Mammalian Biology - Sensory Systems, Vision, Locomotion, and Teeth

Auditory Sensitivity

• Auditory receptors are incredibly sensitive, detecting minute vibrational displacements.

• The basilar membrane's vibrational amplitude is between 10^{-10} and 10^{-11} cm.

• This displacement is smaller than the diameter of a hydrogen atom (10^{-8} cm).

Cochlea Curvature and Sensitivity

• Manoussaki et al. (2008) studied the influence of cochlear shape on low-frequency hearing.

• Observed in various mammals like mice, rats, bottlenose dolphins, sea lions, squirrel monkeys, cats, chinchillas, gerbils, guinea pigs, elephants, humans, and cows.

• An increase of 20 dB was noted.

Hair Cell Length and Frequency Response

• There's a progressive increase in hair cell length from the cochlea's base to its tip.

• In humans, shorter hairs respond to high frequencies (around 20 kHz).

• Longer hairs respond to lower frequencies (around 100 Hz).

• Observed in Guinea pig auditory neurons.

Owl Hearing

• Owls possess a differential hearing ability, distinguishing sounds with a difference of 0.00003 s, equivalent to 1 cm in distance.

• This is facilitated by asymmetric ear placement.

Infrasound Detection

• Infrasound refers to ultra-low-frequency sound.

• Pigeons can detect infrasound, while humans cannot.

• Elephants, whales, and crocodiles use infrasound for communication.

• Figure 13.6 shows thresholds for low-frequency sound detection in pigeons and humans [Kreithen and Quine 1979].

• Loud natural infrasound events are within the hearing level of pigeons but too low for human perception.

Echolocation

• Echolocation (Sonar) is a transmitter-receiver sensory system.

• Water allows for less sound attenuation, making sonar principles the same as echolocation in air but 4x faster.

Mammalian Vision - Rods and Cones

• Rods are mainly in the peripheral regions of the retina and are absent from the fovea.

• The fovea in humans has around 150,000 cones per mm^2. In some hawks, it may have around 1,000,000.

Opsin and Color Perception

• Opsin polypeptide chains vary in sequence, affecting sensitivity and color perception.

• Opsin polypeptide chains are trans-membrane and contain internal Retinal (Vitamin A).

Sensory Receptors and the Retina

• 70% of all sensory receptors are photoreceptors.

• The eye contains 100 million rods and 6 million cones.

• These connect to 1 million ganglion cells.

• Key structures include the cornea, iris, lens, retina, optic nerve, and blind spot.

Vitamin A and Related Molecules

• Vitamin A (Retinol) is converted into Retinal and Retinoic acid.

• ẞ-carotene is a precursor to Vitamin A.

Vertebrate Vision and Light Absorption

• Vertebrates undergo a transition from 11-cis to all-trans, initiating receptor potential.

• This involves bathorhodopsin, lumirhodopsin, and metarhodopsin I.

• Regeneration occurs via enzymatic isomerization, with a half-life of 5 to 30 minutes.

Color Vision

• Most teleost fish, reptiles, and birds have 4 different opsin genes and tetrachromic color vision.

• Mammals have scotopic vision (rods), and some have photopic vision (cones).

• Mammalian color vision is usually dichromatic (cone genes S and L).

• The 'L' gene or 'red' cone is often missing, affecting the ability to see reds and greens.

Trichromatic Vision

• Some mammals, like primates, re-evolved trichromatic vision.

• This involves a duplicated L opsin gene to give L, M, S.

• Up to 2 million colors can be distinguished with overlapping sensitivity between opsin genes.

Color Vision Requirements

• Color vision requires multiple photopigments.

• A single photopigment cannot differentiate between intensity and wavelength of light.

Photopigments and Color Resolution

• At least 2 photopigments are needed.

• Tammar Wallabies have dichromatic vision (539 nm & 420 nm).

• Humans have trichromatic vision (440, 535, 575) with resolution of 0.2/0.3 nm and can distinguish around 1,500 color hues.

• Examples of color composition: Orange - 99% red, 42% green, 0% blue; Yellow - 83%, 83%, 0%; Green - 31%, 67%, 36%; Blue - 0%, 0%, 97% blue.

Nocturnal Mammal Vision

• Nocturnal mammals have greater sensitivity to light.

Factors Affecting Light Sensitivity

• Larger eyes

• S = (\Pi / 4)^2 (D/f)^2 D_r^2 (1-e^{-kl}) ; D=eye diameter, f=focal length, Dr=photoreceptor diameter, l=length/depth of photoreceptive layer

• More rods or absent cones

• Many-to-one connection between photoreceptors and interneurons (tradeoff: lower acuity)

• DNA in nuclei is packaged differently, making them light-focusing rather than scattering.

Mirrors and Light Sensitivity

• Mirrors add sensitivity via the tapetum lucidum.

• Tapetum lucidum is not found in Haplorhini primates.

• Examples: Strepsirrhini (Lemurs + lorises), Carnivores (Yellow to Green), Cats (Riboflavin rodlets), Dogs (Zinc cysteine rodlets), Fruit bats (Phospholipid spheres).

Lenses and Image Formation

• A convex lens converges parallel rays of light onto a single point; the distance from the midline of the lens to the point of focus is the focal length.

• The refractive index of a material is the ratio of the velocity of light in the material to the velocity of light through a vacuum.

• \frac{sin \theta2}{sin \theta1} = \frac{RI1}{RI2}

• Ability to change focal length of lens is called accommodation.

• 1 diopter = 1 meter focal length. Point sources

Refractive Power of the Eye

• Total refractive power of the human eye is 59 diopters (focal length = 16.7 mm).

• Different parts of the eye contribute to focusing power.

• Materials and their refractive indices: Air (1.000293), Glass (1.5), Water (1.33), Diamond (2.42).

• Human eye components: Cornea (1.38), Aqueous humor (1.33), Lens (1.40), Vitreous humor (1.34).

• Diopter in water is 15, whereas in air it is 90.

Focusing in Water

• Aquatic organisms require more spherical lenses.

• When placed back in air, these organisms would be myopic.

• Amphibious animals face difficulties.

• Some adaptations include:

  • Near-flat cornea (e.g., mudskipper)

  • Strong ciliary muscles (e.g., Mergansers)

  • Round lens with focus via lens movement (e.g., Cetaceans)

  • Four-eyed fish (Anableps) with 2 pupils and a pear-shaped lens.

Visual Field and Precision Trade-Off

• Laterally placed eyes give all-round vision (ungulates, rabbits).

• Forward-facing eyes give binocular vision (arboreal marsupials, primates, colugos, cetaceans).

Pupil Shape and Activity

• Pupil shape is related to activity time and foraging mode.

• Examples include sheep, humans, lynx, and cats (Martin S. Banks et al. Sci Adv 2015;1:e1500391).

• Predators vs. Prey.

Mammalian Teeth

• Diphyodonty ('milk' teeth) is the norm.

• Some mammals have