Animals at the extremes: The desert environment

# 8 Questions

## Question 1

Figure 48 illustrates the activity of the antelope ground squirrel Ammospermophilus leucurus during a typical day in the Nevada desert.

Figure 48 Temperature and activity in the antelope ground squirrel, a small desert mammal. The peaks represent periods of activity on the surface, outside the burrow
Figure 48
Willmer, P. Stone, G. and Johnston, I (2000) Environmental Physiology of Animals. Blackwell Science Limited Willmer, P. Stone, G. and Johnston, I (2000) Environmental Physiology of Animals. Blackwell Science Limited

(a) Describe the pattern of activity suggested by the data.

(b) Explain how the behaviour of the ground squirrel relates to thermoregulation.

(c) Is Ammospermophilus an evader, an evaporator or an endurer?

## Answer

(a) During the night Ammospermophilus remains inside the burrow. At about 06.00 h, the squirrel leaves the burrow and is active, presumably collecting seeds. A short period of activity is followed by a return to the burrow. From 06.00 h to 17.00 h the squirrel left the burrow on nine occasions, and was active for a short period before returning to the burrow. Peaks of activity were recorded up to about 17.00 h when the squirrel retired to the burrow for the night.

(b) During the night T b was fairly stable at 36°C. During bouts of daytime activity, T b rose sharply peaking at 43°C. Returning to the burrow enabled the squirrel to cool off, as T b dipped to 38–39°C after only short periods in the burrow.

(c) Use of the burrow suggests Ammospermophilus is an evader, but tolerance of T b as high as 43°C indicates a degree of endurance too.

## Question 2

Classify the following statements as true or false and write a brief explanation of your answer.

(a) Many small desert mammals are able to endure hot and physiologically stressful ambient temperatures by storing heat for a number of hours.

(b) The coat of camels has an insulating effect in hot environments and camels shorn of their coat are likely to have a higher evaporative water loss.

(c) Measurements of BMR in the hoopoe lark have demonstrated that this desert species always has a lower BMR than mesic lark species, and that low BMR in the hoopoe lark is adaptive.

(d) Panting is a thermoregulatory response shown exclusively by large mammals.

(e) When faced with severe heat stress, mammals resort to autonomic responses, whereas reptiles and birds respond behaviourally.

## Answer

(a) False. Most small animals avoid rather than endure heat stress, but a few species, notably ground squirrels, can tolerate T b up to 42–43°C for short periods, so for them relaxed homeothermy is part of their strategy for maintaining activity during the day. Burrow-dwelling rodents can readily unload small amounts of stored heat by returning to cool burrows periodically.

(b) True. Exposure of bare skin to the heat of solar radiation would promote evaporative water loss. The coat insulates the skin from this intake of heat by reflecting solar radiation and in its absence, water loss is likely to increase. Note that the coat must not be so thick that it impedes the effective vaporization of water on the skin and a relevant point here is that many camels have a thin uneven coat (Figure 37) with thicker fur on the back and sparse fur on the sides.

(c) False. Measured BMR for hoopoe larks acclimated to T a 15°C in the laboratory was at 46.8 kJ day–1, close to that measured for a temperate species, the woodlark, 49.4 kJ day–1. This example illustrates the importance of awareness of phenotypic flexibility and the need to draw comparisons between more than two species in order to prove that a feature of a desert species is adaptive.

(d) False. Larger animals tend to rely more on sweating than on panting but panting is observed in some reptiles and is important in most birds, even those species that live in deserts.

(e) False. Although behavioural responses form the major thermoregulatory strategies of reptiles, these animals have autonomic responses too, including vasoconstriction, vasodilation and panting. Birds and mammals also respond to heat stresses by behaviour, thereby making savings of evaporative water loss and energy. In all three groups, reptiles, birds and mammals, both behavioural and autonomic responses have a role.

## Question 3

The following strategies (a)–(f) are adopted by various desert-dwelling species.

(a) the avoidance of heat by burrowing or moving into shade;

(b) the production of concentrated urine and/or uric acid;

(c) a pelage or feathers providing substantial insulation;

(d) a high-threshold T b for onset of sweating;

(e) heat storage;

(f) radiative or conductive heat loss from extensive surfaces of bare skin.

Which of these strategies do the following animals employ?

1. jack rabbit

2. oryx

3. hoopoe lark

4. desert reptiles

5. kangaroo rat

6. camel

## Answer

1. Jack rabbits use (a), (f) and possibly (b). Insulation (c) would not be effective in a relatively small animal, and the heat storage effect (e) would be useful for a short time only. Sweating (d) is not an option for small species because they have a large surface area to volume ratio and would lose too much water.

2. The oryx uses (a) moving into shade during hot summer days, and (e) heat storage. No information is available about whether the species produces concentrated urine, but this is likely. Oryx do not have a thick pelage so (c) is unlikely and lack of bare skin means (f) is not possible.

3. The hoopoe lark uses (a) moving into burrows on hot summer days, (b) production of uric acid which involves very little water loss, and also (f) losing heat by conduction from bare skin surfaces. Insulation (c) and heat storage (e) are not effective for such a small bird. Birds do not sweat (d), although there is some evaporative cooling by evaporation of water from the skin.

