Showing papers in "Integrative and Comparative Biology in 1996"

Maternal effects in animal ecology

Abstract: SYNOPSIS. Maternal effects comprise a class of phenotypic effects that parents have on phenotypes of their offspring that are unrelated to the offspring's own genotype. Although biologists have known about the importance of these effects for decades, maternal effects have only recently been studied widely by evolutionary ecologists. Moreover, the impact of maternal effects on the rate and direction of evolution of other components of the phenotype has only recently been fully elucidated by theoretical genetic models. This paper provides a brief overview of maternal effects research, focusing on research conducted in animal systems. First, I review and summarize definitions and theoretical treatments of maternal effects. Then I survey the diversity of maternal effects and some of the kinds of ecological and evolutionary impacts that maternal effects have been shown to have on offspring performance. Third, I discuss some of the ecological and evolutionary implications of maternal effects in ecological research. In this context, it is argued that the kinds of maternal effects that have been studied, and some of the potential implications of maternal effects that have not been explored are both artifacts of research effort. Hence, I identify several fruitful areas for future maternal effects research. Lastly, I describe the necessity of explicitly incorporating maternal effects in ecological research, whether or not the researcher is focusing on them as the topic of the research.

Homologues, natural kinds and the evolution of modularity

Abstract: Synopsis. The fact that phenotypic evolution can be studied on a char? acter by character basis suggests that the body is composed of locally integrated units. These units can be considered as modular parts of the body which integrate functionally related characters into units of evolu? tionary transformation. These units may either emerge spontaneously by self-organization, or may be the product of natural selection. A selection scenario that could explain the origin of modular units needs to explain the differential suppression of pleiotropic effects between different modules and the augmentation of pleiotropic effects among the elements with? in the module. Four scenarios are discussed: selection for adaptation rate, constructional selection, stabilizing selection and a combination of direc? tional and stabilizing selection. It is concluded that a combination of directional and stabilizing selection is a prevalent mode of selection and a likely explanation for the evolution of modularity.

The Particular Maternal Effect of Propagule Size, Especially Egg Size: Patterns, Models, Quality of Evidence and Interpretations

Abstract: SYNOPSIS. Propagule size is perhaps the most widely recognized and studied maternal effect in ecology, yet its evolution is not well-understood. The large body of extant optimality theory treats parental investment solely as an ecological problem, largely from the perspective of progeny. This approach has had limited success explaining the ubiquitous variation in propagule size within and among natural populations at most temporal and spatial scales. This problem aside, an unassailable gap in propagule size theory is that it pays little heed to the fact that offspring size is a joint phenotype of two individuals- the offspring and its mother. Hence, the ecology of mothers is decidedly as important in shaping the evolution of propagule size phenotypes. There are two reasons to suspect that this gap may account for the lack of success of optimality theory to explain variation in nature. The first is that optimality models of propagule size make no allowance for, nor can they explain, widespread, multivariate correlations between maternal characters and clutch parameters, namely the positive phenotypic covariances of maternal age, size, fecundity, and per-propagule investment found in many organisms. If per-propagule investment is optimized by selection based on the expectation of offspring fitness, then why should that phenotype be a function of maternal age or size when the ecological circumstances of progeny are not changing as a function of maternal age or size? The second gap in current theory is that, like all optimization theory, it is patently non-genetic in that it is assumed that the phenotypes optimized are evolutionarily accessible. Recent maternal effects theory indicates that traits subject to maternal influence behave in unanticipated ways. Specifically, there may be time lags in response to selection, and hence, selection away from the optimum phenotype. This paper explores a suite of issues pertaining to the evolution of propagule size from the broader perspective of propagule size as a maternal effect (PSME) with a goal of widening the lens through which propagule size is viewed by evolutionary ecologists. Two themes are developed. First, I suggest that, to understand egg size variance and its implications for both maternal and offspring fitness, it is necessary to consider explicitly the ecological context in which a mother is producing eggs, not just that into which offspring will enter. I argue that some of the variables that have only been incorporated in pairwise fashion (or not at all) into studies of propagule size might account for the lack of agreement about how this important life history feature evolves. Further, I suggest that failure to consider other sources of selection on maternal phenotypes, driven by a narrow adaptationist view that has historically been taken of PSMEs, has obfuscated many interesting questions surrounding their coevolution with maternal characters. Thus, the second theme is that it is necessary to consider other explanations for why prop-agule size varies apart from those pertaining to offspring fitness per si. Based on a detailed review of the empirical literature, I conclude that the concept of an optimal propagule size is not only an insufficient construct to explain the evolution of propagule size, but that continued reliance on an optimization approach is likely to stifle development of more realistic and predictive theory for the evolution of this key life history trait. Novel theory should incorporate realities from physiology, development and genetics and should accommodate the dynamic nature of the selective environments in which propagule size evolves, all of which have been shown by empiricists to play a role in determining propagule size phenotypes. A key feature of this theory should be the explicit treatment of propagule size as a maternal effect.

