The American Society of Naturalists Forum

In the Eyes of the Beholders: Female Choice and Avian Predation Risk Associated with an Exaggerated Male Butterfly Color

2010-10-13T10:58:00.002-05:00

By Nathan I. Morehouse and Ronald L. Rutowski

The American Naturalist, Volume 176, Issue 6
DOI: 10.1086/657043

Color ornaments are often viewed as products of countervailing sexual and natural selection, because more colorful, more attractive individuals may also be more conspicuous to predators. However, while evidence for such countervailing selection exists for vertebrate color ornaments (e.g., Trinidadian guppies), similar studies have yet to be reported in invertebrates. Indeed, evidence for female mate choice based on extant variation in male coloration is limited in invertebrates, and researchers have not explicitly asked whether more attractive males are also more conspicuous to predators. Here we provide evidence that more chromatic male cabbage white butterflies (Pieris rapae) are more attractive to females but should also be more conspicuous to predators. Female P. rapae preferentially mate with more chromatic males when choosing from populations of males with naturally occurring or commensurate, experimentally induced color variation. Mathematical models of female color vision confirm that females should be able to discriminate color differences between prospective mates. Further, chromatic and luminance contrast scores from female visual system models better predicted male mating success than did measures of male color derived more directly from color spectra. Last, models of avian color vision suggest that preferred males should be more conspicuous to known avian predators.

Evolution of Nutrient Uptake Reveals a Trade-Off in the Ecological Stoichiometry of Plant-Herbivore Interactions

2010-10-13T10:57:00.000-05:00

By Pedro Branco, Maayke Stomp, Martijn Egas, and Jef Huisman

The American Naturalist, Volume 176, Issue 6
DOI: 10.1086/657036

Nutrient limitation determines the primary production and species composition of many ecosystems. Here we apply an adaptive dynamics approach to investigate evolution of the ecological stoichiometry of primary producers and its implications for plant-herbivore interactions. The model predicts a trade-off between the competitive ability and grazing susceptibility of primary producers, driven by changes in their nutrient uptake rates. High nutrient uptake rates enhance the competitiveness of primary producers but also increase their nutritional quality for herbivores. This trade-off enables coexistence of nutrient exploiters and grazing avoiders. If herbivores are not selective, evolution favors runaway selection toward high nutrient uptake rates of the primary producers. However, if herbivores select nutritious food, the model predicts an evolutionarily stable strategy with lower nutrient uptake rates. When the model is parameterized for phytoplankton and zooplankton, the evolutionary dynamics result in plant-herbivore oscillations at ecological timescales, especially in environments with high nutrient availability and low selectivity of the herbivores. High herbivore selectivity stabilizes the community dynamics. These model predictions show that evolution permits nonequilibrium dynamics in plant-herbivore communities and shed new light on the evolutionary forces that shape the ecological stoichiometry of primary producers.

Understanding Rapid Evolution in Predator-Prey Interactions Using the Theory of Fast-Slow Dynamical Systems

2010-09-23T13:49:00.000-05:00

By Michael H. Cortez and Stephen P. Ellner

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656485

The accumulation of evidence that ecologically important traits often evolve at the same time and rate as ecological dynamics (e.g., changes in species' abundances or spatial distributions) has outpaced theory describing the interplay between ecological and evolutionary processes with comparable timescales. The disparity between experiment and theory is partially due to the high dimensionality of models that include both evolutionary and ecological dynamics. Here we show how the theory of fast-slow dynamical systems can be used to reduce model dimension, and we use that body of theory to study a general predator-prey system exhibiting fast evolution in either the predator or the prey. Our approach yields graphical methods with predictive power about when new and unique dynamics (e.g., completely out-of-phase oscillations and cryptic dynamics) can arise in ecological systems exhibiting fast evolution. In addition, we derive analytical expressions for determining when such behavior arises and how evolution affects qualitative properties of the ecological dynamics. Finally, while the theory requires a separation of timescales between the ecological and evolutionary processes, our approach yields insight into systems where the rates of those processes are comparable and thus is a step toward creating a general ecoevolutionary theory.

