OrientationThe behavior of animals is the result of a combination of internal and external factors. Internal physiological states generate the motivation for behavior, while external environmental factors trigger specific actions. For orientation behavior, there is also an orienting factor. For example, when a mouse sees a cat, it quickly runs back to its burrow. The cat is the triggering factor for the running behavior, while the burrow is the orienting factor. When a cat catches a mouse, the mouse serves as both a triggering factor and an orienting factor for the cat.

Sensory Direction Discrimination
Accurate direction discrimination requires two conditions: first, the information from the orienting factor must travel in a straight line. Light travels in straight lines in a uniform medium, so vision becomes the primary sensory modality for direction discrimination in higher animals. In contrast, odors change direction with airflow, making olfaction less accurate for locating food sources. Second, the sensory organ itself must have a structure capable of directional perception.

Animal sensory organs can be roughly divided into three types: single-receptor, dual-receptor, and screen types, with their direction discrimination abilities increasing in that order. For example, when bees search for food, the fragrance drifting from the side forms a concentration gradient, causing unequal stimulation intensities between their two antennae. They immediately turn toward the side with stronger stimulation. However, if the gradient is not obvious and the stimulation intensities perceived by each antenna are identical, they sway their bodies to detect differences in odor concentration between two spatially distant points for orientation. In the former case, they use both receptors simultaneously for orientation; in the latter, the two antennae function as a single receptor, perceiving only the concentration at one point at any given moment. Direction discrimination is the result of integrating sensations obtained at different times and locations.

However, if the incoming information travels in a straight line, some single-receptor organs are also sufficient for orientation. For instance, the heat radiation detector in a rattlesnake’s head is deeply recessed within a small facial pit. The opening of the pit is narrow, allowing only radiation from a certain direction to enter.Therefore, a single cheek pit can be used for orientation. However, the detection ranges of the bilateral cheek pits overlap somewhat along the midline, and using both together is more advantageous for targeting. The auditory receptors of mammals are also deeply recessed within the ears. Some mammals can flexibly move their pinnae to align with the sound source, enabling orientation with a single ear. However, typically both ears are used, either because the same-phase sound waves reach the nearer ear first or because the sound is louder on the near side, thereby determining the direction of the sound source.

The organ type with the highest orientation ability is the screen type, such as the eye. A single eye may contain thousands, millions, or even billions of tiny receptors arranged in an array. In the protruding compound eyes of arthropods, each ommatidium has directionality, receiving light from different angles. The vertebrate eye is concave, with external light being focused and projected onto individual receptors in a one-to-one correspondence. Compound eyes have another advantage: they can discern the polarization direction of light. Animal eyes are generally paired, and when the fields of vision on both sides overlap, the binocular vision also helps in detecting distance.

Stimuli from external orienting factors can also be roughly divided into three categories: mechanical, chemical, and radiative. Mechanical stimuli include touch pressure, balance, and hearing. The chelae of lobsters are covered with a layer of sensory hairs. The different arrangements of these hairs can detect water currents from various directions. These hairs form a screen-like sensory organ capable of discerning various changes in the direction of water flow. Balance perception generally distinguishes the direction of gravity and various accelerated motions, helping to adjust body posture. Among mechanical stimuli, waves are most helpful for orientation. The backswimmer can detect nearby prey based on the direction of water waves, which are subsonic waves below the range of audible sound. Ultrasonic waves, which are above the range of audible sound, have even stronger directionality, enabling navigation in the dark. Bats can catch flying insects in the dead of night or in lightless caves by emitting ultrasonic beams and using their sensory organs to receive the reflected ultrasound for orientation. The principle is similar to radar and is called sonar.

So-called chemical stimuli are mainly various odors perceived through smell. Food, mates, or predators can emit special odors, forming a concentration field that decreases from high to low outward. Animals can use the concentration gradient to discern the direction of the odor source. For example, ants often secrete specific substances along their foraging paths for other nestmates to follow. Additionally, since odors move with airflow, traveling against the wind is an important orientation method for many animals when foraging.

Animals can sense radiation emitted from a distance and locate it accurately. For example, beyond the range of human vision, rattlesnakes can detect weak infrared radiation; certain insects can perceive ultraviolet radiation and the polarization direction of light; a few fish can generate weak electric fields in water, using them to detect other organisms entering the field; some birds can sense the natural magnetic field. The Eurasian robin adopts a certain direction under the influence of the natural magnetic field, but when an artificial magnetic field is added, it shifts to the resultant direction of the two.

Navigation Orientation
All birds and beasts that venture far from home to forage and then return to feed their young must possess certain orientation abilities. When honeybees forage hundreds of meters away from the hive, they fly in winding, circuitous patterns, but when returning to the hive, they can fly straight back, indicating that they can determine the direction of the hive relative to themselves. There are also puzzling examples, such as how salmon, eels, and other long-distance migrators, or migratory birds, orient themselves during long-distance flights. The most studied case is that of homing pigeons. When transported nearly a thousand kilometers away and released, they can return home in less than a day. In unfamiliar locations, they initially fly in circles, gradually expanding the radius, then take a nearly straight path back home, fine-tuning their route to head directly home only after recognizing familiar landmarks. Some ground animals mark their path with secretions along the way, enabling them to retrace their steps.

Experiments have shown that mud daubers can recognize terrain markers around their nests. It’s possible that many animals rely on terrain recognition for daily orientation, as is the case with homing pigeons when approaching their destination. Many migratory birds often follow rivers, mountain ranges, or coastlines during migration, likely using terrain as a navigational aid. However, whether through scent marking or terrain recognition, these methods only guide animals to retrace old paths and do not explain the orientation behavior of homing pigeons when released in unfamiliar locations.

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