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Animal stress and strain: Definition and measurements
Applied Animal Behaviour Science, 20 (1988) 119-126
119
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Animal Stress and Strain: D e f i n i t i o n and Measurements M.K. YOUSEF
Desert Biology Research Center, Department of Biological Sciences, University of Nevada, Las Vegas, N V 89154 (U.S.A.)
Yousef, M.K., 1988. Animal stress and strain: definition and measurements. Appl. Anim. Behav. Sci., 20: 119-126.
INTRODUCTION
The invitation to participate in this symposium has raised some uneasy feelings for me about discussing animal stress in Montreal. It is here, at the University of Montreal, that the late Hans Selye's world-famous experiments on the nature of stress, its causes and effects, were carried out. His pioneering work has been published in many scientific journals and in 30 books, some of which have been translated into various languages. Selye's extensive use of animals in his research work has been bitterly attacked by some in regard to the broad range of issues pertaining to the morality of using animals for experimental purposes. Responding to this criticism, in his book The Stress of Life, Selye (1976) defended his research program by stating that "every effort is made to diminish pain to the absolutely unavoidable minimum" and that "none of them [i.e. the rats] was exposed to unnecessary pain because of carelessness". It is beyond the scope of my presentation to argue the pros and cons of the "unavoidable" suffering caused by the various sorts of stress research programs. Instead, I would like to focus on the essence of what these stress research programs have achieved in terms of measuring stress and its effects. To do this, it is necessary to first answer the question: what is stress? STRESS TERMINOLOGY
To date, animal stress has not been given a universally accepted precise definition. The term stress is very general, and is used quite loosely by various scientists and in everyday speech by the layman. The divergent definitions are unfortunate, leading to confusion in gathering and integrating available data from the literature. This terminological problem represents primarily the bias, 0168-1591/88/$03.50
© 1988 Elsevier Science Publishers B.V.
120
and perhaps a slight difference in the emphasis and training, of the person using the term, as is shown by the following examples. The word stress is precisely defined in physics. A force applied to a spring either elongates or compresses it. This force is termed "stress", and the resulting relative change of the spring length is described as "strain" (Fig. 1 ). If the stressing force is too severe, the spring may be lengthened or compressed to a level where it cannot return to its original length after the stress is removed. In other words, "intolerable" stress results in permanent deformation of the spring. On the other hand, when the term stress is applied to biology, we meet a problem of semantics. For example, the environment (i.e. physical, biotic and psychosocial factors) may be stressing, and thus its components either singly or in concert are often called "stressors". The animal exposed to a stressful environment is described as either "stressed" or "strained". In other words, according to Hans Selye and his school of thought "You are stressed by my words", but according to those who do not depart from the well-established physical concept of stress and strain, "You are strained by my stressful words". The failure of biologists to reach a precise definition of stress has led to the unfortunate expression of animal stress in a negative connotation (i.e. impaired function, reduced efficiency and productivity, reduced fitness, etc.). Nevertheless, available evidence suggests that animal stress should be perceived as a positive influence STRESS IN PHYSICAL TERMS
STRESS/STRAIN RELATIONSHIP FORCE STRESS RESIONSE STRAIN = .
L
/',,
compressed
1
elongated Fig. 1. A diagrammatic representation of a force and its response on a spring to explain the definitions of stress and strain in physical terms.
