Showing papers in "Integrative and Comparative Biology in 1978"

Community Organization Among Neotropical Nectar-Feeding Birds

Abstract: Amer. Zool., 18:779-795(1978). Community Organization Among Neotropical Nectar-Feeding Birds Peter Feinsinger Department of Zoology, University of Florida, Gainesville, Florida 32611 AND Robert K. Colwell Department of Zoology, University of California, Berkeley, California 94720 synopsis. Assemblages of neotropical hummingbirds are organized according to parameters of available resources and morphological-behavioral attributes of particular hummingbird species. We distinguish five types of fiowers relative to hummingbird foraging, and we define six community roles for hummingbirds in exploitation of various flower types. These roles are: high-reward trapliners, which visit but do not defend nectar-rich fiowers with long corollas; territorialists, which defend dense clumps of somewhat shorter fiowers; low- reward trapliners, which forage among a variety of dispersed or nectar-poor fiowers; territory-parasites of two types (large marauders and small filchers); and generalists, which follow shifting foraging patterns among various resources. Simple communities on islands usually contain one species of low-reward trapliner or generalist and one territorial species, and sometimes support one high-reward trapliner; often these species are sexually dimorphic. More complex mainland communities support varying numbers of species in different roles, depending on the relative importance and constancy of different flower types. High-reward trapliners are particularly important in forest under-stories, while forest canopies and open habitats support large numbers of shorter-billed, mobile birds filling the other five roles. We conclude by pointing out the many parallels that exist with other consumer groups. Our geographically diverse studies have been aided by numerous persons and organizations. In addition to those acknowledged elsewhere, we are particularly grateful to: J. A. Wolfe and L. A. Swarm, for their continuing contributions to studies on Trinidad and Tobago; S. Naeem and M. Lentrell for technical assistance; M. C. King, for expert assistance in the field and in the preparation of this paper; J. Ter- borgh, for the generous loan of unpublished data; H. A. Ford, for access to an unpublished manuscript; J. A. Brown and A. Kodric-Brown, for comments on the manuscript; A. B. Bolten, for stimulating discussions on organization among neotropical bees; and M. Bon Saint Come, for hospitality and assistance on Mar- tinique. Feinsinger's field work in summer 1975 was financed by a Venture Grant and a Faculty Research Grant from the University of Denver; his current study on Trinidad and Tobago is financed by NSF grant DEB 76-20371. Colwell's work in Chile was supported by the Ford Foundation and by NSF grant GB 31195; his work in Trinidad and Berkeley was partially financed by an NIH Biomedical Sciences grant and by the Committee on Research at the University of California. INTRODUCTION Terrestrial habitats in the American tropics commonly support several species of birds that feed primarily on nectar. Repeated patterns in the morphology and behavior of coexisting species suggest that certain basic principles are involved in the ecological organization of these as? semblages. Investigation of these princi? ples is facilitated by several factors. Nectar production and nutritional value are pre? cisely measurable. Foraging patterns of birds are easily observed and quantified. The coevolution of bill morphology and flower morphology sets certain limits on the patterns of particular species. Since behavioral interactions between birds are also obvious, it is often possible to infer the effect of interference on resource use. The considerable tactability ofthe bird-flower 779

Functional anatomy of the association between motor units and muscle receptors

Abstract: Muscle spindles and tendon organs occur in most somatic muscles of the mammal and are particularly concentrated in muscles subserving fine movements, including postural muscles and small muscles of the distal extremities. In those mixed muscles in which the different fiber and motor unit types are “compartmentalized,” the spindles, and perhaps tendon organs also, are virtually limited to those compartments predominated by “oxidative” muscle fibers. These morphological observations based on a broad array of muscles in many species, complement electrophysiological studies which have emphasized that (1) the “oxidative” motor units have low reflex thresholds and (2) segmental proprioceptive reflexes may be primarily concerned with the control of finely graded contractions. Consideration of the functional anatomy of the association between motor units and muscle receptors suggests the need for detailed structural-functional analyses of those muscles with specializations in architecture, fiber-type composition and distribution, and in the number and distribution of their muscle spindles and tendon organs. An electrophysiological analysis of the relationship between the spinal cord and such muscles might also reveal certain strategies and mechanisms underlying segmental motor control which are either absent or obscured in the analysis of that select number of “homogenously-mixed” muscles conventionally used in the study of the mammalian segmental motor control system.

