COLORATION IN MAMMALS AND BIRDS. 185 



On Coloration in Mammals and Birds. 



By J. Lewis Bonhote, M.A., E.L.S. 



[Eead 5th May, 1904.] 



Abstract. 



The object of the paper lias been to show, firstly, that the colour 

 of a bird or mammal is primarily due to " activity of nutrition 

 and function," which I have called " vigour," and that where 

 conditions for a high state of vigour exist we shall find the 

 majority of animals brightly coloured and vice versa. 

 " Vigour " is dependent on two causes : — 



(A) Climate, which contains two factors, 



(1) Temperature, 



(2) Eood; 



(B) The Else and Pall of Sexual Activity. 



For Polar Begions, where the two causes closely coincide, 

 the changes are much more marked and violent. 



In Temperate Regions, where the climate is sufl3.ciently severe 

 to aftect the " vigour," but where, at the same time, there is a 

 sufficiently long period of comparative plenty to prevent the 

 sexual activity clashing with climate, the changes are less marked. 



In Tropical Regions the first cause is practically removed, and 

 any changes in colour are due to sexual causes, except in cases of 

 temperate species which have spread into the south. 



Now, the individual " vigour " of various species and groups 

 will differ, and one animal may be able to maintain a full vigour 

 under conditions which would be impossible to another. This 

 will account for animals although Polar becoming brightly 

 coloured, e. g. Musk Ox, Eaven, Penguin, &c. It follows, there- 

 fore, that if they can maintain a full " vigour " in colder regions, 

 they can either (i) maintain an equally full vigour in the tropics, 

 or (ii) in hot climates their metabolism would become too active 

 and they would die. 



Consequently, a dark-coloured animal in Polar regions must 

 either be confined to those regions or be cosmopolitan, e. g. 

 Musk Ox, Eaven. 



By a similar process of reasoning, bright-coloured tropical 

 animals will be found extending northwards, probably becoming 



