Showing papers in "The American Naturalist in 1995"
TL;DR: Application of empirical estimates of the properties of spontaneous deleterious mutations leads to the conclusion that populations with effective sizes smaller than 100 are highly vulnerable to extinction via a mutational meltdown on timescales of approximately 100 generations.
Abstract: Although extensive work has been done on the relationship between population size and the risk of extinction due to demographic and environmental stochasticity, the role of genetic deterioration in the extinction process is poorly understood. We develop a general theoretical approach for evaluating the risk of small populations to extinction via the accumulation of mildly deleterious mutations, and we support this with extensive computer simulations. Unlike previous attempts to model the genetic consequences of small population size, our approach is genetically explicit and fully accounts for the mutations inherited by a founder population as well as those introduced by subsequent mutation. Application of empirical estimates of the properties of spontaneous deleterious mutations leads to the conclusion that populations with effective sizes smaller than 100 (and actual sizes smaller than 1,000) are highly vulnerable to extinction via a mutational meltdown on timescales of approximately 100 generations. We ...
906 citations
TL;DR: It appears that evolution and population dynamics give rise to a feedback mechanism that when double infections are frequent, increased virulence is favored; but when pathogens become more virulent, the force of infection will decrease, favoring lower virulence again.
Abstract: While for pathogen clones singly occupying a host it may pay to adopt a relatively avirulent host exploitation strategy, clones sharing a host have a conflict of interest that favors more virulent strategies. As the number of infections per host depends on the force of infection and the force of infection, in turn, depends on prevailing virulence, evolutionary analysis needs to be integrated with population dynamics. A full-fledged approach requires exceedingly large capacities for bookkeeping of the infection events and is therefore difficult to establish. In this article the host-pathogen interaction is studied for the simple case in which hosts may become at most doubly infected. It appears that evolution and population dynamics give rise to a feedback mechanism. When double infections are frequent, increased virulence is favored; but when pathogens become more virulent, the force of infection will decrease, favoring lower virulence again. Thus, evolutionarily stable strategy (ESS) virulence depends on...
467 citations
TL;DR: In this article, the relation between fitness-associated performance and temperature is modeled by a performance curve defined by two traits: performance breadth (Tbr), a measure of thermal specialization, and the critical maximum temperature for performance (Tmax).
Abstract: Animals and plants often exhibit narrow ranges of thermal preference in variable environments; fitness-enhancing activities such as reproduction and growth tend to be concentrated during times in which body temperature lies within that narrow range. The relation between fitness-associated performance and temperature is modeled by a performance curve defined by two traits: performance breadth (Tbr), a measure of thermal specialization, and the critical maximum temperature for performance (Tmax). Optimality models are used to define the fitness landscape for these two traits under several different patterns of within- and among-generation variation in temperature. In constant environments and environments in which there is significant within-generation variation, specialists with narrow preference ranges are the favored phenotype. In environments in which there is considerable among-generation but little within-generation variation, generalists with broad preference ranges are favored. Specialists in a cons...
465 citations
TL;DR: How the presence of dynamic change in traits affecting inter-specific interactions changes the ways in which interactions between species are distinguished, classified, and measured is discussed.
Abstract: This article discusses how the presence of dynamic change in traits affecting inter-specific interactions changes the ways in which interactions between species are distinguished, classified, and measured. The prevalence of models lacking any trait dynamics has led to methods of identifying and measuring indirect effects that are not valid when phenotypically or evolutionarily plastic traits affect interspecific interactions. When traits are dynamic, the number of links in a dynamic model can no longer be used to distinguish direct and indirect effects, and classifying interactions by a single sign denoting effect on equilibrium density becomes problematic. The presence of trait dynamics also changes the interpretation of manipulative experiments that have been used to measure indirect effects. Because both traits and population densities can transmit indirect effects, a given ordered series of species will often transmit several indirect effects, which may have opposite signs. Because some traits can cha...