Optimal foraging in hummingbirds : testing the marginal value theorem

Abstract: To a hummingbird, clusters of flowers on inflorescences represent patches and provide an ideal situation to test prediction of optimal patch-use. The basic question is what decision rule should a hummingbird use to decide whether or not to leave an inflorescence? The hypothesis is that hummingbirds will adopt the decision rule that maximizes their net rate of energy gain while foraging. This hypothesis leads to an analogue of Charnov's marginal value theorem which determines an optimal decision rule. The optimal decision ruleis then used to predict aspects of the hummingbirds' foraging, and these predictions are compared with field data The optimal decision rule is a function of how much information is used by the hummingbirds. Data indicate that a decision to leave an inflorescence is a function of the number of flowers visited, the number of flowers available on the inflorescence, and the amount of nectar obtained at the last flower. The optimal decision rule was calculated assuming no additional information is used.

Ecological and Evolutionary Implications of Bird Pollination

Abstract: This paper addresses the question of how, and under what ecological circumstances, bird pollination will be optimal for a plant, and which or how many of the available nectar-feeding bird species will be optimal pollen vectors. Pollination by birds is energetically expensive for the plants, and should accur only when birds can mediate optimal patterns of pollen flow and seed set. Each nectar-feeding bird has potential advantages and disadvantages as a pollen vector, related to its size, morphology, and foraging behavior. Which available bird is the optimal pollinator depends on the plant's growth habit, spatial distribution, and breeding system. The various adaptations shown by plants favoring one pollinator over another all revolve around the secretion of nectar and the manner of presenting it to the birds. However, other aspects of plant morphology, physiology, ecology, or life cycle may affect the production and presentation of nectar, and influence plant-pollinator coevolution. Many question remain regarding the interrelations between pollination and the total biology of the plant; bird pollination systems may prove fruitful in yielding meaningful answers.

Conversion of Androgen to Estrogen and Other Steroids in the Vertebrate Brain

Abstract: Aromatase activity has been detected in the central nervous system (CNS) of representatives of each major vertebrate group with the exception of the Agnatha. 5a-reductase and 17/3-oxidoreductase are also present in brain tissues of many vertebrates and in the cerebral ganglion of the lobster. These comparative studies together with autoradiographic, physiological, and behavioral data in mammals and selected nonmammalian species support the view that metabolism in situ is an important component of androgen action and a general characteristic of the vertebrate brain.

On the Nature of the Neurotrophic Phenomenon in Urodele Limb Regeneration

Abstract: The nervous control of regeneration of body parts in the urodele amphibian and other animals has been one of the best model systems for the study of the neurotrophic phenomenon. In the past most of these studies were experimental morphological, but recently the salient problems on the nature of the cellular response to the neurotrophic agent and the nature of the nervous agent itself are also analyzed molecularly. The ensemble of studies reviewed in the present work, which also show that the agent of the nerve is a peptide and defines aspects of its effect on molecular synthesis in regenerate cells, leads me to advance the following theories. I propose that the neurotrophic agent affects only the rate of ongoing events in the cell and not the quality and kind of the events; that the events are already indigenous to the responding cells; that alteration in therate of events, for example increasing the rate of molecular syntheses, yields an increased cell population which by its size and increased cellular interactions has formative and differentiated capabilities which do not exist in a smaller cell population; and finally that the neurotrophic factor (NTF) is one of many “conversational” peptides including nervegrowth factor (NGF) and epidermal growth factor (EGF) which function to alter the absolute rate of ongoing cellular events.

A Neuroendocrine Approach to Brain Function: Localization of Sex Steroid Concentrating Cells in Vertebrate Brains

Abstract: Sex steroid concentrating cells in brain can be demonstrated by the specialized technique of steroid autoradiography. With this technique we have demonstrated the presence and the pattern of sex steroid concentrating cells in a representative species from every major class of vertebrate. This paper discusses the details of the pattern of binding cells found in a teleost, the paradise fish, Macropodus opercularis ; a reptile, the lizard, Anolis carolinemis ; and a carnivore, the mink, Mustela vison . The majority of sex steroid concentrating cells are found in the medial preoptic area, the medial hypothalamus, limbic structures and specific areas of the mesencephalon deep to the tectum. The regions containing the largest numbers of labelled cells have been demonstrated in each species with other techniques to be involved in neuroendocrine regulation or control of sex behavior.

