Behaviour and Morphology
The morphology of heads and their evolutionary origins The concentration of disparate activities that are welded together in vertebrate skulls would seem extraordinary were it not so familiar. The further elaboration of head shapes, especially by horns, antlers, tusks and ossicones, has provided humans with symbols and trophies from the earliest times, yet remain a poorly explored aspect of morphology. It is partly our human fascination with heads and partly the superior survivorship of teeth that has ensured that fossil heads dominate palaeontological collections and provide the data for much of our knowledge of the evolution of heads. Because the fore-ends of primitive chordates and vertebrates were the first to encounter both food and obstacles, basic sense organs and a ‘mouth’ developed there. The polarity of organisms and their symmetrical organization crosses all the higher taxa (Wainwright 1988). The need to respond to light, chemical, electric or vibrational signals led to the differentiation of cells until they developed into eyes, nose and ears, each encapsulated in compartments that shared the upper part of what became the skull. The lower section of this structure became a hinged mandible and eventually both jaws developed teeth. In many predatory vertebrates, from the most primitive to the most advanced, the size of jaws closely reflects the size and toughness of the prey. Furthermore, a gross expression of animal proportions is the size of heads relative to the size of bodies. Apart from whales, the most extreme examples of disproportionately large or small heads are to be found in bats. This is partly because the head and jaws get little or no heavy-duty help from the limbs in the behaviour patterns of capturing and processing prey. Thus Moloney’s Mimic Bat, a species that only takes small, soft-bodied prey, has no need for a large head while the Heart-nosed Bat Cardioderma cor seizes robust, often vertebrate, prey with its large jaws and must quickly subdue it with deep, damaging bites powered by massive jaw muscles. The head of the former is about one-seventh of the combined head–body volume, whereas Cardioderma heads are closer to a quarter! One of the most fundamental expressions of cranial morphology concerns the sizes of sensory activities in the head. It is in the relative
sizes of compartments and in permutations of connecting bridges, struts and welds that the species-specific morphology of mammal heads becomes obvious.Wherever sufficient data exist on the ecology and behaviour of species, we find superb and detailed examples of forms evolving morphological modifications that serve very precise behavioural functions. This is particularly obvious in the relative sizes of, say, orbits, olfactory equipment and auditory bullae in mammal skulls. Even more explicit are the functional shapes of eyes, ears, noses and sensory whiskers in mammals as different as a Galago, a Bat-eared Fox Otocyon megalotis, an Aardvark Orycteropus afer or an Aquatic Genet Genetta piscivora.
Thomas’s Galago Galagoides thomasi
Bat-eared ‘Fox’ Otocyon megalotis
Aardvark Orycteropus afer
Aquatic Genet Genetta piscivora Head/body proportions in Mimic Bat Mimetillus moloneyi and Heart-nosed Bat Cardioderma cor (after Kingdon 1974).
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Heads to illustrate super-development of different sensory faculties.