By Jody Bourton

Earth News reporter

A long neck requires a special heart

For children and scientists alike the extraordinary shape of the giraffe has posed many questions. Why they have such long necks has so far been partly answered. However, exactly how they maintain this neck, and get blood to a head that is two metres from their heart, has remained unknown. Now research reveals that giraffes have a small, powerful, supercharged heart that is different to that possessed by other similar mammals. Scientists have published the discovery in the journal Comparative Biochemistry and Physiology, Part A. Funny long neck "There are not many animals that have evolved to have a very long neck," says giraffe expert Professor Graham Mitchell from the Centre of Wildlife Studies in Onderstepoort, South Africa. Prof Mitchell undertook the study along with Prof John Skinner from the Centre for Veterinary Wildlife Studies at the University of Pretoria South Africa. The heart is smaller than you'd expect in similar-sized animals, but the walls are incredibly thick

Professor Graham Mitchell

Centre of Wildlife Studies, Onderstepoort, South Africa "Giraffes have this very funny long neck, and two questions immediately arise, one is why and the other is how," he says. The answer to the first question, says Prof Mitchell, is that a long neck probably confers a range of advantages, helping the animal feed on different browse, thermoregulate its body and be more vigilant. But he wanted to find out more about how the giraffe (Giraffa camelopardalis) maintains such a long neck and is able to overcome its physiological constraints. "Giraffes have this huge problem of having a head that is 2m away from the heart," Prof Mitchell says. "So in a really big animal, how does it get blood up there?" Under pressure Most mammals have a relatively low blood pressure because their blood needs only move a short distance between head and heart. For the giraffe the distance is significant. That creates two problems: a giraffe's heart must cope with the hydrostatic pressure exerted on it by the amount of blood in such a tall neck. For blood to reach the head, the heart must then beat strongly enough to overcome this significant downward pressure caused by gravity. Previous studies have found the giraffe has an extremely high blood pressure that is twice that found in other animals. But this study is the first to unravel the true nature of the giraffe heart and cardiovascular system. ADVANTAGES OF A LONG NECK Feeding: enables giraffes to eat food that other animals cannot reach Vigilance: communication with other giraffes by sight and seeing predators from a distance Thermoregulation: provides a large surface area to lose heat in the hot sun "For a long time it was thought that the origin of the high blood pressure was a really big heart and that was based on a single measurement based in the 1950s," says Prof Mitchell. The researchers based their results on a range of measurements taken from giraffes culled in south eastern Zimbabwe between 2006 and 2009. "Our concern was partly to explain the origin of high blood pressure and what physiological mechanisms operate to push the blood pressure to the level in the giraffe," he says. "We established that the heart is actually quite small. It's smaller than you'd expect in similar-sized animals, but the walls are incredibly thick," Prof Mitchell says. "You have a small but a very powerful heart delivering the blood pressure." The researchers say giraffes are adapted to the high blood pressure and do not suffer as a consequence. A giraffe's heart has evolved to have thick muscle walls and a small radius, giving it great power. The walls of the blood vessels also thicken with age as the giraffe's neck grows longer, to avoid rupturing under increasing pressure. Expanding vessels The giraffe also has other specialist mechanisms to help deal with the high blood pressure, Prof Mitchell says. "Blood pressure depends on the capacity of the cardiovascular system as well as the efficiency of the pump." "Giraffes have got a way of adjusting the capacity of the cardiovascular system and are able to shrink and expand their blood vessels to change the volume of the cardiovascular system very efficiently." From the data collected on the body dimensions of the dead giraffes, the researchers hope to reveal more about its extraordinary body, including insights into its range of vision and breathing. Prof Mitchell says it will also be exciting to study the physiology of living giraffes using remote devices to collect data. "To measure blood pressure in a free living giraffe doing its thing, that would be really interesting," he says. "For people who study high blood pressure in humans, or people just like me who wonder how giraffes get it right."



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