At the end of this unit, you should be able to:
• explain the concepts of adaptation and acclimatisation
• identify various means by which animals cope with their environments
• describe the concept of environmental energy balance, distribution and transfer which often result in heat or cold stress
• discuss the effect of environmental stress on animal productivity and mechanism of body defence against stress.
3.1 Some Concepts of Adaptation and Acclimatisation
Due to the super-imposing influence of the environment on the survival, behavior, character (expressed as performance, productivity and responses) of farm animals, great consideration is often given to modifying the environment by livestock owner to ensure its suitability.
Equally of primary importance is the need for adaptation and acclimatisation of the hereditary and physical features of the animal itself to the predominating environment if stock must survive under
favourable and harsh environment conditions. Adaptation and acclimatisation of the animal to the environmental conditions form the
basis for any modification or adjustment required on the part of farmer to apply into the management system for survival and improved productivity of the stock. In other words, an animal must first and
foremost undergo natural selection for survivals in an environed before improve growth, reproduction, and other productive traits can be contemplated. By natural selection therefore, the genes favourable for survival in that environment interact, interrelate, associate and coordinate with the environment to ensure the survival of the animal.
This condition is known as adaptive norm which is an array of related genotype capable of adjusting to the demands of the environment. It embodies heterosis, adaptive polymorphism and homeostasis. From the genetic point in view, fitness is the ability to produce offspring which
will survive and produce their likes. Thus, a biologically fit population is one that is capable of adapting to its environment in such a way that it can survive and reproduce. The fitness of animals includes more than their ability to survive but also to withstand the demands of future
environment. These criteria of fitness or ability will depend on adaptation variability, stability and rate of environmental change. It is important to note that while domestication improves on all criteria of
fitness, diseases negate and reduce fitness to almost nil. It may be deduced from the foregoing that living system not only respond to the environment but also control and regulate their reactions towards environmental effects. This is the concept of homeostasis.
Homeostasis
Homeostasis is the constancy of the internal environment of an animal and the mechanism by which such constancy is maintained. It involves regulation of many internal body variables as temperature, PH, salt, water content, nutrient and chemical composition etc. mammals and
birds that occupy the highest evolutionary scale, possess the greatest regulatory capacity and regulate the greatest number of internal factors with the best precision through a deployment of appropriate
physiological adaptation and compensation.
Physiological Adaptation
Any change in the internal condition of an organism which favours its survival during changes in the environment is known as physiological adaptation. Physiological adaptation involves the possession of
mechanisms and capacity that allow organism to adjust itself to other living organisms and to external physical environment. Physiological adaptation may manifest as acclimatisation, acclimation, habituation, learning and conditioning.
Acclimatisation:
Acclimatisation refers to a long term adaptive physiological adjustment which results in an increased tolerance to continuous or repeated exposure to complex climatic conditions
occurring under natural condition.
Acclimation:
Acclimation refers to adaptive changes to a single climatic variable normally produces in an artificial environment e.g. increasing temperature in a climatic chamber.
Habituation: habituation is a gradual change which may lead to a loss of response as a result of repeated stimulation.
Learning:
learning is the acquisition of a new response or a qualitative change of an existing response which may be inform of inhibition or increase of an existing response by a new stimulation.
Conditioning:
Conditioning is the transfer of an existing response of new stimulus. Besides physiological adaptation, other forms of adaptations in organisms include biological and genetic adaptation.
Biological Adaptation
Biological adaptation involves the morphological, anatomical, physiological, biochemical and behaviouralcharacteristic of the animal which promote welfare and favours survival in a specific environment.
Genetic Adaptation
Genetic adaptation refers to the heritable animal characteristics, which favour the survival of a population in a particular environment; such favourable characters are derived from evolutionary changes over many generations.
It is important to note that individual animal has limit to its ability to
regulate its reaction according to genetic make-up. Animal has a defined range of environmental variation within it can live. This is referred to as Zone or Range of Tolerance, beyond which the animal will exhibit some resistance, suffer some damage, and eventually succeed. The Zone or Range of tolerance has upper and lower limits at either ends also known
as Upper and Lower Incident Lethal levels at which death occurs.
The micro-climatic environment has the most profound influence onanimal production, physiology and behavior. It is determined by such meteorological factors as: air temperature, air pressure, relative humidity of air, wind density and air density as well as radiation (defined as the heating and cooling of theatmospheric air). Man can modify his micro�climate by building houses, offices, travelling in cars, planes, ships, wearing clothes etc. animals on the other hand modify the micro-climate by burrowing or moving into or from these meteorological factors in
search of warmth, cold or calm. The behavioural entrance or escape from inclement meteorological conditions is, however, limited in providing the required optimum body conditions at all times. The
primary means by which animal control, or respond to variations in the meteorological factors, and thereby maintain its body conditions, is through the flow of energy or total energy exchange between the animal
and environment.