465 citations
TL;DR: The purpose here is to introduce an unappreciated form of evidence for natural selection that is defined as the repeated independent evolution of the same reproductive isolating mechanism, and to explain the phenomenon and list criteria needed to demonstrate it.
Abstract: Natural selection plays a prominant role in most theories of speciation. Mayr (1963) and others viewed selection as one of the causes of genetic divergence between allopatric populations that yielded reproductive isolation as an incidental by-product. Selection is also invoked in models of parapatric (Endler 1977), reinforcement (Dobzhansky 1951), founder event (Barton 1989), and sympatric (Diehl and Bush 1989) speciation. Yet there is surprisingly ittle evidence from nature in support of its role in any scenario. Our purpose here is to introduce an unappreciated form of evidence for natural selection that we call \"parallel speciation.\" This we define as the repeated independent evolution of the same reproductive isolating mechanism. Below, we explain the phenomenon, list criteria needed to demonstrate it, and identify several possible examples from nature. We discuss broad implications of parallel speciation and review several mechanisms that can produce it. Throughout we use the concept of biological species (Mayr 1963): populations belong to the same species only if they are not reproductively isolated. We add the proviso that reproductive isolation between good species may be imperfect.
451 citations
TL;DR: It is argued that strict separation between chaotic and stochastic dynamics in ecological systems is unnecessary and misleading, and a more comprehensive approach is presented for systems subject to Stochastic perturbations.
Abstract: Chaos is usually regarded as a distinct alternative to random effects such as environmental fluctuations or external disturbances. We argue that strict separation between chaotic and stochastic dynamics in ecological systems is unnecessary and misleading, and we present a more comprehensive approach for systems subject to stochastic perturbations. The defining property of chaos is sensitive dependence on initial conditions. Chaotic systems are "noise amplifiers" that magnify perturbations; nonchaotic systems are "noise mufflers" that dampen perturbations. We also present statistical methods for detecting chaos in time-series data, based on using nonlinear time-series modeling to estimate the Lyapunov exponent λ, which gives the average rate at which perturbation effects grow (λ > 0) or decay (λ < 0). These methods allow for dynamic noise and can detect low-dimensional chaos with realistic amounts of data. Results for natural and laboratory populations span the entire range from noise-dominated and strongl...
440 citations
TL;DR: A stratified diffusion model is constructed, which describes the dynamics of the size distribution of colonies created by long-distance migrants and provides an estimate of range expansion in terms of the rate of expansion due to neighborhood diffusion, the leap distance, and the colonization rate of long- distance migrants.
Abstract: Recent data on biological invasion show that range expansion is driven by various modes of dispersal such as neighborhood diffusion and long-distance dispersal that occur side by side within a species. In such a stratified dispersal process, the initial range expansion mainly occurs by neighborhood diffusion. However, as the range of the founder population expands, new colonies created by long-distance migrants increase in number to cause an accelerating range expansion in the later phase. We classify several well-documented examples of geographi- cal expansions into three major types depending on the nonlinearity of the range-versus-time curve. To examine how long-distance dispersal produces accelerating range expansion, we con- struct a stratified diffusion model, which describes the dynamics of the size distribution of colonies created by long-distance migrants. The model consists of a von Foerster equation combined with a Skellam model. Analyzing the model provides an estimate of range expansion in terms of the rate of expansion due to neighborhood diffusion, the leap distance, and the colonization rate of long-distance migrants. The results explain various types of nonlinear range expansion observed in biological invasions.
409 citations
TL;DR: Results indicate that mammalian grazers have a sufficiently large effect on vegetation and soil moisture that their extinction could have contributed substantially to the shift from predominance of steppe to tundra at the Pleistocene-Holocene boundary.
Abstract: A simulation model, recent experiments, and the literature provide consistent evidence that megafauna extinctions caused by human hunting could have played as great a role as climate in shifting fr...
405 citations
TL;DR: These analyses reveal that seed dispersal syndromes are not entirely interpretable as current adaptations to seed dispersers, and their status as exaptations can be assessed by combining experimental studies of natural selection on fruit size and rigorous comparative and cladistic tests of adaptational hypotheses.
