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Avian Reproduction: A Sociobiological Perspective1
Avian Reproduction: A Sociobiological Perspective1 H. B. GRAVES
Department of Biology and Department of Poultry Science, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received for publication January 26, 1983) (Key words: behavioral adaptation, reproductive success, sexual reproduction) 1983 Poultry Science 62:1667-1670
Behavior is modulated greatly by prior experience and present circumstances acting on and through neural and endocrine mediators. The ultimate basis for the sensory-motor apparatus of an organism is the genetic code carried by each cell. This code sets boundaries on types of stimuli that can be received and, by designing receptor systems and integration circuits, basically establishes not only the "form but also the functional behaviors exhibited. However, continual input from past and present experiences, as well as expectations of consequences of alternatives, influences the ongoing stream of behavior. Information about ways in which neural and endocrine systems operate in concert to receive and integrate internal and external stimuli and modulate ongoing behavior is rapidly accumulating. Unfortunately, much of the information is highly technical and specialized, and interdisciplinary teams are increasingly rare. This symposium represents a limited approach to coordinating presentation of information on one important aspect of the poultry industry, that concerning reproduction of the avian male. My task is simply to introduce the topic by providing a very abbreviated overview of current behavioral biology as it relates to sex and the avian male.
BEHAVIOR AND ADAPTATION At the very heart of social biology are the twin concepts of natural selection and adaptation. Evolution by natural selection may be envisioned as a sort of race to reproduce among
'Article No. 6613 in the Journal Series of the Pennsylvania Agricultural Experiment Station.
individuals in which the score is tallied by relative reproductive success (RS), i.e., by relative numbers of viable offspring contributed to the next generation. Any trait, whether physiological, morphological, behavioral, or otherwise, which is influenced by inheritance may contribute positively or negatively to RS, and such traits are selected for or against by natural or artificial selection. Ethology and sociobiology emphasize the role of behavior in affecting RS. Hence, sex and reproduction are central issues in these disciplines. In 1964, W. D. Hamilton explicitly developed a theory of natural selection that is changing our perception of the evolution of behavior (see Hamilton, 1964, 1970, 1971a,b, 1972). Some aspects of Hamilton's theory, now widely known as "kin selection," are of practical as well as theoretical importance. Hamilton formalized what had been recognized by earlier workers, including Darwin, Haldane, and Lush. Selection for a trait can occur through differential reproduction of a particular individual exhibiting that trait or by differential reproduction of relatives, which, depending on the degree of relationship (r), have a predictable probability of carrying the same genes as the reference individual. Thus, in the case of fulls-sibs, the net loss in reproductive fitness to a brother, who leaves no offspring but who somehow (defense, cooperative display, etc.) increases the number of offspring his sibling has by twofold.is zero. That is, 7 between full-sibs is one-half; his brother leaves twice as many offspring as would otherwise be the case so the total loss in RS for the altruistic brother is 1.0 but the gain for the altruistic brother is two times r, or 1.0. Hence, the altruist experienced no net loss in fitness even though he did not reproduce. The theoretical importance of Hamilton's work cannot be overemphasized;
1667
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
INTRODUCTION
GRAVES
1668
EVOLUTION OF SEX
Curiously enough, sexuality per se is a major problem of evolutionary biology and behavior. As Williams (1975) noted with characteristic clarity, the "immediate advantage of asexual reproduction is generally conceded by those who have seriously concerned themselves with the problem." The basic argument is simple, although many highly sophisticated models exist. The fundamental problem is that sexual females, which produce genetically reduced (meiotic) eggs that must be fertilized by males to restore the diploid genetic complement, have offspring with only one set of the maternal genes. Conversely, asexual females produce offspring from unreduced (mitotic) eggs that do not require fertilization, and they have offspring with both sets of maternal genes. Hence, genetic representation of the asexual female is twice that of a sexual female in the ¥x generation if each produces an equal number of offspring. This "cost of meiosis" (Williams, 1975) disappears if (as presumably is usually the case) the number of offspring produced per unit of resource is equal in sexual and asexual parent(s). In that case, sexual parents, although only related to their offspring by one-half, produce twice as many offspring as do asexual parents because they each invest only one-half the resources per offspring as does an asexual parent. A more serious problem with sexuality is the production of males (Maynard Smith, 1971a,b, 1974, 1978a,b; Treisman and Dawkins, 1976).
