Endotherm - Advantages and Disadvantages of An Endothermic Metabolism

Advantages and Disadvantages of An Endothermic Metabolism

The major advantage of endothermy over ectothermy is decreased vulnerability to fluctuations in external temperature. Regardless of location (and hence external temperature), endothermy maintains a constant core temperature for optimum enzyme activity.

Endotherms control body temperature by internal homeostatic mechanisms. In mammals two separate homeostatic mechanisms are involved in thermoregulation - one mechanism increases body temperature, while the other decreases it. The presence of two separate mechanisms provides a very high degree of control. This is important because the core temperature of mammals can be controlled to be as close as possible to the optimum temperature for enzyme activity.

The overall rate of an animal's metabolism increases by a factor of about two for every 10 °C (18 °F) rise in temperature, limited by the need to avoid hyperthermia. Endothermy does not provide greater speed in movement than ectothermy (cold-bloodedness)—ectothermic animals can move as fast as warm-blooded animals of the same size and build when the ectotherm is near or at its optimum temperature, but often cannot maintain high metabolic activity for as long as endotherms. Endothermic/homeothermic animals can be optimally active at more times during the diurnal cycle in places of sharp temperature variations between day and night and during more of the year in places of great seasonal differences of temperature. This is accompanied by the need to expend more energy to maintain the constant internal temperature and a greater food requirement. Endothermy may also provide a protection against fungal infection. While tens of thousands of fungal species infect insects, only a few hundred target mammals, and often only those with a compromised immune system. A recent study suggests fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures afforded by endothermy might have provided an evolutionary advantage.

Ectotherms will increase their body temperature mostly through external heat sources such as sunlight energy, therefore they depend on the occurring environmental conditions to reach operational body temperatures. Endothermic animals mostly use internal heat production through metabolic active organs (liver, kidney, heat, brain, muscle) or specialized heat producing organs likes brown adipose tissue (BAT). In general, ectotherms therefore have lower metabolic rates than endotherms at a given body mass. As a consequence they would also need higher food intake rates, which may limit abundance of endotherms more than ectotherms.

Because ectotherms depend on environmental conditions for body temperature regulation, they typically are more sluggish at night and in the morning when they emerge from their shelters to heat up in the first sunlight. Foraging activity is therefore restricted to the day time (diurnal activity patterns) in most vertebrate ectotherms. In lizards, for instance, only a few species are known to be nocturnal (e.g. many geckos) and they mostly use 'sit and wait' foraging strategies that may not require body temperatures as high as those necessary for active foraging. Endothermic vertebrate species are therefore less dependent on the environmental conditions and have developed a high variability (both within and between species) in their diurnal activity patterns..

It is thought that the evolution of endothermia was crucial in the development of mammalian species diversity in the Mesozoic period. Their endothermic capabilities provided them with a benefit over the mostly ectothermic dinosaurs that dominated the Mesozoic era. Endothermia gave the early mammals the capacity to be active during night time and avoid higher predation risk during the day. Endothermia therefore may have contributed to the fact that most mammalian taxa were already present at the end of the Mesozoic (65 million years ago) and that most extent mammalian taxa thus went through an evolutionary bottle neck (the nocturnal bottleneck hypothesis).