internal clock to determine direction. Some animals can navigate using celestial cues such as the position of the Sun. Since the Sun moves in the sky, navigation by this means also requires an internal clock. Many animals depend on such a clock to maintain their
circadian rhythm. Animals that use sun compass orientation are
fish, birds, sea-turtles,
butterflies,
bees,
sandhoppers,
reptiles, and
ants. When
sandhoppers (such as
Talitrus saltator) are taken up a beach, they easily find their way back down to the sea. It has been shown that this is not simply by moving downhill or towards the sight or sound of the sea. A group of sandhoppers were acclimatised to a day/night cycle under artificial lighting, whose timing was gradually changed until it was 12 hours out of phase with the natural cycle. Then, the sandhoppers were placed on the beach in natural sunlight. They moved away from the sea, up the beach. The experiment implied that the sandhoppers use the sun and their internal clock to determine their heading, and that they had learnt the actual direction down to the sea on their particular beach. Experiments with
Manx shearwaters showed that when released "under a clear sky" far from their nests, the seabirds first oriented themselves and then flew off in the correct direction. But if the sky was overcast at the time of release, the shearwaters flew around in circles.
Monarch butterflies use the Sun as a compass to guide their southwesterly autumn migration from Canada to Mexico. In 2013, it was shown that dung beetles can navigate when only the
Milky Way or clusters of bright
stars are visible, making dung beetles the only insects known to orient themselves by the galaxy.
Orientation by polarised light shows how polarization of light can indicate direction to bees. Some animals, notably insects such as the
honey bee, are sensitive to the polarisation of light. Honey bees can use polarized light on overcast days to estimate the position of the Sun in the sky, relative to the compass direction they intend to travel.
Karl von Frisch's work established that bees can accurately identify the direction and range from the
hive to a food source (typically a patch of nectar-bearing flowers). A worker bee returns to the hive and signals to other workers the range and direction relative to the Sun of the food source by means of a
waggle dance. The observing bees are then able to locate the food by flying the implied distance in the given direction, though other biologists have questioned whether they necessarily do so, or are simply stimulated to go and search for food. However, bees are certainly able to remember the location of food, and to navigate back to it accurately, whether the weather is sunny (in which case navigation may be by the Sun or remembered visual landmarks) or largely overcast (when polarised light may be used).
Magnetoreception can quickly return to its home, using cues such as the Earth's magnetic field to orient itself. Some animals, including mammals such as blind mole rats (
Spalax) and birds such as pigeons, are sensitive to the Earth's magnetic field. Homing pigeons use magnetic field information with other navigational cues. Pioneering researcher William Keeton showed that time-shifted homing pigeons could not orient themselves correctly on a clear sunny day, but could do so on an overcast day, suggesting that the birds prefer to rely on the direction of the Sun, but switch to using a magnetic field cue when the Sun is not visible. This was confirmed by experiments with magnets: the pigeons could not orient correctly on an overcast day when the magnetic field was disrupted.
Olfaction may use
olfaction to identify the river in which they developed.
Olfactory navigation has been suggested as a possible mechanism in pigeons. Papi's 'mosaic' model argues that pigeons build and remember a mental map of the
odours in their area, recognizing where they are by the local odour. Wallraff's 'gradient' model argues that there is a steady, large-scale gradient of odour which remains stable for long periods. If there were two or more such gradients in different directions, pigeons could locate themselves in two dimensions by the intensities of the odours. However it is not clear that such stable gradients exist. Papi did find evidence that anosmic pigeons (unable to detect odours) were much less able to orient and navigate than normal pigeons, so olfaction does seem to be important in pigeon navigation. However, it is not clear how olfactory cues are used. Olfactory cues may be important in
salmon, which are known to return to the exact river where they hatched. Lockley reports experimental evidence that fish such as minnows can accurately tell the difference between the waters of different rivers. Salmon may use their magnetic sense to navigate to within reach of their river, and then use olfaction to identify the river at close range.
Gravity receptors GPS tracing studies indicate that gravity anomalies could play a role in homing pigeon navigation.
Other senses Biologists have considered other senses that may contribute to animal navigation. Many marine animals such as seals are capable of
hydrodynamic reception, enabling them to track and catch prey such as fish by sensing the disturbances their passage leaves behind in the water. Marine mammals such as dolphins, and many species of bat, == Path integration ==