In asexual reproduction, resources are converted directly to offspring; in sexual reproduction, gametes constitute an intermediate step, and a wasteful one if more male than female gametes are produced. The excess male gametes constitute a cost because they are made from resources not converted to offspring. Advantages of sexuality reside, in large part, in the genetic diversity afforded, especially in a changing environment. PARENTAL INVESTMENT AND MALE REPRODUCTIVE STRATEGIES
Recognition of the fact that males produce smaller and more numerous (usually motile) gametes than the female and that reproductive rates are therefore limited by female rather than by male gametes stimulated Trivers (1972) to formulate a very far-reaching theory on parental investment. Females produce (by definition) a limited number of very large gametes and therefore have a much larger investment in each zygote than do males. Because females invest so much in each gamete, their RS is limited to a large extent by the number of gametes they can produce. By contrast, males invest very little in each zygote, and male RS is limited by the number of zygotes they can gain access to rather than by gamete production. Hence, in the simplest sense, males tend to follow behavioral reproductive strategies based on gaining access to female zygotes (e.g., defending several females from male competitors, defending territories or other resources to attract multiple females, developing morphological and behavioral characteristics enhancing their attractiveness to females, etc.). In that same simplistic sense, females, having a limited number of gametes to use in the evolutionary "game", are choosy in mating, preferring to mate with the highest quality mates available (males can afford to be less discriminating than females). Hence, the optimum number of offspring for males will exceed the optimum number of offspring for females because the cost per offspring is lower for males than for females (Trivers, 1972). This brings about different selection pressures on males and females, promoting sex differences in specialized behavior and morphology as well as in underlying neuroendocrine and other physiological mechanisms. This raises problems of both theoretical and practical importance.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
the practical implications are also readily apparent when one considers that fertility in the avian male is indeed an industry problem and that natural selection has clearly promoted a male reproduction scheme in some birds in which only the dominant male of a brotherhood coalition actually mates. For example, Watts and Stokes (1971) demonstrated that brotherhood groups of wild turkeys cooperate in synchronous display toward females, but subordinate males rarely mate. They found that less than 5% of the wild turkey males on their study site accounted for 100% of the matings. To the extent that such patterns of male sexual behavior have represented genetically adaptive strategies over evolutionary time, alleviation of the problem may not reside in correction of neurophysiological dysfunction but in behavior-management manipulation.
SYMPOSIUM: THE AVIAN MALE BEHAVIOR AND MANAGEMENT
However, the most common domesticated birds exhibit characteristics uncommon in the bird world. Although males and females of the vast majority of avian species look alike, behave similarly, mate on a one-to-one basis, and participate about equally in caring for their young, chickens, turkeys, and most other domestic poultry are usually sexually dimorphic in both appearance and behavior, polygynous, and differ considerably in terms of sex roles related to reproduction and group living. Hence, reproductive behavior in males of domesticated birds more closely resembles that of their mammalian counterparts than it does avian males in general. The model of PI sketched has allowed man to take advantage of what birds, especially chickens, apparently thought was a good idea, i.e., the sharing of "pregnancy and lactation" duties. Not only does the avian model allow the male to share these responsibilities, it provides mankind a golden opportunity to interrupt the system and take over the postzygotic parental investment with modern incubators, hatchers, and artificial rearing systems. However, "re-
sidual" behaviors and their underlying neurophysiological machinery may be counterproductive in modern management systems, and, in that sense, poultrymen may be attempting to modify an antique automobile into a streamlined jet aircraft. That is, the evolutionary history of Gallus has been one in which males were designed to aggressively defend territories, including nest sites, feeding and water sources, and a few (2 to 4) females from other males, traits no longer desirable in commercial poultry enterprises. Male fertility is an increasingly serious problem in the poultry industry, as is male domination of feed troughs and associated overconsumption. Male chickens were unlikely to be faced with problems of inseminating more than a few females during their evolutionary history, and overconsumption of highly nutritious food was similarly an unlikely problem for ancestral fowl. The extent to which the machinery underlying male defense of space, food, females, and mating with relatively few partners has been changed by artificial selection and by modern management is not clear. In some cases, problems created by