4. Reptiles use (a) and excrete uric acid (b). Options (c) and (d) are not possible in reptiles; (e) is unlikely although T b of desert reptiles, e.g. Dipsosaurus, can reach high levels that would not be tolerated by most mammals. (f) is used but all reptilian skin is bare!

5. Kangaroo rats use (a) and (b). A small animal is unlikely to rely on (c) and (e) and kangaroo rats cannot sweat (d). Kangaroo rats are nocturnal and remain in burrows during the day so (f) is not used.

6. The camel uses (b), (c) (d) and (e). Use of shade when possible (a) is probably important too. The coat of camels makes (f) unlikely although short-coated species have sparse fur on their sides through which bare skin can be seen.

## Question 4

(a) Researchers measured significantly lower whole body BMR in wild-caught individuals of a desert-dwelling rodent, compared to BMR measured in a closely related species living in temperate woodland. Why would a lower BMR be an advantage to a desert-dwelling species?

(b) It is tempting to assume that lower measured values of BMR in desert species represent an evolutionary adaptation. What other processes could result in lower BMR for a desert mammal?

## Answer

(a) If BMR is comparatively low, the amount of heat generated internally by metabolism is decreased which lessens the total heat load in a stressful environment. Water loss is reduced as less is evaporated to cool the body. In a hot, arid environment where food may be in short supply, low BMR would reduce daily energy demand.

(b) Phenotypic plasticity, the process by which individual features of physiology are shaped during development, can cause variation in BMR between individuals. Variations resulting from phenotypic flexibility are usually fixed for life. Phenotypic flexibility or acclimatization, the capacity of an animal to vary physiological parameters in response to environmental stresses, may alter BMR. Williams and Tieleman’s work on hoopoe larks showed that at thermoneutral T a , larks acclimated to T a of 15°C had a significantly lower BMR than individuals acclimated to 36°C (Table 5).

## Question 5

Researchers investigated the hypothesis that the skin of a desert lizard shows certain properties that confer thermal resistance, thereby slowing down the rate of heating of the body when the lizard is in full sun. An experiment was designed in which the percentage of solar heat gain in intact live lizards was compared with heat gain in isolated lizard skin preparations. The researchers found that the percentage solar heat gain in the isolated skin preparations was significantly greater than that in the live animals. They concluded that the skin in live animals has physiological and physical properties that confer resistance to solar heat gain.

Write a critique of this experiment and suggest an experimental design that would improve the validity of the results.

## Answer

The problem with the experimental design is that the researchers are comparing solar heat gain in two completely different situations. An isolated skin preparation has a huge surface area to volume ratio and would inevitably warm up very quickly in the sun (and dry up). In the intact live animal, the skin is part of a whole larger body. Therefore, the physical properties of the isolated skin are not the same as those for skin in a live animal. It is inevitable that isolated skin would not have any physiological properties; in contrast, in the whole animal, a number of physiological processes could come into play to reduce solar heat gain. Vasoconstriction would slow transfer of solar heat to the rest of the body. The skin may become paler in colour, which also slows down heat gain.

A better design would involve using either a model of the animal or a mounted preserved specimen of the same size as the live specimen. Thermistors just below the ‘skin’ and in the deep body of the model or preserved specimen, and the intact animal, can be used to monitor heat gain just below the skin and in the body core.

## Question 6

Williams et al. measured BMR and FMR in oryx living in the Mahazat as-Sayd nature reserve in the Arabian desert. The results are summarised in Section 3.4. The researchers then compiled data from the literature for minimum resting metabolic rate for 15 species grouped in the Artiodactyla, ranging from the mouse deer with body mass just 1.61 kg and dik-dik at 43.97 kg to moose weighing 325 kg and camel 407 kg. Values for whole body BMR of each species were plotted against body mass in kg (Figure 49).

Figure 49 Minimum rate of oxygen consumption of Artiodactyla. Red squares represent the oryx (84.1 kg) and the camel (407 kg)
Figure 49
Williams, J.B. et al. (2001) Seasonal variation in energy expenditure, water flux and flood consumption of Arabian oryx, Oryx Leucoryx, Journal of Experimental Biology, 204. Copyright © Company of Biologists Ltd Williams, J.B. et al. (2001) Seasonal variation in energy expenditure, water flux and flood consumption of Arabian oryx, Oryx Leucoryx, Journal of Experimental Biology, 204. Copyright © Company of Biologists Ltd

Describe the data in Figure 49. Do the data for oryx and camel suggest that the desert species have a lower BMR than the other 13 species? Note that the graph is a logarithmic plot, in which the values on the axes increase by factors of 10. You should interpret this graph as you would for any plot.

## Answer

BMR increases linearly with body mass for the 15 species. An artiodactyl weighing 100 kg has a BMR of about 25 litres O2 h–1. The values of BMR and body mass of the oryx are almost on the plotted line suggesting that oryx do not have an unusually low BMR. The camel is a little below the plotted line, suggesting a trend for reduced BMR in this species.

Now that you have completed this course, you may be interested in the companion course Animals at the extreme: hibernation and torpor (S324_2), which builds on and develops some of the science you have studied here.

OpenLearn - Animals at the extremes: The desert environment
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