Abstract: Variation in phenotypic traits of angiosperm fleshy fruits has been explained as the result of adaptations to their mutualistic seed dispersers. By analyzing the information available on fleshy fruit characteristics of 910 angiosperm species, I assess the hypothesis of evolutionary association between fruit phenotypic traits and type of seed disperser (birds, mammals, and mixed dispersers) and address explicitly and quantitatively alternative null hypotheses about phylogenetic effects. Phylogenetic affinity among plant taxa is accounted for by comparative methods including nested ANOVA, phylogenetic autocorrelation, and independent contrasts. Averaging over the 16 fruit traits examined, phylogenetic effects down to genus level explain 61% of total variance. Phylogenetic autocorrelations are strong among close relatives, reaching significance for 11 of the 16 fruit traits examined. When assessed by independent contrast methods, correlated evolution between type of disperser and fruit traits is confined to ...
402 citations
TL;DR: It is shown that an evolutionarily stable signaling system can exist in which the parent obtains accurate information about the resource needs of its young and on which it bases its resource distribution decisions.
Abstract: The young of birds and mammals frequently solicit food and other resources from their parents in ways that appear to be costly either in terms of energetic expenditure or because they may attract predators. Costly solicitation has been explained as a means by which the young manipulate their parents into providing more resources than the parental optimum. Alternatively, communication between parents and young can be interpreted as an evolutionarily stable signaling system. A model is developed of two young in a brood who compete for a fixed amount of resources distributed by their parents. It is shown that an evolutionarily stable signaling system can exist in which the parent obtains accurate information about the resource needs of its young and on which it bases its resource distribution decisions. Young with greater resource requirements solicit at higher levels, but the system is stable because any misrepresentation is selected against. Essential for stability is that signaling must be costly; it is t...
399 citations
TL;DR: It is found that an increase of the level of site saturation increases the dispersal rate, and it is suggested that life-history traits other than dispersal might also experience antagonistic selective forces at the between- and within-deme levels.
Abstract: A Markovian extinction model that takes into account age structure of local populations allows consideration of the effects of demography and successional dynamics on the evolution of migration. Analytical expressions for the evolutionarily stable (ES) rates of dispersal are given for cases in which newly recolonized sites attain carrying capacity within a single season. Using a low-fecundity numerical model, we find that an increase of the level of site saturation increases the dispersal rate. Ecological successions and unequal local extinction rates between newly colonized sites and established populations strongly affect the ES dispersal rate. The frequency of genetic modifiers that enhance the rate of dispersal evolves negative correlations with deme age, with high-migration genotypes predominant among colonizers while progressively declining in frequency as a deme ages. This suggests that between-deme selection (colonization) favors migrants while within-deme selection favors low dispersers, which al...
TL;DR: A model of dispersal tendency is presented in order to explicate the ways in which differences in population density, predation risk, and the distribution of mating opportunities among groups might result in complex dispersal patterns that are consistent with both the results of the present study and the disparate empirical reports in the literature.
Abstract: Young male baboons typically disperse from their group of birth as they near adult size and may continue to migrate between social groups throughout their lives. Long-term data on dispersal and residence patterns of male baboons in Amboseli National Park, Kenya, were available for 110 males in the population, including 43 that were monitored during their natal dispersal. These data enabled us to provide not only a detailed evaluation of the effects of reproductive competition on dispersal but also the first direct estimates of the costs of dispersal in male primates and one of the few direct estimates of fitness costs associated with breeding in the natal group. Males underwent natal dispersal at a median age of 8.5 yr (range, 6.8-13.4 yr) and subsequently remained in nonnatal groups for a median tenure of 24 mo (range, 1-138 mo). Half of the males in the study engaged in moderate to extensive reproductive activity before natal dispersal. Reproductive costs associated with breeding in the natal group were...
