PET brain image of glucose consumption in dolphin brain
Dolphins react to going without food like diabetics—their blood sugar goes up rather than down. And there are other changes (mentioned below) that parallel the pathological condition of human diabetes as the normal status quo of dolphin blood chemistry. Has this anything to do with dolphins being next in line to humans in the size of their brains?
THE DIABETIC CONNECTION?
In selecting for a big brain, evolution therefore created a big glucose problem—the large human brain require a reliable supply of glucose. That involved increasing the rate with which the human liver can produce glucose and also the ability to efficiently carry to the brain. But there are all sorts of questions here.
What about juveniles—their brains are not only much bigger than those of adults but they burn much more energy per unit volume that those of adults (see here for a discussion of this situation). The liver of the children apparently solves this problem by being a superglucose producer. Even so children are much more vulnerable than adults to drops in their blood sugar both in suffering them and in the effects upon of the brain when they happen.
Another problem is "tissue compartment" competition for glucose. At rest the brain does not have rival organs for blood carried glucose but when exercise becomes sustained and intense, skeletal muscle can deplete the glucose in the blood faster than the liver can pump in extra glucose. It has been suggested that children have small bodies (and so so reduced skeletal muscle mass) so that it does not compete against their high glucose using brains.
But what of dolphins and their big brains? Well information is still scant. It is known that like primates but unlike other mammals their adult red blood cells retain the ability found in fetal ones to carry glucose. And it seems that their blood glucose regulation has been adapted in a way that rather oddly makes it akin to that of diabetics in keeping a high level of glucose. When nondiabetic people go without eating their blood sugar goes down not up. Not so in those with diabetes or dolphins. There are other similarities: as with diabetes following fast, dolphins after not eating have raised numbers of blood platelets, and certain blood enzymes such as gamma-glutamyl transpeptidase, and alkaline phosphatase and the uric acid in their blood drops.
The researchers studying dolphins commented "We hypothesize that there may be a common evolutionary reason, based on large brain size, for a diabetes-like state in mammals that is vestigial and pathologic in humans but remains active and essential in marine mammals." This is all puzzling but it suggests that large brains require specialized adaptations outside the cranium so they can be kept well supplied with energy.
Venn-Watson SK, Ridgway SH. 2007. Big brains and blood glucose: common ground for diabetes mellitus in humans and healthy dolphins. Comp Med. Aug;57(4):390-5.
For adult primates red blood cells retaining the glucose carrying capacity of fetuses: Jacquez JA. 1984 Red blood cell as glucose carrier: significance for placental and cerebral glucose transfer. Am J Physiol. Mar;246(3 Pt 2):R289-98.
For dolphins/whales Craik JD, Young JD, Cheeseman CI.1998 GLUT-1 mediation of rapid glucose transport in dolphin (Tursiops truncatus) red blood cells. Am J Physiol. Jan;274(1 Pt 2):R112-9.