Solar - radiation and colour pattern in desert animals. 



In the bare and open desert landscape the intensity of solar- radiation, to which 

 every diurnal animal is exposed is high even if it is often less than on bare moun- 

 tains. For those animals that are active between late morning and late afternoon, 

 this radiation is in summer of the highest ecological importance. It is surprising to 

 find therefore, that no measurements are available on the transmission through the 

 integrement of rays from ultra-violet to infra-red. 



Three types of colour patterns are apparent in desert animals:- 



(1) Mammals and birds show prevalently buff, sandy, pale grey or spotted 

 colours and remain hidden during the day; or when they are diurnal, their chief ene- 

 mies are nocturnal, Buxton, Heim de Balzac and Morrison- Scott have thoroughly 

 destroyed the legend that this type of colouration is primarily protective*. We have 

 to be satisfied with the statement that this 'adaptive' colouration is primarily a phy- 

 siological effect of dry heat on the development of pigments. 



(2) Many Orthoptera in particular, show a very intimate and complicated adapta- 

 tion to the colour of the soil on which they live, imitating the pattern of the pebbles 

 in their environment. Resting Eremiaphila and most Acrididae are usually not to be 

 discerned even by a searching eye. They appear very conspicuous however immedia- 

 tely they fly. Here we obviously have some kind of appreciation of the colour of the 

 environment immediately after the last moult via the eyes, the central nervous sys- 

 tem and endocrine mechanisms (probably connected with the cardiac glands). A 

 similar surprisingly close colour adaptation of the feathers exists in our common 

 desert larks {Ammomanes spp.), as well as in some other desert birds. 



(3) The high proportion of black colouration in diurnal desert animals, was 

 apparently first pointed out by Buxton. This is a rather puzzling phenomenon, as it 

 seems to be a bad adaptation for desert animals and increases the absorption of 

 heat. Some of these black desert animals have black colouration in other biotopes 

 too, so that their blackness is no adaptation; but this only raises the question why 

 black elements prevail so much amongst the diurnal desert animals. Among those 

 are the Tenebrionid beetles which predominate in most deserts of which the noctur- 

 nal species (Blaps) are not less black than are the more common diurnal species. 

 The same can be claimed for the ravens, except that our desert ravens have perhaps 

 a less deep black nuance than have their cousins beyond the deserts. In both these 

 cases black is the common ancestral colour of the group. Wheatears and chats 

 (Oenanthe, Saxicola) are another group of prevalently black desert birds. Buxton 

 points out that their transformation from a buff to a black and white pattern can 

 easily be followed. Here we have apparently a definite adaptation to the desert. 

 The same is true for a number of insects such as the blackish races of Metacerus 

 and other grasshoppers. 



We may also refer to an analogous condition in man. The tents of the Bedouins 

 of the desert are usually black (or dark brown) and their thick overcoats or abbayas 



• c.f. Cott, H. B. 1940. Adaptive coloration in animals. London, p. 154 — Ed. 



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