Energy is the ability to do work. It is regarded biologically as the source of life and movement. All life process involves in one way or other the expenditure of energy through work. An animal cannot continuously gain or lose energy to the environment, otherwise it will die. Thus
animal tolerates energy gains or losses from their immediate environment only within certain limits in the Zone or Range of Tolerance.
An animal exchanges its energy with its environment (micro-climate) through radiation, convention, condition, evaporation and metabolism.
The pathways of energy flow are many and characterised by direct interaction between animal and its environment. In order to survive in a given environment over a long period of time, the energy gain of an animal must be equal to energy loss.
The energy balance of an animal is therefore expressed as:
NB: The positive signs represent gain of energy and the negative signs depict loss of energy. After a long term, the total must add up to zero, but within a short period there can be loss or gain of energy while an animal is cooling down or warming up.
Modes of Energy Transfer
1. Radiation:
Radiation is a form of energy transfer in wavelength by electromagnetic waves. Radiant energy is ubiquitous (i.e.
found everywhere) and is emitted from all objects whenever the surface temperature of the object exceeds absolute zero. Energy loss by radiation is one of the primary sources of heat loss by animals. Objects at ambient temperature (23 – 250C) radiate mostly infrared rays at long wavelength and beyond. Objects at very high temperature e.g. the sun radiate shorter wavelength in the ultraviolet and blue region. The amount of energy radiated is proportional to the 4th power of the surface temperature in absolute degree i.e
Radiation in the natural environment is derived from 2 – main sources: high temperature of the sun (direct source) and the extended source which is the thermal radiation from the ground, trees clouds or atmosphere. An animal exposed to direct solar radiation, absorbs certain quantity of incident energy depending on the surface exposed, angle of surface and absorbance of the surface. The absorbance of the surface determines the greatest
percentage of the incident radiation absorbed. An animal with a black body surface absorbs all the incident energy and its body temperature is much more affected by the quantity of radiation
falling on its surface.
2. Conduction:
When an object is in contact with another, molecular motion can be transferred from one object to another by a process of bombardment similar to diffusion activity. Such transfer of molecular motion is known as thermal conduction. Air for example, is a poor thermal conductor while water is a good
one. Conduction however occurs only from regions of higher to region of lower temperature. It is only the heat that is transferred and not the material.
3. Convection:
this is a special form of heat transfer by conduction.
It occurs where the surrounding medium is fluid e.g. air or water.
Upon increase in temperature air or water rises due to its decreasing density (becoming less dense) and the layer of fluid next to the warm object is replaced by another mass of cooler (more dense) fluid. The exchange of energy by convection is proportional to:
a. Surface area
b. Temperature differential between animal surface and free air beyond the boundary layer
c. Convection coefficient
C α A (Ts – Tc)
C = hcA (Ts – Tc)
Where hc = Coefficient of convection which depends on thermal conductivity and thickness of the boundary.
A = Surface area
Ts – Tc = Temperature differential between animal surface and free air beyond the boundary layer.
The Boundary Layer plays an important role in maintaining body temperature. The concept of Boundary Layer is given as: Just immediately to the surface of the animal is a bound layer of air which is
more or less stationary and it is the transfer zone between the integument temperature (skin temperature) and temperature of the free air beyond the boundary layer. As it is already known heat is always transferred along the temperature gradient occur between the surface temperature of animal and air temperature of short distance away from
the surface. The rate of heat conduction across the boundary layer is dependent on:
(1) the thickness of the layer
(2) the temperature differential between the skin and fluid
(3) the thermal conductivity of the fluid
(4) movement or stillness of air.
Transfer of energy across the boundary involves two methods: by molecular conduction across the boundary layer and into the free air and by mass movement of air. Natural convection occurs when there is no wind or free air while forced convection occurs when there is wind to
transfer energy.
3.2 Adaptive Means of Coping with the Environment
Temporary acclimatisation has to do with adaptation to or tolerance to, a short duration of heat stress rather than to more moderate continuous heat stress. Temporary acclimatisation effect of the heat stress. The wide changes in heat stress between the day and night in three dry zones and between seasons of the year readily prepare animal to acclimatise and adapt easily.