121 when it leads to adaptation. In fact, it is unreasonable to expect animals' lives - in the wild or in captivity - to be free of stress; stress may be the rule rather than the exception. It has been shown that certain levels of stress may be necessary to prepare an animal to elicit the appropriate physiological, biochemical and behavioral adjustments needed to maintain its well-being in the face of unexpected environmental insults. A good example of this is the use of various techniques to acclimatize man before exposure to stressful thermal stresses, as is well known in hot industries such as mining, steel, glass-making, etc. Development of an acceptable and reliable index to measure stress and strain may lead to depiction of stress in terms of a spectrum where the dividing zone between its negative and positive connotations becomes visible. We have yet to develop a universally accepted method to measure stress and strain. Perhaps there is none. INDICES OF STRESS AND STRAIN Extensive effort has been given to the development of an index in which the effects of a given stressful environmental condition is assessed in terms of one or more measurable physiological responses. Relatively good indices have been developed to measure environmental thermal stresses, i.e. wind-chill factor, THI (temperature-humidity index), WBGT (wet-bulb-globe-temperature index), etc. However, the development of an index encompassing climatic, biotic and psychosocial factors has not yet been attempted. The concept of strain in biology refers to the fact that animals survive because of their ability to maintain homeostasis. Any stress (i.e. threat) which may occur to disrupt this homeostasis will produce physiological and biochemical reactions which attempt to either resist any disruption to homeostasis or result in regulation at a new state of homeostasis (Fig. 2). Historically, assessment of strain has emphasized the sympathetic-adrenal medullary axis, known as "the fight or flight" response of Cannon. This concept was followed by Selye, whose work illustrated that no matter what the identity of the stressors, the physiological responses (strain) are relatively the same and are mainly mediated via the pituitary-adrenal axis. This concept was termed the "General Adaptation Syndrome (GAS) ", and consisted of 3 phases: ( 1 ) the alarm stage, which includes the body's initial response; (2) the resistance stage, when adaptation is optimum and stress is overcome; (3) the exhaustion stage, in which acquired adaptation is lost and eventually death results. The 3-stage concept of GAS was widely accepted, but was not experimentally documented. The concept of a non-specific stress response was challenged as the field of neuroendocrinology matured. Experimental work proved that the entire neuroendocrine system was involved in helping the animal to maintain homeostasis in
122 Environmental S t r e s s e s
GT"
New
t
$ta~H~stasis ~t~
Fig. 2. Effects of stress on homeostatic mechanisms. Adequate physiological, biochemical and behavioral responses may overcome the stress, resulting in a new state of homeostasis, i.e. acclimatization. If the force of the stress overcomes the responses, disease and/or death is the final outcome. response to stress. Moreover, the pattern of hormonal responses differed from one type of stressor to another. Given the current understanding of stress biology, models have been developed to incorporate all aspects of the biological responses (defense) to stress. BIOLOGICALDEFENSE AGAINST STRESS Biological defense (strain) against stress is a complex p h e n o m e n o n and can best be phrased in control-system terminology, i.e. sensor, controller and controlled elements. T h e elements involved in sensing include the various internal and external receptors, and thus are dependent on the nervous system. The elements composing the other 2 sub-systems are illustrated in Fig. 3. Coping (or survival) of animals with any stress (threat) is directly associated with maintenance of homeostasis, which requires adjustments and feedback signals between the controlled and controlling elements of the control system. The interrelationship between both sub-systems provide the best biological defense against stress. In general, the central nervous and neuroendocrine systems are the essential components of the controlling elements and the feedback signals. These elements can determine with precision whether or not a given stress represents a challenge to the state of homeostasis. Based on this assessment, the controlled elements, including various physiological, biochemical and behavioral adjustments, are initiated to aid the animal in coping
