Temperature regulation

For many organisms there is a fitness advantage to being warm. Many organisms use behavioural thermoregulation to maintain a high body temperature during the day, basking in the sun to warm up and retreating to the shade to avoid overheating. This option is not open to most aquatic organisms, or those living in soil or sediment. It is also generally not possible for small or nocturnal organisms. A small number of active predatory fish utilise a counter-current heat exchanger (rete mirabile) to retain metabolic heat and warm their muscles, brain or eyes. A few have modified optical muscles as heater organs, and a range of plants generate heat to aid dispersal of scent and attract pollinators. A wide range of larger insects use rapid but unsynchronised muscle contraction to elevate their body temperature prior to flight, or other activity. In hot climates organisms may need to dissipate heat to avoid overheating. The major behavioural mechanism is shade-seeking, or for small organisms stilting or climbing onto objects such as plants to move out of the hottest air net to the ground. Larger mammals may tolerate a limited degree of warming during the day, releasing this in the cool of the night. Evaporative cooling is very effective at losing heat, but because it loses valuable water it can only be used sparingly in arid areas.