Estrogen and Androgen Target Cells in the Brain of Fishes, Reptiles and Birds: Phylogeny and Ontogeny

Abstract: The cellular and subcellular distribution of 3H estradiol in the forebrain of goldfish, platyfish, lizard, turtle, and ring dove was studied by thaw-mount autoradiography. The topographic distribution of estrogen target cells in these non-mammalian species corresponds to the general pattern established earlier in our laboratory for mammals. In all non-mammalians studied, accumulations of estrogen target cells exist in preoptic-septal, central hypothalamic and thalamic areas. While allocortical distribution of estrogen target cells is present in the avian and reptilian species, it appears to be absent in the teleosts. Differences in brain evolution probably account for it. This phylogenetically delayed acquisition of telencephalic steroid target sites seems paralleled by an ontogenetic delay, as observed in the chick embryo: early embryonic appearance of estrogen uptake (receptors) in preoptic and central hypothalamic regions and late appearance of estrogen receptors in the amygdala. Androgen distribution in the forebrain of the dove and lizard overlaps with the estrogen distribution with some exceptions.

Anatomical Distribution of Estrogen, Androgen, Progestin, Corticosteroid and Thyroid Hormone Target Sites in the Brain of Mammals: Phylogeny and Ontogeny

Abstract: The pattern of the anatomical distribution of estrogen target cells in insectivores, rodents and primates is similar It shows relationship to the patterns observed in non-mammalian vertebrates In the forebrain it includes preoptic-septal, central hypothalamic, thalamic and allocortical sites In neonatal and fetal rodents similar target sites can be demonstrated and evolve during embryonic development; however, the nuclear groups are not as well differentiated and the appearance of steroid hormone receptors does not occur simultaneously in them Androgen target cells are accumulated at sites that overlap in part with those of estradiol, but in addition are found extensively in areas associated with psychomotor and somatomotor functions, including floor-plate derivatives in the lower brain stem and spinal cord Glucocorticosteroids show extensive localization in neurons of the allocortex This indicates a phylogenetically recent forebrain acquisition, compared to the sex steroids Thyroid hormones show nuclear concentration in many neurons, in addition to selective uptake in tanycytic ependyma, choroid plexus and certain neuropil The close topographic relationship of hormone target cells to the recess organs of the ventricular system led us to propose the concept of interrelated periventricular secretory units

Correlates and Consequences of Body Size in Nectar-Feeding Birds

Abstract: synopsis. Nectar-feeding birds are among the smallest birds and the largest pollinators. Energetic costs of maintenance, temperature regulation, foraging and reproduction increase in direct proportion to body mass raised to fractional exponents, which may vary from 0.5 to 1.0; overall costs probably vary with an exponent of 0.75. Avian nectarivores acquire most of their energy from flower nectar; in so doing they compete with other nectar feeders and pollinate plants. Larger pollinators are more reliable and move pollen greater distances, but to attract them plants must secrete more nectar and protect it from utilization by smaller animals. Minimum body size of avian nectarivores (2g) appears to reflect both competition with insects and the limited capacity of the smallest birds to acquire and store energy relative to the demands of fasting, temperature regulation, and reproduction. Hummingbirds have attained significantly smaller size than other nectar feeding birds because lower metabolic rates and use of hypothermic torpor reduce their energy expenditure relative to income. Maximum body size of avian nectarivores (approximately 80g) apparently reflects the upper limit of plant energy expenditure for reliable, long distance pollination. Between these limits, size variation reflects divergence to reduce interspecific competition and coevolution with plants to promote specificity.

Gonadal Hormones and Brain Development: Cellular Aspects of Sexual Differentiation

Abstract: Sexual differentiation of the neural control of reproductive function with respect to both gonadotropin secretion and sexual behavior is thought to result from exposure of the brain to testicular androgens during a very restricted or critical period of CNS differentiation and development, when the tissue is competent to respond to the hormone, and after which it is refractory or responds in a reversible manner. This paper reviews the cellular aspects of sexual differentiation with particular emphasis on the morphological expression of the gonadal hormonal effects in the adult brain. It presents experimental evidence for the morphogenetic basis for the observed steroid effects by showing how the addition of steroid to undifferentiated hypothalamic cultures produces a selective neuritic response that is steroid-dependent. These results suggest that since afferent axonal input and temporal factors are critical for dendritic and synaptic differentiation, steroid-induced variations in neuritic development could result in gender-specific responses seen in sexual differentiation of reproductive function.