123 STRAIN-
l
CONTROLLING ELEMENTS NS I
STRESS I
SYNERGISTIC AND/OR INDEPENDENTINPUT
NEUROENDOCRINE ENDOCRINE
CONTROLLED ELEMENTS
FEEDBACK
PHYSIOLOGICAL
BEHAVIORAL
BIOCHEMICAL
1
HOMEOSTATIC ADJUSTMENTS
Fig. 3. A diagrammaticrepresentationof the relationshipbetweenthe controllingand controlled elementsof a control-systemmodelfor stress and strain (NS-- nervoussystem). with the stress. Such adjustments result in either relieving the stress by avoiding it (behavior responses), or by the alteration of various physiological and/ or biochemical responses to re-adjust the animal's biological machinery for a possible new state of homeostasis with a different set-point. It should be recognized that different types of stresses do elicit significantly inconsistent and/or different biological defense mechanisms in the same individuals. Additionally, a single type of stress can trigger variable responses among individuals of the same species. Therefore, monitoring the biological responses to a stress may seem to be a reasonable approach to evaluate its intensity. Yet after the past 7-8 decades of research, we do not have a single universally acceptable index for stress. None of the suggested indices advocating the measurement of a single response, such as plasma corticoids or plasma catecholamines, has proved adequate. Also, to monitor all of the complex biological responses as a potential multiple end-point to measure stress is not only impossible, but is perhaps impractical. Recently, Moberg (1985) suggested a different approach to measure stress by stating; scientists should de-emphasize the traditional approach of measuring discrete physiological responses to stress, e.g. heart rate, or the plasma concentration of adrenal corticoids, and instead examine the effects stress has on reproduction, immunity and metabolism, which would serve as indicators of wellbeing". This alternative is certainly worthy of consideration and should be
124
tested. An additional criterion for stress may include a sensitive measure of animal performance, i.e. growth rate, production level, etc., which is the most important factor in the farm animals' sector. Such indices constitute multiple end-points to measure stress, since such traits as growth, production, reproduction, etc., reflect numerous physiological, biochemical and behavioral functions. CONSEQUENCES OF STRESS AND STRAIN: COMPENSATORY RESPONSES
Significant loss of animal productivity results from various environmental stresses. Unfortunately, most studies to date have focused on the acute and relatively chronic biological responses to stress and have failed to consider the responses following the elimination of the stress. Since in real life animals encounter periodic or seasonal stresses, it would be more realistic if our research work included a sufficient time-span to consider the consequent biological responses to the period of stress. Theoretical biological responses to periodic climatic changes are depicted in Fig. 4. Note that when the stress was removed, animal performance could be characterized by a compensatory response, i.e. performance levels exceeded the norm, rather than by a return to normal level. Thus if one considers the entire spectrum of animal responses, stress may have minimal effects over a productive cycle. The general concept of compensatory responses and capabilities has not been fully addressed by researchers. Evidence of such compensatory capabilities is well demonstrated when animals (in this case cattle ) receiving a less than ideal UNFAVORABLE
l FAVORABLE E~
,,¢
CLIMATE
CLIMATE
FAVORABLE CLIMATE iIh
~) OPTIMUM [--I ~ PROOUCTION ~ LEVEL ,'~ (~
AD-O"O-O. L'~O. ~
~ |I ~ ~,
NEW STEADY 9PCCMPENSATORY STATE | "~p RESPONSE ~,'**"-*"I | ~e 4;eACCLIMATI;IATION'° / ~ ,e"ee CHRONIC] 41,e
/,CO,E
RESPONSE/
RESPONSE
TIME Fig. 4. An example of biological responses in terms of animal performance to periodic climatic changes. {Modified from D.R. Ames, pp. 1-6, in: Moberg et al., 1986.)
125
,0E,L
//
I-3: 0 ill ,,OOE
STR~'~-
Zi
a o
2-"
;