TL;DR: The observation that phytochrome-mediated elongation is advantageous in dense stands, but disadvantageous for uncrowded plants, indicates that a response to foliage shade allows plants to develop an appropriate morphology for the level of competition they experience.
Abstract: Many plants display characteristic phytochrome-mediated stem elongation responses to crowding and vegetation shade, commonly referred to as the shade avoidance syndrome We tested the hypothesis that this elongation is a form of adaptive plasticity by comparing the relative performance at high and low density of wild-type plants and transgenic and mutant plants in which the shade avoidance response was disabled Transgenic tobacco plants in which elongation in response to neighbors was blocked by expression of the oat PHYA gene had decreased relative fitness when grown in competition with elongated wild-type plants In contrast, constitutively elongated Brassica ein mutant plants, deficient in light-stable phytochrome, had lower fitness relative to nonelongated wild type at low density than in competition with elongated wild type at high density The observation that phytochrome-mediated elongation is advantageous in dense stands, but disadvantageous for uncrowded plants, indicates that a response to foli
TL;DR: The description of genetic variation in phenotypic plasticity by the genotype-environment interaction variance is related to the reaction norm model.
Abstract: Phenotypic plasticity has been described by character states and by a reaction norm. In an environment that is heterogeneous with regard to a continuous environmental variable, selection on the character within each environment can be described by two models of selection on phenotypic plasticity: a model of selection on character states and a model of selection on the coefficients of the reaction norm. The two selection models are a linear transformation of each other and mathematically equivalent. The reaction norm model seems to be simpler and to give rise to more predictions. The description of genetic variation in phenotypic plasticity by the genotype-environment interaction variance is related to the reaction norm model.
TL;DR: The patterns of phenotypic, genetic, and environmental correlation among craniofacial traits matched patterns predicted on the basis of developmental and functional relationships among cranial traits.
Abstract: Quantitative genetic theory predicts that functionally and developmentally related characters will be morphologically integrated and tend to be inherited together. Furthermore, coinheritance and environmental integration of functionally related characters may be important in facilitating adaptive evolution. The hypothesis that functionally and developmentally related features of the cranium are phenotypically, genetically, and environmentally integrated in the saddle-back tamarin (Saguinus fuscicollis) cranium is tested. Thirty-nine craniofacial measurements were taken on 209 animals from the Oak Ridge Associated Universities' Marmoset Research Center. Heritabilities and phenotypic, genetic, and environmental correlations were calculated using maximum likelihood methods after the data were corrected for sex and specified environmental effects. The correlation matrices were compared to a series of theoretical intertrait connectivities representing expected patterns of morphological integration using Mantel...
TL;DR: It is proposed that play may not be motor training in the broad sense, but rather it may be behavior designed to influence specific types of development, such as adaptive modification of the developing neuromuscular system.
Abstract: The motor training hypothesis, proposed in its first form nearly half a century ago and broadened subsequently, states that the function of play is adaptive modification of the developing neuromuscular system. Evidence from many mammalian species indirectly supports the motor training hypothesis, but the exact nature of developmental change prompted by play remains unknown. We reviewed literature on the anatomical and physiological effects of exercise in mammals and categorized these as effects available to individuals at any age, versus effects available only during a discrete period of postnatal development, and transitory effects, which decay soon after exercise ends, versus permanent effects. We found that most effects are available at any age and are transitory; we argue that they are not likely primary benefits of play. However, two effects that influence motor performance-modification of cerebellar synaptogenesis and modification of skeletal muscle fiber type differentiation-are available only duri...
TL;DR: Five examples of courtship that illustrate the kinds of studies that can provide evidence of sensory traps are reviewed, which results from deceit by mimicry and the evolution of sensory trap responses before the signals that elicit them as preferences.
