Fundamentals of Water Relations and Thermoregulation in Animals

Abstract

Abstract Animals must maintain their internal environment relatively constant with respect to water, electrolytes and temperature. The process by which they regulate the concentration of their internal water and solute concentrations is called osmoregulation. Osmoregulation is accomplished by physiological, anatomical and behavioural adaptations. These include adaptations that modify the permeability of external surfaces and the development of specialised structures that actively transport solutes and metabolites. Animals face the greatest osmoregulatory challenge in terrestrial environments because concentration gradients there are the greatest. As evaporative cooling is used for thermoregulation in terrestrial environments, animals are often faced with the dilemma of maintaining either water or thermal balance, but not both. Key Concepts: Water and solute balance is maintained by ensuring that the rate of water and solute loss is equivalent to the rate of water and solute coming into the animal. Water and electrolytes can enter or leave the body by exchange across the skin and respiratory surfaces and can be gained through ingestion of food and lost through excretion (faeces or urine). Metabolic processes produce both water and nitrogenous waste products. Nitrogenous wastes are excreted through specialised structures. Desert animals have special adaptations to reduce water loss, which include allowing body temperatures to drift, avoidance of warm temperatures by burrowing or nocturnal behaviour. Aquatic animals primarily excrete their nitrogenous wastes as ammonia, whereas terrestrial animals where water is less available excrete either urea (mammals and amphibians) or uric acid (birds and lizards). Elasmobranchs (sharks and rays) retain urea in their body fluids to maintain a beneficial osmotic gradient. The toxic effects of elevated concentrations of urea are alleviated by the presence of TMAO. Terrestrial animals reduce permeability of their skin by adding layers of lipid, keratin or chitin. Arthropods have the lowest rate of cutaneous water loss. Marine lizards and birds have evolved specialised glands that allow for the excretion of excess salt, whereas mammals have evolved kidneys that can excrete highly concentrated urine. Some animals employ behaviour to find microclimates that allows them to exist within extreme environments, thus allowing them to exist there in the absence of physiological or anatomical adaptations.