A Spectrum of Nectar-Eater Communities

Abstract: Three nectar-eater systems differ in the degree and consistency with which nectar is limiting to the birds. A gradient of nectar limitation exists, ranging from no limitation to limitation some years but not others, to apparent limitation most of the time. The ecological and evolutionary effects of different limitation regimes on aggression and tcrritoriality, on resource partitioning and avian community structure, and on plant pollination strategies are discussed.

Aggressive Social Organization in Nectarivorous Birds

Abstract: I argue that a net benefit model of aggressive social organization is consonant with observed variation between territorial and dominance systems. For nectarivores net benefits are associated with obtaining nectar. Costs are time and energy associated with aggression and possible risks of injury. The fitness criterion probably varies across situations and may be long or short-term. An important problem for behaviorists is to understand the position in a time hierarchy at which particular social interactions are important.

Why Change Gaits? Recruitment of Muscles and Muscle Fibers as a Function of Speed and Gait

Abstract: Muscles shorten, stay the same length and are stretched while they are active during normal modes of terrestrial locomotion. The relative importance of these different types of muscular activity changes as animals change gait. Energy is conserved during a walk by an alternate storage and recovery of gravitational potential energy within each stride, as in an inverted pendulum. In order for this transfer of energy to take place, muscular activity is required to hold the limb rigid while the animal rotates over it. Energy is conserved by a spring mechanism during running, trotting, galloping, and hopping. Energy is stored when active muscles and their tendons are stretched and recovered as they subsequently shorten. The recruitment patterns of motor units as a function of speed therefore, depends on the type of muscular activity as well as the force exerted. Discontinuities in the cross sectional area of active fibers with increasing speed have been observed at the trot-gallop transition. It is suggested that at this point the trunk is recruited as an additional spring enabling more energy to be stored elastically. It is concluded that we must consider what muscles are doing during normal modes of locomotion before we become too involved in designing schemes of motor unit recruitment.

Liquid-Tissue Mechanics in Amphibian Gastrulation: Germ-Layer Assembly in Rana Pipiens

Abstract: During amphibian gastrulation, migrating subsurface germ layers may flow past one another like immiscible viscous liquids in response to tissue surface tension forces. We describe here two physical tests for liquid-tissue morphogenesis in cultured aggregates of subsurface ectoderm (E), mesoderm (M) and endoderm (N) excised from mid-yolk-plug Rana pipiens gastrulae. (i) Liquids are coherent substances in which subunits can slip past one another to relax internal shear stresses. We find, in cross-sections of cell aggregates fixed during compression, that cells within flattened aggregates are intially deformed, but do, as predicted, gradually reassume their original, undistorted shapes, (ii) Surface tensions (γ's) govern ordinary liquid-droplet spreading; e.g. , if equal-sized droplets A and B fuse in medium O, B spreads around A when γAO γBO. When pairs of subsurface aggregates are cultured together, N surrounds M and E, and M surrounds E. To see if γEO > γMO > γNO. we flatten aggregates with quartz fibers calibrated to measure the force of compression. As predicted, under the same flattening force, E aggregates are rounder than M aggregates, which are rounder than N aggregates. Furthermore, a second surface tension relationship can account for the autonomous involution of M between E and N; and these surface tension relationships can also explain the inversion of E, M and N by coated ectoderm to produce normal gastrular germ-layer arrangements. We conclude that, combined with active cell shape changes in solid-like surface cell layers and also with autonomous elongation of dorsal lip mesoderm, tissue surface tension control of liquid-tissue flow in subsurface germ layers is a key morphogenetic mechanism in amphibian gastrulation which might be regulated by changes in intercellular adhesiveness.

