The Garden of Eaden: WHY DO GIRAFFES HAVE LONG NECKS?

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Not only are giraffes the tallest land animal on earth, they also possess the longest neck of any living creature. So long is the giraffes neck that if it want to take a drink it can't simply lower its head, it has to give itself a fighting chance by either spreading its front legs, bend its knees, or kneel on the ground - a risky move by a water hole no matter how big you are!

There are two main hypotheses regarding the evolutionary origin and maintenance of elongation in giraffe necks. The 'competing browsers hypothesis' was originally suggested by Charles Darwin and only challenged 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 (15 ft) high, while even quite large competitors, such as kudu, can only feed up to about 2 m (6 ft 7 in) 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.

The other main theory, the sexual selection hypothesis, proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests -see above film clip - 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. However, this ridiculous objection is as intelligent as questioning the need for the male nipple. If human males did not have the genetic information for nipples then Mendelian theory would dictate that you would have a selection of human offspring displaying one nipple, two nipples, or no nipples at all! Therefore, if female giraffes had short necks then you would end up with progeny with a selection of neck sizes. This is isn't going to be very helpful when it come to impressing your future giraffe 'wife' in the next neck bashing competition!

The circulatory system of the giraffe has several adaptations for its great height. Its heart, which can weigh more than 25 lb (11 kg) and measures about 2 ft (61 cm) long, must generate approximately double the blood pressure required for a human to maintain blood flow to the brain. In order to cope with pumping blood up to such a great height, giraffes have usually high heart rates for their size, at 150 beats per minute.

The Garden of Eaden

WHY DO GIRAFFES HAVE LONG NECKS?

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