This high metabolic rate aids in the maintenance of the high core temperature, however as most of the heat is generated as a by-product of respiration large therefore vast quantities of food must be consumed.
- Herbivores consume plants, which are relatively low in energy and must therefore devote a large portion of their day to eating if they are to maintain this high metabolic rate and internal environment
- Carnivores consume food which contain much higher levels of energy and as a result they eat less often
In the short term, a change in temperature is detected by the temperature receptors situated in the hypodermis layer of the skin. These transfer a signal to the hypothalamus in the brain, which uses negative feedback to rectify the change.
Initially the hypothalamus triggers the higher centres of the brain making the individual aware that it is cold, thus allowing it to make a conscious decision to alter its behavioural pattern. This may involve
- Moving to a warmer or sheltered area
- Changing body position
- Huddling in groups
- Putting on extra layers of clothes
In conjunction with the alterations in behavioural pattern, the hypothalamus also triggers the autonomic nervous system, which brings bout the physiological responses to the cooler temperature:
-
Vasoconstriction of the arterioles supplying the skin with blood. These vessels narrow reducing the blood flow to the surface capillaries by redirecting the majority of blood through a shunt vessel. Thus the amount of heat lost to the surroundings by radiation and conduction is greatly reduced.
-
Piloerection involves the erector pili muscles contracting, and thus pulling on the hair follicle causing it to raise on the body of the mammal. This is a reflex action more commonly used in animals with dense fur. The piloerection makes the fur “thicker” and as a result more air is trapped, reducing heat loss by convection.
-
Shivering is the rapid contraction and relaxation of the muscles of the body, causing the muscular tissue to respire and release more heat, as respiring muscles release approximately 4 to5 times more heat than those at rest.
The thyroid gland also secretes adrenaline further increasing the metabolic rate and as a direct result, heat production is also escalated.
However if an endotherm is to endure cold temperatures for elongated periods of time, the short term behavioural and physiological solutions may be insufficient for the survival of the animal, and as a result adaptations will occur.
As opposed to adrenaline the hormone thyroxine is secreted. This results in a more permanent increase in metabolic rate. In order to support this increased metabolic rate the animal must eat high-energy foods with relatively fewer intervals between meals. The behaviour of the animal may also alter the behavioural pattern; it may hibernate or migrate to avoid the colder months, which it is not physiologically suited to.
Ultimately natural selection will act in a directional way causing the extremes to survive. For example those individuals with:
- Dense hair
- Many brown fat cells
- Few sweat glands
- Light coloured fur
The previous paragraphs have detailed the reactions of an endotherm exposed to cold temperatures; these responses differ dramatically when it is a ectotherm under similar conditions. For example a chameleon exposed to cooler temperatures.
Due to the physiology of an endotherm, the only effective method of temperature regulation that can thus be used is the alteration of behaviour. The chameleon however is a-typical when considering other lizards as it can change the colour of its skin. Thus the lizard can make its skin colour darker, to absorb more heat from the surroundings.
In some tropical countries the temperature near the ground may reach as cold as 0oC, at this temperature the biochemical reactions occurring in the body of the lizard would be so slow it would run the risk of dying. As a result the lizard must find a warmer location so spend the night, usually it has a burrow it can retreat to when it gets too cold. The burrow will usually remain at a constant temperature of approximately 20oC; conditions the lizard will be able to survive.
Lizards will also adjust their body position so when heat needs to be absorbed from the surroundings it is done in the most efficient way, by placing themselves at right angles to the direction of the rays.
Despite these behavioural adaptations most endotherms are ill suited to living in colder climates, as there is insufficient energy available to maintain a suitably high metabolic rate.