Developmental Integration and the Evolution of Pleiotropy

Abstract: The different forms of morphological integration, developmental, functional, genetic, and evolutionary are defined and their theoretical relationships explored. Quantitative genetic models predict that the co-selection of traits involved in a common function will lead to pleiotropic effects at the loci affecting them while functionally-unrelated traits will be affected by separate sets of loci (Wagner, 1996). The patterns of genetic variation produced by these pleiotropic mutations and stabilizing selection for functionally and developmentally interacting traits results in their specific co-inheritance relative to other traits. This in turn leads to their co-ordinated response to selection. Therefore, functional and developmental integration lead to genetic integration which, in turn leads to evolutionary integration. Three examples of how developmental integration structures pleiotropy and morphological variation in non-human primate crania, artificially-modified human crania, and for the effects ofindividual genes on murine mandibular morphology are presented.

Risky Theories—The Effects of Variance on Foraging Decisions

Abstract: SYNOPSIS This paper concerns the response of foraging animals to variability in rate of gain, or risk. Both the empirical and theoretical literatures relevant to this issue are reviewed. The methodology and results from fifty-nine studies in which animals are required to choose between foraging options differing in the variances in the rate of gain available are tabulated. We found that when risk is generated by variability in the amount of reward, animals are most frequently risk-averse and sometimes indifferent to risk, although in some studies preference depends on energy budget. In contrast, when variability is in delay to reward, animals are universally risk-prone. A range of functional, descriptive and mechanistic accounts for these findings is described, none of which alone is capable of accommodating all aspects of the data. Risk-sensitive foraging theory provides the only currently available explanation for why energy budget should affect preference. An information-processing model that incorporates Weber's law provides the only general explanation for why animals should be risk-averse with variability in amount and risk-prone with delay. A theory based on the mechanisms of associative learning explains quantitative aspects of risk-proneness for delay; specifically why the delay between choice and reward should have a stronger impact on preference than delays between the reward and subsequent choice. It also explains why animals should appear to commit the "fallacy of the average," maximising the expected ratio of amount of reward over delay to reward when computing rates rather than the ratio of expected amount over expected delay. We conclude that only a fusion of functional and mechanistic thinking will lead to progress in the understanding of animal decision making.

Potential Abiotic and Biotic Impacts of Zebra Mussels on the Inland Waters of North America

Abstract: SYNOPSIS. The expansion of zebra mussel distribution into inland waterways of North America portends significant abiotic and biotic changes mediated either directly or indirectly by Dreissena . Dreissena fouls a wide array of submerged substrates including rock surfaces, macrophytes, native molluscs, canal and dock walls, and watercraft and motor outdrives. Fouling of water intake pipes and associated installations can severely impair water delivery to hydroelectric, municipal and industrial users, necessitating proactive or reactive control measures. Mussels increase water clarity by removing suspended clay, silt, bacteria, phytoplankton, and small zooplankton. Clear water phases associated with Dreissena grazing may exceed in magnitude and duration those generated by zooplankton grazing. Enhanced water clarity increases light transmittance and growth of benthic plants. Some benthic invertebrates {e.g., unionid molluscs) are adversely affected by Dreissena , whereas others, including amphipod crustaceans, exploit structure associated with or wastes generated by zebra mussels. Dreissena is exploited by a host of predators, most notably waterfowl, fish and crayfish. Waterfowl predators that consume contaminated Dreissena have elevated concentrations of organic pesticides and polychlorinated biphenyl compounds. Invasion of shallow lakes and ponds by Dreissena may divert production and biomass from pelagic to benthic foodwebs, shifting ecosystems to an alternative state.

Maternal Influences on Variation in Egg Sizes in Temperate Marine Fishes

Abstract: SYNOPSIS. We examine the variation in egg sizes of marine teleosts and evaluate the maternal contribution to this variability. At the species level, egg sizes in 309 North Atlantic fishes range from 0.3 to 18.0 mm diameter (median = 1.1), size at hatching varies directly with egg size, and large adult size is associated with large eggs but the relationship is weak. Within populations, egg sizes are distributed normally with a median coefficient of variation of 4% (n = 56 species). Egg size varied among females in all cases for which female-level data were found. Estimates of the variance components of egg size due to females were found for three species and, as a percentage of total variance, are 71 % for capelin, Mallotus villosus , 46% for winter flounder, Pleuronectes americanus , and 35% for Atlantic cod, Gadus morhua . For cod, which spawn multiple egg batches per year, an additional batches-within-females component was estimated to be 26%. Size at hatching also differs among sibgroups and is generally directly related to egg size at the individual level. We modelled fish growth by allowing individuals to grow at exponential rates from a normal distribution of initial sizes. Comparing size variation in model fish to empirical evidence suggests that variation in initial sizes, propagated by growth, could account for a large fraction of the size variation observed months after hatching in natural populations. We view size variation in young marine fishes to be largely of maternal origin and environmentally modulated, which if true has special consequences for fisheries and aquaculture.

