A different type of sociality emerged among mobile complex animals that can individually attain large size. In fact, the largest known living animals, the whales and elephants, comprise two of a very few mammalian orders that contain only social species. The pattern of evolution on Earth has favoured sociality in the smallest and the largest (mostly vertebrates) of animals, albeit for different reasons. The smallest seek the advantages of being large, as protozoans did to form the first animals. The large animals can communicate; they spread out to find food, which all can share, and they protect one another. Among the social groups of large animals, only humans have differentiated their functions to such an extent that their societies begin to behave as individuals. Insect societies show behaviours halfway between societies based on genetically identical members and those created by genetically different individuals; such properties largely reflect their intermediate degree of genetic relatedness. Insects are more cooperative and show a greater degree of altruism than is true of vertebrate societies. Form and function To stay alive, grow, and reproduce, an animal must find food, water, and oxygen, and it must eliminate the waste products of metabolism. The organ systems typical of all but the simplest of animals range from those highly specialized for one function to those participating in many. The more basic functional systems are treated below from a broadly comparative basis. Support and movement A skeleton can support an animal, act as an antagonist to muscle contraction, or, most commonly, do both. Because muscles can only contract, they require some other structure to stretch them to their noncontracted (relaxed) state. Another set of muscles or the skeleton itself can act as an antagonist to muscle contraction. Only elastic skeletons can act without an antagonist; all antagonistic muscles act through a skeleton, which can be either rigid, flexible, or hydrostatic. ♥♥♥Death Rider♥♥♥
Posted on: Sat, 15 Mar 2014 10:24:57 +0000
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