Bulletin 11 July 1980: The Adaptation of Desert Animals to Their Environment - Some Examples
The Adaptation of Desert Animals to Their Environment - Some Examples
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The Adaptation of Desert Animals to Their Environment - Some Examples

by Chris Furley

(Ed. Note: This talk was presented to the ENHG on 19 May. Chris Furley is Veterinarian at Al Ain Zoo.)

Introduction

Desert animals live in an environment that can be very harsh. Major problems include an average 10 to 12 hours hot sunshine a day, a scarcity of water and often of food, and predators. Animals respond to such conditions, however, and have evolved mechanisms to deal with various extremes. Such evolved devices are not found in related species, such as deer, living in more temperate climates.

Obvious ways of coping with heat include seeking shade and burrowing. Red and fennec foxes, and most reptiles, burrow but local hares do not, preferring to rest up under a bush or in rock or sandstone crevice during the day. Some animals may also migrate seasonally.

Evolved mechanisms are largely physiological. To survive in a very arid environment animals must be able to regulate (1) body temperature, (2) water loss, and (3) digestion.

The core of the problem is the ability to cope with very high ambient temperatures. Desert animals, particularly mammals, are able to reduce their metabolic rate to the minimum necessary for functioning efficiently. Given a choice, violent activity is rare, so as not to raise the basic metabolic rate. This is a problem with pregnant animals, where the metabolic rate is necessarily higher, but evolution has brought about adaptation in this area, too. The normal body temperature for larger desert animals, such as the camel and larger oryx, is around 97'F; for smaller animals, such as gazelle, it may be around 101'F.

External sources of heat are conduction (direct heat gained from actual contact with the ground, as when lying down), convection (heat brought by wind and air movements, which can often raise desert temperatures dramatically), and radiation (heat transmitted both from the sun downwards, and reflected from the ground surface upwards into the animals' undersides).

Water balance depends on the relationship between loss and gain. Prime causes of water loss are sweating, evaporation via the lungs, and infection. Maintaining the correct water balance is a constant problem in desert areas for most of the year but animals have adapted to certain eating patterns to counter this. Such eating habits are also important in helping to regulate digestion.

Type Samples

1. Desert Wolf

This animal is a true hunter, constantly on the move tracking and killing prey as well as seeking out carrion. It has the stamina to cover long distances, but also has to cope with heat gain and water loss. It is brown in color and generally a nocturnal animal, holing up during the day in a burrow. The true desert wolf looks very lean but in fact to survive it must carry no excess fat; it is honed down to optimum weight and physique by constant exercise and the ceaseless effort of seeking prey. It travels in small packs.

One unusual feature, in common with foxes, is that the wolf produces two or three cubs annually per litter. Most desert animals produce only single young, to increase the survival chances of both offspring and mother.

Since the wolf, like dogs and foxes, possesses no sweat glands, it helps to control body temperature by evaporation from the lungs by rapid panting.

2. Eland

This animal is not strictly a desert type but has adapted to arid conditions. It regulates body temperature by increasing its respiration rate from a normal 10 to over 70 breaths per minute in a very short time (barely two minutes). It is important to control the body temperature because, in common with all mammals, the brain must receive blood at a constant temperature, without sudden fluctuations. In some animals an increase of 3 - 5 'F at the brain can easily bring about heatstroke. The eland permits its body temperature at the brain rarely exceeds 4'F above normal. This is because the carotid artery takes blood destined for the brain to the nasal area first. Rapid panting serves to cool the blood in the nose area. This is a major physiological adaptation.

The eland regulates water balance by lung evaporation. It only requires approximately four liters per 110 kg body weight per day compared to 25 liters at the same ratio for cows. Of course, water intake becomes more vital as the aridity increases. The major water source in deserts is not rainfall or oasis pools, but the vegetation. Succulent plants in particular are capable of retaining a lot of moisture, and there is the bonus for herbivores of high dewfall levels at dawn in desert environments. Experiments have shown that a plant's moisture content can be up to 30% higher by night than by day, and this is the prime reason for nocturnal grazing. Animals such as the eland also possess an efficient nitrogen recycling mechanism that prevents excess moisture loss through urea.

The eland is capable of focussing on its food when eating while also being able to detect any movement on a horizontal plane ahead and on either side. Such a device enables it to be aware of the early approach of a would-be predator.