management technology can be solved by that same technology, as in artificial insemination in broad-breasted turkeys when body conformation essentially prevents natural mating. A major breakthrough in avian male reproduction may also occur if the neurophysiological machinery underlying two very different components of sexual behavior could be differentiated, i.e., male to male competition for space, females, feed, etc. vs. characteristics associated specifically with male fertility. For example, dominant males typically sire more offspring than subordinates (e.g., Guhl and Warren, 1946; Guhl et ah, 1945; and others). Further, females may be more receptive to dominant than to subordinate males, and females may choose to test the dominance and vigor of males before mating with them. Progress toward improving male fertility may be more rapid if the male to male component of sexuality could be more nearly separated from factors directly associated with sperm quantity and quality and mating vigor per se. This is especially true in turkeys, which often form "brotherhoods" in the wild in which only the dominant male mates, i.e., the dominant male inhibits completed matings in subordinates. The extent to which male to male interactions, which were of great
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
Recent elaboration of theory in behavioral biology emphasizes differences in the amount of parental investment (PI) in time and energy by male vs. female as a driving force in the evolution of sex dimorphism, mating strategies, and parental care. In birds, as in mammals, differences in prezygotic investment of males vs. females in reproduction are skewed in favor of males. Males invest time and energy in mating, but their contribution to a given zygote is relatively small when compared to females, i.e., males have a greater amount of "currency" (e.g., time and energy used in mating and in gamete production) than do females, which are more limited than males in the number of gametes they can produce. Birds are, however, very different from mammals in male vs. female PI. Pregnancy and lactation are strictly limited to females in mammals. In birds, care of the embryo can be shared by both male and female because the embryo is not tied to the mother but equally available to both sexes. This lessens the differential in PI between male and female, drives the optimum number of offspring in male and female toward a common value, reduces sexual selection, promotes cooperation and bonding between parents and decreases differences in mating strategies and promotes sharing in care of offspring.
1669
GRAVES
1670
REFERENCES Guhl, A. M., N. E. Collias, and W. C. Allee, 1945. Mating behavior and the social hierarchy in small flocks of White Leghorns. Physiol. Zool. 18: 365-390. Guhl, A. M., and D. C. Warren, 1946. Number of offspring sired by cockerels related to social dominance in chickens. Poultry Sci. 25:460— 472. Hamilton, W. D.,1964. The genetical theory of social behaviour, I, II. J. Theor. Biol. 7:1-52. Hamilton, W. D., 1970. Selfish and spiteful behaviour
in an evolutionary model. Nature 228:1218— 1220. Hamilton, W. D., 1971a. Geometry for the selfish herd. J. Theor. Biol. 31:295-311. Hamilton, W. D., 1971b. Selection of selfish and altruistic behavior in some extreme models. Pages 57—91 in Man and Beast: Comparative Social Behavior. J. F. Eisenburg and W. S. Dillon, ed. Smithsonian, Washington, DC. Hamilton, W. D., 1972. Altruism and related phenomena, mainly in social insects. Annu. Rev. Ecol. Syst. 3:193-232. Maynard Smith, J., 1971a. The origin and maintenance of sex. Pages 163—175 in Group Selection. G. C. Williams, ed. Aldine-Atherton Press, Chicago. Maynard Smith, J., 1971b. What use is sex? J. Theor. Biol. 30:319-335. Maynard Smith, J., 1974. Recombination and the rate of evolution. Genetics 78:299-304. Maynard Smith, J., 1978a. Optimization theory in evolution. Annu. Rev. Ecol. Syst. 9:31—56. Maynard Smith, J., 1978b. The Evolution of Sex. Cambridge Univ. Press, Cambridge, England. Treisman, M., and R. Dawkins, 1976. The "cost of meiosis:" Is there any? J. Theor. Biol. 63: 479-484. Trivers, R. L., 1972. Parental investment and sexual selection. Pages 136—179 in Sexual Selection and the Descent of Man, 1871-1971. B. Campbell, ed. Aldine Press, Chicago. Watts, C. R., and A. W. Stokes, 1971. The social order of turkeys. Sci. Am. 224:112-118. Williams, G. C , 1975. Sex and Evolution. Monographs in Population Biology, No. 8. Princeton Univ. Press, Princeton, NJ.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
adaptive significance over evolutionary t i m e , are maladaptive u n d e r m o d e r n managem e n t systems is largely u n k n o w n . However, it does seem certain t h a t male t o male behavior and male t o female behavior interact, often in c o m p l e x and little u n d e r s t o o d ways. Links b e t w e e n female receptivity t o males and male behavior or m o r p h o l o g y , links b e t w e e n male defense of space o r of females and male fertility, and links b e t w e e n o t h e r behavioral traits and economically i m p o r t a n t characteristics m u s t be b e t t e r u n d e r s t o o d at behavioral, genetic, and n e u r o e n d o c r i n e levels. This t y p e of understanding requires input from behaviorists as well as from n e u r o e n d o c r i n ologists. This s y m p o s i u m represents a step t o w a r d such a c o n c e r t e d effort.