i i
~e'~-O--II ~~ S E V E R E S T R E S S - "= o . L l . ~ " ~ . LEVEL
/
l_S RESS-- I I DURATION I
TIME
,,--,-
5. Illustration of compensatory growth in animals. (Modified from G.L. Hahn, pp. 7-10, in: Moberg et al., 1986. ) Fig.
nutritional level for optimum growth rate are placed in feedlots and provided with high-energy diets; a fact appreciated in the beef cattle industry. Recently, experimental evidence for compensatory growth in chickens and pigs exposed to nutritional a n d / o r thermal stresses was reported (Fig. 5). Note that upon exposure to a s e v e r e level of stress, animals cannot fully compensate. What are the stress threshold limits, i.e. duration or intensity, for compensatory responses? This important question remains unanswered. Future research should focus on defining the limits governing compensatory responses to provide a better understanding of how far it is necessary to modify adverse environments to acquire optimum animal performance and guard the animal's health, safety and well-being. STRESS RESEARCH NEEDS AND ANIMAL EXPERIMENTATION
What the foregoing discussion shows is that our knowledge of stress mechanisms and measurement and compensatory responses remains inadequate. No simple formula or computer program will give us answers to many of the questions which block the path to full understanding of animal stress. To fill the gaps in our knowledge, animal experiments must continue. To decide whether animal experiments are ethically justifiable in relation to their resulting benefits for humans or animals is no easy matter. Some critics argue that no knowledge, however fruitful, is worth the agony to which the test animals have been subjected. However, once one goes beyond superficial media reports
126 and anti-vivisectionist outcries, the important implications and applications of the results to health sciences and the well-being of m a n and other animals can be seen in a different light. I have no doubt that in some animal experiments suffering, and perhaps torment, was inflicted on the experimental animals. Such cases are few and far between, since the majority of research using animals is supported by governmental granting agencies that operate in accordance with a dual screening procedure; the ethics review committees, usually at the researcher's home institution, and the peer review committees under the auspices of the funding agency. This screening process is not infallible. Recent animal welfare legislation, added to the fact that funding has become more competitive, will dramatically reduce research projects that seem lacking in scientific merit or are dubious from a h u m a n e standpoint. In fact the scientific c o m m u n i t y is aware t h a t addressing the rights of experimental animals can lead to better research, since the animals will be healthier and more normal once certain rights are satisfied, i.e. optimum housing, bedding, exercise, etc. In closing, the scientific c o m m u n i t y must continue to take strong measures to ensure t h a t animal experiments are justifiable on a moral basis and on potential scientific merit. Also, it is time for the animal protectionists to look beyond the emotional outcries stemming from rumors regarding certain research procedures. All will be well served if reasoned and cautious evaluation of research work is u n d e r t a k e n without emotional rhetoric and hostility.
REFERENCES Moberg, G.P. (Editor), 1985. Animal Stress. American PsychologicalSociety, Bethesda, MD. Moberg, G.P., Swanson, L.V. and Weibold, J. (Editors), 1986. Limiting the Effects of Stress on Cattle. Utah AgriculturalExperiment Station Research Bulletin No. 512, Utah State University, Logan. Selye, H., 1976. The Stress of Life. McGraw-Hill,New York, 515 pp.
119
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Animal Stress and Strain: D e f i n i t i o n and Measurements M.K. YOUSEF
Desert Biology Research Center, Department of Biological Sciences, University of Nevada, Las Vegas, N V 89154 (U.S.A.)
Yousef, M.K., 1988. Animal stress and strain: definition and measurements. Appl. Anim. Behav. Sci., 20: 119-126.
INTRODUCTION
The invitation to participate in this symposium has raised some uneasy feelings for me about discussing animal stress in Montreal. It is here, at the University of Montreal, that the late Hans Selye's world-famous experiments on the nature of stress, its causes and effects, were carried out. His pioneering work has been published in many scientific journals and in 30 books, some of which have been translated into various languages. Selye's extensive use of animals in his research work has been bitterly attacked by some in regard to the broad range of issues pertaining to the morality of using animals for experimental purposes. Responding to this criticism, in his book The Stress of Life, Selye (1976) defended his research program by stating that "every effort is made to diminish pain to the absolutely unavoidable minimum" and that "none of them [i.e. the rats] was exposed to unnecessary pain because of carelessness". It is beyond the scope of my presentation to argue the pros and cons of the "unavoidable" suffering caused by the various sorts of stress research programs. Instead, I would like to focus on the essence of what these stress research programs have achieved in terms of measuring stress and its effects. To do this, it is necessary to first answer the question: what is stress? STRESS TERMINOLOGY
To date, animal stress has not been given a universally accepted precise definition. The term stress is very general, and is used quite loosely by various scientists and in everyday speech by the layman. The divergent definitions are unfortunate, leading to confusion in gathering and integrating available data from the literature. This terminological problem represents primarily the bias, 0168-1591/88/$03.50
© 1988 Elsevier Science Publishers B.V.