Oviposition Site Choice and Life History Evolution

Abstract: SYNOPSIS. Studies of life history evolution, as well as much of life history theory, have typically focused on "hard" components of life histories; phenotypic characteristics that can be readily observed, quantified, and ultimately, connected rather directly to fitness. Typical of these are propagule size, propagule number, and age and size at maturity. What is largely missing from the study of life history evolution is consideration of the role of behavior, principally female oviposition site choice, in the evolution of life histories. For oviparous organisms, natural selection cannot produce locally optimized "hard" components of life history phenotypes without a consistent environmental context (whether invariant or variable); in a variable environment, that consistent environmental context can be most effectively provided by interactive oviposition site choice. I present a model of selection on oviposition site choice in the context of the evolution of "hard" components of life history phenotypes, along with some experimental data illustrating oviposition site choice in response to predators. The model and data are then related to the overall question of the role of oviposition site choice in life history evolution. The conclusion is that oviposition site choice must be under equally strong selection with egg size, egg number and the other hard components of life histories in order to generate and optimize locally adapted or ecologically specialized life history phenotypes, and must therefore, play a significant role in the evolution of life histories.

Transitions from Drag-based to Lift-based Propulsion in Mammalian Swimming

Abstract: Synopsis. The evolution of fully aquatic mammals from quadrupedal, terrestrial mammals was associated with changes in morphology and swimming mode. Drag is minimized by streamlining body shape and appendages. Improvement in speed, thrust production and efficiency is accomplished by a change of swimming mode. Terrestrial and semiaquatic mammals employ drag-based propulsion with paddling appendages, whereas fully aquatic mammals use lift-based propulsion with oscillating hydrofoils. Aerobic efficiencies are low for drag-based swimming, but reach a maximum of 30% for lift-based propulsion. Propulsive efficiency is over 80% for lift-based swimming while only 33% for paddling. In addition to swimming mode, the transition to high performance propul? sion was associated with a shift from surface to submerged swimming providing a reduction in transport costs. The evolution of aquatic mam? mals from terrestrial ancestors required increased swimming performance with minimal compromise to terrestrial movement. Examination of modern analogs to transitional swimming stages suggests that only slight modification to the neuromotor pattern used for terrestrial locomotion is re? quired to allow for a change to lift-based propulsion.

Exponential Versus Hyperbolic Discounting of Delayed Outcomes: Risk and Waiting Time

Abstract: Frequently, animals must choose between more immediate, smaller rewards and more delayed, but larger rewards. For example, they often must decide between accepting a smaller prey item versus continuing to search for a larger one, or between entering a leaner patch versus travelling to a richer patch that is further away. In both situations, choice of the more immediate, but smaller reward may be interpreted as implying that the value of the later reward is discounted; that is, the value of the later reward decreases as the delay to its receipt increases. This decrease in value may occur because of the increased risk involved in waiting for rewards, or because of the decreased rate of reward associated with increased waiting time. The present research attempts to determine the form of the relation between value and delay, and examines implications of this relation for mechanisms underlying risk-sensitive foraging. Two accounts of the relation between value and delay have been proposed to describe the decrease in value resulting from increases in delay: an exponential model and a hyperbolic model. Our research demonstrates that, of the two, a hyperbola-like discounting model consistently explains more of the variance in temporal discounting data at the group level and, importantly, at the individual level as well. We show mathematically that the hyperbolic model shares fundamental features with models of prey and patch choice. In addition, the present review highlights the implications of a psychological perspective for the behavioral biology of risksensitive foraging, as well as the implications of an ecological perspective for the behavioral psychology of risk-sensitive choice and decision-making.

A Review of the Biology and Ecology of the Quagga Mussel (Dreissena bugensis), a Second Species of Freshwater Dreissenid Introduced to North America

Abstract: Synopsis. North America's Great Lakes have recently been invaded by two genetically and morphologically distinct species of Dreissena. The zebra mussel (Dreissena polymorpha) became established in Lake St. Clair of the Laurentian Great Lakes in 1986 and spread throughout eastern North America. The second dreissenid, termed the quagga mussel, has been identified as Dreissena bugensis Andrusov, 1897. The quagga occurs in the Dnieper River drainage of Ukraine and now in the lower Great Lakes of North America. In the Dnieper River, populations of D. poly? morpha have been largely replaced by D. bugensis', anecdotal evidence indicates that similar trends may be occurring in the lower Laurentian Great Lakes. Dreissena bugensis occurs as deep as 130 m in the Great Lakes, but in Ukraine is known from only 0-28 m. Dreissena bugensis is more abundant than D. polymorpha in deeper waters in Dneiper River reservoirs. The conclusion that North American quagga mussels have a lower thermal maximum than zebra mussels is not supported by obser? vations made of populations in Ukraine. In the Dnieper River drainage, quagga mussels are less tolerant of salinity than zebra mussels, yet both dreissenids have acclimated to salinities higher than North American pop? ulations; eventual colonization into estuarine and coastal areas of North America cannot be ignored.