Abstract: Sensory traps affect mate choice when male courtship signals mimic stimuli to which females respond in other contexts and elicit female behavior that increases male fertilization rates. Because of the supernormal stimulus effect, mimetic signals may become quantitatively exaggerated relative to model stimuli. Viability selection or a decrease in responsiveness to signals that are exaggerated beyond their peak supernormal effect may limit signal elaboration. Females always benefit by responding to models and they may often benefit by responding to mimetic courtship signals. If the response as a preference is costly, it may be maintained by frequent and strong selection for the response to the model. I review five examples of courtship that illustrate the kinds of studies that can provide evidence of sensory traps. The strategic designs of mimetic courtship signals arise not from selection of responses to them but from selection for responses to models. This results from deceit by mimicry and the evolution ...
TL;DR: Using the evolutionarily stable strategy (ESS) approach in a model for the evolution of seed size, it is shown that small-scale spatial variation in seedling density favors the evolution in seed size within individual plants if competition among seedlings is sufficiently asymmetric in favor of larger seeds.
Abstract: Using the evolutionarily stable strategy (ESS) approach in a model for the evolution of seed size, I show that small-scale spatial variation in seedling density favors the evolution of variation in seed size within individual plants if competition among seedlings is sufficiently asymmetric in favor of larger seeds. A single seed size is found to be never evolutionarily stable. I always find at least some continuous adaptive variation in seed size. The model generates the following predictions. At least some continuous variation in seed size is adaptive. Plants with many resources have more variable seeds than plants with few resources. Plants with low juvenile mortality have more variable seeds than plants with high juvenile mortality. Plants with aggregated seed dispersal have seed size distributions that are more biased toward small seeds compared to plants with more even seed dispersal. An explicit formula for the evolutionarily stable seed size distribution was obtained for Poisson-distributed seed nu...
TL;DR: A direct effect of uterine retention on offspring viability offers a plausible selective advantage for the evolution of viviparity in squamate reptiles and possibly in other vertebrates and invertebrates also.
Abstract: Viviparity has evolved many times within squamate reptiles, mostly in cool climates, but the selective advantages of uterine retention of eggs remain obscure. Previous analyses have assumed that intrauterine incubation enhances offspring survival because of early hatching or protection of the young in utero. I suggest instead that prolonged uterine retention directly enhances hatchling viability, because eggs incubated at maternal body temperatures produce "better" hatchlings than do eggs incubated at normal nest temperatures. To test this idea, I incubated eggs of two species of montane scincid lizards from southeastern Australia (Bassiana duperreyi and Nannoscincus maccoyi) under thermal regimes designed to simulate temperatures in nests and maternal oviducts. Hatchling phenotypes were substantially affected by incubation temperatures. The variables affected by incubation at maternal versus nest thermal regimes include the hatchling's morphology (body size, relative tail length), running speed in a labo...
TL;DR: All of the patterns documented here should be viewed as guidelines for hypothesis testing with the understanding that much unexplained variation in survival exists among herbivorous insect species.
Abstract: Five hundred thirty life tables for 124 holometabolous, herbivorous insects were compiled from the literature and studied to uncover demographic patterns. Survival and sources of mortality of the preadult stages were correlated with their ecological setting, and several patterns emerged. Exophytic herbivores suffer a 5%-10% greater risk of preadult mortality than endophytics but are also nearly twice as large (pupal length, 11.8 mm vs. 6.4 mm; adult body length, 13.5 mm vs. 5.5 mm) and twice as fecund (206.1 vs. 99.3 eggs per female). Attack by natural enemies is the most frequent mortality source for immature herbivores (frequency = 48%). Plant factors are not nearly as frequent (9%) but have probably been underestimated in past life tables. The influence of plant factors is greatest in the early stages, and the influence of enemies is greatest in the later stages of development. Plant factors kill endophytic species more frequently than exophytic species (frequencies = 15% vs. 2%), whereas enemies kill ...
TL;DR: In this article, the authors used data from the literature and their own collections to show that colony sizes of tropical ant species are about one-tenth the average size of temperate species.
