Chapter 5: Sensory Systems and Behavior

Overview of Animal Sensory Systems

• Fundamental Principles: Animals acquire essential environmental information through diverse sensory systems. Biotic and abiotic variables in the environment change constantly, requiring specialized detection mechanisms.

• Types of Sensory Receptors: Specific receptors are evolved to detect different stimuli:

  • Chemicals: Detected via chemoreception.

  • Light: Detected via photoreception.

  • Vibrations: Detected via mechanoreception.

  • Electric Fields: Detected via electroreception.

  • Magnetic Fields: Detected via magnetoreception.

• Defining Sensory Receptors: These are specialized neurons or cells that respond to environmental stimuli and transmit information via neural signals.

• Umwelt: A concept introduced by Jakob von Uexküll (1909), referring to the "self-centered world" or the specific environment as perceived by an organism. Its sensory systems determine its perception and define what can be detected and responded to.

Neural Components and Signal Transmission

• Anatomy of a Neuron:

  • Dendrites: Branch-like structures that detect a stimulus.

  • Cell Body (Soma): Integrates the incoming information.

  • Axon: A long fiber that conducts the neural signal (action potential) away from the cell body.

  • Terminal Buttons (Axon Terminals): Responsible for the transmission of the signal to another neuron, organ, or muscle.

• Synaptic Transmission:

  • Presynaptic Neuron: The neuron sending the signal.

  • Postsynaptic Neuron: The neuron receiving the signal.

  • Synapse: The gap between neurons where neurotransmitter molecules are released from the presynaptic neuron and taken up by the postsynaptic neuron.

  • Neural Signal Path: Dendrites → Cell Body → Axon → Synapse → Postsynaptic Dendrites.

Chemosensory Systems: Gustation and Olfaction

• Chemoreception: The process of detecting chemical stimuli where a chemical molecule binds to a receptor on a neuron.

• Gustation (Taste): The detection of dissolved chemicals, usually within the mouth or on the body surface.

• Olfaction (Smell): The detection of volatile (airborne) stimuli.

  • Odorant: A gaseous compound that is perceived as a smell.

  • Pheromone: Species-specific chemical compounds that affect the behavior or physiology of others of the same species.

• Flehmen Response: A behavioral movement (curling the upper lip) in many mammals that facilitates the transfer of pheromones/odorants into the vomeronasal organ.

• Physiological Basis of Chemosensation:

  • Odor-binding proteins: Found in the mucus of vertebrates or the sensilla of invertebrates; they increase the ability of the medium to carry hydrophobic chemicals over receptors.

  • Receptor Diversity: The number of olfactory receptor genes varies wildly by species:

    • Fruit Fly: 130 receptors.

    • Humans: 400 receptors.

    • Mice: 1,100 receptors.

    • Roundworm: 1,200 receptors.

    • African Elephant: 2,000 receptors.

• Research: Sweet and Umami Perception in Rodents (Zhao et al. 2003):

  • The Five Basic Tastes: Sweet, sour, salty, bitter, and umami (savory).

  • Hypothesis: Receptors $T1R1$, $T1R2$, and $T1R3$ are responsible for sweet and umami perception.

  • Methods: Created knockout (KO) mice lacking functional DNA for one of the three receptors. Measured neural activity and lick rates when presented with sucrose (sugar) or amino acids (umami).

  • Results:

    • $KO\text{-}1$ and $KO\text{-}3$ mice showed weak response to amino acids.

    • $KO\text{-}2$ and $KO\text{-}3$ mice showed weak response to sucrose.

  • Conclusion: $T1R1 + T1R3$ combination is required to detect umami. $T1R2 + T1R3$ combination is required to detect sweetness.

• Research: Cuttlefish Response to Odors (Boal et al. 2010):

  • Question: How do solitary cuttlefish find breeding aggregations?

  • Methods: Presented individuals with seawater (control), old eggs (36–48 h), and new eggs ($< 1\text{ h}$). Measured ventilation rates and maze movement.

  • Results: Individuals responded most strongly to new eggs; 16 out of 22 individuals moved toward the arm of a Y-maze containing new egg odor.

  • Conclusion: Cuttlefish can detect the odor of freshly laid eggs to find spawning sites.

• Application: Mosquito Victim Location: Mosquitoes use antennas, maxillary palps, and labial palps to detect carbon dioxide ($CO_2$) and the chemical octenol to locate hosts.

Photoreception: Light and Image Perception

• Photoreceptors: Specialized neurons sensitive to light that function as visual receptors.

  • Opsins: Photosensitive proteins that change shape when struck by light, triggering a neural signal.

  • Rods: Photoreceptors specialized for sensitivity in low-light conditions.

  • Cones: Photoreceptors specialized for color vision and high acuity.

• Wavelength Sensitivity: Species differ in the range of light they detect. Humans typically see between $400\text{ nm}$ and $700\text{ nm}$. Bees see shorter wavelengths (ultraviolet, starting around $300\text{ nm}$) but less of the red spectrum.

• Research: Color Vision in Monarch Butterflies (Blackiston, Briscoe, & Weiss 2011):

  • Methods: Trained butterflies to associate a specific color Artificial flower (red, purple, blue, or yellow) with a food reward. Butterflies were then tested against gray flowers of varying reflectance.

  • Results: Individuals spent over 95\text{%} of trial time on the learned color, regardless of the brightness of gray alternatives.

  • Conclusion: Monarchs possess true color discrimination.