The Physiological Ecology of the Zebra Mussel, Dreissena polymorpha, in North America and Europe

Abstract: Synopsis. The zebra mussel, Dreissena polymorpha (Pallas), was intro? duced into North America in 1986. Initial North American (N.A.) studies suggested that physiological responses varied between N.A. and European populations. However, literature review indicates agreement on most as? pects of physiological adaptation including: respiratory responses; hypoxia/anoxia tolerance; salinity limits; emersion tolerance; freezing resis? tance; environmental pH limits; calcium limits; starvation responses; and bioenergetic partitioning. The main differences among N.A. and European mussels appear to be elevated upper thermal limits and temperatures for optimum growth among N.A. populations. N.A. zebra mussels probably originated from the northern shore of the Black Sea in the warmest por? tion of the mussel's European range. However, most European physio? logical data come from northern Europe where populations may be adapt? ed to colder temperatures. Alternatively, N.A. research suggests that mus? sels may have a capacity for seasonal temperature acclimatization such that responses recorded in warmer N.A. waters may be different from those recorded in northern Europe even after short-term laboratory accli? mation. Studies of genetic variation and physiological response among European and N.A. D. polymorpha populations are required to elucidate the basis for physiological differentiation. Recently evolved D. polymor? pha has poor resistance adaptations compared to unionacean and sphaeriid bivalves with longer freshwater fossil histories. Poor resistance adapta? tions make it less suited for stable habitats, instead, its high fecundities, early maturity, and rapid growth are adaptations to unstable habitats where extensive resistance adaptations are of little value.

The Importance of Body Stiffness in Undulatory Propulsion

Abstract: During steady swimming in fish, the dynamic form taken by the axial undulatory wave may depend on the bending stiffness of the body. Previous studies have suggested the hypothesis that fish use their muscles to modulate body stiffness. In order to expand the theoretical and experimental tools available for testing this hypothesis, we explored the relationship between body stiffness, muscle activity, and undulatory waveform in the mechanical context of dynamically bending beams. We propose that fish minimize the mechanical cost of bending by increasing their body stiffness, which would allow them to tune their body's natural frequency to match the tailbeat frequency at a given swimming speed. A review of the literature reveals that the form of the undulatory wave, as measured by propulsive wavelength, is highly variable within species, a result which calls into question the use of propulsive wavelength as a species-specific indicator of swimming mode. At the same time, the smallest wavelength within a species is inversely proportional to the number of vertebrae across taxa ( r 2 = 0.21). In order to determine if intact fish bodies are capable of increasing bending stiffness, we introduce a method for stimulating muscle in the body of a dead fish while it is being cyclically bent at physiological frequencies. The bending moment (N m) and angular displacement (radians) are measured during dynamic bending with and without muscle stimulation. Initial results from these whole body work loops demonstrate that largemouth bass possess the capability to increase body stiffness by using their muscles to generate negative mechanical work.

Zebra mussel infestation of unionid bivalves (unionidae) in north america

Abstract: SYNOPSIS. In 1989, zebra mussels received national attention in North America when they reached densities exceeding 750,000/m 2 in a water withdrawal facility along the shore of western Lake Erie of the Laurentian Great Lakes. Although water withdrawal problems caused by zebra mussels have been of immediate concern, ecological impacts attributed to mussels are likely to be the more important long-term issue for surface waters in North America. To date, the epizoic colonization (i.e., infestation) of unionid bivalve mollusks by zebra mussels has caused the most direct and severe ecological impact. Infestation of and resulting impacts caused by zebra mussels on unionids in the Great Lakes began in 1988. By 1990, mortality of unionids was occurring at some locations; by 1991, extant populations of unionids in western Lake Erie were nearly extirpated; by 1992, unionid populations in the southern half of Lake St. Clair were extirpated; by 1993, unionids in widely separated geographic areas of the Great Lakes and the Mississippi River showed high mortality due to mussel infestation. All infested unionid species in the Great Lakes (23) have become infested and exhibited mortality within two to four years after heavy infestation began. Data indicate that mean zebra mussel densities >5,000-6,000/m 2 and infestation intensities >100-200/unionid in the presence of heavy zebra mussel recruitment results in near total mortality of unionids. At present, all unionid species in rivers, streams, and lakes that sympatrically occur with zebra mussels have been infested and, in many locations, negatively impacted by zebra mussels. We do not know the potential consequences of infestation on the 297 unionid species found in North America, but believe zebra mussels pose an immediate threat to the abundance and diversity of unionids.

Unsteady mechanisms of force generation in aquatic and aerial locomotion

Abstract: tion. First, analyses have become much more comparative, both across and within species. The great diversity of teleost fishes provides one of the best backgrounds for exploring the evolution of locomotor sys? tems at both the physiological and anatomical levels (see papers by Fish [1996], Westneat [1996], Gillis [1996], and Lauder and Jayne [1996] in this volume). By doing so, researchers are discovering new and diverse patterns of locomotory behavior. In addition to cross species comparisons, researchers are attempting to focus on the more subtle but equally important changes in swimming behavior within the ontogeny of single spe? cies (see paper by Hale [1996] in this vol? ume). The second reason for an alteration in perspective is that researchers are not simply satisfied with treating locomotion as a means of getting efficiently from one point to another, but also as a complex and flexible system of behavior required for feeding, courtship, and escape. In this new perspective, maneuverability is often con? sidered a more important design criterion than energy efficiency. And finally, the availability of powerful desktop microcomputers makes it possible to develop more sophisticated mathematical models of aquatic locomotion (see papers by Jordan [1996] and Fauci [1996] in this volume). Whether focused on juvenile salmon or