Sex Steroids and the Differentiation of Avian Reproductive Behavior

Abstract: Experiments in which avian embryos are treated with sex steroids or steroid antagonists suggest that sexual differentiation of reproductive behavior (and thus differentiation of the brain mechanisms for such behavior) is controlled by steroids produced by the embryonic gonads. In chickens and Japanese quail, males hatched from eggs treated with estradiol or testosterone during incubation are feminized (demasculinized); they fail to exhibit masculine sexual behavior as adults, and no longer are behaviorally distinguishable from females. Some evidence suggests that testosterone may mimic the feminizing action of estradiol by being converted to an estrogen in the embryonic brain. Genetic female quail exposed to an antiestrogen during embryonic development are masculinized; they exhibit an increased ability to display the masculine copulatory pattern. Thus the behavior of these species is feminized by embryonic exposure to sex steroids, the anhormonal (neutral) sex for behavioral differentiation appears to be the male, and females appear to result from estrogen produced by the embryonic ovaries. In contrast, sex steroid treatment of mammals early in development masculinizes behavior, the female is the neutral sex, and males result from fetal androgen secretion. These opposite patterns of psychosexual differentiation in birds and mammals are correlated with a major difference between the avian and mammalian sex-determining mechanism. Implications for other vertebrates are discussed.

Mammalian Muscle Fiber Types and Their Adaptability

Abstract: The neuromuscular system can adapt to a variety of different kinds of chronic workloads. The specificity of these adaptations is reflected in the specific components of the neuromuscular system that are overloaded. An examination of changes that occur during normal development can serve as a basis for efforts to understand the mechanisms which are responsible for the maintenance of a given histochemical, biochemical and physiological profile of a motor unit. The normal process of muscle development can be retarded by spinal transection or immobilizing a neonatal muscle and some of the neonatal features can be reintroduced in adult animals by these same procedures. In general the speed related properties of fast skeletal muscle are not altered markedly by any type of physiological overload. There is some evidence that immobilization or intense endurance trained slow muscle is faster than the normal muscle. The metabolic properties related to maintenance of tension during prolonged work are readily adaptable to chronic low resistance-highly repetitive contractions. Factors that determine the specificity of the response to neuromuscular overload seem to be motor unit recruitment frequency, the tension produced within the musculature and the duration of the tensions.

Motor Units: Physiological/Histochemical Profiles, Neural Connectivity and Functional Specializations

Abstract: This brief review considers muscles as ensembles of motor units, the viewpoint usually taken by motor systems physiologists. The morphological, histochemical and mechanical properties of muscle units are discussed in relation to the intrinsic properties of the motoneurons that innervate them, and in connection with the organization of synaptic inputs that play a significant role in determining functional usage. These factors, from synaptic organization to muscle fiber physiology and biochemistry, are all precisely interrelated. The overall design of the soleus (SOL) and medial gastrocnemius (MG) motor unit populations in the cat hindlimb seems ideally suited to the functional roles played by these contrasting muscles. As more information accumulates about these populations, and about others with different functional roles, we should have increasingly clear ideas about the fundamental question of why different muscles look and act as they do in various animal species.

An Experimental Approach to the Effects of Predictability on Community Structure

Abstract: This paper considers the effects of different levels of resource predictability on niche widths, competition and diversity in a community of three hummingbird species. Three pairs of hypotheses are tested: whether decreasing predictability (a) increases or decreases niche width, (b) increases or decreases competition and (c) increases or decreases diversity. The results show that complementary increases and decreases in niche width occur with decreasing predictability, while competition and diversity decrease, at least with extreme unpredictability. A model, which assumes the species to have similar resource preferences, and its predictions are examined. The dominant species, Lampornis clemenciae , excludes a subordinate species, Archilochus alexandri , from preferred resources. This defense becomes unprofitable with decreasing predictability and Archilochus invades the resources once vigorously defended by Lampornis . When the spectrum of resources is altered in the direction of decreasing suitability for both species, Lampornis becomes even more specialized and sensitive to the effects of unpredictability. A third species, Eugenes fulgens, steals resources undefended by Lampornis . These characteristics seem to be common to a number of communities in very different taxonomic groups, and characteristic of communities in which species share a common preferred resource. The question of how a community organized with distinct resource preferences responds to decreasing predictability remains open.

Neurotrophic Contribution to a Proposed Tripartite Control of the Mitotic Cycle in the Regeneration Blastema of the Newt, Notophthalmus (Triturus) viridescens

Abstract: My work has shown that the neural dependence of the limb regenerate (blastema) for growth, is retained in vitro and it follows the normal in vivo pattern. Implanted dorsal root ganglia promote localized growth in cultured blastemata whereas in the absence of nerves little or no development ensues. Our transfilter studies show that the neurotrophic effect, which appears to be mediated by a factor capable of transmission through thin filters of low porosity, is manifested in a proximo-distal gradient of mitotic activity which diminishes with distance from the nerve source. This mitogenic effect appears to require at least 48 hr for expression, which corresponds to one revolution of the cell cycle. In the absence of nerves, both DNA synthesis and mitotic index in the blastema decline significantly; however, a basal level is maintained which may be nerve independent. In this context and in the light of pertinent recent literature, we present a model in which nerves, the apical epidermis and insulin (tripartite control) all play essential roles in the mitotic cycle.

