Mammalian Characteristics and Adaptations

Mammals: Characteristics - Adaptations - Endothermy

Endothermy

• Endothermy is not exclusive to mammals; birds also exhibit this trait.

• It is associated with mammalian characteristics like:

  • Hair
  • Sweat glands
  • Four-chambered heart (related to higher metabolism)
  • Specialized teeth

• Energetic cost of endothermy is high.

  • Absolute energy cost increases with body mass (positively correlated).
  • Mass-specific energy cost is negatively correlated with body size (energy cost does not increase proportionally to body size).
  • Kleiber's Law: When log body size increases by 1 unit, log energy cost increases by 0.75.

• Not all mammals are fully endothermic.

  • Examples: elephant shrew, naked mole rats exhibit variations.
  • Ectotherm: Body temperature varies with ambient temperature.
  • Heterotherm: Body temperature regulation varies.

Heterotherms

• Pygmy possums are heterotherms.

Facultative Torpor

• Big brown bat (Eptesicus fuscus):
  • Weighs 16g.
  • Found in USA and hibernates in winter in the North.
• Mexican free-tailed bat (Tadarida mexicana):
  • Weighs 10g.
  • Found in southern USA and migrates south in winter.
• Body temperature (Tb) can drop as low as 7^{\circ}C when torpid, saving energy up to 40-fold.

Mammals: Characteristics - Adaptations - Lactation

Lactation

• Nipples are not always present.
  • Monotremes secrete milk onto a flattened milk patch.
• No direct correspondence between the number of mammary glands/lobes and nipples.
  • Most mammals have fewer young than nipples (except opossums).
  • Humans have 20-40 glands/lobes.
  • Probably modified sweat glands.

Evolution of Lactation

• Monophyletic origin is suggested by the similarity of glands in different groups (Capuco & Akers 2009 J. Biol. 8, 37).
• Lactation allows:
  • Less burdening of the mother
  • Independence from environmental resource availability
  • Delay in the development of teeth
• Milk has antimicrobial properties, including maternal immunoglobulins (IgG) via Brambell receptors (FcRB).

Lactation Details

• Secreting milk evolved from specialized sweat glands.
• Only one species has lactating males: Dayak fruit bat (Dyacopterus spadiceus) (Fagan, Constable & Law (2024) Nature Comms 15 (5342).
• Milk composition:
  • Water
  • Fats
  • Proteins
  • Lactose (40% of calories)
  • Glucose & Galactose
  • Minerals (CaHPO4, Vit B6, B12, K)
• Hormonal control by prolactin & oxytocin (stimulated by suckling).

Extreme Milks

• Humans: 4% fat, 7% sugar, 0.9% protein
• Cows: 3.5% fat, 5% sugar, 3.3% protein
• Hooded Seal: 60% fat (intermittent feeding in cold environments) (Oftedal et al. 1993)
• Black Rhino: 0.2% fat (slow reproductive cycle in warm climate) (Skiviel et al. 2013)
• Tammar Wallaby: 14% sugar (asynchronous content lactation) (Nicholas et al. 1997)
• Eastern Cottontail Rabbit: 14% fat, 16% protein (intermittent feeding)

Mammals: Characteristics - Adaptations - Skin & Hair

Mammalian Skin Structure

• Layers (from outside in):
  • Epidermis (contains melanin)
  • Dermis (double-layered, contains armadillo dermal plates)
  • Hypodermis (fat layer)
• Other structures:
  • Basement membrane
  • Sweat gland (modified as mammary gland)
  • Pore of sweat gland
  • Sebaceous gland
  • Hair follicle (epidermal)
  • Hair papilla (dermal)

Skin Exocrine Glands

• Exocrine glands secrete through a duct.
• Dermal and subcutaneous.
• Main types:
  • Mammary: milk-producing
  • Sebaceous: oil-secreting sebum (lipids and waxes)
  • Wax producing: in ears
  • Sweat: thermoregulation, excretion, communication
• Presence/distribution varies:
  • Sweat glands in paws of cats, snout in platypus
  • Scent glands in temporal region of elephants, anal region of rodents and cats.
  • Musth - Temporin (Phenols)
  • Skunk - Anal glands (Thiols – sulphur).

Evolution of Hair

• Theories (Meng & Wyss. 1997. Nature 385:712-714):
  • Arose as sensory structures first
  • Arose as insulation in primitive mammalian endotherms
• Evolved in the therapsid lineage.
  • Early therapsids lacked scales, but no evidence of hair.
  • Hair by 210 mya, possibly earlier (Multituberculates Late Triassic).