Abstract: In eusocial species, the size of the superorganism is the summed sizes of its component individuals. Bergmann's rule, the cline of decreasing size with decreasing latitude, applies to colony size in ants. Using data from the literature and our own collections, we show that colony sizes of tropical ant species are about one-tenth the average size of temperate species. This pattern holds when species or genera are sample units. Further, this trend is shown in 17 of 19 genera and five of six subfamilies. Bergmann's rule may arise if seasonal famine favors larger organisms, given their increased energy reserves. We constructed three colony sizes of the ant Solenopsis invicta. We deprived these colonies of food, or food and water. Queens, when surrounded by 102 workers or 104 workers, survived longer than solitary queens. When deprived only of food, days of queen survival had an allometry of M0.21 (where M is mass), not significantly different from the predicted M0.25 for unitary organisms. We propose that sho...
TL;DR: This work derives the optimal strategy that maximizes Y subject to a prescribed risk of extinction, and shows that Y is maximized, with unlimited harvesting capability, when the critical size equals the carrying capacity of the population and with limited harvesting capability when c < K.
Abstract: Optimal harvesting theories based on the concept of maximum sustained yield assume a stationary distribution of population size, ignoring that extinction is the eventual fate of all populations. We analyze the dynamics of populations at risk of extinction from demographic and environmental stochasticity as well as harvesting. Diffusion theory is used to derive approximate formulas for the mean time to extinction, T, the expected cumulative harvest before extinction, Y, and the mean and standard deviation of annual harvest, ȳ and σy. In numerical examples we compare the performance of different harvesting strategies in terms of these statistics. We derive optimal harvesting strategies that maximize Y or ȳ. The optimal strategies always involve a threshold function in which harvesting occurs at the maximum possible rate above a critical population size, c, with no harvest below c. For a broad class of stochastic population models we show that Y is maximized, with unlimited harvesting capability, when the cr...
TL;DR: It is found that taxonomic membership statistically accounted for the majority of seed size variation within data sets, and differences among data sets were also largely associated with shifts in taxonomic composition.
Abstract: Seed size is correlated with a number of other plant attributes, and such correlations are commonly interpreted as adaptations to different life histories. However, seed size is also strongly related to species phylogeny. In the current comparative literature, correlations between trait variation and taxonomic membership are often interpreted as evidence for phylogenetic constraints, and such phylogenetic explanations of variation are presented as alternatives to adaptive explanations. However, phylogenetic patterns of trait variation should not be discussed exclusively in terms of constraints. Neither should phylogenetic and adaptive explanations for variation be automatically presented as alternatives. Another explanation for similarity among related taxa is phylogenetic niche conservatism (PNC). This process is well known to exist-indeed, it is an inevitable consequence of evolution by natural selection-and is explicitly adaptive. Under PNC, radiation within a lineage is shaped by phylogeny, by the env...
TL;DR: It is confirmed that, in relation to body mass, hole-nesting species develop more slowly than related species from continents, and it is concluded that predation has been an important selective factor in the evolution of developmental rates in altricial birds.
Abstract: Birds develop rapidly compared with other vertebrates, and it has been proposed that such rapid development serves to minimize the period of exposure to predators of eggs and nestlings. Evidence is, however, scant. We test the proposition that nest predation has been an important selective factor in the evolution of the rapid developmental rates characteristic of altricial birds. We compare the incubation and nestling periods of altrical land birds from islands that lack native mammalian and reptilian nest predators with those of phylogenetically related species from areas with predators. We show that open-nesting native species from islands take longer to hatch and fledge than related species from continents, controlling for body mass. In contrast, species that have been recently introduced to islands have similar incubation and nestling periods there compared to those in their region of origin. In addition, we show that statistically different regression equations describe the interspecific allometric relationships be- tween incubation and nestling period and body mass in hole-nesting and open-nesting species, the latter of which presumably face higher predation pressure. We confirm that, in relation to body mass, hole-nesting species develop more slowly. We conclude that predation has been an important selective factor in the evolution of developmental rates in altricial birds.
TL;DR: The hatching times of 46 species worldwide support the contention that predation, primarily on newly hatched larvae, and not other sources of mortality, selects for synchronous hatching by crabs.