Maternal Effects on Offspring Quality in Poeciliid Fishes

Abstract: SYNOPSIS. We evaluated the effects of maternal environment on offspring size and composition in three species of poeciliid fishes. We chose food availability as the environmental factor for study. Mature females were assigned to either high or low food for an interval of time, then randomly reassigned to high or low food, with the restriction that there be equal numbers in each of four treatments: high-high, high-low, lowhigh, and low-low food availability. The three species chosen for study differ in the pattern of maternal provisioning. Poecilia reticulata and Priapichthys festae mothers provide all resources necessary for development as yolk, prior to fertilization. In contrast, Heterandria formosa mothers continue to provision the young throughout development. These species also differ in whether or not they have superfetation, or the ability to carry multiple broods of young in different stages of development. P. reticulata does not have superfetation while the other two species do. We were interested in whether the pattern of maternal provisioning or superfetation influenced the maternal effect. The two lecithotrophic species responded to low food by producing larger young with greater fat reserves. H. formosa, the matrotrophic species, responded to low food by producing smaller young. We propose that the production of large young in the face of low food availability might represent adaptive plasticity; matrotrophy might represent a constraint that prevents such an adaptive response. Superfetation had no impact on this maternal effect.

Comparative Biology of Zebra Mussels in Europe and North America: An Overview

Abstract: Synopsis. Since the discovery of the zebra mussel, Dreissena polymor? pha, in the Great Lakes in 1988 comparisons have been made with mussel populations in Europe and the former Soviet Union. These comparisons include: Population dynamics, growth and mortality rates, ecological tol? erances and requirements, dispersal rates and patterns, and ecological im? pacts. North American studies, mostly on the zebra mussel and a few on a second introduced species, the quagga mussel, Dreissena bugensis, have revealed some similarities and some differences. To date it appears that North American populations of zebra mussels are similar to European populations in their basic biological characteristics, population growth and mortality rates, and dispersal mechanisms and rates. Relative to European populations differences have been demonstrated for: (1) individual growth rates; (2) life spans; (3) calcium and pH tolerances and requirements; (4) potential distribution limits; and (5) population densities of veligers and adults. In addition, studies on the occurrence of the two dreissenid species in the Great Lakes are showing differences in their modes of life, depth distributions, and growth rates. As both species spread throughout North America, comparisons between species and waterbodies will enhance our ability to more effectively control these troublesome species.

Maternal Condition and Nest Site Choice: An Alternative for the Maintenance of Environmental Sex Determination?

Abstract: SYNOPSIS. Egg size and nest site selection are two potential effects that can have a persistent influence on the phenotype of offspring. In this paper, I develop the maternal condition-dependent choice hypothesis for the maintenance of environmental sex determination. The hypothesis stipulates three conditions: 1) there must be variation in the maternal effect, 2) the variation in the maternal effect must influence fitness of the offspring differently between the sexes, and 3) female reproductive behavior is determined by her condition or how her condition will influence her offspring's fitness. Females with the ability to recognize environments that have a higher probability of producing the sex that would benefit the most from maternal condition will have an advantage. Using egg size as a maternal effect, I test this hypothesis in the diamondback terrapin, an emydid turtle with temperature-dependent sex determination. Terrapins have large variation in egg size among clutches and little variation within clutches. Egg mass is the primary determinant of hatchling mass and can result in as much as a three year difference in reaching minimum size of first reproduction in females, but may not affect age or size of first reproduction in males. Finally, terrapins select open nesting sites with warmer incubation conditions and place larger eggs there. Females place smaller eggs in cooler sites. Terrapin reproduction is consistent with the prediction of the maternal condition-dependent nest site choice hypothesis. The model and supporting data demonstrate how maternal effects can be an important factor to consider in studies of environmental sex determination.

A New Look at Locomotion in Microorganisms: Rotating and Translating

Abstract: SYNOPSIS. The locomotion and orientation of free-swimming microorganisms have been widely studied for more than a century. With few exceptions, only the two-dimensional translational velocity of the organism in question is ever reported, yet a complete description of motion requires a three-dimensional description of both the translational and rotational velocities. Recent theoretical work, considering both the translational and rotational velocities, has demonstrated that a previously unrecognized orientation mechanism exists—helical klinotaxis. Efforts to test the predictions of this theory are described, concluding that helical klinotaxis is probably utilized by a diverse assemblage of free-swimming microorganisms.

