Fundamentals of Water Relations and Thermoregulation in Animals

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.

Growth in marine mammals: a review of growth patterns, composition and energy investment

“…The deposition of lean muscle tissue has an asymptotic relationship with increasing energy input, as there is a limit to how much lean mass can be deposited within a given timeframe ( Fuller and Chen, 1997 ). In mammals, metabolized protein is excreted as nitrogenous waste in the form of urea ( Reeds et al, 1980 ; Costa et al, 2013 ), while protein not metabolized can be deposited as structural tissue, and may also be used later as an energy source ( Crocker et al, 1998 ).…”

Chipping in: functional morphology of the American beaver under range expansion

“…(2017). Rodent thermoregulation is partly achieved through their nasal turbinates, therefore, increased surface area of the rostrum, which may be accompanied by a higher complexity of the structure of the turbinates, improves that process (Costa et al., 2013; Stumpp et al., 2018). Both precipitations and temperature may also have indirect effects on beaver skull morphology.…”

Convergent evolution in lemur environmental niches

“…In primates, increases in glycogen content and capillarization of the sweat glands are positively correlated with temperature and negatively related to precipitation, indicating that adaptations in skin physiology are also associated with climatic niches, though data are limited for lemurs (Best & Kamilar, ). Desert rodents have exceptionally long nephridia in their kidneys which allow them to extract water that would otherwise be lost in urine (Costa, Houser, & Crocker, ). Another avenue of future investigation should examine kidney physiology in primates.…”

“…In primates, increases in glycogen content and capillarization of the sweat glands are positively correlated with temperature and negatively related to precipitation, indicating that adaptations in skin physiology are also associated with climatic niches, though data are limited for lemurs (Best & Kamilar, 2018). Desert rodents have exceptionally long nephridia in their kidneys which allow them to extract water that would otherwise be lost in urine (Costa, Houser, & Crocker, 2013) Convergent evolution in physiognomy, physiology and behaviour in response to arid climates is observed in many communities of similarly adapted organisms from disparate parts of the evolutionary tree. The repeated independent evolution of sclerophyllous, shrubby vegetation in Mediterranean-like climates around the world (Mooney & Dunn, 1970), and succulent, water-conserving plants in desert climes (Arakaki et al, 2011) are clear examples of how limitations set by the climate drive evolution of similar physiological and morphological adaptations in distantly related lineages.…”