Fundamentals of Water Relations and Thermoregulation in Animals


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.

Keywords: osmoregulation; metabolism; electrolytes; nitrogenous wastes; evaporative water loss; urea; uric acid; kidney; salt gland

Figure 1.

A simple schematic of the primary processes important to maintain cellular homeostasis.

Figure 2.

The avenues of water intake and loss and electrolyte intake and loss are shown for freshwater and marine teleost fish.

Figure 3.

Schematic of where water and electrolytes enter and leave the body for a marine mammal and in this case excretion of electrolyte and water as milk only occurs when females are lactating.

Figure 4.

Skull of a California sea lion with an exploded view showing the nasal turbinates. Photo by J. Hiatt.

Figure 5.

Nitrogen as a waste product is excreted in three different chemical forms depending on the organism.

Figure 6.

The overall shape of mammalian kidneys varies with marine mammals showing the greatest development of a reniculate or lobulate kidney. Adapted from Slijper .

Figure 7.

Diagram of a mammalian kidney. A single nephron is shown. Blood passes through a capillary network called the glomerulus and an ultrafiltrate passes into the Malpighian body, active transport occurs in the tubules and within the loop of Henle; a concentration gradient is created that allows resorption of water as the filtrate passes through the collecting duct. Reprinted with permission from Schmidt‐Nielsen . Copyright 1990 Cambridge University Press.

Figure 8.

Cape ground squirrels are active during the day, periodically retreating to their burrows to cool down. Their tail is used as a parasol helping to shade the body from the sun. Note the position of the shadow, indicating that the animal is facing with its back towards the sun. Photo by Dan Costa.

Figure 9.

Large mammals like this African black‐backed jackal find shade under trees and shrubs during the hottest part of the day. This saves water by reducing the need to use evaporative cooling. Photo by Dan Costa.



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Costa, Daniel P, Houser, Dorian S, and Crocker, Daniel E(Nov 2013) Fundamentals of Water Relations and Thermoregulation in Animals. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003216.pub2]