Biology:Sign Stimulus
The term Sign Stimulus also known as a releaser, is the determining feature of a stimulus that produces a response. Sign stimuli are often found when observing a Fixed Action Pattern (FAP) which is an innate behavior with very little variance in the manner in which the actions are executed. Several examples of sign stimuli can be seen through the observation of animal behavior in their natural environment. Experimenters have gone into these natural environments to better assess the stimuli and determine the key features of them that elicit a fixed action pattern. Scientists have also observed direct exploitation of sign stimuli in nature among different species of birds.
Examples
Greylag Geese
An example of this process can be seen in graylag geese. In the case that a female greylag goose's egg rolls away from her nest, she instinctively extends her neck and reaches out her bill to transport the egg back into the nest.[1][2] At first glance this may look like the goose has simply noticed a problem (the absence of the egg from her nest) and taken steps to solve it. Yet, because of the distinct repetition of the motion scientists have defined the action as FAP and have defined the egg as its sign stimulus. The sight of the egg outside of the nest serves as the stimulus in this particular instance because it is only after the recognition of the eggs displacement that the Fixed Action Pattern occurs.[1][3]
The manipulation of the sign stimulus through a series of experiments can allow scientists to understand what specific component of the stimulus is responsible the innate behavioral sequence. If the egg were to be picked up and taken away after it is displaced from the nest, the goose still exhibits the same head moving motion even though there is no egg present.[1] This was put to the test by using objects such as beer cans, and baseballs. Experimenters found that the stimulus merely had to be an object that was large enough in size, convex enough in shape, and comfortable enough for the goose to lay its neck around the edges of the object.[1]
These features that the stimulus has to obtain in order to trigger a resulting FAP were then given the official term of Sign Stimuli. Scientists came to the realization that there must be an innate deciphering method that the goose goes through in order to determine a suitable sign stimulus. This was defined as an Innate Releasing Mechanism (IRM). The goose's IRM when put to the test in the natural world not being manipulated by scientific experimentation is almost always efficient in getting the desired item of an egg back into the nest.[1]
Three-spined Stickleback Fish
Another example of a sign stimulus comes from the three-spined stickleback fish. Male sticklebacks have a red underbelly that helps them during the mating season. Males however have a very aggressive innate response toward other males when they observe this red coloring. The fixed action pattern consists of the male aggressively attacking the stimulus as to scare off the other fish.[4]
To confirm that the red color is indeed the sign stimulus, researchers allowed male fish to be exposed to objects that were not fish themselves but had a similar coloring pattern to the males during breeding season. The same innate behaviors were exhibited toward objects with a red underside. Yet, when the male fish were approached by a similar looking fish painted all white, no elicit behavior was observed, confirming the color as the sign stimulus.[4]
Other Examples
More examples of Sign Stimuli include;
- The red mark on the mother's beak recognized by kelp gull chicks.
- Light polarization patterns recognized by mayflies when they are deciding where to drop their eggs.
- The presence of a female Sage Grouse that causes males to exhibit a strut pattern display in order to attract the female during breeding season.[5]
Exploitation
Brood Parasitism
The exploitation of sign stimuli can be seen when exploring the concept of brood parasitism. Other animals will learn the sign stimuli of different species and use it to manipulate the other animal into doing something for its own benefit. An example of this can be seen in the European Cuckcoo. This particular species of bird benefits by handing off the task of raising their young to other species of birds who will care for any egg that is found in its nest. In this case the sign stimulus is the presence of the eggs in the nest. If the other bird does not recognize the other bird's egg as foreign, it will continue on caring for it as its own by incubating it and hatching it. Once it is hatched, the young cuckoo ensures for itself a solid upbringing by instinctively pushing its neck around anything that is solid in the nest and pushing it over the edge. Thus classifying the sign stimulus as any solid object. This allows for the cuckoo's now foster parents to have plenty of time to devote to caring for it without any other distractions.[6]
Further Experiments
Supernormal Stimuli
When conducting further experiments, scientists came across the idea of Supernormal Stimuli. A scientist by the name of Tinbergen found that incubating geese when given the choice between performing the egg-rolling FAP with an egg versus a full sized volleyball, they chose the volleyball. These supernormal stimuli, although not naturally occurring, gave insight into the process of natural selection and how it may be that some stimulus features such as the size of the egg result in and increased ability to trigger a FAP.[1]
Significance
Sign Stimuli are used to identify the one or many key features of an otherwise complex stimulus that will result in a Fixed Action Pattern. The exact features that will trigger the pattern are unknown and it is only through various tedious experiments that manipulate the stimulus that will indicate the determining features. The recognition of a sign stimulus seems to be something that is hard wired into the brain rather than learned. Since the resulting fixed action pattern is an innate response with little to no variability, the sign stimulus itself must also be a trigger that's brain recognition process is innate to the animal. [1] [3] [4]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Gould, James (1983). Ethology: The Mechanisms and Evolution of Behavior. John Wiley & Sons Ltd.
- ↑ "Fixed Action Pattern - Greylag Goose". http://www.pigeon.psy.tufts.edu/psych26/fap.htm.
- ↑ 3.0 3.1 OpenStax College Biology. "Behavioral Biology: Proximate and Ultimate Causes of Behavior". https://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction. Retrieved 20 November 2018.
- ↑ 4.0 4.1 4.2 Rowland, William (February 3, 2000). "Habituation and development of response specificity to a sign stimulus: male preference for female courtship posture in stickleback". Animal Behaviour 60 (1): 63–68. doi:10.1006/anbe.2000.1462. PMID 10924204.
- ↑ Haven Wiley, R. (1973). "The Strut Display of Male Sage Grouse: A "Fixed" Action Pattern". Behaviour 47 (1/2): 129–152. doi:10.1163/156853973X00319.
- ↑ Ehrlich, Dobkin, Wheye, Paul, David, Darryl (1988). "Brood Parasitism". https://web.stanford.edu/group/stanfordbirds/text/essays/Brood_Parasitism.html.
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