Sensory filtering

Animals receive much more sensory information than they could possibly register in their brain and respond to. Therefore brain has to be selective and filter out certain information that is not so necessary. Sensory filtering or stimulus filtering takes place at several levels, namely, at the level of sense organs, nerves or different parts of brain.

Sensation is the basic data sent by sense organ to brain, and sense organs have their limitation and hence filter out much of the information. For example, human eye filters out ultraviolet and infrared rays from the spectrum.

Peripheral filtering is done by receptors because of their mechanical ability to receive and transmit information. Receptors are often highly specialized and respond to a narrow range of stimulus. For example caloreceptors can perceive sense of heat but not cold and frigidireceptors can only transmit the sense of cold. Bats can perceive ultrasonic sounds for echolocation but sense organs of other mammals do not possess that ability.

CNS filtering is done by different parts of brain by selective attention or because the part is not well developed. Perception is the interpretation by brain of sensory information in the light of earlier experience. A lot of information is received by brain but is not perceived.

Reticular Activating System located inside the medulla oblongata if inactivated, stops lots of nerve impulses coming through the cranial nerves.

Stimuli that reach respective areas of brain such as optic lobes, auditory lobes etc. can get filtered out if not important. Epithalamus, which functions as the central switch board, selects and sends only necessary nerve impulses to cerebral hemispheres.

When the nerve impulses arrive in different areas of cerebral cortex, they are analysed and interpreted and if found worthless can be rejected without perception.

Only information that is considered important is selected by the areas of cerebral cortex and interpreted, and motor action travels through the nerves to muscles to act. Neurochemical information coming from cerebral cortex affects hypothalamus, which stimulates endocrine system to alter the behaviour of the animal.

Muller’s Law of specific nerve energies: Sensation perceived depends on the part of nervous system activated, and not on the sense organ stimulated.

Examples: The male of the South American tree frog (Eleutherodactylus coqui) produces co-qui call to attract female and also to repel other competing males. The tympanic membranes of male and female are adapted differently. Males can hear only the co note and get warned and repelled. Males cannot hear the qui part of the call, while females hear only the qui part of the call and therefore get attracted to males.

Olfactory cells located on the antennae of male moths (Lepidoptera) can perceive only specific pheromones which are released by the female of the same species. These pheromones cannot be perceived by the receptors of males of another species.

European Robin (Erithacus rubicula) attack red-breasted male robins. Only red feathers of the competitors are perceived and the other colours are filtered out for attack and for defending the territory.


Naked Mole Rat

The naked mole rat (Heterocephalus glaber) is a burrowing social rodent found in the grasslands of East Africa, mainly in Ethiopia, Kenya and Somalia. The unusual anatomy and social life of this bizarre animal was first studied by the German naturalist, Eduard Rüppell in 19th century, although he thought them to be mutated and deformed individuals.

Anatomy. Normally the individuals are 3 to 4 inches long but the dominant queen is larger and has longer body. Fertile males are also larger than the workers and possess abdominal testes. All individuals possess almost hairless and wrinkled pinkish to yellowish skin, hence the name, naked mole rat. Very little scattered hairs and some whiskers are present on the body. They have lost hairs because of underground life, where they do not need hairs to protect them from sun rays. Eyes are quite small or rudimentary as they are not needed in dark tunnels where they spend their entire lives. Their sense of smell and touch are very well developed to compensate for the loss of eyes.  Legs are thin and short but are well adapted to move backward as well as forward with ease. The prominent organs of their body are oversized incisors that are used to dig underground tunnels. Their lips close tight just behind the teeth to prevent soil from entering mouth while digging. Their jaw muscles are strong and large as they spend most of their time digging burrows. They have no external ear pinna.

Naked mole rat has long life for a rodent as it can survive up to 30 years, while the other rodents survive only 3 years. This unusual longevity is attributed to the social structure and protected life of this species in tunnels. They escape excessive heat of the surface by remaining in cool, moist burrows and when it is very cold in night, they huddle together in a pile to conserve one another’s body heat.

Social system. The naked mole-rat is the only mammal that exhibits social life similar to termites in which members of the colony are anatomically modified into different castes to perform various duties and also spend their entire lives underground.   Only one female called the queen is fertile that breeds and dominates and controls the activities of the colony. One to three males are fertile to fertilize the queen, while the rest of the members of colony of both sexes are sterile and function as workers. There are two types of workers; the smaller ones focus on gathering food, tunnelling and maintenance of the nest, while the larger workers defend the nest from intruders and predators. Larger individuals do less tunnelling work and food gathering but spend their time in scouting and defending the colony from invaders. Smaller individuals are more active and do all the household duties. Colony of 75 to 80 individuals is normal but up to 300 members can be found in some healthy colonies that live in a complex system of tunnels which may reach a cumulative length of up to 3-5 kilometres.

