In a research letter to this journal on the physiology of the giraffe, McCalden et al. suggested that cranial blood flow in the giraffe is regulated by an autoregulatory response of the cranial vasculature. Their conclusion was based on two points: 1) that carotid flow remains constant over a wide range of carotid pressures, and 2) that the giraffe exhibits certain postural behaviour.
The argument for (2) is that giraffes always remain resting sternally for a few seconds before standing to their full height, apparently to allow for autoregulatory adjustments by the cranial resistance vessels to a cranial postural hypotension of less than 100 mm Hg. Apart from the fact that the compensatory pause noted by Von Citters, as Warren points out, is not in any way unique to giraffes but occurs in very many mammals including man, several other arguments suggest that the wide auto-regulatory range of 70-210 mm Hg reported by McCalden et al. is not the essential basis of proper cerebral blood flow in the giraffe