Mammalian Adaptations: Locomotion, Metabolism, and Reproduction

Posted on Dec 5, 2025 in Biology

L8 – Mammalian Locomotion I: Terrestrial Adaptations

  • Cursorial: Running adaptation characterized by long limbs and reduced digits (e.g., pronghorn, cheetah).
  • Advantages of Cursorial Locomotion: Increased range, successful migration, and endurance in predator–prey interactions.
  • Cursorial Adaptations: Longer distal limbs, reduced or fused bones, and loss of the clavicle.
  • Horse Evolution: Extreme digit reduction resulting in a single toe, optimizing efficient endurance.
  • Ricochetal: Bipedal hopping utilizing elastic energy (e.g., kangaroo rat, jerboa). Elastic recoil provides energy savings and aids predator evasion.
  • Semi-fossorial: Partial burrowers (e.g., marmot, badger).
  • Fossorial: Full burrowers (e.g., mole, naked mole rat). Challenges include managing low O₂, high CO₂, and heat.
  • Ecosystem Engineer: An organism that physically alters its environment (e.g., prairie dog activity increases soil nutrients and biodiversity).
  • Keystone Species: A species whose removal causes the collapse of the ecosystem structure.

L9 – Mammalian Locomotion II: Aerial Adaptations

  • Gliding: Evolved at least 30 times in vertebrates and 9 times in mammals (e.g., colugos, 2x Rodentia flying squirrels, 3x Marsupialia sugar gliders).
  • Gliding Adaptations: Patagium (wing membrane), styliform cartilages, and a flattened tail.
  • Why Gliding? Related to habitat structure, predator avoidance, locomotor economy, safety (less impact upon landing), and foraging efficiency.
  • Bat Flight: Characterized by elongated digits 3–5, which form the chiropatagium.
  • Bmp2 Up-regulation and Digit Elongation: An example of rapid regulatory evolution.
  • Structural vs. Regulatory Evolution: Structural involves changes in coding sequences; regulatory involves mutations that alter the timing or location of gene expression, such as Bmp2.
  • Wing Loading (WL): Body weight ÷ wing area. Lower WL indicates slower flight and more control.
  • Aspect Ratio (AR): Wingspan² ÷ wing area. High AR indicates fast, long-distance flight.
  • Molossids (Free-tailed Bats): High WL/AR, making them fast, open-air flyers.
  • Hover-gleaner: Bats that hover near vegetation to hunt (e.g., Plecotus).
  • Ground-gleaner: Bats that hunt prey on the ground (e.g., Antrozous pallidus).

L10 – Feeding and Digestion in Mammals

  • Diet–Trait Links: Diet influences traits such as teeth, skull morphology, gut structure, microbiome composition, behavior, and metabolism.
  • Easy Diets: Insects, meat.
  • Challenging Diets: Plants, ants/termites.
  • Diet Evolution: Herbivores show the greatest diversification; omnivory is often gained secondarily.
  • Onychomys (Grasshopper Mouse): A scorpion-eating mouse. Its venom binds to the Nav1.8 channel, blocking pain (an example of coevolution).
  • Herbivory: Cellulose digestion is achieved via symbiotic microbes.
  • Foregut Fermenters (Slow, Efficient): Digestion occurs in a multi-chambered stomach before the small intestine (e.g., ruminants, sloths, colobus monkeys).
  • Hindgut Fermenters (Fast, Less Efficient): Digestion occurs primarily in the large intestine or caecum after stomach digestion (e.g., horses, rabbits, elephants).
  • Microbiome: The community of symbiotic microbes in the gut; varies significantly by diet and phylogenetic lineage.
  • Phylogenetic Constraint: The panda is a herbivore that retains a carnivore-like gut structure.
  • Convergence: Horse and elephant exhibit similar hindgut microbiomes despite distant ancestry.
  • Insectivore: Eats insects (considered a primitive mammalian diet).
  • Myrmecophagous: Specialized diet of ants and termites (e.g., anteater, pangolin).