Tunnelling system. The naked mole rats spend all their lives underground in dark tunnels and never come out to see the light of the day. Their tunnel system is intricate and often spreads to several kilometres, two to three feet below the surface. It has several chambers for queen, juveniles, breeding males and workers for feeding, sleeping, reproducing and other activities of the colony. There are also chambers where food collected during burrowing is stored for future use by the colony. There is also a separate isolated communal defecation area in the tunnels to prevent infectious diseases. Tunnel systems are sealed from the outside world for protection of the colony and to keep the intruders away, but this causes the burrows to be oxygen deficient. However, the naked mole-rat is well adapted for low oxygen environment found within the tunnels. Mole rats expand their burrows after rains when the soil becomes soft for easy tunnelling.

Food. Naked mole rats feed on the underground portions of plants that include large tubers, bulbs and rhizomes that are exposed during tunnelling. As their diet is composed of chiefly cellulose, they possess symbiotic bacteria in their gut to digest it, as in termites. Vertebrates cannot digest cellulose and must take the services of micro-organisms to digest it. The young individuals must feed on the faeces of other individuals to get the cellulose digesting bacteria in their guts.

Vampire Bats – The Blood Suckers


Bats are the only flying mammals in the world that roost upside down.   Their wings are made of excessively long fingers between which membranous skin called patagium is stretched.   Vampire bats are blood sucking or sanguivorous flying mammals that live solely on blood of vertebrates. Three species of vampire bats occur in central and South America: the common vampire bat (Desmodus rotundus), the hairy-legged vampire bat (Diphylla ecaudata) and the white-winged vampire bat (Diaemus youngi).

Unlike other bats, vampire bats can walk, jump and even run by using both forelimbs and hind limbs.   A recent study found that common vampire bats can, run at a speed of up to 72 meters per minute.

Contrary to the prevailing stories about the large blood sucking bat-man or Dracula, vampire bats are tiny creatures, the size of human thumb that emerge from their roosting places in the darkness of night to suck blood of sleeping cattle and horses which are their usual victims.

Common vampire bats are tailless, medium-sized mammals, with a length of 7–9 cm and weighing only 50 grams but can double their weight in one feeding. They have very strong hind legs and a strong thumb of fore limb that helps them in running as well as in taking off after feeding.


The excessively cruel Romanian General, Prince Vlad III, the Impaler (1431-1476) was the original “human vampire” and was also called Dracula (son of dragon). Bram Stoker’s (1897) novel Dracula combined the discovery of vampire bats in South America with Vlad the Impaler horror stories, to create the main character of his novel, Count Dracula who was a century old blood thirsty vampire. Stoker had read newspaper articles on vampire bats and decided to include the blood-sucking creature in his book. Vampire bats rarely bite humans but once they suck human blood, they are known to come back the next night to feed again.

In Transylvania, the area of Bram Stoker’s Dracula, there are stories of sightings of the vampires which are believed to hang around crossroads on St. George’s Day, April 23, and on the eve of St. Andrew, on November 29, when people are known to have close encounters with them. Some believe the Dracula hallucination happens because of the fact that the area is situated on one of Earth’s strongest magnetic fields and people there have some kind of extra-sensory perception.


The lesser false vampire bat (Megaderma spasma) is an insectivorous bat found in Southeast Asia from India to Indonesia and the Philippines. It has a wing span of 30 cm and body length of 10 cm.

The greater false vampire bat (Megaderma lyra) is native to Asia. It is also known as the Indian false vampire bat that preys upon other bats, small birds, reptiles, fish and insects.

Desmodus draculae is an extinct species of vampire bats that inhabited South America during the Pleistocene. The species has also been called the giant vampire bat because of its large size.

The genus Vampyrum contains only one species, the spectral bat or false vampire bat (Vampyrum spectrum). This species is the largest bat native to the Americas and also the largest carnivorous bat in the world. The wingspan of this bat can reach up to 100 cm.


Vampire bats locate a suitable host using infrared sensors which lead them to their victims on the ground. They land near their prey and walk stealthily using fore as well as hind limbs till they find a suitable spot to suck blood. Vampire bats have heat sensors on the nose that help it to locate warm blood just beneath victim’s skin. The bats have razor-sharp teeth with which they make a painless incision to draw blood. Vampire bats’ saliva contains a substance called draculin which is a plasminogen activator that ensures that the blood from the wound of their prey doesn’t clot either while flowing from the wound or in the stomach of bat.

Contrary to popular belief, Vampire Bats do not suck blood but lap it up using their grooved tongue as it flows out of the wound.   Certain chemicals in the Vampire Bat’s saliva numb the area of skin around the bite to prevent the host from feeling pain.