Abstract: The adaptive significance of synchronous larval release by marine animals has been elusive. The hatching times of 10 species of intertidal crabs were determined and compared to those expected if crabs release larvae when predation on females, embryos, and newly hatched larvae is least. The safest time to release larvae occurs during the largest-amplitude nocturnal high tides of the lunar month. Crabs throughout the intertidal zone can release larvae near their refuges, and larvae will be transported rapidly at night from shorelines where diurnal planktivores abound. High and middle intertidal species released larvae at this time, which suggests that predation on all three life stages ultimately may synchronize reproduction. Unlike these species, low intertidal crabs are inundated every day, and therefore females could release larvae near refuges daily. Although one low intertidal species did release larvae every day and often during the daytime, two other species released larvae during the safe period. Th...
TL;DR: A new proximate explanation for this phenomenon is discussed: individuals may learn site-specific serial motor programs that enhance their ability to move rapidly, safely, and efficiently around obstacles and barriers in familiar areas.
Abstract: Judging from studies of homing and territorial behavior, many animals value familiar home ranges or territories. This article discusses a new proximate explanation for this phenome- non: individuals may learn site-specific serial motor programs that enhance their ability to move rapidly, safely, and efficiently around obstacles and barriers in familiar areas. The literature on motor learning in humans and on hurdle race training in humans and horses yields a number of specific predictions on how animals should behave, if they practice and learn serial motor programs that facilitate high-speed locomotion along complicated routes or pathways. Support for some of the predictions of the motor learning hypothesis is already available in the literature on animal play, exploration, maze learning, and spatial orientation, and other predictions of this hypothesis should be readily testable using small mammals.
TL;DR: Growth was enhanced at incubation temperatures that produced males, and variation in growth plasticity among clutches was consistent with Charnov-Bull predictions, in this TSD species.
Abstract: Abstrcact.-We examined a critical component of the Charnov-Bull hypothesis of temperaturedependent sex determination (TSD) by determining the reaction norms of hatchling growth to embryonic incubation temperature in the common snapping turtle, Chelydra serpentina. Hormone manipulations of eggs produced females at male temperatures and vice versa, which thereby permitted same-sex comparisons of hatchling growth across a range of incubation temperatures. In this way, the normally confounded effects of incubation temperature and sex were dissociated experimentally. The resultant hatchlings, including controls and experimentals, exhibited normal gonadal structure and sex steroid profiles. The subsequent growth of hatchlings monitored for 6 mo was strongly affected by embryonic incubation temperature but not by sex. As predicted, growth was enhanced at incubation temperatures that produced males. Clutch effects and interaction effects (clutch by incubation temperature) on growth were significant. In addition, there was a positive genetic covariance among incubation temperatures, but incubation temperature effects varied among clutches. The variation in growth plasticity among clutches was consistent with Charnov-Bull predictions. In this TSD species, incubation temperature is likely to have differential fitness effects on the sexes mediated via differences in growth.
TL;DR: It is illustrated that ecological differences in tadpoles correlate with behavioral and morphological phenotypes and that phenotypic traits can be used to predict ecological performance in manipulative experiments in natural settings.