Department of Biology and Department of Poultry Science, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received for publication January 26, 1983) (Key words: behavioral adaptation, reproductive success, sexual reproduction) 1983 Poultry Science 62:1667-1670
Behavior is modulated greatly by prior experience and present circumstances acting on and through neural and endocrine mediators. The ultimate basis for the sensory-motor apparatus of an organism is the genetic code carried by each cell. This code sets boundaries on types of stimuli that can be received and, by designing receptor systems and integration circuits, basically establishes not only the "form but also the functional behaviors exhibited. However, continual input from past and present experiences, as well as expectations of consequences of alternatives, influences the ongoing stream of behavior. Information about ways in which neural and endocrine systems operate in concert to receive and integrate internal and external stimuli and modulate ongoing behavior is rapidly accumulating. Unfortunately, much of the information is highly technical and specialized, and interdisciplinary teams are increasingly rare. This symposium represents a limited approach to coordinating presentation of information on one important aspect of the poultry industry, that concerning reproduction of the avian male. My task is simply to introduce the topic by providing a very abbreviated overview of current behavioral biology as it relates to sex and the avian male.
BEHAVIOR AND ADAPTATION At the very heart of social biology are the twin concepts of natural selection and adaptation. Evolution by natural selection may be envisioned as a sort of race to reproduce among
'Article No. 6613 in the Journal Series of the Pennsylvania Agricultural Experiment Station.
individuals in which the score is tallied by relative reproductive success (RS), i.e., by relative numbers of viable offspring contributed to the next generation. Any trait, whether physiological, morphological, behavioral, or otherwise, which is influenced by inheritance may contribute positively or negatively to RS, and such traits are selected for or against by natural or artificial selection. Ethology and sociobiology emphasize the role of behavior in affecting RS. Hence, sex and reproduction are central issues in these disciplines. In 1964, W. D. Hamilton explicitly developed a theory of natural selection that is changing our perception of the evolution of behavior (see Hamilton, 1964, 1970, 1971a,b, 1972). Some aspects of Hamilton's theory, now widely known as "kin selection," are of practical as well as theoretical importance. Hamilton formalized what had been recognized by earlier workers, including Darwin, Haldane, and Lush. Selection for a trait can occur through differential reproduction of a particular individual exhibiting that trait or by differential reproduction of relatives, which, depending on the degree of relationship (r), have a predictable probability of carrying the same genes as the reference individual. Thus, in the case of fulls-sibs, the net loss in reproductive fitness to a brother, who leaves no offspring but who somehow (defense, cooperative display, etc.) increases the number of offspring his sibling has by twofold.is zero. That is, 7 between full-sibs is one-half; his brother leaves twice as many offspring as would otherwise be the case so the total loss in RS for the altruistic brother is 1.0 but the gain for the altruistic brother is two times r, or 1.0. Hence, the altruist experienced no net loss in fitness even though he did not reproduce. The theoretical importance of Hamilton's work cannot be overemphasized;
1667
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
INTRODUCTION
GRAVES
1668
EVOLUTION OF SEX
Curiously enough, sexuality per se is a major problem of evolutionary biology and behavior. As Williams (1975) noted with characteristic clarity, the "immediate advantage of asexual reproduction is generally conceded by those who have seriously concerned themselves with the problem." The basic argument is simple, although many highly sophisticated models exist. The fundamental problem is that sexual females, which produce genetically reduced (meiotic) eggs that must be fertilized by males to restore the diploid genetic complement, have offspring with only one set of the maternal genes. Conversely, asexual females produce offspring from unreduced (mitotic) eggs that do not require fertilization, and they have offspring with both sets of maternal genes. Hence, genetic representation of the asexual female is twice that of a sexual female in the ¥x generation if each produces an equal number of offspring. This "cost of meiosis" (Williams, 1975) disappears if (as presumably is usually the case) the number of offspring produced per unit of resource is equal in sexual and asexual parent(s). In that case, sexual parents, although only related to their offspring by one-half, produce twice as many offspring as do asexual parents because they each invest only one-half the resources per offspring as does an asexual parent. A more serious problem with sexuality is the production of males (Maynard Smith, 1971a,b, 1974, 1978a,b; Treisman and Dawkins, 1976).