120
and perhaps a slight difference in the emphasis and training, of the person using the term, as is shown by the following examples. The word stress is precisely defined in physics. A force applied to a spring either elongates or compresses it. This force is termed "stress", and the resulting relative change of the spring length is described as "strain" (Fig. 1 ). If the stressing force is too severe, the spring may be lengthened or compressed to a level where it cannot return to its original length after the stress is removed. In other words, "intolerable" stress results in permanent deformation of the spring. On the other hand, when the term stress is applied to biology, we meet a problem of semantics. For example, the environment (i.e. physical, biotic and psychosocial factors) may be stressing, and thus its components either singly or in concert are often called "stressors". The animal exposed to a stressful environment is described as either "stressed" or "strained". In other words, according to Hans Selye and his school of thought "You are stressed by my words", but according to those who do not depart from the well-established physical concept of stress and strain, "You are strained by my stressful words". The failure of biologists to reach a precise definition of stress has led to the unfortunate expression of animal stress in a negative connotation (i.e. impaired function, reduced efficiency and productivity, reduced fitness, etc.). Nevertheless, available evidence suggests that animal stress should be perceived as a positive influence STRESS IN PHYSICAL TERMS
STRESS/STRAIN RELATIONSHIP FORCE STRESS RESIONSE STRAIN = .
L
/',,
compressed
1
elongated Fig. 1. A diagrammatic representation of a force and its response on a spring to explain the definitions of stress and strain in physical terms.
121 when it leads to adaptation. In fact, it is unreasonable to expect animals' lives - in the wild or in captivity - to be free of stress; stress may be the rule rather than the exception. It has been shown that certain levels of stress may be necessary to prepare an animal to elicit the appropriate physiological, biochemical and behavioral adjustments needed to maintain its well-being in the face of unexpected environmental insults. A good example of this is the use of various techniques to acclimatize man before exposure to stressful thermal stresses, as is well known in hot industries such as mining, steel, glass-making, etc. Development of an acceptable and reliable index to measure stress and strain may lead to depiction of stress in terms of a spectrum where the dividing zone between its negative and positive connotations becomes visible. We have yet to develop a universally accepted method to measure stress and strain. Perhaps there is none. INDICES OF STRESS AND STRAIN Extensive effort has been given to the development of an index in which the effects of a given stressful environmental condition is assessed in terms of one or more measurable physiological responses. Relatively good indices have been developed to measure environmental thermal stresses, i.e. wind-chill factor, THI (temperature-humidity index), WBGT (wet-bulb-globe-temperature index), etc. However, the development of an index encompassing climatic, biotic and psychosocial factors has not yet been attempted. The concept of strain in biology refers to the fact that animals survive because of their ability to maintain homeostasis. Any stress (i.e. threat) which may occur to disrupt this homeostasis will produce physiological and biochemical reactions which attempt to either resist any disruption to homeostasis or result in regulation at a new state of homeostasis (Fig. 2). Historically, assessment of strain has emphasized the sympathetic-adrenal medullary axis, known as "the fight or flight" response of Cannon. This concept was followed by Selye, whose work illustrated that no matter what the identity of the stressors, the physiological responses (strain) are relatively the same and are mainly mediated via the pituitary-adrenal axis. This concept was termed the "General Adaptation Syndrome (GAS) ", and consisted of 3 phases: ( 1 ) the alarm stage, which includes the body's initial response; (2) the resistance stage, when adaptation is optimum and stress is overcome; (3) the exhaustion stage, in which acquired adaptation is lost and eventually death results. The 3-stage concept of GAS was widely accepted, but was not experimentally documented. The concept of a non-specific stress response was challenged as the field of neuroendocrinology matured. Experimental work proved that the entire neuroendocrine system was involved in helping the animal to maintain homeostasis in
122 Environmental S t r e s s e s
GT"
New
t
$ta~H~stasis ~t~
