Giraffes (Giraffa camelopardalis) are the largest ruminant and the tallest living terrestrial mammal species on Earth! The tallest specimen ever scientifically recorded was a bull (male) of the Masai subspecies (Giraffa camelopardalis tippelskirch), that stood a staggering 20 feet (6.09m) high! Weighing in at 545 to 1,905 kg (1,200 to 4,200 lbs), Giraffes still manage to run at up to 35 mph (56 km). The Giraffes’ head is about 3 meters (9 ft) above the heart which makes it very difficult for the heart to pump blood to the brain. To cope with this the Giraffe has the highest blood pressure of all mammals and its heart can weigh up to 10 kg (22 lbs)! Besides their long necks, Giraffes also have long, grasping tongues, which can extend 45 cm (18 in) to reach tasty inner tree leaves.
Evidence from the fossil record suggests that Giraffes evolved from deer like animals with a much shorter neck. By about 1 million years ago, modern animals we’d recognise as Giraffes had appeared on the African savannah. Evidence has shown that the Giraffes long neck is a real hindrance obviously costing a lot of energy to grow and maintain but that it also has a large survival cost. For example studies have shown that male Giraffes, with their larger necks, were about twice as likely as females to be killed by a predator – mostly Lions (Panthera leo).
So why though did these long necks evolve? Until fairly recently, the most commonly suggested theory involved finding food. Individuals that were born with longer than average necks were thought to have a feeding advantage, since in times of adversity, they could reach higher into trees to forage for leaves. Longer necked individuals were more successful at surviving, and passed their long neck genes onto their offspring. Over many generations, the modern long necked Giraffe we are familiar with evolved.
In the 1980s, scientists in Africa decided to compare this theory to the realities of Giraffe life. A study showed that the main criterion that determines the dominance among male Giraffes is neck size. Other studies found that despite their long necks, Giraffes spent most of the dry season when food was scarce feeding in low bushes rather than in tall trees. In the rainy season, when leaves were green and plentiful, Giraffes were more likely to turn their attention tree ward. Also, observations of Giraffes have shown that over 50% of the time, they feed with their necks horizontally. So while a long neck obviously allows Giraffe to reach more of a range of food, it didn’t seem to provide enough of a survival advantage in scarce times to account for its evolution. Obviously some other evolutionary selection pressure was at work.
Evidence found that a long neck did give an individual Giraffe an advantage, but not in the way that was first thought. An average male Giraffe’s neck weighs 90kg (200 lbs) and can stretch 1.8 m (6 ft). Giraffes fight over females by swinging their necks and heads like a medieval ball and chain. The longer and heavier the neck, the more momentum behind the often bone-shattering head slams. Research has found that males with the longest, most massive necks tended to win the mating contests, obviously, allowing their genes to be passed down to future generations. It’s been suggested that competition for mates that pushed the evolution of the Giraffe’s neck, with longer-necked animals more successful at reproducing. Female Giraffes have many of the same genes, so their necks are long, too. But the females’ necks stop growing in adolescence – while male Giraffes go on to add nearly 45kg (100 lbs) of neck weight as they reach adulthood.
The standard story about why Giraffes have evolved their incredibly long necks states that this trait has helped them in reaching to higher leaves. This story, however, is probably wrong. Giraffes are obviously capable of feeding on higher leaves than other animals but this advantage doesn’t seem to be sufficiently great to justify the costs of having such a long neck, the advantage in getting mates (sexual selection) seems to be a much more likely explanation for this incredible wonder of the animal kingdom!