Question 2 of 6: Body-Temperature Regulation Below and Above the Thermoneutral Zone

Question 2: Body-Temperature Regulation Below and Above the Thermoneutral Zone (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Ambient temperature below the thermoneutral zone (below the LCT). Once ambient temperature falls below the lower critical temperature, peripheral thermoreceptors in the skin and central thermoreceptors in the hypothalamus, spinal cord and viscera signal a falling core-to-set-point error to the hypothalamic integrating centre, which drives two classes of response. Physical responses act first and are the cheapest: sympathetic vasoconstriction of cutaneous arterioles sharply cuts blood flow to the skin, raising the animal's effective peripheral insulation, and piloerection thickens the trapped air layer in the coat, both reducing the rate of heat loss to the environment without costing any extra metabolic energy. As temperature falls further and physical insulation alone cannot hold the balance, the hypothalamus engages metabolic (chemical) thermogenesis: somatic motor output drives shivering — rapid, largely uncoordinated skeletal-muscle contraction that converts most of its ATP turnover directly to heat — and sympathetic/hormonal output (catecholamines, thyroid hormone) drives non-shivering thermogenesis, principally via uncoupling protein-1 (UCP1) in brown adipose tissue in neonates and cold-adapted species, which short-circuits the mitochondrial proton gradient to release heat instead of synthesizing ATP. Behaviourally, the animal also increases voluntary feed intake (extra substrate for thermogenesis), huddles with conspecifics, and seeks shelter from wind and precipitation. Together these responses raise metabolic heat production above the basal/resting level, and the lower the ambient temperature falls below the LCT, the more heat production must rise to hold core temperature steady.

(b) Ambient temperature above the thermoneutral zone (above the UCT). Above the upper critical temperature, the same sensor/hypothalamic pathway drives the opposite effector set. Sympathetic vasoconstrictor tone to the skin is withdrawn, producing cutaneous vasodilation: skin blood flow rises sharply, carrying core heat to the body surface for radiative and convective loss (the reverse of the cold-side response). Because radiative and convective loss become progressively less effective as ambient temperature approaches or exceeds core temperature, the hypothalamus additionally drives active evaporative cooling: sympathetic cholinergic output to eccrine sweat glands in species with functional sweat glands (humans, horses, cattle), and/or increased respiratory rate and depth (panting) in species with few or non-functional sweat glands (pigs, dogs, poultry). Evaporation of water from the skin or respiratory mucosa removes a large amount of latent heat per gram evaporated and, unlike radiative/convective loss, remains effective even when ambient temperature is close to or above core temperature. Behaviourally, the animal reduces voluntary activity and, critically, reduces voluntary feed intake — digesting and metabolizing feed itself generates heat (the heat increment of feeding), which is disadvantageous when the animal is already struggling to shed heat — and it seeks shade, water, or airflow. Unlike the cold-side response, these heat-defence mechanisms carry their own metabolic and water costs (panting muscle work, water/electrolyte loss), so above the UCT the animal is trading one thermoregulatory burden for another rather than simply "doing less."