Adaptation

     The term adaptation implies that there is some sort of norm from which the body or system deviates in response to changes in the normal environment. Within the normal population a range of values is seen for any particular criterion that is examined, whether it be, say, activity of an enzyme, a blood parameter or body weight. Thus there is the statistical concept of the normal distribution. Adaptation implies a shift in the normal distribution or in the values for a particular individual. The former may be a long-term phenomenon in response to, e.g., climatic change where those animals best suited genetically to the change will survive. Short-term adaptation implies that the physiological systems can respond to changes in external factors. These factors include environmental temperature, light cycle or intensity, stocking density, the physical environment and nutrition (particularly in relation to energy or protein intake). In general, the term can relate to a modification that lessens the negative impact of imposed change or takes advantage of an opportunity afforded. 

     One major aspect relates to changes in environmental temperature. Homeothermic animals tend to have a defined range of temperature – the thermoneutral zone – within which core body temperature remains constant without any change in heat production. The thermoneutral zone varies for different species and stages of development and may also be modified by adaptation of an animal to prolonged exposure to an environment that falls outside the thermoneutral zone. However, within the zone, different species have a wide range of mechanisms by which they can adapt to maintain homeostasis. For example, poultry can increase heat loss in warm environments by increasing blood flow to the comb, wattles and shanks and, conversely, can reduce heat loss by reducing blood flow, changing posture and piloerection, thus improving body insulation. Pigs, individually housed, alter posture to increase or decrease heat loss and, in groups, can significantly reduce heat loss by huddling together. Environmental temperatures below the thermoneutral zone result in shivering, which is a rapid noradrenaline-induced mechanism for increasing heat production. Prolonged exposure to low temperature results in an increase in basal metabolic rate, due to non-shivering thermogenesis. This adaptation takes several weeks to complete in response to a permanent reduction in environmental temperature.

     Feed intake is increased at low temperatures and reduced at temperatures close to or above the upper limit of the thermoneutral zone. In the case of domestic fowl, food intake declines linearly across the normal range of environmental temperature (15–30°C). Stocking density and availability of trough space can also lead to marked changes in food intake. In pigs, for example, it has been observed that intakes are 10–15% higher with individually housed animals compared with those in groups. It is unclear whether this is a behavioural adaptation to boredom on the part of individual pigs or depression of intake due to competition in groups. However, there is a wide range of behavioural adaptations associated with changes in the physical environment etc. For example, stereotypic behaviours such as bar-biting by sows tethered in stalls and reductions in tail-biting and aggression by pigs provided with the opportunity to root are negative and positive examples of such adaptations.

     Of particular importance is the ability of the body systems to respond to changes in nutrition, especially in relation to energy and protein. One of the most extreme examples of response to undernutrition relates to studies by McCance and Mount (1960) on young pigs. These pigs were maintained for long periods on just sufficient quantities of a normal diet to maintain body weight. Whereas the maintenance requirement (MR) of normal piglets would be around 550 kJ kg 1 metabolic body weight (W0.75), these undernourished pigs showed an MR of 250 kJ kg 1 W0.75. The speed with which such changes occur in response to energy or protein deprivation was demonstrated by McCracken and McAllister (1984), who observed a reduction of approximately 25% in calculated maintenance requirement over a 3-week period. Changes in organ size relative to body weight have been observed during undernutrition of a wide variety of species, including poultry, pigs, cattle and sheep, and can be considered as contributing to the improved economy of the system. Conversely, increases in energy intake during lactation are associated with increased digestive organ capacity and increased metabolic rate. Similarly, offering a high-fibre (less digestible) diet to non-ruminants results in increased digestive organ size and weight, particularly in the hindgut, and increased energy supply from microbial fermentation.

     In summary, the human or animal body has a wide range of mechanisms for coping with external stressors and a multitude of short-term and long-term adaptations have been reported, of which only a few examples have been discussed above. (KJMcC) See also: Energy intake; Thermoregulation; Voluntary food intake.