Abstract: This study relates community ecology to phenotypic design in two species of hylid tadpole that share larval habitat on the rocky shoreline of Isle Royale, Michigan. The ecological results indicate that the two species partition a competition-predation gradient on the shore. Pseudacris crucifer is mostly high on the shore at low density in persistent pools with predators (dragonfly larvae, Aeshnajuncea); tadpoles have low individual growth rates and metamorphose late at large body size. Pseudacris triseriata is more variable in growth and development but typically grows more rapidly and metamorphoses earlier. All P. crucifer that attain metamorpho- sis emerge from pools with Aeshna, whereas P. triseriata metamorphose from a mixture of pools with and without predators. A reciprocal transplant experiment showed that both species survived poorly in the upper level pools with predators, which indicates that predation is more important there. The experiment did not show strong competition between the two species in either kind of pool. The phenotypes, including plasticity, of the two species are consistent with their ecological differences. Pseudacris triseriata has traits that functional arguments suggest should assist in processing food: in natural populations it hatches at large size, feeds actively and conspicuously, and has a relatively round body and small tail muscle and tail fin. Pseudacris crucifer has the opposite suite of characters, which functional arguments suggest favor predator avoidance. The variation in larval habitat correlated with differences in plasticity. In the trans- plant experiment, P. crucifer showed little change in phenotype, whereas P. triseriata reduced activity and increased tail fin and tail muscle size in the presence of Aeshna. This study illustrates that ecological differences in tadpoles correlate with behavioral and morphological phenotypes and that phenotypic traits can be used to predict ecological performance in manipulative experi- ments in natural settings. One of the underlying theses of ecology asserts that we can understand the control of communities in terms of interactions, notably competition and preda- tion, between the component populations. A rich experimental tradition has tested this proposition, focusing on interactions among populations. The impor- tance of causal factors is judged according to the impact on population dynamics, judged by experimentally excluding or adding member species in the community (Gause 1934; Connell 1961; Wilbur 1972). This experimental approach, termed phenomenological, has established the causal bases for an increasing number of natural patterns in communities, including the differences in habitat among closely related species.
TL;DR: General models incorporating search behaviors are used to demonstrate the parallels between attack rates in host-parasitoid systems and transmission processes in sexually and vector-transmitted diseases.
Abstract: General models incorporating search behaviors are used to demonstrate the parallels between attack rates in host-parasitoid systems and transmission processes in sexually and vector-transmitted diseases. Density-dependent transmission, in which the probability of an individual's becoming infected is a function of the density of infectives, I, is the usual assumption in disease models. Frequency-dependent transmission, in which the probability of an individual's becoming infected is a function of the proportion of infectives, IIN, is often considered charac- teristic of venereal and vector-based systems. These two characterizations of the transmission process are shown to represent extremes of the Type II functional response curve. When there is vector-based transmission, and depending on the details of vector behavior, the probability of an uninfected host's becoming infected may range from being predominantly a function of I to being proportional to IIN2. With a limited number of hosts visited per vector, transmission may decline with increasing overall density of the host population; this was observed in empirical data for a pollinator-transmitted disease. Unified, general models of the transmission process are essential for comparison of dynamic processes in different systems and for studies of the evolution of the transmission process itself.
TL;DR: This work treats territory settlement in the long-lived oystercatcher Haematopus ostralegus as a career decision with long-term fitness consequences through its effect on social status, and finds no evidence for a phenotypic superiority of owners of high-quality territories.
Abstract: For an individual, the decision on when to settle (i.e., at what age) cannot be separated from the decision on where to settle (i.e., in which habitat). We tackle both problems simultaneously by treating territory settlement in the long-lived oystercatcher Haematopus os- tralegus (in which territories differ strikingly in quality and many individuals delay settlement) as a career decision with long-term fitness consequences through its effect on social status. We reject the hypothesis that oystercatchers are not able to judge territory quality or that short-term advantages of a high-quality territory are offset by long-term costs. We estimate the expected future reproductive success (EFRS) of birds of different social status on the basis of a Markov model. A simple contest model shows that owners of high-quality territories must have a consid- erable advantage that allows them to despotically exclude other birds. We find no evidence for a phenotypic superiority of owners of high-quality territories. Also, the act of breeding, even in a poor territory, does not enhance the probability of subsequently settling in a good territory. The available data support the queue hypothesis that unsettled individuals must develop site dominance in order to overcome the owner advantage. By queuing for a specific territory, an individual reduces its chances of becoming established elsewhere. At evolutionary equilibrium, the local queues for high-quality territories are longer, which implies longer waiting times and a higher probability of premature death. This trade-off between territory quality and waiting time links the despotic distribution to deferred maturity and provides a partial explanation for both.