Original: A narrower definition specifies the requirement for irreversibly distinct behavioral groups or castes (with respect to sterility and/or other features), and such a definition excludes all social vertebrates (including mole rats), none of which have irreversible castes.
Simplified: A narrower definition specifies the requirement for irreversibly distinct behavioral groups or castes (with respect to sterility and/or other features). The newer definition excludes all social vertebrates (including mole rats), because they do not have distinct castes that are different.