african bark barking black-&-white black-coats black-skin Bray braying Camouflage common-zebra contrast contrasting equids equus-quagga-boehmi erect-manes family-groups fast grant’s-zebra grazers harems herbivores herbivorous herd herds individual mammals mohawk-like-manes patterns plains-zebra social stiff-manes stripes subspecies ungulates unique unique-stripe-patterns white-fur white-stripes zebra