Variation in Resource Acquisition and Use among Host Clones Creates Key Epidemiological Trade-Offs

2010-09-21T14:57:00.000-05:00

By Spencer R. Hall, Claes R. Becker, Meghan A. Duffy, and Carla E. Caceres

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656523

Parasites can certainly harm host fitness. Given such virulence, hosts should evolve strategies to resist or tolerate infection. But what governs those strategies and the costs that they incur? This study illustrates how a fecundity-susceptibility trade-off among clonally reared genotypes of a zooplankton (Daphnia dentifera) infected by a fungal parasite (Metschnikowia) arises due to variation in resource acquisition and use by hosts. To make these connections, we used lab experiments and theoretical models that link feeding with susceptibility, energetics, and fecundity of hosts. These feeding-based mechanisms also produced a fecundity-survivorship trade-off. Meanwhile, a parasite spore yield-fecundity trade-off arose from variation in juvenile growth rate among host clones (another index of resource use), a result that was readily anticipated and explained by the models. Thus, several key epidemiological trade-offs stem from variation in resource acquisition and use among clones. This connection should catalyze the creation of new theory that integrates resource- and gene-based responses of hosts to disease.

Recycling-Mediated Facilitation and Coexistence Based on Plant Size

2010-09-21T14:55:00.000-05:00

By Antonio J. Golubski, Katherine L. Gross, and Gary G. Mittelbach

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656493

We introduce nutrient recycling into a model where competitors differ in the scale at which they perceive their environment. In a two-resource system with both external nutrient inputs and recycling, larger consumers ("integrators") often generate resource distributions that favor their smaller ("nonintegrator") competitors, and vice versa. This occurs because recycling of integrator biomass reduces between-patch resource heterogeneity, whereas recycling of nonintegrator biomass does not. Combined, recycling and throughput can allow coexistence when it is not possible with either alone. With recycling, the presence of an integrator also may facilitate higher biomass of a co-occurring nonintegrator. Our model provides a context where recycling can generate negative feedback between competitors that differ in size and so promote coexistence. This is opposite to the positive recycling-mediated feedback commonly expected on the basis of litter chemistry differences between competitors. Effects of recycling and homogenization on nonintegrators may also be negative in our model, depending on the conformation of the system's resource supply points and the species' relative resource requirements. Our model suggests that the effects of plant size on competitive outcomes may depend critically on the degree of resource recycling found in the system and, reciprocally, that the effects of recycling may depend on plant size.

Tactical Population Movements and Distributions for Ideally Motivated Competitors

2010-09-21T14:54:00.000-05:00

By Jonathan T. Rowell

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656494

The spatial distributions of populations are a reflection of underlying rules for movement behavior in the context of the environment encountered by individuals. Here I study how ideal directed movement--in which individuals travel in the direction offering the most immediate perceived improvement to their personal fitness--dictates the spatial position of two populations occupying the same relative niche and engaged in competition via interference to an individual's ability to gather resources. Drawing on the analytic derivation of equilibria, numerical simulations, and graphical assessments, I provide conditions under which sympatry, parapatry, or regional exclusion is expected during different phases of the community's development. I also demonstrate that specific competitive asymmetries produce distinguishable distributions and invasion patterns and identify which populations are found centrally or peripherally. Dynamic and dispersal equilibria were examined for differences in the sensitivity to spatial variations in fitness, per capita mortality, metabolic efficiency, the strength of interspecific interference, resource collection speed, and the optimal location of each population along an environmental cline. These asymmetries were studied both in isolation and pairwise in fitness trade-off scenarios.

