Among the drivers of behaviour, external stimuli play a highly significant role, evoking responses that are often essential for an individual’s survival. Their importance is such that selective adaptation processes have favoured stereotyped and immediate responses to certain types of stimuli, classifying them as innate behaviours that the individual does not need to learn through experience. In ethology, these types of stimuli are called “key stimuli,” with the central nervous system mechanism that triggers the corresponding motor responses compared to a lock that can only be opened with the “key” of the appropriate stimulus. These stimuli can be of various types: olfactory, chromatic, acoustic, or configurational.
Their ability to evoke innate responses makes them active in both newborns and young animals, as well as in adults of various species, with essential functional value in contexts of pure survival and in territorial or reproductive situations. Key stimuli are, above all, a cornerstone of nonverbal communication in the animal world.
Chromatic stimuli, such as the red patch on the chest of the robin (Erithacus rubecula), are a classic example of key elements: an individual seeing a conspecific invading its breeding territory will immediately and vigorously chase it away, but will futilely pursue a red-stained cotton ball that also attracts it while ignoring a stuffed robin stripped of its red feathers. This shows that the stimulus is the red patch alone, regardless of the shape or form of the bearer, which is irrelevant.
Equally well-known are the experiments Niko Tinbergen conducted in the 1950s with the three-spined stickleback. The male of this small fish, when breeding, develops a large red spot on its belly, which attracts mature females into its territory but aggressively repels mature males, also with red bellies. Wax decoys without the red spot were ignored, but models that did not resemble a small fish yet had red undersides were attacked when introduced into their aquariums.
Sticklebacks can always recognise a female ready to breed by her swollen belly filled with eggs, which noticeably alters her appearance. This also plays a key role in mate recognition.
Key auditory stimuli are essential for the recognition of their own chicks in female turkeys during the first brood: they respond to any object that emits their characteristic chirping call as if it were a chick, disregarding visual cues. Conversely, if deafened, they may even kill their own chicks because they cannot hear the key auditory stimuli that trigger maternal behaviour.
Specific chemical “alarm substances” have been identified in fish, released by the laceration of the skin of animals injured by predation. When released into the water, these substances induce a strong fear response and sudden flight in their conspecifics. This behaviour is not exhibited if the blood of another species is introduced into the water.
In moths, chemicals (pheromones) released by females are capable of attracting males even at extremely low concentrations, always with a specific key effect.
Innateness helps the Young
The fact that key stimuli trigger innate responses gives newborns significant advantages in overcoming critical moments in their early days. One of the most compelling examples is provided by experiments conducted by Gerard Baerends on cichlid fish that rear their eggs in their mouths. This parental care strategy is practised by both females and males of various species. Their young take refuge in their mouths in times of danger, escaping predators. However, they are also attracted to simple decoys that vaguely resemble the head of a fish with its mouth open, as well as decoys with dark spots or indentations, such as a half-blackened white disc or a simple test tube open at the bottom, since orienting themselves toward dark spots is what would, under natural conditions, lead them to the parent’s mouth.
Experiments with Tilapia mossambica, a mouth-brooding cichlid. Above is a diagram showing how the young move toward the mouth to seek shelter. Below are tests with flat discs (a-c, a’); a notched disc (d); and a test tube with a hole in the bottom. When frightened, the young seek refuge on the underside of the decoys, attracted by the dark spots that resemble the parent’s mouth (from Baerends, 1957).
Rabbit chicks, rather than mallard, pheasant, partridge, or turkey chicks, run for shelter in burrows or among vegetation when a bird of prey passes by, alerted by alarm calls from their parents, especially the females. The same effect is also produced by a decoy flying low above them, provided its direction of motion mimics the shape of a bird of prey. Looking at the model in the following figure, you can understand why: if moved to the right, it resembles the silhouette of a bird of prey (short neck and long tail), which induces flight;
If moved to the left, it resembles a goose (long neck and short tail), in which case flight does not occur. Here, we are dealing with a key stimulus that consists not only of a shape but also of a shape-movement interaction. This experiment, the result of collaboration between Lorenz and Tinbergen, has been criticised, questioning whether a response to such a complex configuration of stimuli could be hereditary and therefore innate, as they claimed. Furthermore, in this type of experiment, a habituation effect to the stimulus is likely to occur, depending on the species tested, and therefore, learning phenomena may come into play.
Selective response to a key stimulus has extremely high adaptive value for animals, especially in species whose behaviour is dominated by instinctive factors. This is true even if the individual occasionally faces useless situations. For a male stickleback, keeping rivals away from his territory is so vital that he must always be ready to respond to red objects. Anything red that appears before his eyes could be a usurping male, even if this forces him to futilely attack a red flower petal that has fallen into his pond. Tinbergen reported that some sticklebacks that could see the road in front of the laboratory from their aquariums would become inexplicably active, so much so that they would suddenly and briefly bump into the wall of the aquarium facing the road. Then someone realised that all this was caused by passing postal trucks, which in Holland are red. The fleeting spot that darted before their eyes was mistaken for a sudden competitor!
Credits
Author: N. Emilio Baldaccini, Former Professor of Ethology and Conservation of Zoocenotic Resources at the University of Pisa. He has published over 300 scientific papers in national and international journals. Actively engaged in scientific education, he is also a co-author of academic textbooks on Ethology, General and Systematic Zoology, and Comparative Anatomy.
Translated by Maria Antonietta Sessa