Integration of Craniofacial Structures During Development in Mammals

Abstract: The integration of craniofacial elements during development in mammals is studiedin a phylogenetic approach. Developmental series of four metatherian (marsupial) and five Eutherian (placental) taxa are examined, and the sequence of emergence of major characters of the central nervous system, the cranial skeleton and the cranial musculature is reconstructed. These sequences are transformed into a series of characters that are then mapped onto the phylogeny of the taxa. This phylogenetic approach makes it possible to distinguish between characters thatare uniform across all mammals, and those that differentiate marsupials and placentals. The most significant difference between the two groups involves the relative timing of the developmentof the central nervous system and the somatic system. The central nervous system differentiates far in advance of the bones and muscles of the head in eutherians. In metatherians, somatic elements, particularly of the face, are accelerated. Additional other differences between the two groups of mammals are discussed. Many elements, however, are unchanged, and develop in a consistent relation despite overall shifts in development. These data are used to test several hypotheses on the ways that cranial development in mammals is integrated.

The Use of Gait Transition Speed in Comparative Studies of Fish Locomotion

Abstract: SYNOPSIS. Physiological and biomechanical inquiries into the principles of vertebrate locomotion require comparison among animals of different size, habitat and phyletic association. In designing comparative studies of locomotion, a major challenge is to isolate the effects of experimentally imposed variation from the confounding effects of variation in animal activity level associated with differences in scale and life history. For swimming vertebrates, traditional measures of speed used for comparison, including sprint speed and critical swimming speed, should in theory each elicit similar efforts from different animals but have practical shortcomings that can limit their usefulness. This paper presents an alternative approach, adapted from the work of mammalian physiologists, which controls for differences in relative activity level among swimming animals of different size and habitat through comparison at gait transition speeds. The method is illustrated with examples from study of the teleost fish family Embiotocidae, whose members exhibit a distinct transition from exclusively pectoral fin oscillation to combined pectoral and caudal fin propulsion with increasing swimming speed. The pectoral-caudal gait transition speed, or any percentage thereof, is shown to be 'biomechanically equivalent' for swimmers of different size. When this performance limit is expressed in terms of body lengths traveled per unit time, a common normalization of swimming speed, it varies markedly across size and habitat within the family. This finding has the important implication that length-specific speeds may not induce comparable degrees of exercise from different fishes, and thus kinematic and physiological comparisons at such speeds can yield misleading results. The comparative approach described for pectoral fin swimmers, and the limitations of length-specific speed, should be generally applicable to studies of other swimming vertebrates.

Physiological Aspects of Zebra Mussel Reproduction: Maturation, Spawning, and Fertilization

Abstract: SYNOPSIS. The prolific reproductive capabilities of the zebra mussel, Dreissena polymorpha , have facilitated the rapid spread and high densities of this biofouling organism since its accidental introduction into North America less than 10 years ago. Research on its reproductive mechanisms and capabilities may be valuable not only in predicting its further spread, but also in investigating basic mechanisms of reproduction and development and in developing new strategies to mitigate its impact. Since zebra mussels are dioecious and fertilization occurs externally, coordinated maturation, spawning, and other mechanisms have evolved to increase the probability of successful fertilization. The zebra mussel undergoes an annual cycle of gonadal growth and gamete maturation, culminating in one or more spawning events in late spring or early summer. Temperature, rates of temperature change, food availability, and effects of neighboring mussels seem to be critical variables that determine reproductive responses. Serotonin is a biogenic amine which is implicated in spawning behavior and can reliably trigger spawning. Serotonin is present in the gonad in neural varicosities that encircle groups of gametes, and specific serotonergic ligands can mimic or block spawning caused by serotonin. In females, serotonin reinitiates meiosis causing maturation from prophase I to metaphase I prior to spawning. Spawned oocytes contain substances that are species specific sperm chemoattractants. The sequence of binding, entry, and subsequent nuclear movements have been observed with fluorescence and scanning microscopy. Despite their negative ecological and economic impacts, zebra mussels have also provided a new and easily obtainable resource for studies of reproductive mechanisms.

Trade-offs in Energy Allocation During Lactation

Abstract: SYNOPSIS. During lactation, mothers require energy to meet both maternal and offspring requirements. If a mother exports too much energy to dependent offspring (in milk), her weight loss may be excessive and maternal risk may increase. Conversely, too little energy allocation to offspring may reduce the growth rate or induce mortality of dependent offspring. This paradigm was evaluated in cotton rats ( Sigmodon hispidus ) supporting small (3 pup) and large (6 pup) litters from early to late lactation. Several types of evidence indicate that physiological constraints limit the ability of mothers with large litters to provide resources to offspring. Mothers with large litters produced a dilute, energy-poor milk and their rates of food intake, weight loss and energy export per litter appeared to approach physiological maxima. Whereas the energy exported to pups in small litters increased from early to late lactation, the energy flow per pup in large litters was consistently low; consequently, offspring in large litters had low growth rates. An increase in either maternal food intake or weight loss (catabolism of maternal tissue) could have provided additional energy to offset the low growth rate of pups in large litters. However, mothers with large litters did not substantially increase their food intake or weight loss compared with mothers supporting small litters. These results indicate that the maternal support of offspring in large litters is limited. The pattern of energy allocation shown by cotton rats with large litters likely reflects a compromise between meeting maternal and offspring energy requirements ( cf. , Parker and Macnair, 1979). The energy flow is greater than optimal for the parent but less than optimal for the offspring. Less maternal-offspring conflict occurs in small than large litters because offspring in small litters maintain a high growth rate at a relatively low maternal cost. Yet, under favorable environmental conditions, the reduction in maternal-offspring conflict has no apparent fitness benefit.