3. Arabian Dorcas Gazelle

Whereas large animals tend to congregate in small groups, smaller species such as gazelle are frequently found in herds of up to 40 or more. There is usually a dominant bull, with other males lower down the hierarchical scale straggling on the fringes of the main herd which consists typically of females and young.

The Arabian Dorcas gazelle is all white underneath to reflect heat radiated upwards from the ground surface. It is brown on top for desert camouflage. Its hair is short but dense enough to act as a shield against direct heat.

4. Arabian Gazelle

This animal may be distinguished from the previous gazelle by its black horizontal stripe along the body. Its hooves are small and hard, not splayed out; thus it is not well suited to an all-sand terrain.

This gazelle has a fatty layer beneath the skin to prevent not heat loss, but heat gain. It should be noted that animals like gazelle have a large surface area in proportion to body weight; hence the need for such a fatty layer as an extra shield besides the body coat. Arctic animals, by comparison, usually carry extra poundage to compensate.

Pregnancy places great demands on would-be mothers, and gazelles produce single offspring at two-year intervals, not annually. This helps the overall survival rate of the herd, which tends to frequent localities where good and abundant grazing is not easily come by. If necessary, an imminent birth can be delayed by up to 12 hours.

5. Fringe-Eared Oryx

The water intake requirement for this oryx is 80% less than that for a normal cow. That is to say, the loss rate is much less. This oryx assimilates some seven or eight liters per day compared to the domestic cow's 25 or so.

The coat is overall brown, indicating that the fringe-eared oryx is not a true desert animal. However, the hide is still light enough to reflect some 20% of direct heat.

6. Scimitar-Horned Oryx

This animal is white in color and well adapted. It is native to Chad and Central Africa. Its coat is capable of reflecting some 40% of direct heat, and there also exists the fatty layer shield. The hair is short to keep the coat clean and reasonably free of parasites, in order to maintain the efficiency of the reflection mechanism.

This oryx can go for nine months without drinking partly due to its efficient nitrogen recycling system. The energy it can absorb from given plants is much higher than that absorbed by goats from the same vegetation.

It is a ruminant, like all herbivores being unable to digest cellulose before it is sufficiently broken down by bacteria in the stomach.

Body temperature may be raised by up to 6'F without physiological precautions being necessary. If the body temperature goes beyond this level, the sweating mechanism is activated. In contrast to most desert animals, the body temperature of the scimitar-horned oryx can go higher than the ambient temperature around it.

7. Addax

This North African gazelle is small with an all-white coat and is well suited to an arid environment. By day it spends much time lying or sitting in any available shade thus effectively reducing direct heat-load by as much as two-thirds. Radiated heat is also considerably reduced by keeping to shade. The hair of this animal is longer than that of some gazelles, but combined with its color, it can reflect up to 60% of direct heat.

8. Arabian Oryx

The Arabian oryx is the specialist of the local desert environment. It has a white coat consisting of dense, longish hair. The hooves are wider than those of other similar animals, enabling it to take to the sands. Under the coat is a very thick fatty layer, and in between the skin is black which cuts down the absorption of ultraviolet rays. Brain temperature is well regulated and the kidneys are supremely efficient.

In the desert ecosystem, a group hierarchy system reduces stress in individuals. Eight or 10 is the usual number for a group of Arabian oryx in the wild, with a maximum of five or six groups in any given locality. Recently almost extinct, this oryx has been successfully bred in captivity and efforts are being made to reintroduce it to its original habitats. Apart from man, predators are not a problem.

The Arabian oryx is a shade seeker. It must regulate its body temperature to within 3'F of normal. It tends to feed at dusk because digestion is more effective during the cooler nocturnal hours. It can smell water miles away and make towards it over any kind of terrain. Unlike most herbivores, it can detect succulent root tubers up to half a meter deep in the ground and root them out. When disturbed it has no qualm about heading directly into areas like the Empty Quarter. The calves are brown and tend to crouch and hide while the adults flee from predators, to return later. However, within a fortnight of birth, the young are essentially prepared for long distance travel with the group.

(All of these animals are represented at the Al Ain Zoo. Other animals, such as the camel, are also well adapted to desert environments, but the adaptations are specific to one species. The physiological changes mentioned above occur in types, and not only in the species described in this article.)





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