Permanent acclimatisation to climate stress may be due to changes in the
behavior of the animal or to changes in physiological relations that may or may not be inherited. Natural or artificial selection for morphological characteristics that assist the animal to acclimatised may be needed for permanent acclimatisation such changes in behavior of domestic livestock ought to form good management aims and indicators to facilitate the adaptation. Common adaptive behaviours to heat stress by tropical livestock include sluggish movement to reduce muscular heat production, raising of the wings among poultry to allow for air circulation and heat loss; tendency of livestock to graze at night and more often seek to stay under shade in the hot day; drinking of large volume of water; slow rate and reduced consumption of feed; pigs
Walloons in water ponds or moist environment and often stretch themselves in lying position. Physiological adaptations to tropical environment on the other hand are several and, often, not easily
observed. These include:
• decreasing the body metabolic rate and varying the body temperature
• varying coater turnover (loss) by concentration of waste products in urine to conserve body water • recycling of urea in the saliva to conserve nitrogen in period when forage is lacking
• varying tolerance to salt concentration in drinking water
• decreasing certain hormonal activity, for example, the thyroid and adrenal hormones
• panting or reduction of body heat through the short and quick breath.
There are also morphological adaptations or characteristics that help to achieve environmental acclimatisation especially to temperate animal to tropical condition. These include:
• possession of large skin area in relation to live weight especially develop or large comb
• skin pigmentation, with short and light coloured hair to reduce heat and light radiation absorption
• tendency to have thin layer of subcutaneous fat deposit
• possession of long leg by Desert sheep or goats to cover long distance in search of feed and water.
Most adaptative features are derived from natural selection and they often form the basis for ultimate development of a new and more adapted breed or strain with minor manipulation by man, as the
manager. Breeding of animal for higher productivity must recognise important adaptive traits that may help to achieve good performance.
Similarly, importation of livestock from a distinct environment to another need to take into consideration adaptation to major climatic factors and disease and parasite criteria, feed situation, prices of inputs and products as well.
3.3 Concept of Energy Balance in the Physical Environment
Since mammals and avian that form bulk of farm animals are homeotherms and hence maintain a constant body temperature, they possess means for producing and losing heat during extreme cold or heat conditions respectively.
Heat Production: Animal produce heat when transforming chemical energy of food into work. Under normal circumstances a grazing animal while in the sun may gain heat directly or indirectly from solar radiation.
The added solar heat and metabolic heat generated from food and muscular activities form the animal heat gain.
The heat gains in one animal vary from another as a result of:
(a) the intensity of heat production by different organs varies depending of the weight of the organ. On net weight basis, the brain generates heat faster than the muscular tissue due to higher density of the former
(b) the body size also affects heat requirement, for instance, smaller
sized animals require a greater heat production per unit weight than larger sized animals if the same temperature is to be maintained. This is because the smaller the animal, the greater the surface area and the more the heat loss
(c) specific surface area which is the ratio of surface to volume determines heat loss. This is because with increasing body size,
the surface to volume of the animal increases, and therefore the relative surface from which is dissipated increased. When heat production is better expressed per unit surface area, the effect of body size is largely eliminated. Heat production is better expressed in terms of body surface area.
Evaporation: Evaporative water loss occurs from the skin after it has been secreted by the sweat glands. Other areas of evaporative of water losses include respirative water loss and sweating. The two forms of water are two major processes used in temperature regulation in animals.
The process of evaporation of water requires a large amount of energy and is therefore used to cool the body of animal. Evaporation occurs only when the air is not saturated already with water.
Metabolic and Chemical Heat Transfer
With increasing environmental temperature body temperature for
homeotherm is constant while that for poikilotherm increases. Increasing
ambient temperature for poikilotherms increases the metabolic rate and decreases with decreasing ambient temperature similar normal chemical
reaction. However, with homeotherms, the metabolic rate decreases with increasing temperature and increases with decreasing temperature. Other chemical reactions like muscular activities, shivering and metabolic reaction of the liver are also involved e.g. liver apart from producing heat also releases glucose into circulation which is a basic requirement
for chemical temperature regulation.
Factors Affecting Heat Production
Ingestion of food leads into increase in heat production which varies greatly with the type of nutrients ingested. Also in ruminants the rumen micro-organism constitutes an auxiliary source of heat (about 10 per cent) in addition to the animal’s moral heat production. The foetuses as well as lactation add up to the amount of heat production by the dam.
During severe cold or active physical exercise, the heat generation by muscular activities increases while the heat from abdominal organ decreases. In cattle and sheep, heat production is about 10 per cent greater in standing position than in lying position. In pregnant animal, foetus metabolism together with the acceleration of body processes of the dam result in an increment in the total heat production.
