We are all familiar with these behaviours: a few months old human baby grasps objects placed in their palms; a parent herring gull taps its beak on the ground and its chick will peck at the red spot on the parent gull’s beak several times. We refer to these behaviours as natural instincts in animals. Natural instincts, also known as innate or instinctive behaviours, are inherited behaviours exhibited in animals reared in isolation, independent of any experience. [1] An animal can perform its innate behaviour the first time it is exposed to the stimulus without learning or practicing. There several types of innate behaviours, including reflexes, fixed action patterns, movements and migrations, controlled by various neural mechanisms.
Reflexes are the simplest type and are controlled by the most basic unit in any body activity under nervous system control: the reflex arc. A reflex arc consists of input, integration and output. Take the knee-jerk reflex for example: input occurs when your patellar tendon is hit by a hammer and stretched. [2] The stretch is then detected by sensory neurons that travel to your spinal cord. The signal is then passed on from sensory neurons to motor neurons through synapses. These motor neurons cause immediate contraction of the quadriceps muscle to produce movement.
More complex than reflexes, fixed action patterns are innate behavioural sequences, which usually involves a network of neurons. Once triggered, it will inevitably go to completion even if the stimulus is removed in the meantime. [3] In Lorenz and Tinbergen’s egg-rolling experiment, when an incubating greylag goose notices an egg near the nest, it extends its neck and rolls the egg back up into the nest with its bill. [4] The goose would go through the rest of the behaviour even if the egg was removed after the goose had begun neck extension. Fixed action patterns vary from species to species and, therefore, they are regulated by different neurological mechanisms. For example, the mechanism of escape swimming behaviour in nudibranch Tritonia, a soft-bodied molluscs gastropod, has been extensively studied. This behaviour can be triggered by bursts of impulses in 2 bilaterally symmetrical groups of cells in the pleural ganglion. [5] When this organism needs to escape, dorsal and ventral flexors neurons are excited simultaneously. Alternating burst of impulses generated from the central pattern generator coordinate swimming movement by alternating between ventral flexion and dorsal flexion. After several repetitions, an inhibition signal is sent to the flexions and the animal returns to a relaxed position. [6]
Movements in response to stimuli are innate behaviours known as kinesis and taxis. Kinesis occurs when an organism changes its movement in an undirected way, for example pillbugs increase their speed of locomotion as the temperature increases. [7] On the other hand, taxis is a type of directed motion of an organism in response to a stimulus, such as light (phototaxis), chemical signal (chemotaxis) and gravity (geotaxis). Different organisms have different neurological mechanisms for kinesis and taxis, for instance in chemotaxis, C. elegans worms determine whether to move towards or away from an odor source at the level of sensory neurons and use a biased random walk to move up or down the gradient; while organisms with bilaterally symmetric chemosensory organs, such as fruit fly larvae and mammals, use a stereo approach that compare the neuronal responses on the left and right to determine the distribution of food source in space [8][9]
More complex innate behaviours typically require integration of multisensory neuronal responses. Migration is a type of variable innate behaviour in response to the availability of resources, characterized as long-range seasonal movement of animals. [10] Young birds use a clock-and-compass strategy during their first autumn migration journey. [11] Birds have endogenous circadian and circannual clocks. [12] The circadian clock is based on a cyclic transcriptional feedback loop, triggered by light-dark cycle through photoreceptor molecules in the retina, the pineal gland and other brain regions. [13] The circannual clock is not as well understood as the circadian clock, but its existence has been proven. As for the compass, most birds can use two compasses: a sun compass and a star compass. Night-migration birds have the intrinsic ability to look for rotating light dots in the sky and to interpret the center of rotation as north.[14] To establish a sun compass, young birds need to learn to associate the position of the sun with their circadian clock. [15] The celestial rotation plays a crucial part in both compasses so they may be parts of a single celestial compass system. Celestial cues are detected by birds’ eyes and processed by visual brain pathways.[16] Studies show it is almost impossible for bird’s eyes to detect slow celestial motion. Nevertheless, the birds could incorporate the current star pattern to a snapshot of some previous pattern and fixed landmarks.[17] Another well-known compass used by migratory birds is the magnetic compass. Young birds inherit the ability to sense the Earth’s magnetic field, most likely with sensors in their eyes and the main sensory molecule is likely a cryptochrome protein, although calibration is needed for the compass to become functional. [18]
Innate behaviours are important because there is no risk of an incorrect behaviour being learned. They are evolutionarily advantageous and increase the species chances of survival. Nevertheless, humans, a highly complex organism, have far fewer innate behaviours compared to other animals, of which most are reflexes. Why does evolution sometimes favour a learned behaviour, a complex and potentially costly adaptation? It allows an organism to better adapt to changes in the environment and are modified by previous experience. Therefore, innate and learned behaviours are both vital for a species continuance.
Author; Gracia Gu.
References
[1] Innate Behavior. [online] Available at: <https://medical-dictionary.thefreedictionary.com/Innate+behavior> [Accessed 8 October 2020].
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