Skull Morphology and Diet

  • Carnivore: Typically features a small masseter muscle and a large temporalis muscle.
  • Herbivore: Exhibits the opposite pattern (large masseter, smaller temporalis).

L11 – Bat Ecology and Conservation Threats

  • Ecosystem Service: Bats provide natural pest control (e.g., the Texas Tadarida brasiliensis colony eats 150 tons of insects per night).
  • White-Nose Syndrome (WNS): Caused by the fungus Pseudogymnoascus destructans. It disrupts hibernation, leading to fat loss and death.
  • WNS Origin and Spread: Originated in Europe/Asia; now widespread across North America, approaching California.
  • WNS Prevention: Decontamination procedures and cave closures.
  • Other Threats to Bats: Habitat loss, pesticide use, wind turbines, and human disturbance.
  • Additional Ecosystem Services: Pest control, pollination (e.g., Leptonycteris pollinates agave), and seed dispersal (by fruit bats).
  • Hibernaculum: A winter roost site.
  • Maternity Colony: A group of female bats raising pups.

L12 – Principles of Mammalian Endothermy

  • Endothermy: Internal heat generation primarily via metabolic processes.
  • Homeothermy: Maintaining a stable body temperature (Tb) regardless of the ambient temperature (Ta).
  • Benefits of Endothermy: Allows activity at night or in cold conditions, enabling wider ecological niches.
  • Primary Heat Sources: Mitochondria-rich organs (liver, heart, brain).
  • Behavioral Thermoregulation: Using behavior (e.g., huddling, seeking sun or shade) to control body temperature.
  • Basal Metabolic Rate (BMR): The minimum energy required to maintain basic physiological functions at rest.
  • Mass-Specific Metabolic Rate (msMR): Metabolic rate per unit body mass; typically higher in small mammals.
  • Allometric Scaling of BMR: Large mammals have a lower mass-specific metabolic rate (the scaling slope is approximately 0.75).
  • Pygmy Shrew: Near the lower size limit for mammals; must eat constantly due to high msMR.
  • Thermal Neutral Zone (TNZ): The range of ambient temperatures where body temperature remains constant and metabolic rate is minimal.
  • Below Lower Critical Temperature (LCT): Responses include shivering, non-shivering thermogenesis (using Brown Adipose Tissue, BAT), and increased muscle activity.
  • Above Upper Critical Temperature (UCT): Responses include sweating, panting, and saliva spreading.
  • Fine-Scale Control Mechanisms: Behavior (huddling, posture), vasoconstriction/vasodilation, and piloerection.
  • Thermal Radiator: A body part specialized for heat loss (e.g., large ears, wings).
  • Countercurrent System: An arrangement of arteries and veins that minimizes heat loss by retaining heat in the core, especially in limbs.

L13 – Mammalian Adaptations to Cold Climates

  • Cold Stress: Highly dependent on body size (due to surface area to volume ratio).
  • Adaptations of Large Mammals: Thick coats (40–80 mm insulation).
  • Adaptations of Small Mammals: Dense fur, use of burrows, and temporal heterothermy (torpor).
  • Seasonal Coats: Changes in thickness and color (e.g., snowshoe hare, fox).
  • Subnivean Space: The insulated layer under the snow, providing a warm microclimate for small mammals like voles and lemmings.
  • Regional Heterothermy: Different temperatures maintained in various body regions (e.g., cold limbs).
  • Temporal Heterothermy: Body temperature varies over time (e.g., torpor).
  • Torpor: Involves three phases: entry, maintenance (often supported by BAT), and arousal (via shivering).
  • Hibernation: Extended seasonal torpor lasting weeks or months.
  • Why Small Animals Use Torpor: High surface area to volume (SA/V) ratio leads to rapid heat loss.
  • Migration: An adaptive strategy to “escape in space” from cold stress.
  • Adaptive Mismatch: Seasonal coat color change misaligned with snow cover, resulting in low survival (e.g., snowshoe hare).
  • Grinnell Resurvey (Yosemite): Found that temperatures increased by 4°C; low-elevation species moved upward, and high-elevation species contracted their range.
  • Conservation Strategy: Protecting environmental gradients (elevation and latitude) is crucial to preserve diversity under climate change.