The presence of brown fat is another source of heat production. Brown fat is found in rodents but has been found in other animals including man. It is especially useful in homeotherms exposed to cold and hibernators. The brown adipose tissue is distributed around vital organs of the thorax, along sympathetic ganglia of the central nervous system,
around the cervical and thoracic segment of the spinal cord to prevent loss of heat and excessive cold from inactivating the function of the vital organs. Both the metabolic and thermo-genic actions of the brown fat are by stimulation from sympathetic nervous system under cold
condition.
Heat Loss:
Heat loss from animal body is by two means:
(1) sensible heat loss i.e. through radiation, convection and conduction
(2) insensible heat loss which is through evaporation of water.
Heat loss by means of sensible heat loss offers little or no control for the
animal to regulate unlike insensible heat loss in which animal exerts marked control. Heat transfer by sensible heat can be in either direction of loss or gain, while insensible heat transfer is only along one direction i.e. through loss from animal to the environment.
Heat transfer involves two forms of gradient:
• Inner gradient and out gradient. Inner gradient concerns with heat flows from the core of the body to the surface of the body.
• Outer gradient on the other hand refers to the heat that goes from the surface of the body to the environment. Heat transport along the inner gradient is affected by conduction across the tissues and
by convection by the blood. Along the outer gradient, heat transport is by the following:
1. Convection across the hair coat and boundary layer of still air surrounding the body
2. Convection from the boundary layer of air to the fully moving air
3. By radiation from the tips of the air across the boundary layer
4. Evaporation across the hair coat and boundary layer.
3.4 Effect of Stress on Animal Productivity
Generally heat result in increase in the blood volume and a decline in red blood count. However, longtime heat stress at moderate level leads to haemo-concentration as a result of heat loss. Heat loss also creates an acid surplus due to formation of lactate, metabolic or respiratory acidiosis which may exhaust the bicarbonate buffer system of the blood resulting in a fall of PH. The increase in blood volume also leads to a drop in total protein concentration. The decline in tyroxine secretion is accompanied by increase in ACTH (growth hormone) released and also induces the release of vasopressin from the posterior pituitary gland.
Other effects of heat stress on specific productive parameters are discussed as follows:
1. Effect of heat of reproductive Function:
In the male, heat stress impairs testicular function resulting in depressed
spermatogenesis, lower testosterone function before leydig cells, delayed puberty, and decrease libido. There is also an increase in sperm abnormalities and impaired integrity of sertoli cells.
Heating the testises to abnormally high temperature cause a complete cessation of spermatogenesis. There is also a decline in sperm mobility, sperm density and fluctuation in seminal PH.
In female animals there is delayed puberty, reduced ovulation rate, increased incidence of silent heat. Short oestrous and prolonged oestrous cycle. Other effects are reduced conception
rate, increased rate of embryonic mortality, increased litter abnormality, increased incidences of abortion and therefore
depressed little size, low birth rate, poor growth rate and poor lactation.
2. Effect of Light on Reproduction:
Certain animals are seasonal breeders while other breed throughout the year. Experiments have shown that exposing sheep to constant photoperiod reduces
spermatogenesis and if the photoperiod is reduced to 13 hours daily there is increase in ovulation rate. Seasonal fluctuation in
day length is an important factor affecting the length of breeding in some other parts of the world except the tropics. In ewes the constant photoperiod is not as efficient as reducing photoperiod in inducing oestrus.
3. Effect of Climate on Egg Production:
It has been demonstrated that among all factors affecting egg production, temperature, humidity and light play a major role. An increase in temperature
above 270 C, reduces the number of egg laid per year and egg shell thickness. If temperature is above 270C the shell thickness is reduced and the egg quality is reduced. Of all the parts produced, the egg yolk is the least susceptible to heat stress while the albumen is most susceptible to high relative humidity also lower egg production.
In birds, the control of sexual maturity depends largely on the pattern of photoperiod. Exposing bird to increasing day length is known to hasten
sexual maturity while shorter day length delays sexual maturity.
Increasing the day length also causes an increase in the number of egg laid per year.
However, beyond certain period of day length, increasing the photoperiod will make the bird photo-refractory i.e. resistant to photoperiod with cessation of reproductive.
3.5 Mechanism of Body Defence against Stress
Mechanism of Body Defence against Cold
The animal body can defend itself against cold by three means namely:
storing or conserving heat, through insulation and by increasing heat production or a combination of all. Increasing the body insulation against cold is more economical considering energy expenditure involved.