• Research: UV Plumage and Mate Choice in Birds (Siitari et al. 2002):

  • Observation: Birds have four types of cones and can see ultraviolet light.

  • Methods: In an aviary, females were presented with two males: one control and one treated with UV-absorbing sunscreen to reduce reflectance.

  • Results: 11 of 13 females chose the control (UV-reflective) male.

  • Conclusion: Female mate choice is affected by UV plumage; they prefer higher UV reflectance.

• Infrared Detection: Some species detect wavelengths longer than visible light ($> 700\text{ nm}$), such as some beetles, vampire bats, and snakes.

  • Snake Study (Safer and Grace 2004): Pit vipers (Crotalinae) vs. true vipers (Viperinae). Crotaline snakes showed high tongue-flick rates and strikes at warm targets ($32\text{--}35^{\text{ o}}C$) vs. cool targets ($20\text{--}25^{\text{ o}}C$). Viperinae showed no difference.

  • Conclusion: Crotaline snakes use specialized pit organs to detect infrared radiation; non-crotaline vipers cannot.

Mechanoreception: Vibration and Sound

• Mechanoreceptors: Detect physical vibrations through air, water, or substrates. Found on diverse locations including the head, thorax, wings, or legs.

  • Near-field: Detection of air/water particle displacement close to the source.

  • Far-field: Detection of pressure waves (sound) at a distance.

• Research: Ultrasonic Song in Moths (Nakano et al. 2006):

  • Context: Corn borer moths produce pulses higher than human hearing ($> 20\text{ kHz}$).

  • Methods: Compared mating success of deafened females vs. sham-treated/control females.

  • Results: 84\text{%} of control females mated within 2 minutes, compared to only 59\text{%} of deafened females.

  • Conclusion: Male ultrasonic pulses are critical for successful female courtship and mating.

• Research: Infrasound in Elephants (McComb et al. 2003):

  • Infrasound: Frequencies below human hearing ($< 20\text{ Hz}$). These attenuate slowly and travel long distances.

  • Methods: Played familiar vs. unfamiliar elephant vocalizations from up to $2.5\text{ km}$ away.

  • Results: Groups recognized familiar members up to $1\text{ km}$ away (responding with contact calls) and bunched defensively to unfamiliar calls up to $1.5\text{ km}$ away.

• Lateral Line System in Fish:

  • Contains neuromasts (mechanoreceptors) that detect water velocity, acceleration, and direction.

  • Catfish Study (Pohlmann et al. 2001/2004): Catfish track prey (guppies) in the dark by following their hydrodynamic wake. When the lateral line was rendered nonfunctional using cobalt chloride ($CoCl_2$), tracking ability failed. When gustation (taste) was surgically removed, tracking was unaffected.

  • Conclusion: The lateral line is the primary system for wake-tracking in catfish.

• Substrate-Borne Vibrations: Many species detect vibrations through silk, vegetation, or ground.

  • Antlion Larvae: Build sand pits. Research by tossing sand or using needles showed that antlions precisely detect where prey falls based on sand vibrations. 26 of 30 antlions successfully tossed sand at the vibration source.

Electroreception and Magnetoreception

• Electroreception: The capability to detect weak electric fields. Found in elasmobranchs (sharks/rays), bony fish, amphibians, and some mammals (platypus).

  • Ampullae of Lorenzini: The specialized organs in sharks for electrodetection.

  • Hammerhead vs. Sandbar Sharks (Kajiura & Holland 2002): Tested if the wider head of the hammerhead provides better detection. Both species were attracted to active electric dipoles. Results showed no significant difference in detection threshold between species, suggesting wider heads don't necessarily provide "greater" sensitivity but perhaps a wider search area.

• Magnetoreception: Detecting the Earth's magnetic field for orientation/navigation.

  • Information types: Polarity (North/South), Angle of Inclination (angle with the horizon, varies with latitude), and Intensity (strength).

  • Mechanism: Certain cells in the beak or head contain magnetite (a permanent magnet). The visual system may also contribute.

  • Pigeon Study (Mora et al. 2004): Pigeons were trained to go to specific feeders based on magnetic field polarity. Sectioning the trigeminal nerve caused a significant decline in correct choices; sectioning the olfactory nerve had no effect.

  • Conclusion: The trigeminal nerve is the primary conduit for magnetic information to the brain.

Evolutionary Arms Race: Predator and Prey

• Co-evolution: An adaptation in one species leads to a counter-adaptation in another.

• Insect Tympanal Organs: Specialized "ears" that have evolved independently at least 18 times to detect the ultrasonic pulses of hunting bats.

  • Bats use low pulse rates for searching and high pulse rates (terminal buzz) for attacking.

  • Mating/flying insects with these organs perform evasive maneuvers (power dives) when ultrasound is detected.

• Mothers and Bats Research:

  • Aposematism: Some distasteful (noxious) moths produce ultrasonic clicks. Bats learn to associate these clicks with bad taste, reducing attacks (Hristov & Conner 2005).

  • Sonar Jamming: The palatable moth Bertholdia trigona produces complex acoustic sounds that increase in intensity just before an attack.

  • Results: Control moths (silenced) were attacked over $10\text{ times}$ more frequently than sound-producing moths (Corcoran et al. 2009).

  • Conclusion: These ultrasonic sounds effectively "jam" the bat’s ability to localize the prey, representing a sophisticated antipredator adaptation.