This impressive photo shows a curious behavior sometimes seen in wild giraffes called osteophagia. It literally means "bone eating."
Giraffes don't consume the entire bone. Instead they just gnaw on it a bit, wearing it down, before moving on to a new section of bone. It is thought that the bones provide an additional source of both calcium and phosphorous, beyond what's typically found in the plants they consume. Both minerals are critical for maintaining their own skeletal health.
Giraffes aren't the only bone eaters out there. It turns out that it's actually a fairly common behavior for lots of ruminants. A 2013 paper in the Journal of Archaeological Science provides a handy list of critters known to munch on the occasional bone:
A number of medium to large ungulate species are reported to gnaw on bones, antlers, and horns, including caribou/reindeer (Rangifer tarandus), red deer (Cervus elaphus), fallow deer (Dama dama), axis deer (Axis axis), mule deer (Odocoileus hemionus), elk (Cervus canadensis), and bighorn sheep (Ovis canadensis). The south Asian muntjac, eastern Sahara camel (Camelus dromedarius), and a number of African bovids, most notably giraffe (Giraffa camelopardalis) and sable (Hippotragus niger), and also including gemsbok (Oryx gazelle), kudu (Tragelaphus strepsiceros), and wildebeest (Connochaetes spp.), exhibit this peculiar behavior. Even domestic stock, including sheep (Ovis aries), cattle (Bos taurus), and pigs (Sus scrofa), are known to gnaw and ingest portions of skeletal elements.