Differences in species nurtured by adaptation have favoured economic
ways of supporting higher body insulation to animals living in cold climates. The body insulation is in three classes:
1. Peripheral Tissue: This act by vasoconstriction of the coetaneous and sub-coetaneous to reduce thetemperature
gradient from the skin surface to the environment and also by the aid of subcutaneous fat.
2. Hair Coat Insulation: This depends entirely on trapped air which occupies over 95 per cent of volume of the air coat. The
insulating capacity increases with thickness and air density of the air coat. For example, temperature and arctic species of animals tend to develop thick air coat while most tropical animal have
thin air coat. There is also a non-linear fall of temperature along the hair coat, so that as the body size of animal decreases below certain level, the level of the hair coat decreases. However, wind
and rain greatly reduce the efficiency of hair coat as insulating mechanism. But the impeding effect of wind diminishes with
increasing hair coat density.
3. Insulation of the Air: This insulation is caused by the layer of air or boundary layer adhering to the surface of the hair coat in the hairy species and to surface of the body in non-hairy species.
It varies from one specie to another and is almost independent of the body size. The insulating mechanisms of the boundary layer decreases with increasing air speed.
Mechanism of Body Defence against Heat
This can be effected by:
1. Behavioural means e.g. moving away from heat source, drinking more water, looking for shed or cold surface
2. Reduction in body insulation e.g. (a) vasodilatation to the ears, legs and tongues as more blood flows there to dissipate heat by taking advantage of hairlessness of the body parts. (b) Shedding of hair: If environmental temperature is equals to body temperature, vasodialation ceases to be very effective.
3. Increase in temperature loss: This occurs either from the skin or respiratory tract. The evaporation from skin is by sweating through sensible and insensible heat loss. Loss of heat energy from respiratory tract is by panting as often noted in chicken or dog.
4. By lower rate of heat production if exposed to heat stress. The appetite drops and animal consume less feed. It also reduces its motor and thyroid activities. The thyroid gland regulates basal metabolism for homeotherm.
5. Increase in the reflectance of hair coat to solar radiation. Animal with lighter hair coat reflect more heat than those with darker coat colour. The relative importance of cutaneous and respiratory
evaporation varies from specie to another. A sweating animal controls the amount of water while a panting animal controls the
amount for larger proportion of total evaporation than European type of cattle. Also within a breed, heat tolerant animal have higher cutaneous and lower respiratory evaporation than heat intolerant counterpart.
4.0 CONCLUSION
Animals living in different ecologies of the world have for several decades and for every moment of the day developed means for coping with their environment as a matter of survival. Farm animals expectedly must go beyond survival to improve their productivity notwithstanding
the degree of stress to contend with. Breeding or introduction of animal into an environment should recognise important adaptive traits and concepts that may help to achieve good performance. Consideration must be given to effects and adaptive mechanisms for different stress factor.
5.0 SUMMARY
The concept of fitness of farm animal extends from ability to survive now and withstand environmental demands in future, to ability to produce sufficiently to justify cost of domestication. Homeostasis, physiological, biological and genetic adaptations are concepts in
understanding the means by which animal cope with their environment.
The concept of energy balance forms the central pivot which tilts the environmental stress in different directions for animal to respond.
Effective responses of animals to environmental stress often result in
depressed productivity even in attempt to apply mechanisms to ward off the pervading stress condition. The responsibility of the producer is to
understand these concepts in the management of the stock for survival
and higher productivity by controlling the overbearing influence of the environment.
6.0 TUTOR-MARKED ASSIGNMENT
1. Define and explain the following concepts.
(a) Heterosis
(b) Homeostasis
(c) Habituation
(d) Conditioning
(e) Learning
2. List and discuss the various modes of energy transfer
3. Describe the influence of heat stress on five specific animal performance traits.
7.0 REFERENCES/FURTHER READING
Adeyemo, O. Heath, E., Adadevoh, B. K., Steinbach, J. and Olaloku,
E.A. (1979). “Some Physiological and Behavioural Responses in
Bos indicus and Bos taurus Heifers Acclimatised to the Hot,
Humid Seasonal Equatorial Climate.” International Journal of
Biometerology 23 (3): 231 – 237.
Bligh, J., Cloudsley – Thompson, J. L. & MacDonald, A.G. (eds.).
Environmental Physiology of Animal. Oxford, U.K: Blackwell
(Publ.)
Findlay, J.D. (1954). Climate Physiological of Farm Animals. Met,
Monogr 2: 19 – 29.
Johnson, H.D. (1985). “Physiological Responses and Productivity of
Cattle.” In: Yousef, M.K. (ed). Stress Physiology in Livestock.
Vol. II: Ungulates. Boca Raton: CRC Press (Publ.)
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