Neural Control of Cell Cycle Events in Regenerating Salamander Limbs

Abstract: Nerves, wound epidermis, and injury are indispensable for salamander limb regeneration, but their mechanism of action is not understood. A hypothesis has been presented (Tassava and Mescher, 1975) which suggests that injury is important to dedifferentiation and entry of limb stump cells into the cell cycle, nerves are required for one or more G2 events in order that cells can proceed to mitosis, and the wound epidermis maintains the daughter cells in the cell cycle. The resultant cells accumulate to form the blastema. Complete and partial denervation experiments, which attempted to test this hypothesis, are discussed. Blastema cell cycle parameters, measured after complete denervation, did not vary greatly from innervated controls, even though denervated blastemas were resorbed. Blastema cell cycle parameters of partially denervated limbs, which exhibited delayed regeneration, were likewise not lengthened when compared to completely innervated controls. These results are consistent with the view that after either complete or partial denervation, some blastema cells continue to cycle and reach the M phase in the same time as controls. Other blastema cells block completely, never reach M, and are then removed. A possible mechanism for resorption of denervated blastemas is presented.

Genie Control of Axolotl Metamorphosis

Abstract: SYNOPSIS. Neoteny in the Mexican axolotl, Ambystoma mexicanum , is caused by homozygosity for a single recessive gene. The dominant allele causing physical metamorphosis is found in the closely related species, Ambystoma tigrinum , with which it can hybridize. Despite the failure of axolotls to undergo physical metamorphosis, they do undergo a cryptic metamorphosis. A larval-to-adult hemoglobin form change, serum protein changes and other physiological events usually associated with amphibian metamorphosis occur during early larval life at ages comparable to the age at which Ambystoma tigrinum undergoes both the cryptic and external metamorphic events. Axolotl cryptic metamorphosis can be induced precociously by immersion of the larvae in low concentrations of thyroxine; physical metamorphosis can be induced with higher thyroxine concentrations. The site of action of the gene responsible for axolotl neoteny has not been identified. A change in the sensitivity of external metamorphic processes to thyroxine, or reduced hormonal stimulation by the pituitary or hypothalamus may be responsible. A comparison of these functions in Ambystoma tigrinum and the axolotl may identify the lesion.

Estrogen and Androgen Receptor Proteins in Embryonic and Neonatal Brain: Hypotheses for Roles in Sexual Differentiation and Behavior

Abstract: We have demonstrated and partially characterized putative estrogen and androgen receptors from mouse hypothalamus for a range of perinatal ages. For the first time, estrogen and androgen receptors from embryonic mouse and rat hypothalamus are described and characterized; they display similar parameters as the receptor proteins of adult mice and rats. The ontogeny of these proteins is discussed in the context of models for the control of the “critical period” of sexual differentiation of the brain. The androgen-binding proteins, presumed to be receptors, are compared for hypothalamus and kidney and for the androgen-resistant mutant mouse, testicular feminization ( Tfm ). The putative receptor forms that are observed help to define the possible function of brain androgen receptors during sexual differentiation. Development and modification of DNA—cellulose chromatography for the affinity separation of steroid receptors of brain is described. The methods allow complete separations of receptor proteins from non—receptor, steroid-binding proteins and subsequent analysis of the resultant receptors.