Fig. 2. Effects of stress on homeostatic mechanisms. Adequate physiological, biochemical and behavioral responses may overcome the stress, resulting in a new state of homeostasis, i.e. acclimatization. If the force of the stress overcomes the responses, disease and/or death is the final outcome. response to stress. Moreover, the pattern of hormonal responses differed from one type of stressor to another. Given the current understanding of stress biology, models have been developed to incorporate all aspects of the biological responses (defense) to stress. BIOLOGICALDEFENSE AGAINST STRESS Biological defense (strain) against stress is a complex p h e n o m e n o n and can best be phrased in control-system terminology, i.e. sensor, controller and controlled elements. T h e elements involved in sensing include the various internal and external receptors, and thus are dependent on the nervous system. The elements composing the other 2 sub-systems are illustrated in Fig. 3. Coping (or survival) of animals with any stress (threat) is directly associated with maintenance of homeostasis, which requires adjustments and feedback signals between the controlled and controlling elements of the control system. The interrelationship between both sub-systems provide the best biological defense against stress. In general, the central nervous and neuroendocrine systems are the essential components of the controlling elements and the feedback signals. These elements can determine with precision whether or not a given stress represents a challenge to the state of homeostasis. Based on this assessment, the controlled elements, including various physiological, biochemical and behavioral adjustments, are initiated to aid the animal in coping
123 STRAIN-
l
CONTROLLING ELEMENTS NS I
STRESS I
SYNERGISTIC AND/OR INDEPENDENTINPUT
NEUROENDOCRINE ENDOCRINE
CONTROLLED ELEMENTS
FEEDBACK
PHYSIOLOGICAL
BEHAVIORAL
BIOCHEMICAL
1
HOMEOSTATIC ADJUSTMENTS
Fig. 3. A diagrammaticrepresentationof the relationshipbetweenthe controllingand controlled elementsof a control-systemmodelfor stress and strain (NS-- nervoussystem). with the stress. Such adjustments result in either relieving the stress by avoiding it (behavior responses), or by the alteration of various physiological and/ or biochemical responses to re-adjust the animal's biological machinery for a possible new state of homeostasis with a different set-point. It should be recognized that different types of stresses do elicit significantly inconsistent and/or different biological defense mechanisms in the same individuals. Additionally, a single type of stress can trigger variable responses among individuals of the same species. Therefore, monitoring the biological responses to a stress may seem to be a reasonable approach to evaluate its intensity. Yet after the past 7-8 decades of research, we do not have a single universally acceptable index for stress. None of the suggested indices advocating the measurement of a single response, such as plasma corticoids or plasma catecholamines, has proved adequate. Also, to monitor all of the complex biological responses as a potential multiple end-point to measure stress is not only impossible, but is perhaps impractical. Recently, Moberg (1985) suggested a different approach to measure stress by stating; scientists should de-emphasize the traditional approach of measuring discrete physiological responses to stress, e.g. heart rate, or the plasma concentration of adrenal corticoids, and instead examine the effects stress has on reproduction, immunity and metabolism, which would serve as indicators of wellbeing". This alternative is certainly worthy of consideration and should be
124
tested. An additional criterion for stress may include a sensitive measure of animal performance, i.e. growth rate, production level, etc., which is the most important factor in the farm animals' sector. Such indices constitute multiple end-points to measure stress, since such traits as growth, production, reproduction, etc., reflect numerous physiological, biochemical and behavioral functions. CONSEQUENCES OF STRESS AND STRAIN: COMPENSATORY RESPONSES
Significant loss of animal productivity results from various environmental stresses. Unfortunately, most studies to date have focused on the acute and relatively chronic biological responses to stress and have failed to consider the responses following the elimination of the stress. Since in real life animals encounter periodic or seasonal stresses, it would be more realistic if our research work included a sufficient time-span to consider the consequent biological responses to the period of stress. Theoretical biological responses to periodic climatic changes are depicted in Fig. 4. Note that when the stress was removed, animal performance could be characterized by a compensatory response, i.e. performance levels exceeded the norm, rather than by a return to normal level. Thus if one considers the entire spectrum of animal responses, stress may have minimal effects over a productive cycle. The general concept of compensatory responses and capabilities has not been fully addressed by researchers. Evidence of such compensatory capabilities is well demonstrated when animals (in this case cattle ) receiving a less than ideal UNFAVORABLE
l FAVORABLE E~
,,¢
CLIMATE
CLIMATE
FAVORABLE CLIMATE iIh
~) OPTIMUM [--I ~ PROOUCTION ~ LEVEL ,'~ (~
AD-O"O-O. L'~O. ~
~ |I ~ ~,
NEW STEADY 9PCCMPENSATORY STATE | "~p RESPONSE ~,'**"-*"I | ~e 4;eACCLIMATI;IATION'° / ~ ,e"ee CHRONIC] 41,e
/,CO,E
RESPONSE/
RESPONSE
TIME Fig. 4. An example of biological responses in terms of animal performance to periodic climatic changes. {Modified from D.R. Ames, pp. 1-6, in: Moberg et al., 1986.)