Undulatory Locomotion in Elongate Aquatic Vertebrates: Anguilliform Swimming since Sir James Gray

Abstract: Anguilliform locomotion is the mode of undulatory swimming used by eels and other elongate vertebrates in which waves (of shorter length than the body of the animal) are propagated posteriorly along the length of the animal, propelling it forward. Although a broad array of vertebrate taxa convergently use this locomotor mode, much of what we know regarding the kinematics of anguilliform swimming derives from the classic work of Sir James Gray in the 1930s on the European eel, Anguilla anguilla. To better understand the variability within this phylogenetically widespread mode of locomotion, I have reviewed recent work on the swimming kinematics of elongate fishes, salamanders and snakes. The amplitude and relative speed of the undulatory waves propagated along the body during swimming differs among elongate vertebrate tax, and can also vary with swimming speed. In addition, the lateral velocity and orientation of tail segments can reach their maxima at different times within a tailbeat cycle (in contrast to Gray's suggestion that these two events occurred simultaneously). Finally, the angle of attack of tail segments is negative during a consistent portion of each tailbeat cycle in the elongate salamander Siren . While this has yet to be examined in other anguilliform taxa, it is possible that they will also show this pattern. Hence, although Gray's descriptions of anguilliform swimming are generally accurate, gaining a broader understanding of this mode of locomotion requires the characterization of the diversity and complexity found among animals using this undulatory mode.

Springs in Swimming Animals

Abstract: Animals can lower the metabolic cost of swimming by using appropriately tuned, elastic springs. Jet-powered invertebrates use springs that lie in functional parallel to their swimming muscles to power half the locomotor cycle. The parallel geometry constrains the spring to be non-linearly elastic; muscle power is diverted to load the spring only when swimming muscles are not capable of producing maximal hydrodynamic thrust. The springs of jellyfish and scallops are forced at or near their resonant frequency, producing large energy savings. Measuring the contribution of elastic energy storage to jet-powered locomotion has been facilitated by the relatively simple geometries of invertebrate locomotor systems. In contrast, complex musculoskeletal systems and kinematics have complicated the study of springs in swimming vertebrates. Skins, tendons and axial skeletons of some vertebrate swimmers have appropriate mechanical properties to act as springs. To date, though, there exist just a handful of studies that have investigated the mechanical behaviors of these locomotor structures in swimming vertebrates, and these data have yet to be integrated with measures of swimming power. Integrating mechanical, kinematic, hydrodynamic and metabolic data are required to understand more fully the role of elastic springs in vertebrate swimming energetics.

Variations in the reproductive cycle of dreissena polymorpha in europe, russia, and north america

Abstract: Synopsis. The reproductive cycle of the zebra mussel (Dreissena poly? morpha) is highly variable throughout its range in Europe, Russia2, and North America. The environmental factors influencing this variation are poorly understood, but successful reproduction is occurring in areas where it was initially believed that adult zebra mussels could not survive (i.e., southern United States). The differences in mussel reproduction occurring from site-to-site make it difficult to predict timing of specific events, such as the start of larval production, that are important in initiating containment or control procedures. For example, the amount of time required for a fertilized egg to develop into a juvenile mussel can be as short as 8 days, or as long as 240 days. Release of gametes by adults can be a highly synchronized event, focused over a 1-2 week period, or it can be completely non-synchronized, occurring throughout the year. Zebra mus? sels in some localities start spawning at water temperatures of 12-13?C, but do not start until water temperatures reaches 22?C at other sites. While some of this variability in reproductive behavior stems from mussel ad? aptation to local conditions, part is due to difficulties in sampling these events. It is difficult to determine reproductive success of a specific pop? ulation because of the problems in separating locally produced larvae from larvae drifting in from other areas. Further research is needed not only on the relationship between reproduction and environment at the community level, but also on the variability in response of individual mussels.

An Introduction to Risk Sensitivity: The Use of Jensen's Inequality to Clarify Evolutionary Arguments of Adaptation and Constraint

Abstract: Synopsis. I review the basic rationale for risk-sensitive foraging, and present a few of the most common expressions of risk-sensitivity theory. A simple heuristic, the energy-budget rule, and the z-score model are explained. These functional models of risk sensitivity assume or explicitly model fitness as a nonlinear function of an animal's energy state. For such nonlinear relationships, Jensen's inequality predicts that the fitness realized by an animal with a more constant energy state may be higher or lower than the fitness resulting from a variable energy state with the same mean, depending on the shape of the fitness function. Predictive func? tional models of behavior, like the energy-budget rule and the z-score model, depend on specific features of the function relating energy state to fitness, which may or may not be general features for most organisms. While behavioral ecologist have studied the functional significance of animal responses to variation in foraging rewards, psychologists have long studied the psychological mechanisms by which preferences for vari? able or constant rewards are expressed. Jensen's inequality is applied here too; psychologists propose nonlinearities in the perception and processing of information. The growing number of different relationships that might account for risk-sensitive behavior is a potential source of confusion. I advocate "returning to the basics," Le., that hypotheses to explain risksensitive behavior should specify precisely the assumed nonlinear rela? tionships.

