Theme 3 Animals PDF

**Theme 3 (animals)** **Terrestrial Animals:** - Animals originated in the oceans - Terrestrial (land-dwelling) animal lineages are descended from ancestors that independently left the aquatic environment to live on land - Earliest recorded one in 428 My old -- a myriapod -- but fi...

**Theme 3 (animals)** **Terrestrial Animals:** - Animals originated in the oceans - Terrestrial (land-dwelling) animal lineages are descended from ancestors that independently left the aquatic environment to live on land - Earliest recorded one in 428 My old -- a myriapod -- but first transition must have taken place earlier - Relatively few taxa - Gastropods, arthropods (insects, arachnids, myriapods, crustaceans) nematodes, annelids, amniote vertebrates, etc - Some of these taxa encompass many species -- terrestrial environments provided many evolutionary opportunities - Velvet worm - Onychophora -- the only completely terrestrial animal phylum **Conditions on Land:** - Terrestrial environment makes different demands from the aquatic environment - Main factors affect terrestrial animals - Gravity -- maintenance of posture, locomotion - Air -- obtaining oxygen, much less dense than water - Locomotion, sensory modes, and thermal properties - Water -- always a limiting resource - Sunlight -- exposure to UV **Life on Land:** - Some requirements for a terrestrial - Desiccation avoidance - Mechanisms -- reduce water loss, replace lost water - Tolerance (aestivation, life cycles) - Excretion with limited water loss - Gas exchange with air - Internalized gas exchange organs to avoid desiccation (lungs, book lungs, tracheal systems) **Desiccation and The Environment -- Terrestrial Animals: Part I** - Terrestrial animals are all, to a greater or lesser extent, constrained by the availability of water - This has wide-reaching effects upon their anatomy, behaviour, and physiology - Constant water loss through evaporation - Across we respiratory membrane - Across surface of skin - Water loss in urine and feces - Some species lose water through thermoregulatory methods (sweating, panting) - Requires - Waterproofing of outer layer of body (keratin, wax) - Minimal exposure of gas-exchange and digestive surfaces to air (internal placement) **Nitrogenous Wastes:** - Toxic ammonia produced in every cell of the body by catabolism of amino acids and nucleic acids - Reptiles, birds, and insects convert ammonia to uric acid -- very low solubility, semi-solid nitrogenous wastes can be excreted while conserving water **Desiccation and The Environment -- Terrestrial Animals: Part II** - Mammals are ureotelic -- convert ammonia to less toxic urea, but must lose water in excretion - Loop of Henle aids in conservation of water in mammals - Produces concentrated urine -- hyperosmotic to blood - The longer the loop, the better for water conservation - Kangaroo rats: - Desert adapted rodents with very long loop of Henle - Produces small quantity of highly hyperosmotic urine - Metabolic water is very important **Desiccation and The Environment -- Terrestrial Animals: Insects** - Must deal with small size (cube-square relationship favors desiccation by evaporative water loss from the body surface) and inevitable evaporative loss from wet respiratory surfaces in **trachea.** - Waxy outer layer of **cuticle** minimizes evaporative water loss from the body surface. - Spiracles permit closing of the **tracheal system**, cutting down on evaporative water loss. ### **Desiccation Tolerance** - ### **Terrestrial tardigrades** live in water films in damp environments. - **Cryptobiosis:** Formation of a resistant stage (tun) in response to environmental challenges (dehydration, sub-zero temperatures). - **Anhydrobiosis:** When slowly desiccated, a resistant tun is formed -- when rehydrated, the tardigrade returns to its active state. ### **Rotifer Life Cycle** ### Unstressed Environment (Moist): - ### Parthenogenesis (Clones) - Adult female (2N) → Diploid (2N) egg Stressed Environment (Dry): - Sexual Reproduction (Genetic Variability): - Adult male (1N) → Haploid (1N) sperm - Haploid (1N) egg → Diploid (2N) zygote (desiccation-resistant) ### **Aestivation** - Desert-dwelling **spadefoot toads** spend most of their adult lives buried deeply: - **Metabolism depressed.