125
,0E,L
//
I-3: 0 ill ,,OOE
STR~'~-
Zi
a o
2-"
;
i i
~e'~-O--II ~~ S E V E R E S T R E S S - "= o . L l . ~ " ~ . LEVEL
/
l_S RESS-- I I DURATION I
TIME
,,--,-
5. Illustration of compensatory growth in animals. (Modified from G.L. Hahn, pp. 7-10, in: Moberg et al., 1986. ) Fig.
nutritional level for optimum growth rate are placed in feedlots and provided with high-energy diets; a fact appreciated in the beef cattle industry. Recently, experimental evidence for compensatory growth in chickens and pigs exposed to nutritional a n d / o r thermal stresses was reported (Fig. 5). Note that upon exposure to a s e v e r e level of stress, animals cannot fully compensate. What are the stress threshold limits, i.e. duration or intensity, for compensatory responses? This important question remains unanswered. Future research should focus on defining the limits governing compensatory responses to provide a better understanding of how far it is necessary to modify adverse environments to acquire optimum animal performance and guard the animal's health, safety and well-being. STRESS RESEARCH NEEDS AND ANIMAL EXPERIMENTATION
What the foregoing discussion shows is that our knowledge of stress mechanisms and measurement and compensatory responses remains inadequate. No simple formula or computer program will give us answers to many of the questions which block the path to full understanding of animal stress. To fill the gaps in our knowledge, animal experiments must continue. To decide whether animal experiments are ethically justifiable in relation to their resulting benefits for humans or animals is no easy matter. Some critics argue that no knowledge, however fruitful, is worth the agony to which the test animals have been subjected. However, once one goes beyond superficial media reports
126 and anti-vivisectionist outcries, the important implications and applications of the results to health sciences and the well-being of m a n and other animals can be seen in a different light. I have no doubt that in some animal experiments suffering, and perhaps torment, was inflicted on the experimental animals. Such cases are few and far between, since the majority of research using animals is supported by governmental granting agencies that operate in accordance with a dual screening procedure; the ethics review committees, usually at the researcher's home institution, and the peer review committees under the auspices of the funding agency. This screening process is not infallible. Recent animal welfare legislation, added to the fact that funding has become more competitive, will dramatically reduce research projects that seem lacking in scientific merit or are dubious from a h u m a n e standpoint. In fact the scientific c o m m u n i t y is aware t h a t addressing the rights of experimental animals can lead to better research, since the animals will be healthier and more normal once certain rights are satisfied, i.e. optimum housing, bedding, exercise, etc. In closing, the scientific c o m m u n i t y must continue to take strong measures to ensure t h a t animal experiments are justifiable on a moral basis and on potential scientific merit. Also, it is time for the animal protectionists to look beyond the emotional outcries stemming from rumors regarding certain research procedures. All will be well served if reasoned and cautious evaluation of research work is u n d e r t a k e n without emotional rhetoric and hostility.
REFERENCES Moberg, G.P. (Editor), 1985. Animal Stress. American PsychologicalSociety, Bethesda, MD. Moberg, G.P., Swanson, L.V. and Weibold, J. (Editors), 1986. Limiting the Effects of Stress on Cattle. Utah AgriculturalExperiment Station Research Bulletin No. 512, Utah State University, Logan. Selye, H., 1976. The Stress of Life. McGraw-Hill,New York, 515 pp.