In asexual reproduction, resources are converted directly to offspring; in sexual reproduction, gametes constitute an intermediate step, and a wasteful one if more male than female gametes are produced. The excess male gametes constitute a cost because they are made from resources not converted to offspring. Advantages of sexuality reside, in large part, in the genetic diversity afforded, especially in a changing environment. PARENTAL INVESTMENT AND MALE REPRODUCTIVE STRATEGIES
Recognition of the fact that males produce smaller and more numerous (usually motile) gametes than the female and that reproductive rates are therefore limited by female rather than by male gametes stimulated Trivers (1972) to formulate a very far-reaching theory on parental investment. Females produce (by definition) a limited number of very large gametes and therefore have a much larger investment in each zygote than do males. Because females invest so much in each gamete, their RS is limited to a large extent by the number of gametes they can produce. By contrast, males invest very little in each zygote, and male RS is limited by the number of zygotes they can gain access to rather than by gamete production. Hence, in the simplest sense, males tend to follow behavioral reproductive strategies based on gaining access to female zygotes (e.g., defending several females from male competitors, defending territories or other resources to attract multiple females, developing morphological and behavioral characteristics enhancing their attractiveness to females, etc.). In that same simplistic sense, females, having a limited number of gametes to use in the evolutionary "game", are choosy in mating, preferring to mate with the highest quality mates available (males can afford to be less discriminating than females). Hence, the optimum number of offspring for males will exceed the optimum number of offspring for females because the cost per offspring is lower for males than for females (Trivers, 1972). This brings about different selection pressures on males and females, promoting sex differences in specialized behavior and morphology as well as in underlying neuroendocrine and other physiological mechanisms. This raises problems of both theoretical and practical importance.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
the practical implications are also readily apparent when one considers that fertility in the avian male is indeed an industry problem and that natural selection has clearly promoted a male reproduction scheme in some birds in which only the dominant male of a brotherhood coalition actually mates. For example, Watts and Stokes (1971) demonstrated that brotherhood groups of wild turkeys cooperate in synchronous display toward females, but subordinate males rarely mate. They found that less than 5% of the wild turkey males on their study site accounted for 100% of the matings. To the extent that such patterns of male sexual behavior have represented genetically adaptive strategies over evolutionary time, alleviation of the problem may not reside in correction of neurophysiological dysfunction but in behavior-management manipulation.
SYMPOSIUM: THE AVIAN MALE BEHAVIOR AND MANAGEMENT
However, the most common domesticated birds exhibit characteristics uncommon in the bird world. Although males and females of the vast majority of avian species look alike, behave similarly, mate on a one-to-one basis, and participate about equally in caring for their young, chickens, turkeys, and most other domestic poultry are usually sexually dimorphic in both appearance and behavior, polygynous, and differ considerably in terms of sex roles related to reproduction and group living. Hence, reproductive behavior in males of domesticated birds more closely resembles that of their mammalian counterparts than it does avian males in general. The model of PI sketched has allowed man to take advantage of what birds, especially chickens, apparently thought was a good idea, i.e., the sharing of "pregnancy and lactation" duties. Not only does the avian model allow the male to share these responsibilities, it provides mankind a golden opportunity to interrupt the system and take over the postzygotic parental investment with modern incubators, hatchers, and artificial rearing systems. However, "re-
sidual" behaviors and their underlying neurophysiological machinery may be counterproductive in modern management systems, and, in that sense, poultrymen may be attempting to modify an antique automobile into a streamlined jet aircraft. That is, the evolutionary history of Gallus has been one in which males were designed to aggressively defend territories, including nest sites, feeding and water sources, and a few (2 to 4) females from other males, traits no longer desirable in commercial poultry enterprises. Male fertility is an increasingly serious problem in the poultry industry, as is male domination of feed troughs and associated overconsumption. Male chickens were unlikely to be faced with problems of inseminating more than a few females during their evolutionary history, and overconsumption of highly nutritious food was similarly an unlikely problem for ancestral fowl. The extent to which the machinery underlying male defense of space, food, females, and mating with relatively few partners has been changed by artificial selection and by modern management is not clear. In some cases, problems created by management technology can be solved by that same technology, as in artificial insemination in broad-breasted turkeys when body conformation essentially prevents natural mating. A major breakthrough in avian male reproduction may also occur if the neurophysiological machinery underlying two very different components of sexual behavior could be differentiated, i.e., male to male competition for space, females, feed, etc. vs. characteristics associated specifically with male fertility. For example, dominant males typically sire more offspring than subordinates (e.g., Guhl and Warren, 1946; Guhl et ah, 1945; and others). Further, females may be more receptive to dominant than to subordinate males, and females may choose to test the dominance and vigor of males before mating with them. Progress toward improving male fertility may be more rapid if the male to male component of sexuality could be more nearly separated from factors directly associated with sperm quantity and quality and mating vigor per se. This is especially true in turkeys, which often form "brotherhoods" in the wild in which only the dominant male mates, i.e., the dominant male inhibits completed matings in subordinates. The extent to which male to male interactions, which were of great
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
Recent elaboration of theory in behavioral biology emphasizes differences in the amount of parental investment (PI) in time and energy by male vs. female as a driving force in the evolution of sex dimorphism, mating strategies, and parental care. In birds, as in mammals, differences in prezygotic investment of males vs. females in reproduction are skewed in favor of males. Males invest time and energy in mating, but their contribution to a given zygote is relatively small when compared to females, i.e., males have a greater amount of "currency" (e.g., time and energy used in mating and in gamete production) than do females, which are more limited than males in the number of gametes they can produce. Birds are, however, very different from mammals in male vs. female PI. Pregnancy and lactation are strictly limited to females in mammals. In birds, care of the embryo can be shared by both male and female because the embryo is not tied to the mother but equally available to both sexes. This lessens the differential in PI between male and female, drives the optimum number of offspring in male and female toward a common value, reduces sexual selection, promotes cooperation and bonding between parents and decreases differences in mating strategies and promotes sharing in care of offspring.