** - Only emerge when it rains to breed. - Some desert-dwelling frogs: - Secrete a **cocoon** while aestivating -- only nostrils left open. - Metabolism depressed. - Can spend up to 2 years buried. ### **More Requirements for a Terrestrial Life** - **Desiccation Avoidance:** - Mechanisms to reduce water loss and replace lost water. - **Desiccation Tolerance:** - Aestivation and life cycles. - **Excretion with Limited Water Loss:** - Internal bulk flow of fluids and gases. **Gas Exchange with Air:** - Internalized gas exchange organs to avoid desiccation (lungs, book lungs, tracheal systems). ### **Gas Exchange with Air** Disadvantages of breathing air: - CO₂ does not diffuse into air as easily as into water. - Inevitable **evaporative water loss** from the internal respiratory surface, which must remain wet. Advantages of breathing air: - 21% O₂ -- much greater than water. - Atmospheric O₂ diffuses much more rapidly. - **Bulk flow of air (ventilation):** Requires less muscular effort. ### **Gas Exchange with Air - Trachea** - **Insect tracheal system:** Delivers air directly to tissues via interstitial fluid. - Moist exchange surfaces are internal. ### **Gas Exchange with Air - Vertebrate Lungs** - Vertebrate lungs use **bulk flow of air** to the respiratory membrane. - Moist exchange surfaces are internal. - Requires muscular effort (**ventilation**). ### **More Requirements for a Terrestrial Life** - **Protect gametes from desiccation:** Fertilization without water (internal). - **Protect embryos from desiccation:** - Aquatic larvae. - Thick covering on eggs/embryos. - Amniote vertebrates (birds, reptiles, mammals): Amniotic membrane. - **Temperature extremes:** - Avoid: Thermoregulation. - Tolerate. - **Constraints on sensory systems:** - Chemosensors. - Mechanosensors (e.g., tympanal organ, vertebrate middle ear). - **Support body weight:** - Robust skeleton. - SA/V relationships, size, stance. ### **Amphibians -- Reproduction in Water** - In vertebrates: - Amphibians lay **anamniotic eggs** in the water. - Embryos can exchange gases and wastes with the aquatic environment. - Adult form can live on land. ### **Reproduction on Land -- The Amniotic Egg** - In vertebrates, the **amniotic egg** provides an aqueous environment for the developing embryo. - Requires **internal fertilization.** - **Uricotely:** Semi-solid nitrogenous wastes to conserve water. - **Extraembryonic membranes** support the developing embryo. - Shell porous to air and possibly to water. ### **Reasons for Thermoregulation** - Most terrestrial animals **regulate** their body temperature when possible: - Through **metabolic activity.** - Through **behavior.** - Animal body temperature range: \~4°C to 40°C. ### **Why Expend Energy on Thermoregulation?** - Formation of ice crystals (\ - **Heterothermy:** Allowing body temperature to vary. - **Homeothermy:** Tightly regulating body temperature around an unvarying mean. - These define two axes across which thermoregulatory strategies form a continuum. - **Endotherms:** Metabolic rate changes with temperature to maintain a constant body temperature (energetic cost). - **Ectotherms:** Metabolic rate changes directly with body temperature, which changes with environmental temperature (potential liability -- loss of performance). ### **Heat Exchange with the Environment** - **Conduction:** Direct heat transfer by contact. - Air conducts heat poorly; water conducts heat well. - Gill-breathing aquatic organisms tend to be **isothermic** with the water they swim in. - **Radiation:** Transfer of heat as long-wave light. - Effective for heating up (e.g., basking in the sun). - **Convection:** Transfer of heat by a moving medium (air or water flowing over an organism). - **Evaporation:** Energy consumed by the change from liquid to gas; effective for heat removal. ### **Countercurrent Heat Exchange** - Cold-climate terrestrial endotherms conserve heat through **countercurrent heat exchange**: - Warm blood in efferent vessels heats cool blood in afferent vessels. - Regional heterothermy is facilitated by this mechanism. ### **Torpor and Hibernation** - **Torpor:** Reduces energy demands in small endotherms during periods of low/high environmental temperatures or resource unavailability. - Body temperature set point drops. - Metabolism depressed. - **Hibernation:** A seasonal version of torpor, undertaken during periods of low temperature. ### **Endothermy in Insects** - Bees and some other flying insects are **heterothermic endotherms.