Thermoregulatory Adaptations to Heat in Mammals

How Desert Animals Maintain Body Temperature in Harsh Climates

May 22, 2009

In harsh climates, desert mammals have evolved to cope with potential water loss and heat stress. Here are some of the ways animals use thermoregulation to deal with heat

The habitat an animal lives in can often be quite harsh. In the desert, heat stress and dehydration are serious issues faced on a daily basis, placing demands and limitations on an animal's functions and development and even causing death. Many desert mammals, ranging from small rodents to Bighorn sheep use thermoregulation as a way of keeping their body temperature at an optimal level.

What is Thermoregulation?

The main purpose of thermoregulation is to maintain body temperature homeostasis. Mammalian body temperature can vary widely between core and the periphery. Mammals have temperature sensitive neurons and nerve endings in the brain, skin, spinal cord and body core. The hypothalamus is considered to be the body’s ‘thermostat’ and receives messages from the thermoreceptors located in the body, which act in a negative feedback system to control the animal’s body temperature.

How do Desert Animals Use Thermoregulation to Cope With Hot Climates?

Rodents living in arid environments are much more efficient in conserving energy and water in comparison to their counterparts in mesic (moderately moist) habitats. Reduced resting metabolic rate and the use of non-shivering thermogenesis are important adaptations in the desert rat. The resting metabolic rate for desert rats at their thermoneutral zone are lower then predicted for their body mass according to allometric equations. Deviations from allometric equations can be due to environmental conditions as well as food quality and quantity.

One desert-adapted species, Acomys cahirinus is nocturnal and loses relatively high amounts of water by evaporation. Its diet includes snails and green plants, both high in water contents, as well as camel and goat feces. A.cahirinus' resting metabolic rate is again lower then the predicted value based on its body mass. When in cool Mediterranean habitats, A.cahirinus can lower its body temperature for several hours, even becoming hypothermic. This hypothermia is a well-regulated daily torpor (a state or inactivity or dormancy), resulting in reduced energy expenditure.

A study comparing desert species Acomys russatus with a mesic species Apodemus mystacinus shows significant differences in daily energy expenditure. The lower resting metabolic rate in desert species, in addition to being crucial to water and energy conservation, it thought to increase their lifespan.

Non-shivering thermogenesis in desert rodents is shown to be high, and believed to be linked to their ability to maintain their low resting metabolic rates for several hours each day.

Bighorn sheep in arid climates also use a number of mechanisms to deal with heat conditions to avoid dehydration and heat stress. The most common being the use of shade during the midday heat, by means of caves or tree canopy. These behavioural adaptations work in conjunction with physiological and morphological mechanisms in maintaining water balance and body temperature.

The amount of time spent in caves or under the tree canopy increases with increasing temperature. Studies have shown that bighorn sheep spend around 7 hours per day bedded in shade during hot periods compared with only 0.75 hours per day in winter. Using shaded microhabitats reduces heat load and helps the sheep maintain a temperature gradient that is conducive for conductive heat loss, thus the need for evaporative cooling in minimized which in turn reduces water loss.

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.

Cain, J.W, Jansen, B.D, Wilson, R.R & Krausman, P.R, 2008, ‘Potential thermoregulatory advantages of shade use by desert bighorn sheep’ Journal of Arid Environments, vol 72, pp. 1518-1525.

Haim, A & Izhaki, I, 1994, ‘Comparative physiology of thermoregulation in rodents: adaptations to arid and mesic environments’, Journal of Arid Environments, vol 31, pp. 431-440.

The copyright of the article Thermoregulatory Adaptations to Heat in Mammals in Anatomy & Physiology is owned by Roberta Goli. Permission to republish Thermoregulatory Adaptations to Heat in Mammals in print or online must be granted by the author in writing.