1669
GRAVES
1670
REFERENCES Guhl, A. M., N. E. Collias, and W. C. Allee, 1945. Mating behavior and the social hierarchy in small flocks of White Leghorns. Physiol. Zool. 18: 365-390. Guhl, A. M., and D. C. Warren, 1946. Number of offspring sired by cockerels related to social dominance in chickens. Poultry Sci. 25:460— 472. Hamilton, W. D.,1964. The genetical theory of social behaviour, I, II. J. Theor. Biol. 7:1-52. Hamilton, W. D., 1970. Selfish and spiteful behaviour
in an evolutionary model. Nature 228:1218— 1220. Hamilton, W. D., 1971a. Geometry for the selfish herd. J. Theor. Biol. 31:295-311. Hamilton, W. D., 1971b. Selection of selfish and altruistic behavior in some extreme models. Pages 57—91 in Man and Beast: Comparative Social Behavior. J. F. Eisenburg and W. S. Dillon, ed. Smithsonian, Washington, DC. Hamilton, W. D., 1972. Altruism and related phenomena, mainly in social insects. Annu. Rev. Ecol. Syst. 3:193-232. Maynard Smith, J., 1971a. The origin and maintenance of sex. Pages 163—175 in Group Selection. G. C. Williams, ed. Aldine-Atherton Press, Chicago. Maynard Smith, J., 1971b. What use is sex? J. Theor. Biol. 30:319-335. Maynard Smith, J., 1974. Recombination and the rate of evolution. Genetics 78:299-304. Maynard Smith, J., 1978a. Optimization theory in evolution. Annu. Rev. Ecol. Syst. 9:31—56. Maynard Smith, J., 1978b. The Evolution of Sex. Cambridge Univ. Press, Cambridge, England. Treisman, M., and R. Dawkins, 1976. The "cost of meiosis:" Is there any? J. Theor. Biol. 63: 479-484. Trivers, R. L., 1972. Parental investment and sexual selection. Pages 136—179 in Sexual Selection and the Descent of Man, 1871-1971. B. Campbell, ed. Aldine Press, Chicago. Watts, C. R., and A. W. Stokes, 1971. The social order of turkeys. Sci. Am. 224:112-118. Williams, G. C , 1975. Sex and Evolution. Monographs in Population Biology, No. 8. Princeton Univ. Press, Princeton, NJ.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 22, 2015
adaptive significance over evolutionary t i m e , are maladaptive u n d e r m o d e r n managem e n t systems is largely u n k n o w n . However, it does seem certain t h a t male t o male behavior and male t o female behavior interact, often in c o m p l e x and little u n d e r s t o o d ways. Links b e t w e e n female receptivity t o males and male behavior or m o r p h o l o g y , links b e t w e e n male defense of space o r of females and male fertility, and links b e t w e e n o t h e r behavioral traits and economically i m p o r t a n t characteristics m u s t be b e t t e r u n d e r s t o o d at behavioral, genetic, and n e u r o e n d o c r i n e levels. This t y p e of understanding requires input from behaviorists as well as from n e u r o e n d o c r i n ologists. This s y m p o s i u m represents a step t o w a r d such a c o n c e r t e d effort.