The Evolution of Horn-Like Organs

Abstract

Hornlike organs evolved independently in a number of mammalian families. Though these organs assumed great diversity they did evolve into several general functional types. A short review of the structure and development of hornlike organs is given. Some views on horn function and evolution are critically discussed. The evolution of hornlike organs is visualised as follows: In primitive large mammals the head blow became effective as a fighting form due to increased mass and inertia of the heads. Some forms grasped this potential. Combats were carried out from the broadside while opponents delivered head blows on each others body. Skull protuberances now became adaptive. Concurrently, defensive mechanisms evolve, decreasing the effectiveness of these protuberances. Foremost among them is a thick, heavy hide or specialised dermal shield. These adaptive syndromes gave no impetus towards larger and more complex horns. This impetus arose with the appearance of a new method of defense - catching the opponent's blows with the horned head. This leads to the evolution of heavy skulls and horns capable of catching and holding the opponent's head. The target area of attack remained the body. Frontal engagements resulted from the opponents' attempts to control each others horned head. It is shown that bovids and suids followed similar evolutionary roads in their mode of combat. The tusks of the suidae fulfill the same function and were subject to similar selection as the horns of short horned bovids. Thus Sus and Oreaisinos, and Bos and Phacochoerus are entirely similar in their mode of combat, hornlike organs and defense mechanisms. The primitive frontal engagement gave rise to two different modes of combat, ramming and wrestling. The wrestlers evolved complex horns, whose function is to bind opponents together, thus allowing them to develop full strength wrestling and pushing matches. The rammers evolved a heavy skull and heavy horns. A trend towards cephalisation of display organs can be detected in ruminants and suids. Display organs are interpreted as attention guiding adaptations. It is shown that the distribution of display organs correlates with the display. A broadside body display is found in ruminants with a diffuse distribution of display organs. If display organs are cephalised, the broadside display disappears. This is found in the North American wild sheep. Display centers about one heavy, very large horns of males. Next it is verified that the rams horns are display organs. It is shown, the horn display is the prerogative of the dominant; sheep can differentiate horn size classes; large horned males enjoy reproductive advantage; horn size carries a priori dominance status. Hence horns evolved to function as weapons inflicting damage; as defense organs shielding their owner; as binding organs allowing opponents a secure lock in battle; as display organs having an a priori intimidating effect on certain conspecifics. Some species of ruminants possess hornlike organs which change progressively in size with the age of their owner. It is postulated that, as in North American wild sheep, horn size parallels dominance order. Conspecifics learn to associate horn size and agonistic potential of opponents. Hence horn size signals to conspecifics predictable social relationships. In large cervids from temperate and cold climates this system must save stored energy during the rut, which males need for the severe winter. Graded horn sizes allow wild sheep to live in an open society in which strange individuals may meet and fit into the dominance hierarchy with a minimum of combat. Graded horn sizes hence serve as rank indicators. It is next made plausible that territories in ruminants function in conjunction with displaying males as rank indicators. Outside the territories males mix freely. Hence graded horn sizes and territoriality may enable open societies, by acting as communication mechanisms and creating a predictable, social environment. A hypothesis is presented explaining the selective forces for the evolution of display. It predicts that, antagonistic selection forces shape display and display-organs, that a potential exists for rapid spread in a population of differences in behaviour and morphology, that intimidation mechanisms evolved for all sensory pathways, that external appearance af animals changes more repidly than their primary adaptations.