L14 – Desert Adaptations and Water Economy

  • Desert Environment: Characterized by low rainfall, extreme temperatures, and unpredictable food resources.
  • Desert Formation: Occurs primarily at horse latitudes (30°N/S) and due to rain shadows.
  • Primary Challenges: Managing intense heat and dehydration.
  • Rain Shadow Effect: A dry area formed on the leeward side of mountains where moist air is blocked.
  • Relative Medullary Area (RMA): A kidney structural feature that correlates directly with the ability to concentrate urine.
  • Adaptive Hyperthermia: Allowing body temperature (Tb) to rise temporarily to conserve water by avoiding evaporative cooling (sweating/panting).
  • Rete Mirabile (Carotid Rete): A network of arteries and veins that uses countercurrent exchange to cool the brain selectively.
  • Water Economy Index: A ratio indicating an animal’s net water gain or loss; a positive index means a water surplus.

Desert Survival Strategies

  • Evader: Avoids heat by being nocturnal and using burrows (e.g., Dipodomys, kangaroo rat).
  • Evaporator: Uses panting or sweating for cooling (e.g., jackrabbit, coyote).
  • Endurer: Tolerates high heat and dehydration (e.g., Oryx, camel).

Specific Adaptations

  • Dipodomys Adaptations: Long Loops of Henle, high RMA, highly concentrated urine, dry feces; can survive solely on metabolic water from seeds.
  • Kelso Dunes Study: Found that Dipodomys did not use torpor but maintained a flexible diet (insects/vegetation) and a positive water balance.
  • Ammospermophilus (Antelope Squirrel): Diurnal; uses adaptive hyperthermia and cools down periodically in burrows.
  • Oryx Strategy: Allows body temperature to rise during the day (Tb↑) and dumps excess heat nocturnally (Tb↓).

L15 – Mammalian Reproduction I: Anatomy and Cycles

  • Reproductive Mode: Dioecious (separate sexes) with internal fertilization.
  • Male Anatomy: Testes position is variable (scrotal, seasonal, or internal).
  • Spermatogenesis: Continuous process occurring in the seminiferous tubules, supported by Sertoli cells.
  • Accessory Glands: Produce secretions that form seminal fluid and may contribute to copulatory plugs.
  • Copulatory Plug: Hardened seminal material that aids sperm retention and may block rival sperm (e.g., Tgm4 KO mice show fewer litters).
  • Baculum (Os Penis): A bone found in the penis; its presence is often linked to sperm competition intensity.

Female Reproductive Tract Variation

  • Monotremes: Possess a cloaca.
  • Marsupials: Feature paired vaginas and a pseudovaginal canal.
  • Eutherians (Placentals): Tract structure is highly variable, ranging from duplex to simplex.

Uterus Types

  • Duplex, Bipartite, Bicornuate, Simplex.

Oogenesis and Estrous Cycles

  • Oogenesis: Starts during the fetal stage, pauses until puberty, and completes upon fertilization.
  • Estrous Cycle: Cyclic period of female receptivity (Estrus = “heat”).
  • Menstrual vs. Estrous: Primates shed the endometrium; most other mammals reabsorb it.
  • Monoestrous: One cycle per year (e.g., panda, wolf).
  • Polyestrous: Multiple cycles per year (e.g., mouse, horse).
  • Estrus Signals: Olfactory, visual, and acoustic cues (e.g., pandas chirp).

Key Reproductive Hormones

  • FSH (Follicle-Stimulating Hormone): Promotes follicle growth.
  • LH (Luteinizing Hormone): Triggers ovulation and corpus luteum formation.
  • Estrogen: Thickens the uterine lining.
  • Progesterone: Maintains the uterine lining post-ovulation.

Ovulation Types

  • Spontaneous Ovulation: Hormonally timed (e.g., humans).
  • Induced Ovulation: Triggered by mating (e.g., rabbits, cats).