Water Balance in Terrestrial Animals

Recurring themes in physiological ecology Economy: balancing gains and losses water, heat, and energy Effects of body size and shape importance of surface area-to-volume ratio Effects of climate adaptations to extreme conditions Differences among terrestrial animal taxa mammals, birds, reptiles, amphibians, arthropods

Variation in Water Availability Concept 4.3: The water balance of organisms is determined by exchanges of water and solutes with the external environment. Water is the medium in which all biochemical reactions necessary for life occur. Water has unique properties that make it a universal solvent for biologically important solutes.

Water Availability The tendency of water to move down concentration gradients, and the magnitude of those gradients, determine whether an organism tends to lose or gain water from its environment. Must consider an organism’s microclimate in order to understand its water relations.

Variation in Water Availability Maintaining optimal water content is a challenge for freshwater and terrestrial organisms. Ocean waters maintain the water balance of marine organisms. Terrestrial species lose water to a dry atmosphere. Freshwater organisms lose solutes to and gain water from their environment.

Adaptations to environmental conditions can be anatomical, morphological, physiological, or behavioral Anatomy = internal structure of an organism, including size, shape, and position of it organs Morphology = external structure of an organism, including its overall size and shape Physiology = study of the functions of living cells, tissues, and organs

Effect of body size on rate of evaporative water loss Surface area-to-volume ratio increases as body size decreases Evaporative water loss increases as surface area-to-volume ratio increases

Example: compare 1 cm x 1 cm x 1 cm cube with 10 cm x 10 cm x 10 cm cube Large animal 10 1 Small animal 1 1 10 Surface area = 1x1x6 = 6 cm2 Volume = 1x1x1 = 1 cm3 SA:V ratio = 6:1 = 6.0 Surface area = 10x10x6 = 600 cm2 Volume = 10x10x10 = 1000 cm3 SA:V ratio = 600:1000 = 0.6

Sources of Water Gain 1. Free liquid water = found in lakes, rivers, puddles, dewdrops, etc. 2. Preformed water = obtained through food with a high moisture content 3. Metabolic water = produced through breakdown of nutrients in an animal’s diet C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

Water Gain Some insects can extract water vapor from the air. mechanism not fully understood e.g. grasshoppers, lice

Water Regulation on Land - Animals Wia= Wd + Wf + Wa - We - Ws Wia= Animal’s internal water Wd = Drinking Wf = Food Wa = Absorbed by air We = Evaporation Ws = Secretion / Excretion

Water Regulation on Land - Animals

Water Balance Inputs Outputs Cutaneous Drinking Uptake through roots Uptake through body wall (amphibians) Water in food Metabolic water Where does this come from? Outputs Cutaneous leaking through outer surface (skin, exoskeleton) Respiratory panting, breathing, transpiration Excretory Feces Urine Other secretions: sweat, tears, milk

Reducing Water Loss Cutaneous water loss: Stay in cool/moist microclimate; venture out when relative humidity is high. Use relatively impermeable materials on outer surface, such as scales, waxes, etc. Don't sweat Decrease surface area (e.g., small leaves) Reduce surface temperatures of body by shading (hairs/spines, etc.)

Reducing Water Loss Respiratory Water Loss Stay in moist microclimate (e.g., burrow); venture out when relative humidity is high. Kangaroo rats plug the burrow entrance in daytime to reabsorb moisture from their breath. Nasal water condensation: evaporative cooling in the nasal passageways cools exhaled air. Minimize the length of time stomates are open See photosynthesis notes (later)

Reducing Water Loss Excretory Water Loss: Concentrate urine (e.g., The Kangaroo Rat is capable of producing urine twice as concentrated as seawater). Dry feces (e.g., Kangaroo Rat droppings are five times drier than lab rat feces). Produce uric acid rather than urea -- uric acid requires 10x less water than urea to rid the same amount of waste.

Reducing Water Loss Efficient kidneys that shunt water from waste products, resulting in water conservation. Highly concentrated urine or pellets of uric acid Large intestine reabsorbs most water and produces very dry feces.

Amount of water lost through excretion depends on type of excretory product Birds and most reptiles excrete uric acid which doesn’t require much water to eliminate Mammals and most amphibians excrete urea, which requires more water to eliminate Tradeoff: more energy required to produce uric acid than to produce urea

Water balance example: Kangaroo Rat Water gains: Free water 0.0 ml Preformed water 6.0 ml Metabolic water 54.0 ml Total 60.0 ml Water losses: Urine 13.5 ml Feces 2.6 ml Evaporation 43.9 ml Total 60.0 ml

Adaptations for Desert Life Behavioral Adaptations

Drink more water

Burrowing Organisms are exposed to little or no sunlight. Therefore cooling strategies that use water such as sweating, panting, etc. need not be employed, and water is thus conserved. Some species of rodents purposely pant inside the burrows in order to make the water available for a second round of consumption. Use of shady micro-habitats is similar in function to burrows in that they enable an organism to reduce overall exposure to high light and temperature levels.

Gular/tongue fluttering; a cooling method.