White-winged vampire bats also suck blood of birds that are perched on trees. The bat approaches the bird by crawling on the underside of the branch thus staying out of sight of bird and then makes a cut on the hind toe to suck blood. Vampire bat can suck half of its body weight in blood during 20-30 minutes of feeding session.


Vampires are known to share meals with one another since they roost in colonies either in caves or in tree holes. Mother bats regurgitate blood from their stomach for their offspring to lap it up. If some bats can’t find meal some day, they may beg others to regurgitate some blood for them. Such favours are always reciprocated for the social system to work and to ensure survival of the colony. Vampire bat colony typically has one reproducing male, about 20 females and their offspring. Vampire bat female almost always has only one offspring in a breeding season.


Bats rest by hanging on a branch of tree or on a rock cliff because their legs are not strong enough to support their body weight in an upright posture. When bats relax, the weight of their body pulls down the tendons of muscles that are connected to digits, making the digits to clench tightly around the branch. The talon joints lock in that position preventing the bat to fall. This system is so effective that even if a bat dies while roosting, it continues to hang for quite some time.

Why bats roost in hanging position? Hanging posture is an ideal position for takeoff because the bat simply falls down in air to fly rather than jumping like birds. Contrary to birds, bats can’t jump into the air from the ground or branch to take off because they have weaker hind limbs that are attached to patagium of the wings. Their wing muscles are also not strong enough to produce sufficient lift to take off from the ground upward.   Owing to their weak hind limbs they can’t even run to build up necessary lift force. Instead, they use the claws of fore limbs to climb to a higher spot and then drop down into flight.

By perching in hanging position, bats hide effectively among foliage and escape from predators by quickly launching into flight. In hanging position they also remain close to the hanging fruits on which the fruit-eating bats feed. However, unlike other species of bats, vampire bats can walk on the ground, can also run and jump using both fore limbs and hind limbs, which is a blood sucking adaptation.

Polar Bear – Largest land carnivore

Polar bear’s zoological name is Ursus maritimus, which means “sea bear”. Polar bears are restricted to icy Arctic region where temperature falls as low as -50°C. They are well adapted to the freezing temperatures, which restricts them to the circumpolar region where 19 populations exist. Over forty percent of polar bears live in the north of Canada.

Polar bears have the distinction of being the largest land carnivores. Adults are about 8 ft long and weigh 300-700 kg, larger than the fearsome grizzly bear. Fifty percent of body weight is due to fat and blubber that makes 10 cm thick layer under their blackish skin.

Polar bear’s whitish fur is extremely dense, oil coated and water repellent, so much so that water does not reach the skin even when they are swimming in sea. Each hair is transparent with a hollow core that refracts and reflects visible light to make the fur appear snow white.

Polar bear paws measure up to 12 inches and have fur and foot pads with papillae on the soles that help distribute weight of body over larger area and to get firm grip on the slippery ice surface. Polar bear claws are two inches long, curved, sharp and strong to prevent slipping on ice and to help in climbing over icy slopes.


Polar bears are strong swimmers and divers which enables them to swim from one iceberg to another and to hunt seals in sea. They can also swim underneath ice sheet in search of food. Polar bears can swim continuously for hundreds of kms at speeds of about 10 km/hr. They are known to be the only terrestrial animals to be very comfortable in the sea, often covering long distances with ease helped by their fatty body and waterproof fur. There are instances of polar bears covering marathon 400 km by swimming continuously for several days without finding a resting place. These long journeys enable bears to reach long distances via the sea route.


Polar bears do not hibernate in true sense in dens like the brown and black bears do. Instead they remain active throughout winter in spite of freezing cold. However, pregnant females dig a protective den in snow where they give birth to 2-3 cubs in November- December. During denning cubs subsist on mother’s milk and mother on her fat reserves. They remain in the den for about three months, safe and protected from the cold and wind. Mother bears do not enter deep hibernation because they need to maintain higher body temperature in order to meet the demands of pregnancy, birth, and nursing. Polar bear families generally emerge from their dens in March-April when the cubs are strong enough to survive outside.


Polar bears hunt their favourite prey, the seals while they are basking in the sun or through the breathing holes made by them in the ice sheet. They also eat plants and berries and also hunt fish and other animals. They also act as scavengers of dead whales, walrus or fish. Generally they consume skin and fat of the prey in order to accumulate reserve fat in the body. No other animal can eat and digest such fat-rich diet as do these bears.

Pests of crops

A large number of insects attack crops in various stages of growth and cause immense economic losses. Some of them are host specific while others are polyphagous and can migrate from one crop to another. Owing to their immense capabilities to destroy crops, these pests must be studied in detail and their control measures devised. The following crop pests are given in detail here. Click on the title to open.