Osmoregularity Adaptations to Heat in Mammals
Temperature Has One of the Greatest Influences On Animal Metabolism
May 22, 2009
Avoiding the daytime heat, producing dry fecal pellets, concentrating urine to reduce water loss as well as keeping their daily energy expenditure low are some of the mechanical and behavioural adaptations desert animals have to cope with harsh environments.
What is Osmoregulation?
Temperature has one of the greatest influences on animal metabolism and energetics. Low body temperature requires a higher metabolic rate due to the temperature dependence of enzymatic reactions and the high metabolic rates can result in overheating due to high rates of heat production. It is also important for desert mammals to minimize water loss to maintain ionic and osmotic balance.
How do Desert Animals Use Osmoregulation to Cope With Hot Climates?
Desert mammals not only need to limit heat exposure, but also water loss, particularly when water may not be readily available in their environment. Evaporative cooling is an important way of excreting excess heat from the body. Due to the fact that desert mammals need to conserve water, they have adapted a way of cooling their bodies without losing too much water.
One example is the kangaroo rat. To minimize water loss and excess heat, the rat remains in a burrow during the day and comes out at night when the temperature is cooler. The temperature of the burrow needs to be significantly lower then the core temperature of the animal for the nasal countercurrent mechanism to conserve respiratory moisture. The cooling properties of the nasal epithelium are reduced if the animal leaves the cool burrow to go into the warmer temperature outside, and this increases its water loss.
Another adaptation in the kangaroo rat are its efficient kidneys. It excretes highly concentrated urine resulting in a dry fecal pellet due to rectal absorption of water. Water is gained through the dry seeds that form its diet.
Small desert mammals can use different strategies to deal with periods of food shortage or limited water supply. The desert mouse opossum (Thylamys pusilla), which occurs in temperate drylands of the neotropics in Argentina, Paraguay and Bolivia, is nocturnal and insectivorous (its diet also includes fruit). It has the ability to store fat and shows torpor. As with many desert mammals, it does not rely on drinking water as its only source of water and has an efficient urine system.
T. pusilla is able to maintain its body weight on a diet containing approximately 70% water and their urine concentration was found to be independent of drinking water availability as long as they had access to food with high water content (such as fruit). The kidney morphology of the T. pusilla is similar to that of the Australian desert marsupials Ningaui ridei and Pseudantechinus macdonnellensis.
For other animals such as the camel, which is too large to retreat to a burrow when temperatures get too high, has adapted other methods to retain water. Camels don’t sweat and therefore don't lose water by excessive sweating like other animals can. They also allow their body temperature to rise during the day, then during the night, when it is cooler, their body temperature drops. Due to the camels large size, its temperature only rises again slowly the next day. Camels also have a thick fur, which acts as a heat shield.
Large desert mammals like the camel, orient their bodies in a way that minimizes surface exposure to sunlight and limbs of large animals are highly vascularised and have large surface area to aid in heat dissipation.
You may also be interested in reading Thermoregulatory Adaptations to Heat In Mammals and Digestive And Metabolic Adaptations to Heat.
Resources and further reading:
Bozinovic, F & Gallardo, P, 2006, ‘The water economy of South American desert rodents: From integrative to molecular physiological ecology’ Journal of Comparative Biochemistry and Physiology, part C, vol. 142, pp. 163-172.
Diaz, G.B, Richardo, A.O & Dacar, M, 2001, ‘Water conservation in the South American desert mouse opossum, Thylamys pusilla (Didelphimorphia, Didelphidae), Journal of Comparative Biochemistry and Physiology, part A, vol. 130, pp. 323-330.
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