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).
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.
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.
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 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 (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.
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 polypeptide chains vary in sequence, affecting sensitivity and color perception.
Opsin polypeptide chains are trans-membrane and contain internal Retinal (Vitamin A).
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 (Retinol) is converted into Retinal and Retinoic acid.
ẞ-carotene is a precursor to Vitamin A.
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.
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.
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 requires multiple photopigments.
A single photopigment cannot differentiate between intensity and wavelength of light.
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 mammals have greater sensitivity to light.
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 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).
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
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.
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.
Laterally placed eyes give all-round vision (ungulates, rabbits).
Forward-facing eyes give binocular vision (arboreal marsupials, primates, colugos, cetaceans).
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.
Diphyodonty ('milk' teeth) is the norm.
Some mammals have