Burrowing Owls Desert Tortoise

Shade Microhabitats Desert Horned Lizard Cactus Rat

Timing of Activity Nocturnal or crepuscular (dawn/dusk) activity reduces the amount of exposure to light and heat extremes. Scorpions Aardvark

Body “Acrobatics" Body positions are altered in order to reduce surface contacting the hot surface or to increase surface area where moisture may be collected from morning fogs common to many desert mornings.

Shovel-snouted Lizard (Meroles anchietae)

This beetle (Stenocara sp) is collecting moisture falling from a thorny branch in the Namib Desert. They also collect water from coastal fog. See article link on web page.

Adaptations to desert life Behavioral: Including food items with high preformed water content in diet, when possible Aestivation = seasonal dormancy during hottest and driest time of year

Aestivation

Adaptations for Desert Life Morphological Adaptations

Morphological Adaptations Some desert animals have body parts specially adapted for fat storage (e.G. Camel’s hump, Gila Monster’s tail) Fats when digested yield greatest amount of metabolic water of any nutrient

Morphological Adaptations Water can be stored in the roots, stems, and/or leaves of plants (plants that do this are called succulents).

Morphological Adaptations Long nasal passages Nasal water condensation: evaporative cooling in the nasal passageways cools exhaled air. Warm body temp and hi moisture level in lungs Air cools in nostrils as it leaves and water is reabsorbed Because cooler air holds less water than warmer air, water in the cooled, exhaled air condenses along the nasal passages. The longer, narrower, nasal passages found in many desert animals cools air further and condenses more water.

Sable Antelope

Adaptations for Desert Life Physiological Adaptations

Adaptations to desert life Physiological Kidneys produce highly concentrated urine (long loops of Henle) Large intestine reabsorbs most water and produces very dry feces Exhaled air is cooled in the nasal cavity before leaving the body Some animals can use metabolic water to meet all of their water needs

Tolerate Dehydration Many of the desert plants (e.g., this prickly pear cactus) and animals (e.g., desert toads) can tolerate great losses of water out of their bodies without dying.

Tolerate Dehydration Toleration of temperature changes where the organism's internal temperature rises and falls along with outside temperatures is another adaptation to extreme environmental changes. This conserves water that would be lost with sweating or panting.

Gloger’s rule Animals of a given species tend to be darker in color in humid environments and lighter in arid environments Pacific Northwest merlin (wet climate) Prairie merlin (dry climate)

Marine air-breathing vertebrates Includes groups of mammals, birds, and reptiles, but no amphibians Marine environment presents challenges similar to desert because drinking salt water causes dehydration unless excess salt is eliminated

Adaptations to marine environment Salt glands of reptiles and birds secrete concentrated sodium chloride solution Marine mammals produce concentrated urine and avoid drinking sea water Milk of lactating marine mammals is very concentrated Albatross excretes excess salt through tubes on its bill

Water Balance in Aquatic Organisms Body concentration can be similar to ambient env. Many marine invertebrates use osmosis to equilibrate solute concentrations by moving water. (review isotonic, hypotonic, hypertonic) very efficient - no energy expenditure

Water Balance in Aquatic Organisms Sharks are hyper-osmotic to seawater so water diffuses in by decreasing osmotic gradient b/t body and env, they cut the cost of osmoregulation Bony fish are hypo-osmotic to seawater lose water constantly must drink seawater and eliminate extra salt

Water Balance in Aquatic Organisms Marine Fish and Invertebrates Isomotic organisms do not have to expend energy overcoming osmotic gradient. Sharks, skates, rays - Elevate blood solute concentrations hyperosmotic to seawater. Slowly gain water osmotically. Marine bony fish are strongly hypoosmotic, thus need to drink seawater for salt influx.

Water Balance in Aquatic Organisms Freshwater Fish and Invertebrates Freshwater organisms must minimize water gain impermeable body surfaces - cheap efficient excretory systems - energetically costly e.g. contractile vacuoles of protozoa Hyperosmotic organisms that excrete excess internal water via large amounts of dilute urine. Replace salts by absorbing sodium and chloride at base of gill filaments and by ingesting food.

Contractile vacuole full empty

Genetically Based Differences Identification of Mechanisms of Natural Selection (population-level studies) Breeding Experiments: Are the traits heritable? Selection Experiments: Determine which combination of features results in higher fitness in a specific, experimental environments.

Adaptations to desert life Anatomical Some desert animals have body parts specially adapted for fat storage (e.g. camel’s hump, Gila monster’s tail) Fats when digested yield greatest amount of metabolic water of any nutrient

Dissimilar Organisms with Similar Approaches to Desert Life Camels Can withstand water loss up to 20%. Face into sun to reduce exposure. Thick hair: Increased body temperature lowers heat gradient. Saguaro Cactus Trunk / arms act as water storage organs. Dense network of shallow roots. Reduces evaporative loss.

Two Arthropods with Opposite Approaches to Desert Life Scorpions Slow down, conserve, and stay out of sun. Long-lived Low metabolic rates Cicadas (Diceroprocta apache) Active on hottest days. Perch on branch tips (cooler microclimates). Reduce abdomen temp by feeding on xylem fluid of pinyon pine trees.