Thermoregulation in Endotherms

Thermoregulation in Endotherms

• Endotherms produce their own body heat from metabolism and maintain a constant body temperature, unlike ectotherms that conform to their environment.
• Endothermic homeothermy involves maintaining a constant body temperature, whereas ectotherms exhibit poikilothermy, or a variable body temperature.

Body Temperature Variation

• Body temperatures vary among different groups of mammals, with a range of over 10°C within each group.
• Birds have higher body temperatures on average, with a high range between passerines and non-passerines.

The Scholander-Irving Model

• The model depicts the relationship between resting metabolic rate (RMR) and ambient temperature in endothermic animals.
• The assumption is that animals maintain a constant body temperature, with a thermoneutral zone (TZ) where energy expenditure is minimal.
• The model is based on measurements from animals in metabolic chambers, with air temperatures identical to operative temperatures.

Thermoregulation in Natural Environments

• Body temperatures remain constant until extreme temperatures are reached, at which point animals' physiological systems become overwhelmed.
• Animals in dry environments lose water more rapidly than those in humid environments due to the "steepness" of the [H₂O vapour] gradient.

Humidity

• Humidity refers to the amount of water vapour in the air, which depends on the temperature.
• Absolute humidity is the vapour density (mg H2O L-1), partial pressure (kPa), or the amount of water vapour in the air.
• Relative humidity is the percentage of water vapour when the air is saturated.
• Saturation deficit is the difference between the capacity and the actual amount of water vapour in the air.

Physical Performance and the "Thermal Cliff"

• CTmin refers to the critical minimum temperature, below which an animal can no longer maintain its body temperature and dies.
• The critical factor for physical performance is the optimal temperature, and even small increases above the optimal range can be dangerous.
• Lethal hyperthermia occurs when body temperature rises above the optimal range, and evaporative cooling mechanisms are crucial for preventing this.

Body Temperature Variation

• There is a circadian cycle in normothermic body temperature, with typically lower temperatures at rest.
• The amplitude of the circadian cycle (RT) scales negatively with body mass (Mb), with RT = 11.066 * Mb-0.37.
• Body temperature variation is seen in species like the crested barbet, with a 4°C decrease at night and rewarming during the day.
• Starving responses can slightly increase the amplitude and decrease body temperatures in response to food limitation.

Heterothermy

• Heterothermy refers to the ability of some endotherms to exhibit facultative hypothermic responses, or reversible, controlled reductions in body temperature.

• This allows for energy conservation, as metabolic rates are lowered.

• Categories of heterothermic responses include hibernation, daily torpor, and shallow rest-phase.

• Hibernation involves a reduction in body temperature for days/weeks, with a pronounced drop in temperature (up to 40°C).

• Daily torpor involves a reduction in body temperature for hours/days, with a pronounced drop in temperature.

• Shallow rest-phase involves a reduction in body temperature for hours/single rest phase, with a shallow drop in temperature.