How Do Animals Perceive the World?• Jakob von Uexküll’s Tick• "...this eyeless animal finds the way to her watchpoint [e.g. at the top of a tall grass blade] with the help of only its skin’s general sensitivity to light. The approach of her prey becomes apparent to this blind and deaf bandit only through her sense of smell.
Jakob vonUexkullThe odor of butyricacid, whichemanates from thesebaceous folliclesof all mammals,works on the tick asa signal that causesher to abandon herpost (on top of theblade of grass/bush)and fall blindlydownward towardher prey.
A Tick’s World (Cont’d)• If she is fortunate enough to fall on something warm (which she perceives by means of an organ sensible to a precise temperature) then she has attained her prey, the warm-blooded animal, and thereafter needs only the help of her sense of touch to find the least hairy spot possible and embed herself up to her head in the cutaneous tissue of her prey.• She can now slowly suck up a stream of warm blood.
A Tick’s Three Sensory Cues• Von Uexkull shows that the world of a female tick is reduced to three sensory cues: the smell of butyric acid, the warmth of a mammals skin, and the feel of a warm liquid.• At sexual maturity the tick mates with a male, climbs to the tip of a branch, and waits.
A Tick’s “Umwelt”• No stimulus other than the smell of butyric acid is detected. No light, no sound, no vibration, no taste, no other smell.• Amazingly, the female tick may sit dormant for as long as 18 years sensing nothing and doing nothing until molecules of butyric acid reach her olfactory sense. When butyric acid is detected, the tick drops off the branch. If it senses warmth, the tick begins to burrow. If it senses a warm liquid, it drinks, falls to the ground, lays its eggs, and dies.
A Tick’s “Umwelt” (Cont’d)• Jakob von Uexkull (1934) who argued that to truly understand animal behavior one must appreciate the animals "umwelt" or self-world. This self-world is determined by the animals sensory systems, the means by which sensory information is processed and perceived, and its action systems.• To illustrate this approach, von Uexkull asks his readers to "blow, in fancy, a soap bubble around each creature to represent its own world, filled with the perceptions which it alone knows."
The Sense Organs: Windows to the World• Sensory receptors are dendrites (one of three parts of nerve cells) specialized to detect certain types of stimuli.• The five best known: taste, smell, hearing, vision, and touch.• Others involve: balance (rotational motion and gravity), temperature, pain, electricity in some fish, polarized light in birds, ultraviolet light in some birds and insects, the earth’s magnetic field in birds and sea turtles, and infrared in snakes.
The Sense Organs (Cont’d)• Each type of sensory receptors detects a particular kind of stimulus. When stimulation occurs, sensory receptors initiate nerve impulses that are transmitted to the spinal cord and/or brain. Sensation occurs when nerve impulses reach the cerebral cortex. Perception is an interpretation of the meaning of sensations.• Sense of Hearing in humans: The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear, and each consists of specialized hair cells that are sensitive to mechanical stimulation (mechanoreceptors).
Sense Organs (Cont’d)• Sense of Taste and Smell in humans: taste and smell are due to chemoreceptors that are stimulated by molecules in the environment. After molecules bind to receptor proteins of taste cells and olfactory cells, nerve impulses go to the cerebral cortex which determines taste and odor according to the pattern of stimulation. ---since humans can respond to a range of sweet, sour, salty and bitter tastes, the brain appears to survey the overall pattern of incoming sensory impulses and takes a “weighted average” of their taste messages as the perceived taste.
Sense Organs (Cont’d)• Sense of Touch in humans: The skin has receptors that are sensitive to touch, pressure, pain and temperature (these are mechanoreceptors, chemoreceptors and thermoreceptors).• Sense of Vision in humans: Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex. The eyes’ rod cells are sensitive to dim light and the cone cells are sensitive to both bright light and colors (they are both photoreceptors).
Senses in Animals (Niko Tinbergen’s Comments, 1965)• “…All [animals] may be said to live in different worlds, since each perceives best only that part of the environment essential to its success. Thus, how an animal behaves has much to do with what its sense organs are and whether these are few or man, simple or complex.”• “What sort or stimuli do animals receive? …they are not necessarily the same as those to which a human might react. …animals, including man, have different ‘windows to the world.’ Some have sensory equipment that in some respects is much poorer than ours; in others, the senses are far superior. There are even animals that react to stimuli which we cannot detect at all—sights or sounds or smells which we could not discover without artificial extensions to our own sense organs”
Extraordinary Senses in Animals• Bat & The Moth
Human Vision vs. Snake Infrared “Vision”• The human visual system is sensitive to a portion of the electromagnetic spectrum (in wavelength, the typical way of talking about light waves, from a bit less than 400 nanometers to a bit more than 700 nm).• Note that although human beings cannot see ultraviolet rays or infrared rays, other creatures are sensitive to those portions of the electromagnetic spectrum.
Pit Vipers• For example, pit vipers (e.g., rattlesnakes, copperheads, water mocassins) have a pit organ near each eye.• These organs allow the snakes to detect infrared radiation with greater precision than one might expect.• For example, when blinded (humanely, for example, using electrical tape), these snakes will strike at a rat in back of its head—thereby avoiding its sharp teeth. Thus, the infrared world "seen" by these snakes must be fairly detailed.