Comparative Endocrinology of Piscine Hypothalamic Hypophysiotropic Peptides: Distribution and Activity

Abstract: Fishes display a variety of anatomical relationships between brain and pituitary to a degree unique among vertebrates. This group is pivotal for understanding evolution of functions of hypophysiotropic peptides. We review information concerning occurrences, distributions and physiological activities of three identified peptides in fish brain, and biological properties of fish brain extracts. Thyrotropin releasing hormone may be present universally in piscine central nervous tissue; however, this peptide has not been clearly demonstrated to have hypophysiotropic activity in fishes. Somatostatin also has been shown to occur in fish brains; studies of actions of this substance are virtually absent. Gonadotropin releasing hormone is apparently of broad occurrence in fishes; its hypophysiotropic activity is well established for several teleostean species. Anatomical relationships between brain and pituitary are particularly varied among elasmobranchs. Investigations involving additional elasmobranch representatives, as well as other fishes, are needed before generalizations can be made. Widespread extrahypothalamic distribution of hypophysiotropic peptides in lower vertebrates and neurotransmitter (or related) functions of neurones containing these peptides provide a basis for proposals concerning evolution of hypothalamic control of the pituitary gland.

Hypophysiotropic Centers in the Brain of Amphibians and Fish

Abstract: The subject is the localization of three different hypophysiotropic centers in the brain of amphibians and fish. The thyrotropic hormone-releasing hormone (TRH) in Xenapus may originate from the dorsal magno-cellular neurons of the preoptic nucleus. This hypothesis is based on correlative changes between these cells and alterations in thyroid activity during metamorphosis. Experimental data are in support of a functional relationship between certain preoptic neurons and the thyrotropic activity of the pituitary. The MSH inhibiting activity of the hypothalamus is effected by means of an aminergic innervation of the pars intermedia in amphibians, teleosts and elasmobranchs. In amphibians the aminergic fibers originate from the caudal part of the paraventricular organ (PVO); in elasmobranchs probably from the nucleus medius hypothalamicus (NMI); in teleosts the origin still has to be investigated. Two centers producing gonadotropic hormone-releasing hormone (GRH) have been demonstrated. Lesion experiments lead to the hypothesis that GRH is produced in the caudal hypothalamus, i.e., in the nucleus infundibularis ventralis of amphibians and in the nucleus lateralis tuberis of fishes. ImmunoHuorescence studies indicate in both groups the presence of neurons, in front of the preoptic area in the telencephalon, and these neurons are immuno-reactive with anti-mammalian LH-RH.

On the Development of Pigment Patterns in Amphibians

Abstract: synopsis. Pigmentary mutants in amphibians provide important vehicles for studying basic problems in development. Some of these mutants exert influences on the tissue environ? ment in which the chromatophores differentiate and others involve the expression of pigment in specific types of pigment cells. Melanophores are the best known of all chromatophores, and albinism has been much studied. Genes controlling its expression may operate at different levels. Some are involved in the production of tyrosinase, while others affect the melanosomal matrix. In contrast, melanoid mutants are characterized by an overproduction of eumelanin, usually through the differentiation of an excessive number of melanophores. Melanoid mutants of the axolotl also exhibit a great diminution in xanthophore and iridophore number, and the same seems to be true of some melanoid-like mutants of leopard frogs. The pteridine pigments of xanthophores and purines of iridophores are often products of xanthine dehydrogenase (XDH) activity, and it is suspected that the expression of the melanoid phenotype may result from a genetic defect involving this enzyme. This is supported by experiments involving the administration of an XDH inhibitor, allopurinol, to normal larvae. This inhibitor results in the production of partial phenocopies of the melanoid condition. Blue mutants involving either partial or complete diminution of xanthophore and iridophore pigments may also be based upon deficient XDH activity.

Embryonic Pituitary Adrenal Axis, Behavior Development and Domestication in Birds

Abstract: ACTH and corticosterone exert opposite effects on the approach and imprinting behavior of newly hatched ducklings. Wild mallard and domesticated Pekin ducklings differ in the early posthatch period in both plasma corticosterone levels and approach/avoidance behaviors. Injection of Pekin duckling embryos with pituitary—adrenocortical hormones alters both later adrenal function and certain aspects of posthatch behavior. These birds have behavioral and hormonal characteristics which resemble those of wild mallards. The hypothesis that behavioral differences in wild and domesticated ducklings result from a higher level of pituitary adrenal function in the wild embryo is explored. Although adrenocortical function changes during domestication in many species, evidence that the hormonal changes mediate the concomitant changes in approach and avoidance behavior remains inconclusive. Factors which cause adrenal function and early behaviors to differ in wild and domesticated genotypes must be sought in the gene action during embryonic development. Since imprinting behavior is modulated by pituitary—adrenal hormones, any factor which affects post—hatch adrenal function may potentially affect imprinting. Later behavior development in the adult is strongly dependent on neonatal experiences; and, therefore, hormonal modulation of early imprinting behavior may constitute an important determinant of adult social behavior.

