Terrestrial Animals PDF

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 ta...

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 o Gastropods, arthropods (insects, arachnids, myriapods, crustaceans) nematodes, annelids, amniote vertebrates, etc o Some of these taxa encompass many species – terrestrial environments provided many evolutionary opportunities Velvet worm o Onychophora – the only completely terrestrial animal phylum Conditions on Land: Terrestrial environment makes different demands from the aquatic environment Main factors affect terrestrial animals o Gravity – maintenance of posture, locomotion o Air – obtaining oxygen, much less dense than water o Locomotion, sensory modes, and thermal properties o Water – always a limiting resource o Sunlight – exposure to UV Life on Land: Some requirements for a terrestrial o Desiccation avoidance ▪ Mechanisms – reduce water loss, replace lost water ▪ Tolerance (aestivation, life cycles) o Excretion with limited water loss o 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 o Across we respiratory membrane o Across surface of skin Water loss in urine and feces Some species lose water through thermoregulatory methods (sweating, panting) Requires  o Waterproofing of outer layer of body (keratin, wax) o 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: o Desert adapted rodents with very long loop of Henle o Produces small quantity of highly hyperosmotic urine o 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) o Adult female (2N) → Diploid (2N) egg Stressed Environment (Dry):  Sexual Reproduction (Genetic Variability): o Adult male (1N) → Haploid (1N) sperm o Haploid (1N) egg → Diploid (2N) zygote (desiccation-resistant) Aestivation Desert-dwelling spadefoot toads spend most of their adult lives buried deeply: o Metabolism depressed. o Only emerge when it rains to breed. Some desert-dwelling frogs: o Secrete a cocoon while aestivating – only nostrils left open. o Metabolism depressed. o Can spend up to 2 years buried. More Requirements for a Terrestrial Life Desiccation Avoidance: o Mechanisms to reduce water loss and replace lost water. Desiccation Tolerance: o Aestivation and life cycles. Excretion with Limited Water Loss: o 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: o Aquatic larvae. o Thick covering on eggs/embryos. o Amniote vertebrates (birds, reptiles, mammals): Amniotic membrane. Temperature extremes: o Avoid: Thermoregulation. o Tolerate. Constraints on sensory systems: o Chemosensors. o Mechanosensors (e.g., tympanal organ, vertebrate middle ear). Support body weight: o Robust skeleton. o SA/V relationships, size, stance. Amphibians – Reproduction in Water In vertebrates: o Amphibians lay anamniotic eggs in the water. o Embryos can exchange gases and wastes with the aquatic environment. o 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. o Requires internal fertilization. o Uricotely: Semi-solid nitrogenous wastes to conserve water. o Extraembryonic membranes support the developing embryo. o Shell porous to air and possibly to water. Reasons for Thermoregulation Most terrestrial animals regulate their body temperature when possible: o Through metabolic activity. o Through behavior. Animal body temperature range: ~4°C to 40°C. Why Expend Energy on Thermoregulation? Formation of ice crystals (