The document explores stimulus filtering, a process by which sensory information is prioritized, allowing animals and humans to focus on significant stimuli. It discusses sign stimuli and innate releasing mechanisms that trigger fixed action patterns in response to specific cues, such as mating behaviors among fish and aggressive responses in other animals. The document also highlights examples of these mechanisms across various species, illustrating how certain stimuli prompt instinctual behaviors.

1. ASSIGNMENT
Stimulus filtering, sign stimulus, innate releasing
mechanism, fixed action pattern
SUBMITTED BY
RESHMA AJITH
II MSc Zoology

2. STIMULUS FILTERING
The process of separating useful sensory information from the many thousands of stimuli present in the external
environment, so that only potentially useful information is sent to the brain. Many sensory organs are adapted
so that they receive stimuli only within a certain range. For example, the human eye only detects colours in the
visible region of the spectrum and not in the ultraviolet or infrared regions. Another method of stimulus filtering
involves sign stimuli, which are more important than other stimuli in eliciting a response and, once detected,
may lead to other stimuli being ignored.
PHYSIOLOGICAL MECHANISM
When particular signals are received by the animal, the superior-ranking neurons determine which signals are
important enough to preserve and which signals are insignificant and can be ignored.This process essentially
works as a filter as the synapses of the neural network enhance certain signals and repress others, with simple
stimuli receiving attention from lower-level neurons, and more complicated stimuli receiving attention from
higher level neurons.
Relation to humans
Stimulus filtering is also seen humans from a day-to-day basis. The phenomenon is called the cocktail party
effect. When in a crowded room people tend to ignore other conversations and just focus on the one they are
participating in. This effect also works in that when an individual hears their name in another's conversation they
immediately focus on that conversation.
EXAMPLES
Female midshipman fish undergo stimulus filtering when it comes time to mate with a male. Midshipman fish
use stimulus filtering when listening to sounds produced by underwater species.Dominant signals underwater
range between 60–120 Hz, which is the most normally the most sensitive to the fish's auditory
receptor.However, the female auditory system changes seasonally to acoustical stimuli in the songs of male
midshipman fish. In the summer when female midshipman fish are reproducing they listen to a male humming
song that can be produce a frequency level of 400 Hz.The summer is reproducing season for the females so their
hearing is more sensitive to the high frequency of the male humming.
SIGN STIMULI
Sign stimuli, also called releasers or key stimuli, are those stimuli that are capable of releasing Fixed
Action Pattern (FAP) or consummatory behaviour of the animal. They are signals that evoke instinctive
patterns of behaviour in animals, such as fighting behaviour in the territorial animals, triggered by the
entry of another male. Lehrman found that courting male dove began to bow and coo to a stuffed
model of female in the absence of a living female.

3. Konrad Lorenz (1972) was the first biologist to identify sign stimuli which he called key stimuli because
they function as keys to release and unlock the fixed action pattern of the animal. He proposed the
Innate Release Mechanism in response to sign stimuli.
Niko Tingbergen (1952) conducted experiments on stickleback fish in which bright red colour of the
belly and neck is a sign stimulus for the other males to attack it, while it elicits attraction in female fish.
Bird chicks respond to jerks in the nest, which is a sign stimulus for them to open their beak for eating
food. Similarly distress calls given by chicks are a sign stimuli for hens to release rescuing behaviour.
Bright red colour of the oral cavity of a cuckoo chick is a sign stimulus for the foster parent, warbler to
feed it. Otherwise the chick is distinctly different from the foster parents.
Sign stimulus is often not one character but a combination of many stimuli, namely, shape of the bill,
colour and patches on body or even actions and auditory signals.
Sign stimuli or releasers can be of three types:
Visual Releasers. They are morphological characters that are displayed to elicit response, as display of
feathers or dancing in birds. Nest making behaviour in birds such as weaver bird, not only attracts
females but also repels other males. Fire flies emit light signals that bring about response from
females. In three-spined stickleback fish (Gasterosteus) males get nuptial coloration during breeding
season in which neck and belly become bright red in colour to attract females and also to warn other
males not to enter the territory.
Auditory Releasers. The sign stimulus is auditory signal like humming sound of male insects. Song birds
such as cuckoos, nightingales, starlings establish their territory and attract the female for mating by
singing, often for several days. The song is a signature stimulus of individual bird as it is specific. Birds
of different areas sing in different dialect and elicit different response.
Stridulatory organs of some insects, such as cicadas and crickets are strong sign stimuli during breeding
season.
Chemical Releasers. Pheromones are different types of volatile chemicals released in the atmosphere
that act as releasers on the individuals of the same species or other species. Pheromones affect the
individuals of the same species while allomones affect other species.
Sex pheromones in insects are produced by the females and perceived by the enlarged antennae of
males of the same species, sometimes from the distance of a couple of kilometers.
In vertebrates, the sex pheromone, Copulin is secreted by the female in estrus.
Alarm pheromones are produced by skin cells of several species of fishes that have schooling or
shoaling behaviour. Alarm pheromones are also released by the stinging honey bees and wasps which
attract other individuals of hive to attack.

4. Ants and termites produce trail pheromones from the posterior end of abdomen which help them to
follow each other.
INNATE RELEASING MECHANISMS
The ethological explanation proposes that aggression can be the result of an evolved
automatic biological response in the brain. It is believed that animals have a built-in
neural structure (a network of neurons) which, when exposed to specific stimuli (signs
or releasers) such as facial expressions, will cause the release of an automatic behavioral
response (a fixed action pattern). This inbuilt biological structure or process is called the
innate releasing mechanism (IRM). For example, when a dog sees a cat running away
from them, they have an instinctive response to chase the cat. When the cat is still, the
innate releasing mechanism is not activated; however, it is the cat running that activates
the innate releasing mechanism. Consequently, the dog chasing the cat is an example of
a fixed action pattern which is activated by an innate releasing mechanism. The dog’s
desire to chase is automatic and instinctual in response to a cat or any other small
animal running away.
FIXED ACTION PATTERN
Fixed action pattern is a series or sequence of acts that occur behaviorally in animals.
This sequence is unchangeable and will be carried out to completion once started,
regardless of changes in the original stimulus. A stimulus is some external factor that will
cause an organism to respond to it. Fixed action patterns are triggered by a type of
external stimulus called a sign stimulus. The job of the sign stimulus is to start off the
fixed action pattern of a particular organism.
FIXED ACTION PATTERN EXAMPLES
Gull chicks are fed by both parents. As soon as a gull chick hatches from its egg, it pecks
at whatever bill is nearby. Pecking, in turn, elicits the parent to regurgitate food into
their chick’s mouth.
MALE STICKLE BACK FISH
During the breeding season, male sticklebacks develop a red belly and display innate
aggressive behavior towards other males.
When a male stickleback spots another nearby male, he will launch into a fixed action
pattern involving aggressive displays designed to scare off the stranger. The specific
stimulus that triggers this fixed action pattern is the red belly coloration pattern

5. characteristic of males during breeding season.
REFERENCES
https//www.tutor2u.net
www.ncbi.nlm.nih innate releasing mechanisms
www.iaszoology.com