Proximate Costs of Competition for Nectar

Abstract: Food competition among coexisting nectarivorous birds is conspicuous and often intense, affecting patterns of flower choice, daily behavior budgets, and timing of successful reproduction. Exploitative competition involves loss of accumulated nectar to other individuals that visited a flower first. Preliminary data support the use of Poisson models of the frequencies of point-source visitation and overlap for determining the probabilities of actual competitive events. Nectar losses from monitored flowers can be estimated in terms of time intervals between visits weighted by flower-specific nectar production and bird-specific nectar removal capabilities. Foraging time budgets then provide a meaningful common denominator for assessing the impacts of competitive nectar losses, because compensatory increases in foraging time are required to maintain a balanced energy budget. Flexibility in foraging time budgets made possible by high efficiency foraging and predictably low competitive losses may be an important determinant of reproductive timing and success in nectar feeding birds. Aggressive displacement of competitors and territorial defense of flowers are common forms of interference competition in nectar-feeding birds. Aggression has definable caloric costs that ultimately must relate to caloric gains. Defense of flowers increases the aggressor's exclusive use of nectar, increases the predictability of a nectar supply, and increases the average amount of nectar obtained per flower. Simple cost-benefit models of territoriality define conditions when net benefits of territoriality are greater than those of alternatives.

Feeding: Models of Costs and Benefits in Energy Regulation

Abstract: Models to predict feeding behavior at the level of consumption and use of energy involve either details of internal (physiological) controls or economic principles of regulation based on optimal (evolutionary) foraging theory. These two approaches will ultimately be related, but the former requires more information for specific predictions. The latter can provide predictions based on selected criteria for regulation. Meal sizes and feeding frequencies of hummingbirds are examined relative to two regulatory, criteria: maximizing rate of net energy gain and maximizing efficiency (intakes/expenditures) through a “crop emptying” model that incorporates energy intake from food and energy expenditures for short-term (meal to meal) maintenance and longer-term (overnight) energy storage. Experimental results suggest that the feeding behavior of hummingbirds is differentially sensitive to short-term and daily uses of energy. Changes in overnight energy storage requirements result primarily in changes in meal size, while changes in meal to meal maintenance requirements result primarily in feeding frequency changes. The economic models predict these responses. The feeding behavior of hummingbirds also appears to be sensitive to food quality, time spent flying to and from a food source, and costs associated with the weight of ingested food.

Organization of the Morphogenetic Field in Regenerating Amphibian Limbs

Abstract: The amphibian limb is an example of a secondary embryonic field that can be reactivated during larval or adult life so that amputated parts are regenerated. Two major questions are: (1) what is the origin of the morphogenetic field of the regeneration blastema, and (2) what is the nature of this field and how does it specify the spatial pattern of blastemal redifferentiation? Evidence is analyzed here which leads to the following propositions: (1) the field is represented in latent form by properties of the mature limb cells, and these properties are activated and inherited by the blastemal cells after amputation and dedifferentiation. At the same time, the inherited field is sensitive to the mature stump tissues and its spatial organization can be altered by a stump pattern alien to the one from which it was derived. (2) The properties of the mesodermal limb tissues represent positional values that are arranged in gradients along the proximal-distal, anterior-posterior and dorsal-ventral axes. These properties allow dedifferentiated mesodermal cells to change their positional value to any value between their original one in the limb and the value of any neighboring cell after creation of a discontinuity. The direction of change is always from proximal to distal in the PD axis; it is uncertain as to whether change can take place only centripetally or both centripetally and centrifugally along the AP and DV axes. (3) Epidermal cells have the same positional value everywhere in the limb and act as the distal and circumferential boundaries up to which the mesodermal cells may change their positional values. The proximal boundary is represented by the level-specific properties of the mesodermal cells at the maximum extent of distal to proximal dedifferentiation. Normal regeneration can then be visualized as occurring in the following way. When deletions are made in the limb pattern, cells with widely different positional values are confronted. During regeneration, blastema cells increase in number and continually interact with their neighbors to adjust their positional values within the boundaries until discontinuities are eliminated. The multiple limbs resulting from rearrangement of stump tissue patterns can also be accounted for by using these propositions. It is suggested that positional information is encoded on the cell surface and/or in the extracellular matrix.