Sunday, June 19, 2011
The giraffe - a magnificent animal
Both Dublin zoo and Fota wildlife park have had newly born giraffes during the last few months. This will gladden the heart of anyone who is at all familiar with these magnificent animals. Their grace and power is only matched by the wonder they inspire for what can be achieved by the evolutionary process.
The giraffe seems to be a docile creature but its sheer size means that, apart from the lion, it has no serious predators. Even he, or far more likely she when it comes to hunting, the much vaunted occupant of the top of the wilderness food chain, is very respectful of the tallest member of the animal kingdom. A kick from a giraffe can kill. To defend its young in the wild the mother will place the calf underneath its body where its legs act as barriers to attack. While this can be an effective technique it is not perfect. Only about 50% of calves born reach maturity.
Its neck is, of course, this animal’s defining visible characteristic, but it also has very long legs. The combination results in the ability to feed in the higher reaches of trees, above almost all of its competitors for this type of food. Evolution is responsible for this state of affairs and it is nothing short of mind boggling to contemplate how it has come about. Individual giraffes did not stretch themselves to gain increased height that they could pass on to their offspring, although a certain Chevalier de la Marck, who pre-dated, and helped pave the way, for Charles Darwin in the study of evolution thought that such a thing was possible. Indeed he and his followers believed that it was the only way it could work. Later writers have pointed out that this would mean that an individual who had a leg amputated and who afterwards became a parent would have at least the possibility of producing a one-legged son or daughter.
The gene theory of inheritance, discovered by the Catholic monk, Gregor Mendel, showed how the process really worked. Over many generations, taller giraffes that were born as the result of random genetic mutations survived to the point where they were able to pass on their genes, including the gene that defined tallness, as they produced young, while their shorter siblings and cousins did not. Time, lots and lots of it, is an essential element of this progression, as is the rate at which the animals reproduce. The gestation period of the giraffe, at significantly more than one year, makes the evolutionary change all the more remarkable.
More ways than one
There are always in nature different strokes for different folks. The other animal that has had its eye on the leaves at the tops of the trees is the elephant. Evolution solved its problem by giving it a long proboscis, or trunk. The solution in this case meant arranging for the food to be brought down to the point of delivery, the mouth, for processing, while the giraffe had the point of delivery repositioned to achieve the same result. The swan is another example of this answer – its long neck allows it to feed off the bottom of many of the waterways it inhabits while its body floats on the surface, whereas its relation and frequent companion, the duck, is happy to dive for its food.
Nature has had much more to do in the case of the giraffe to make it a viable species than provide it with long legs and an extended neck. Its overall height has severe consequences for the ability of its cardiovascular system to do its job of delivering blood to its extremities and making sure it stays there for as long as it is needed. Firstly, the giraffe has a very large heart – much larger as a proportion of its body weight than any other animal. In the case of the upper neck there is a complex series of valves that mean blood will be retained in this location when the head is lowered, while at the lower reaches the legs and lower body have a thick, tight-fitting covering of skin which is there to contain blood that would otherwise simply burst out through the blood vessel walls if the same pressure was present in any other animal.
While the giraffe is adapted for feeding high up in its habitat and does this superbly, even elegantly, things are not so agreeable when it comes to carrying out certain other functions that are also essential for survival. Foremost among these is drinking. Lapping water from a pond or stream means spreading out the front legs and lowering the neck in way that is ungainly and obviously stressful for the individual. It also seems calculated to place the animal in a highly vulnerable position in the event that Ms. Lioness happens to be in the vicinity. Here evolution has also provided at least a partial solution. It turns out that the giraffe has the capacity to store large amounts of water in its body so that the number of occasions it finds it necessary to drink is minimised. This has the beneficial side effect of allowing it to live for long periods in areas where water is scarce.
Evolution’s outcome with regard to the physiology of the giraffe is as noteworthy as its development of the cognitive facility in humans.
The giraffe is altogether a truly remarkable animal. A trip to Fota or Dublin zoo to see them is well worth while, especially now that there are new, native-born, additions to the families.