Hair

• Hair has “grain” (lost in moles and burrowers).
• Specialized hairs:
  • Sensory function (Vibrissae): navigation (rats), trail following (seals) (Seals – Dehnhardt et al. (1998) Nature 394
  • Defensive (Spines)

Mammals: Characteristics - Adaptations - Hearing

Jaw and Middle Ear Structure

• Evolutionary transition from reptiles to mammals involves changes in jaw and middle ear structure.
• Reptiles:
  • Jaw joint formed by quadrate and articular bones.
  • Stapes present in the middle ear.
• Cynodonts (advanced therapsids):
  • Reduction in postdentary bones.
  • Dentary bone becomes more prominent in the jaw.
• Mammals:
  • Jaw joint formed by dentary and squamosal bones.
  • Quadrate becomes incus, articular becomes malleus in the middle ear.
• Dimetrodon (early mammal-like reptile)
• Thrinaxodon (cynodont, advanced therapsid)
• Didelphis (marsupial)

Why Did This Happen?

• Later therapsids/early mammals:
  • Masseter muscle allowed greater use of jaws in chewing.
  • Jaw joint under greater stress.
  • Postdentary bones required strengthening.
  • Conflict with role as vibrating auditory ossicles.
  • Resolved by transfer of jaw insertion to dentary.

Akinetic Skull

• Lack of movement between the upper jaw and braincase.
• Not a defining feature of mammals.
• Secondary palate formed from processes of the premaxillae, maxillae, and palatines.
• Probably a result of young mammals’ need to suckle.
• Allows precise, strong tooth occlusion [contact between teeth].

Mammalian Ear

• The maculae of sacculus and utriculus are specialized for detecting gravity.
• The cristae ampullarae are specialized for detecting body movements.
• Semi-circular canals [Balance]

The Human Ear

• The external ear preferentially transmits wavelengths around 4 times its own length (approx. 4 kHz). [range 0.03 to 20 kHz]
• The middle ear ossicles (stapes, incus, malleus) transmit sound-induced vibrations from the large tympanic membrane (about 45 mm^2) to the small oval window in the bony wall of the inner ear (about 3 mm^2).

Vertebrate Hair Cells and Auditory Sensors

• Most vertebrate mechanoreceptors are modified hair cells.
• These are non-neural secondary sensory cells and make up the so called acoustico-lateralis system of vertebrates.
• The activation effects are asymmetric as seen in the invertebrate hair sensilla.

Trapdoor-Spring Mechanism

• Diagrammatic representation of stereocilia, kinocilium, and tip links.
• Mechanical stimuli act directly on ion channels in the stereocilia, by increasing tension in a 'gating spring' attached to the channel gate.
• At any one time channels are distributed between open and closed states, but an increase in tension (dashed line) makes the open state energetically more favorable, and shifts the distribution towards the open state.
• The sensitivity curve, relating displacement of the bundle (measured at the tips) to the probability of the channel being open, shows the very narrow operating range of hair cells.
• A displacement of about a third of a micrometre - approximately the diameter of one stereocilium - is a saturating stimulus.
• The curve also shows that some channels are open in the resting position, suggesting some resting tension in the tip links.

Hair Cell Polarization

• Shearing towards the longest stereocilium depolarizes the cell by around 20 mV.
• Shear in the opposite direction hyperpolarizes by around 5 mV.
• Shear at right angles has little effect (asymmetric).

Organ of Corti

• Transduces vibrational fluid movement and encodes them to represent acoustic parameters of frequency and intensity to the CNS.
• The Organ of Corti is located on the basilar membrane.
• The stereocilia of the hair cells are covered by a gelatinous structure called the tectorial membrane.

Sensitivity

• Auditory receptors can detect incredibly small vibrational displacements.
• The vibrational amplitude of the basilar membrane is between 10^{-10} and 10^{-11} cm.
• This is less than the diameter of a hydrogen atom (10^{-8} cm!!).

Cochlea Curvature Ratio - Sensitivity

• Manoussaki et al. (2008) the influence of cochlear shape on low-frequency hearing PNAS105, 6162-6166.
• Increased of 20 dB!
• Examples: mouse, rat, Bottlenose dolphin, Sealion, Squirrel monkey, cat, chinchilla, gerbil, Guinea pig, elephant, human, cow

Hair Cell Length and Frequency

• There is a progressive increase in the length of the hair cells from the base of the cochlea to the tip.
• In humans, the shortest hairs respond preferentially to high frequencies (around 20 kHz) while the longer hairs respond preferentially to lower frequencies (around 100 Hz).
• Guinea pig auditory neurons

Owl Hearing

• Differential hearing ability can distinguish 0.00003 s - Equivalent to 1 cm in distance!
• Asymmetric ear placement

Infrasound

• Infrasound – ultra-low frequency sound detection in birds
• Elephants, whales and crocodiles all use infrasound for communication

Echolocation

• Echolocation or Sonar is an example of a “transmitter-receiver” sensory system.
• Water – The principles of sonar are the same as for echolocation in air but there is less attenuation of sound in water. (4x faster)