Electroreception• Electroreception, which is the detection of weak electric fields, is widespread among vertebrates, with cases in all classes of fishes, two orders of amphibians and even mammals (the duck-billed platypus). This exotic sense seems to be an ancestral vertebrate trait, as it is present in lampreys and cartilaginous fishes.• Its spotty presence in particular vertebrate groups indicates that electroreception has evolved (been reinvented) a number of times during vertebrate evolution. Particularly compelling evidence for the independent evolution of this sense is its presence in certain species of African and South American fishes.
Electroreception (Cont’d)• Electroreception is also found the duck-billed platypus, a primitive, egg-laying, monotreme mammal.• In all cases, electroreception does not seem to be the ancestral condition. Most modern boney fish species are not electroreceptive. Similarly, electroreception in the duck-billed platypus is probably a derived trait because it is not characteristic of reptilians (from which mammals evolved).• Electrogenic fish produce electric signals by discharging their electric organs, which consist of columns of modified muscle cells (electrocytes).
Electroreception (Cont’d)• Some electric organs generate strong discharges (hundreds of volts) that are useful for stunning prey (the electric eel), whereas others produce weak discharges (millivolts) that are used for social communication and electrolocation.• Species that have electric organs of the weak discharge type produce either intermittent (pulse species) or periodic (wave species) discharges. Both types of weakly-electric fish also have electroreceptors that are tuned to the species-specific higher frequencies found in their discharges.
Electric Eel Uses EODs to Stun Prey for Capture and Eating
Electric “Eels”• Despite their serpentine appearance, electric eels are not actually eels. Their scientific classification is closer to carp and catfish.• These famous freshwater predators get their name from the enormous electrical charge they can generate to stun prey and dissuade predators. Their bodies contain electric organs with about 6,000 specialized cells called electrocytes that store power like tiny batteries.
Electric Eels (Cont’d)• When threatened or attacking prey, these cells will discharge simultaneously, emitting a burst of at least 600 volts, five times the power of a standard U.S. wall socket.• They live in the murky streams and ponds of the Amazon and Orinoco basins of South America, feeding mainly on fish, but also amphibians and even birds and small mammals.• They have poor eyesight, but can emit a low-level charge, less than 10 volts, which they use like radar to navigate and locate prey.
Electric Organ Discharge (EOD) in Fish• Electric fish can use electricity as a communicative device, much as humans use auditory signals. Using its electric organ, the fish produces an electric organ discharge (EOD), which is broadcast through the surrounding water and received by other fish in the environment.• Detecting these signals other fish process various aspects of the signal to determine its significance. Fish constantly emit EODs, which can express a fishs species, gender, reproductive intent, social status, and even level of aggression.• Decoding electrocomminicative "fish speak" is a difficult process, and much remains to be discovered. Each species of electric fish varies its EOD differently to communicate different cues.
Echolocation in Whales and Porpoises• Toothed whatles (dolphins, porpoises, river dolphins, orcas and sperm whales) use echolocation (or biosonar) in their underwater habitat becaise it has favourable acoustic characteristics and vision is extremely limited.• Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. Most toothed whales use clicks in a series, or click train, for echolocation, while the sperm whale may produce clicks individually. Different rates of click production in a click train give rise to the familiar barks, squeals and growls of the bottlenose dolphin.
Echolocation in Whales and Porpoises• Echoes are received using the lower jaw as the primary reception path, from where they are transmitted to the inner ear via a continuous fat body. Lateral sound may be received though fatty lobes surrounding the ears with a similar acoustic density to bone.• Some researchers believe that when they approach the object of interest, they protect themselves against the louder echo by quieting the emitted sound. In bats this is known to happen, but here the hearing sensitivity is also reduced close to a target.
Echolocation Apparatus in a Bottlenosed Dolphin
Sensing the Earth’s Magnetic Field• Both migratory birds and sea turtles are able to sense the earth’s magnetic field and appear to be able to use it in navigation.• An experiment with a migrating Australian bird, the silvereye, provides evidence.--1. Scientists subjected migrating silvereyes to a strong magnetic pulse; the result was that the orientation of their subjects different significantly to that of untreated control birds; it took ten days for most of the treated birds to correct their direction.--2. Conclusion: adult silvereyes rely upon magnetic field information during their migratory journeys.Graphic: G. Scott Fig. 4.18, p. 89.
Sensing the Earth’s Magnetic Field• Another conclusive experiment involved green sea turtles that migrate over long distances.• Fig. 4.46, p. 139, Alcock: experimental manipulation of the magnetic field affects the orientation of green sea turtles.• Turtles that experience the magnetic field associated with an area to the north of their actual location swim south; turtles that sense the magnetic field of an area to the south of their actual location swim north.
Green Turtle Annual Migration(to/from Ascension Is. to the Coast of Brazil)
Sensing Ultraviolet and Polarized Light• Hypotheses: monarch butterfly navigation is dependent on ultraviolet radiation as well as polarized sunlight (p. 136-137, Alcock).• A monarch flight cage was covered with a UV interference filter, which screened out this component of sunlight (which humans can’t see).• The monarchs became confused and many stopped flying altogether; most individuals resumed flight, however, as soon as the filter was removed.• A similar experiment with polarized light with monarchs yielded similar results: monarchs can orient to polarized light, using it to consistently orient to the southwest—the direction of their annual migration