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Elephant hierarchical social structure is characterized by four social levels: basic, first-tier mother–calf units (displayed here as discrete circles); second-tier core groups comprised regularly associating mother–calf units (similarly shaded clusters of discrete first-tier units); third-tier or bond groups (separated clusters of second-tier groups with shaded background); and fourth-tier or clan groups (all units). See Wittemyer et al. (2005b) for more details.
