Giraffes

Order: Artiodactyla (Greek: ἄρτιος (ártios), "even", and δάκτυλος (dáktylos), "finger/toe”) 

Family: Giraffidae
Genus: Giraffa
Species: G. camelopardalis

Giraffes are the tallest ungulate (hoofed like cows) mammals on Earth. Adults typically have a height of around 5–6 m ( about a metre or so taller than African Elephants). 

Giraffe_Ithala_KZN_South_Africa_Luca_Galuzzi_2004

Two male giraffes establishing dominance by 'necking' image: wikipedia

They are herbivores and are adapted to eating leaves (often acacias) that are well beyond the reach of other shorter ungulates.

A close relative of the giraffe is another ungulate called the okapi which has markings similar to a zebra but more features of a giraffe.

Their long neck results from a the lengthening of the cervical vertebrae (growing mostly longer after birth to make it easier to give birth), not from the addition of more vertebrae. Each cervical vertebra is over 28 cm  long.

Evolutionary explanations of their long necks or why do giraffes have long necks anyway?

There several hypotheses regarding the evolutionary origin and maintenance of elongation in giraffe necks. 

The "competing browsers hypothesis" was originally suggested by Charles Darwin and challenged only recently. It suggests that competitive pressure from smaller browsers, such as kudu, steenbok and impala, encouraged the elongation of the neck, as it enabled giraffes to reach food that competitors could not. This advantage is real, as giraffes can and do feed up to 4.5 m high, while even quite large competitors, such as kudu, can feed up to only about 2 m  high.

There is also research suggesting that browsing competition is intense at lower levels, and giraffes feed more efficiently (gaining more leaf biomass with each mouthful) high in the canopy. 

However, scientists disagree about just how much time giraffes spend feeding at levels beyond the reach of other browsers,  and a 2010 study found that adult giraffes with longer necks actually suffered higher mortality rates under drought conditions than their shorter-necked counterparts. This study suggests that maintaining a longer neck requires more nutrients, which puts longer-necked giraffes at risk during a food shortage.

Another theory, the sexual selection hypothesis, proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests to establish dominance and obtain access to sexually receptive females. 

In support of this theory, necks are longer and heavier for males than females of the same age, and the former do not employ other forms of combat. However, one objection is that it fails to explain why female giraffes also have long necks.  It has also been proposed that the neck serves to give the animal greater vigilance.

Skull and ossicones

Both sexes have prominent horn-like structures called ossicones, which are formed from ossified cartilage, covered in skin and fused to the skull at the parietal bones. Being vascularized, the ossicones may have a role in thermoregulation,  and are also used in combat between males. 

Appearance is a reliable guide to the sex or age of a giraffe: the ossicones of females and young are thin and display tufts of hair on top, whereas those of adult males end in knobs and tend to be bald on top.


See also


Reference:

http://en.wikipedia.org/wiki/Giraffe