Heat Loss by Convection Occurs When the Body Warms Air or Water
When the air or water in contact with the skin is heated, it flows away, thereby exposing the skin to cooler fluids. Because it takes more heat to warm water than it does to warm an equivalent mass of air, water-dwelling animals lose more heat by convection than do terrestrial mammals.
The amount of heat lost by convection depends on the thermal gradient (temperature difference) between the skin of the animal and the fluid overlying the skin; a greater thermal gradient results in more heat loss. In natural convection the warmed air or water rises from the surface of the animal because it is less dense than the cooler fluid. In forced convection, cooler fluid is moved over the skin surface by a breeze or current or simply because the limbs and animal are moving. Forced convection is more effective than natural convection as a mode of heat loss because the thermal gradient is maintained by the constant renewal of the cooler air or water that blankets the surface of the skin. Young or small animals left in a cool draft can quickly lose much body heat by convection and should be protected from such situations.The thermal gradient for heat loss can be altered by changes in skin blood flow and the amount of insulation separating the animal from the environment. Increasing blood How to the skin raises skin temperature and therefore heat loss, whereas a reduction in skin blood flow reduces heat loss. Hair traps air and impairs convection. The thickness of the layer of hair can be altered by piloerection (making the hair stand up) and by growing a thicker hair coat in preparation for winter. The thick layer of blubber in sea mammals also provides a layer of insulation. Reducing the area of the exposed body surface area also reduces convective heat loss. Animals accomplish this by curling up in a ball or by huddling with Iittermates.