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The family Giraffidae has two extant members: the savannah-dwelling giraffe
(Giraffa camelopardalis) and the forest-living okapi (Okapia
johnstoni). Native to Africa, this family is highly adapted for
browsing - although the two species inhabit very different habitats, both
feed at a level higher than any other sympatric terrestrial herbivore. The
giraffe is the tallest living mammal, towering up to 5.9 meters above the
ground. While giraffes are renowned for their long necks, they possess
the same number of cervical vertebrae - seven - as most other mammals. The
forequarters of both species are overdeveloped, and the back slopes downward
to the rump.
The fossil record of the Giraffidae begins in Africa during the Miocene,
extending to the present on this continent. Giraffes also ranged
widely in Eurasia from the middle Miocene to the Pleistocene. Some
of these giraffes bore highly developed, branching ossicones (see
below). The modern-day okapi, on the other hand, closely
resembles the ancestral form of the early Pliocene giraffids
(Paleotragus).
Two subfamilies (or tribes) are sometimes recognized:
-
Giraffinae (or Giraffini) - the giraffe
-
Palaeotraginae (or Okapini) - the okapi
The legs are long to very long, and have only two digits. Giraffids
walk with an ambling or pacing gait, with the front and rear legs on one
side of the body moving forward together - since the body is relatively short,
this ensures the legs do not interfere with each other. The extreme
length of the legs means that giraffids have to straddle or bend their front
legs in order for their heads to reach the ground to graze or drink.
Both the giraffe and okapi possess a long, prehensile tongue with which they
browse. Leaves form the principal diet of both species; giraffids select
for high-quality foliage, and have a very efficient digestive system with
which to process it (the stomach of the giraffe is half the size of that
of the grazing African buffalo, Syncerus caffer). Tongue-like
papillae in the stomach provide the largest absorptive surface of any ruminant.
The teeth of this family are characteristically rugose - the enamel
is not smooth as in all other mammals. The Giraffidae maintain the
typical ruminant dentition: I 0/3, C 0/1, P 3/3, M 3/3 x 2 = 32.
A pair of skin-covered bony knobs (known as ossicones) are present in both
species: in Okapia they grow from the frontal bones in males only;
in Giraffa both sexes have ossicones on the parietal bones.
Ossicones of modern giraffes are short and unbranched, and are never
shed. In Giraffa, layers of bone are often laid along the suture
between the frontal and nasal bones, forming a third protuberance which is
especially prominent in adult males. At birth, the ossicones are present
as cartilaginous knobs, which ossify, grow, and fuse with the skull with
age. |