** - Generate sufficient heat through the action of flight muscles to maintain a high constant temperature in the thorax. - Tend to be furry. ### **Freeze Tolerance and Freeze Avoidance** - **Freeze Avoidance:** - Some ectotherms can supercool their extracellular fluid (ECF) below 0°C without freezing (mainly marine). - **Freeze Tolerance:** - Some terrestrial ectotherms allow the bulk of their ECF to freeze for extended periods. - High intracellular osmolality depresses the freezing point. - Control of ice nucleation in ECF. ### **Support Body Weight** - Terrestrial animals are subject to gravity: - Volume (mass) of a terrestrial organism is a function of (linear dimension)³. - Cross-sectional area of the limb (support and strength) is a function of (linear dimension)². - Implications: - As animals get larger, body mass increases faster than the cross-sectional area of the limbs for support. - Limbs must grow disproportionately larger as animals increase in size (**allometric growth**). ### **Support Body Weight -- Allometry** - **Allometry:** Characteristic of most animals. - Different parts of the body grow at different rates with an increase in overall size. - Also an evolutionary phenomenon associated with trends in increasing or decreasing body size in a lineage. ### **Allometric Relationships -- Example** - The **ant** exhibits isometric growth in the diagram. - In the real world, limbs of larger specimens would not be of sufficient thickness for efficient locomotion. - Limbs must grow **allometrically** to support increasing body weight with size. ### **Skeleton Types in Aquatic and Terrestrial Animals** Two types of hard skeletons: 1. **Exoskeletons (external)** 2. **Endoskeletons (internal)** **Functions of Skeletons:** - Provide attachment points and leverage for muscles (force transmission). - Transmit compressive stress to the substrate. - Provide a framework for tissues of the body. - Act as a **mineral bank** (vertebrates). - Protection for delicate organs or the whole body. ### **Endoskeletons (Terrestrial and Aquatic)** - Terrestrial Endoskeletons: - Firmly attached girdles. - Enclosed ribcages. - Aquatic Endoskeletons: - Loosely attached girdles. - Ribcages not enclosed. ### **Hard Skeletons -- Endoskeletons** - Bone: - Collagenous matrix mineralized by calcium phosphate (CaPO₄) crystals. - Highly vascularized; matrix architecture supports scattered osteocytes. - Metabolically active. - Bears compressive stress well but shear stress poorly. - The arthropod exoskeleton: - Consists of **chitin** -- a complex polysaccharide. - May be impregnated with calcium carbonate. - Composed of plates (**tergae**) with joints between them. - Limb joints are mobile. - Muscles are located within the skeleton. ### **Hydrostatic Skeletons** - Volume of fluid enclosed by a muscular wall. - Fluid is incompressible but pressurized when muscles contract. - The shape changes with contraction of different muscle layers (longitudinal and circular). - Found in organs or the whole body of some animals. ### **Aquatic Animals** - Water supports the body, reducing the need for structural adaptations for weight. - Affects size, stance, and skeleton. - Desiccation is less of a threat. - Stable and mild temperatures in water. - Metabolic waste can be removed efficiently. - Sound transmits well from water to body. ### **Challenges of Living in Aquatic Environments** - **Water is dense:** Requires energy to displace. - **Water is viscous:** A layer clings to the body, resisting movement. - **Low oxygen content:** \~1--2% compared to \~21% in air. - **High thermal conductance:** 25x that of air, requiring aquatic animals to conserve or generate heat. ### **Salt and Water Balance** - Excess salt in the diet is a problem for marine tetrapods. - **Marine birds and reptiles** excrete excess salt through specialized **salt glands** near the upper respiratory tract or eyes. - **Marine mammals:** Produce highly concentrated, **hyperosmotic urine.** ### **Being Warm in Aquatic Environments** - Water is a good heat conductor, so aquatic organisms are mostly **heterothermic ectotherms.** - **Aquatic homeothermic endotherms:** Insulate with blubber or waterproof pelage. - **Respiratory Medium:** Breathing air allows a higher metabolic rate, and air is a poor conductor of heat compared to water. ### **Aquatic Endotherms -- Countercurrent Heat Exchange** - Retains heat through **countercurrent heat exchange,** allowing outbound blood to heat inbound blood along the length of an organ.