Functional morphology of aquatic flight in fishes : kinematics, electromyography, and mechanical modeling of labriform locomotion

Abstract: Synopsis. Labriform locomotion is the primary swimming mode for many fishes that use the pectoral fins to generate thrust across a broad range of speeds. A review of the literature on hydrodynamics, kinematics, and morphology of pectoral fin mechanisms in fishes reveals that we lack several kinds of morphological and kinematic data that are critical for understanding thrust generation in this mode, particularly at higher velocities. Several needs include detailed three-dimensional kinematic data on species that are pectoral fin swimmers across a broad range of speeds, data on the motor patterns of pectoral fin muscles, and the development of a mechanical model of pectoral fin functional morphology. New data are presented here on pectoral fin locomotion in Gomphosus varius, a labrid fish that uses the pectoral fins at speeds of 1-6 total body lengths per second. Three-dimensional kinematic data for the pectoral fins of G. varius show that a typical "drag-based" mechanism is not used in this species. Instead, the thrust mechanics of this fish are dominated by lift forces and acceleration reaction forces. The fin is twisted like a propeller during the fin stroke, so that angles of attack are variable along the fin length. Electromyographic data on six fin muscles indicate the sequence of muscle activity that produces antagonistic fin abduction and adduction and controls the leading edge of the fin. EMG activity in abductors and adductors is synchronous with the start of abduction and adduction, re? spectively, so that muscle mechanics actuate the fin with positive work. A mechanical model of the pectoral fin is proposed in which fin morphometrics and computer simulations allow predictions of fin kinematics in three dimensions. The transmission of force and motion to the leading edge of the fin depends on the mechanical advantage of fin ray levers. An integrative program of research is suggested that will synthesize data on morphology, physiology, kinematics, and hydrodynamics to under? stand the mechanics of pectoral fin swimming.

Homology, Hox Genes, and Developmental Integration

Abstract: SYNOPSIS. The establishment and inheritance of individualized structural units is a key feature of morphological evolution, embodied in the concept of homology. In current debates, homology is often equated with identical genetic encoding. The empirical evidence for this assumption is ambiguous. Genetic identity can indicate morphological identity in some cases, but several examples show that gene expression patterns and regulatory systems of development may be highly conserved while morphological characters undergo dramatic evolutionary innovation. This indicates some independence of structural homology from its genetic and developmental makeup. It is proposed that phenotypic evolution depends strongly on the epigenetic context in which genetic redundancy becomes available for the control of new developmental interactions. The integrated character of developmental systems may represent an important factor in the origin and identity of morphological characters and can stabilize incipient structures before their full genetic integration. The origin of the autopod section of the tetrapod limb is an example which suggests that novel homologues can arise in evolution as a consequence of changing the epigenetic context of conserved gene function.

Pectoral Fin Locomotion in Fishes: Testing Drag-based Models Using Three-dimensional Kinematics

Abstract: Paired fin propulsion in fishes has classically been divided into two categories which represent biomechanical extremes in the use of appendages for propulsion: lift-based and drag-based mechanisms of thrust production. Theoretical models predict that fishes using drag-based propulsion should have wedge-shaped fins with relatively blunt distal edges, a fin beat cycle that is oriented along the anteroposterior (x) axis, feathering of the fin to reduce drag during the protraction phase, and maximal fin area during the retraction phase as the fin sweeps posteriorly perpendicular to the body. In this paper we use a three-dimensional analysis of pectoral fin propulsion in the largemouth bass, Micropterus salmoides , to (1) evaluate the extent to which bass pectoral fin kinematics fit predictions of drag-based propulsion, and (2)demonstrate the complexity of fin movement when the traditional two-dimensional analysis is extended into three dimensions. We attached small markers to visualize the diaphanous distal fin edge, and we videotaped lateral and ventral views from which we could measure x, y, and z coordinates from the fin and body. We divided the fin into two triangular elements for which we calculated planar (three-dimensional) angles relative to each of three reference planes (XY, YZ and XZ) during the fin beat cycle. We show how angles of attack based only on two-dimensional data may result in gross errors that severely compromise understanding of the mechanics and hydrodynamics of pectoral propulsion. Furthermore, three-dimensional analysis revealed that bass fin kinematics are much more complex than expected on a rowing model of drag-based propulsion, and that the pectoral fins may produce drag-based thrust even during protraction. Three dimensional kinematic data are critical to understanding the hydrodynamics of aquatic animal propulsion. Such data are a necessary foundation for reconstructing patterns of movement, modeling (both theoretical and empirical), and for assessing the extent to which motion is under active control or a passive consequence of fluid resistance.