african bark black coats black skin Bray common zebra equids fast grant’s zebra grazers herbivores herbivorous herd herds individual mammal patterns plains zebra social stripes ungulates unique white fur white stripes zebra black-&-white black-coats black-skin common-zebra grant’s-zebra greyscale monochrome plains-zebra striped white-fur white-stripes erect manes harems mohawk-like manes stiff manes erect-manes mohawk-like-manes stiff-manes gray grey patterned b&w bird black and white feather feathers finch tail zebra finch animal black couple Ears equus grevyi Grévy's zebra horse mane nature pair two white zoo