Ecology of the brontosaurs.
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Because it is so tall, the giraffe, Giraffa camelopardalis, provides an important animal model for investigating adaptive mechanisms to orthostatic (gravitational) pressure changes. Previous physiological studies of the giraffe have concentrated on arterial blood pressures in the heart and neck. Briefly, these investigations revealed that arterial pressure near the giraffe heart is about twice that in humans, to provide more normal blood pressure and perfusion to the brain. Another important question is that of how giraffes avoid pooling of blood and tissue fluid (oedema) in dependent tissues of their extremities. As monitored by radiotelemetry, the blood and tissue fluid pressures that govern transcapillary exchange vary greatly with exercise. These pressures, combined with a tight skin layer, move fluid upward against gravity. Other mechanisms that prevent oedema include precapillary vasoconstriction and low permeability of capillaries to plasma proteins.
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The sequences of proteins from ancient organisms can be reconstructed from the sequences of their descendants by a procedure that assumes that the descendant proteins arose from the extinct ancestor by the smallest number of independent evolutionary events ('parsimony'). The reconstructed sequences can then be prepared in the laboratory and studied. Thirteen ancient ribonucleases (RNases) have been reconstructed as intermediates in the evolution of the RNase protein family in artiodactyls (the mammal order that includes pig, camel, deer, sheep and ox). The properties of the reconstructed proteins suggest that parsimony yields plausible ancient sequences. Going back in time, a significant change in behaviour, namely a fivefold increase in catalytic activity against double-stranded RNA, appears in the RNase reconstructed for the founding ancestor of the artiodactyl lineage, which lived about 40 million years ago. This corresponds to the period when ruminant digestion arose in the artiodactyls, suggests that contemporary artiodactyl digestive RNases arose from a non-digestive ancestor, and illustrates how evolutionary reconstructions can help in the understanding of physiological function within a protein family.
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