Why do Giraffes have such long necks?

01. May 2012 · 2 comments · Categories: African Animals · Tags: , ,
Giraffes (Giraffa camelopardalis) the tallest living terrestrial species on Earth!

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!

 

Why do Zebra have stripes?

01. January 2012 · 2 comments · Categories: International Wildlife · Tags: , ,
Plains Zebra (Equus burchelli) (WC)

It’s a question that has puzzled me since childhood, I know, I should have had more things to worry about! There are many different theories as to why Zebra have stripes. The three most common theories are firstly is that they act as markers allowing individual Zebra to recognise each other in large herds. On its own this theory doesn’t hold much water with me, however, as other herd animals, such as the Wildebeest, still recognise each other despite the fact they do not possess these fingerprint like stripe patterns.

The second theory is that the black stripes on a Zebra will absorb a lot of heat from the fierce African sunlight, whereas the white stripes will reflect a lot of heat. The result of this is the downward movement of air from the black stripes and an upward movement of air from the white stripes, this difference in air movement then creates many small whirling winds on the Zebra body. This movement of air is said to help cool the Zebra down.

The third, and perhaps most quoted theory, is that the stripes help the Zebra to evade predators. The stripes help to camouflage the animals as the stripes break up the body against its background, in herds the action of running together also causes confusion to predators, such as the Lion, as they cannot distinguish between individual animals easily.

New research has now discovered another factor that may have influenced the stripes of the Zebras coat. The new study suggests that the stripe pattern scrambles the vision of a much smaller predator, namely the bloodsucking Horsefly. Horseflies, the females of which feed on blood, are attracted to light waves that are oriented in a particular direction, which we experience as glare. This glare lures these vampire bugs, it is theorised, because it resembles light reflected off water, where they lay their eggs. The black and white pattern is thought to help to disrupt the signal of light waves to the Horseflies, acting as a kind of natural fly repellant!

When I read this study I wondered why other Equine species were not striped in a similar fashion, but then I remember reading a book about horse breeds, I know I need a life, and remembering that some ancient horse breeds, as well as horse species in cave paintings sometimes exhibit faint stripes on their legs, although not as distinct as the Zebra’s. The Przewalski Horse also exhibits some striping on the leg. The fact there are more Horseflies, and more Horsefly species in Africa in comparison to more temperate areas maybe a reason why the striping is reduced or absent, not to mention the intense selective breeding of modern horse breeds. Although why other African species have not adopted this strategy remains a mystery.

Perhaps we will never know exactly why a Zebra has stripes, although if I had to put my life on it I’d say it’s probably a combination of all these factors and probably a dozen we clever humans haven’t even thought of yet, although not knowing doesn’t subtract, for me, the astonishing spectacular these unusual animals make in the African savannah.