Blood Vessels in the Hind Limb of the Mallard (Anas platyrhynchos): Anatomical Evidence for a Sphincteric Action of Shunt Vessels in Connection with the Arterio-venous Heat Exchange System

Birds are better at regulating heat loss through their legs than their bills: implications for body shape evolution in response to climate

Endotherms use their appendages-such as legs, tails, ears and bills-for thermoregulation by controlling blood flow to near-surface blood vessels, conserving heat when it is cold, and dissipating heat in hot conditions. Larger appendages allow greater heat dissipation, and appendage sizes vary latitudinally according to Allen's rule. However, little is known about the relative importance of different appendages for thermoregulation. We investigate physiological control of heat loss via bird bills and legs using infrared thermography of wild birds. Our results demonstrate that birds are less able to regulate heat loss via their bills than their legs. In cold conditions, birds lower their leg surface temperature to below that of their plumage surface, retaining heat at their core. In warm conditions, birds increase their leg surface temperature to above that of their plumage surface, expelling heat. By contrast, bill surface temperature remains approximately 2°C warmer than the plumage surface, indicating consistent heat loss under almost all conditions. Poorer physiological control of heat loss via bird bills likely entails stronger selection for shorter bills in cold climates. This could explain why bird bills show stronger latitudinal size clines than bird legs, with implications for predicting shape-shifting responses to climate change.

Selective brain cooling and its vascular basis in diving seals

Brain (Tbrain), intra-aorta (Taorta), latissimus dorsi muscle (Tm) and rectal temperature (Tr) were measured in harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals during experimental dives in 4°C water. The median brain cooling was about 1°C during 15 min diving, but in some cases it was as much as 2.5°C. Cooling rates were slow for the first couple of minutes, but increased significantly after about 5 min of diving. The onset of cooling sometimes occurred before the start of the dive, confirming that the cooling is under cortical control, like the rest of the diving responses. Taorta also fell significantly, and was always lower than Tbrain, while Tm was fairly stable during dives. Detailed studies of the vascular anatomy of front flippers revealed that brachial arterial blood can be routed either through flipper skin capillaries for nutritive purposes and return through sophisticated vascular heat exchangers to avoid heat loss to the environment, or, alternatively, through numerous arterio-venous shunts in the skin and return by way of large superficial veins, which then carry cold blood to the heart. In the latter situation the extent to which the brain is cooled is determined by the ratio of carotid to brachial arterial blood flow, and water temperature, and the cooling is selective in that only those organs that are circulated will be cooled. It is concluded that Tbrain is actively down-regulated during diving, sometimes by as much as 2.5°C, whereby cerebral oxygen requirements may be reduced by as much as 25% during extended dives.

Responses to catecholamines and nerve stimulation of the perfused rete tibiotarsale and associated blood vessels in the hind limb of the Mallard (Anas platyrhynchos)

The rete tibiotarsale, which is a functional arterio-venous heat exchanger, and its associated blood vessels were perfused in situ and the responses to epinephrine (E), norepinephrine (NE), and perivascular nerve stimulation (NS) were studied. The vasoconstrictor response of the arterial shunt, which bypasses the rete, was significantly greater than in adjacent segments of the same artery. The constrictor responses to E, NE, and NS were reversibly blocked by the alpha-blocker phentolamine. The rete arteries were also capable of vasoconstriction, but during simultaneous perfusion of the arterial shunt and the rete arteries, stimulation with NE and E stopped perfusate flow in the shunt while the flow in the rete arteries increased. Segments of the large vein and rete veins were unresponsive to NE, E, and NS. The results support the previously reported morphological evidences for a sphincteric action of the shunt artery, but not of the shunt vein.