Ecology of Sexual Dimorphism and Clinal Variation of Coloration in a Damselfly

2010-09-21T14:18:00.000-05:00

By Idelle A. Cooper

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656491

Sexual selection, more so than natural selection, is posited as the major cause of sex differences. Here I show ecological correlations between solar radiation levels and sexual dimorphism in body color of a Hawaiian damselfly. Megalagrion calliphya exhibits sexual monomorphism at high elevations, where both sexes are red in color; sexual dimorphism at low elevations, where females are green; and female-limited dimorphism at midelevations, where both red and green females exist. Within a midelevation population, red females are also more prevalent during high daily levels of solar radiation. I found that red pigmentation is correlated with superior antioxidant ability that may protect from UV damage and confer a benefit to damselflies in exposed habitats, including males, which defend exposed mating habitats at all elevations, and females, which are in shaded habitats except at high elevation. This study characterizes the ecology of sexual dimorphism and provides a new, ecological hypothesis for the evolution of female-limited dimorphism.

Phylogenetic Targeting of Research Effort in Evolutionary Biology

2010-09-21T13:47:00.000-05:00

By Christian Arnold and Charles L. Nunn

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656490

Many questions in comparative biology require that new data be collected, either to build a comparative database for the first time or to augment existing data. Given resource limitations in collecting data, the question arises as to which species should be studied to increase the size of comparative data sets. By taking hypotheses, existing data relevant to the hypotheses, and a phylogeny, we show that a method of "phylogenetic targeting" can systematically guide data collection while taking into account potentially confounding variables and competing hypotheses. Phylogenetic targeting selects potential candidates for future data collection, using a flexible scoring system based on differences in pairwise comparisons. We used simulations to assess the performance of phylogenetic targeting, as compared with the less systematic approach of randomly selecting species (as might occur when data have been collected without regard to phylogeny and variation in the traits of interest). The simulations revealed that phylogenetic targeting increased the statistical power to detect correlations and that power increased with the number of species in the tree, even when the number of species studied was held constant. We also developed a Web-based computer program called PhyloTargeting to implement the approach ([URL STATUS="OKAY"]http://phylotargeting.fas.harvard.edu[/URL]).

Phylogenetic Metrics of Community Similarity

2010-09-21T13:19:00.000-05:00

By Anthony R. Ives and Matthew R. Helmus

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656486

We derive a new metric of community similarity that takes into account the phylogenetic relatedness among species. This metric, phylogenetic community dissimilarity (PCD), can be partitioned into two components, a nonphylogenetic component that reflects shared species between communities (analogous to Srensen's similarity metric) and a phylogenetic component that reflects the evolutionary relationships among nonshared species. Therefore, even if a species is not shared between two communities, it will increase the similarity of the two communities if it is phylogenetically related to species in the other community. We illustrate PCD with data on fish and aquatic macrophyte communities from 59 temperate lakes. Dissimilarity between fish communities associated with environmental differences between lakes often has a phylogenetic component, whereas this is not the case for macrophyte communities. With simulations, we then compare PCD with two other metrics of phylogenetic community similarity, _ST and UniFrac. Of the three metrics, PCD was best at identifying environmental drivers of community dissimilarity, showing lower variability and greater statistical power. Thus, PCD is a statistically powerful metric that separates the effects of environmental drivers on compositional versus phylogenetic components of community structure.

Incorporating Clade Identity in Analyses of Phylogenetic Community Structure: An Example with Hummingbirds

2010-09-17T12:35:00.000-05:00

By Juan L. Parra, Jimmy A. McGuire, and Catherine H. Graham

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656619

An important challenge in community ecology is to determine how processes occurring at multiple spatial, temporal, and phylogenetic scales influence the structure of local communities. While indexes of phylogenetic structure, which measure how related species are in a community, provide insight into the processes that shape species coexistence, they fail to pinpoint the phylogenetic scales at which those processes occur. Here, we explore a framework to identify the species and clades responsible for the inferred patterns of phylogenetic structure within a given community. Further, we evaluate how communities that share the nonrandom representation of species from a given clade in the phylogeny are distributed across geography and environmental gradients. Using Ecuadorian hummingbird communities, we found that multiple patterns of phylogenetic structure often occur within a local assemblage. We also identified four geographic regions where species from certain clades exhibit nonrandom representation: the eastern Amazonian lowlands, the western dry lowlands, the Andes at middle elevations, and the Andes at high elevations. The environmental gradients along which changes in the local coexistence of species occurred were mainly elevation, annual precipitation, and seasonality in both temperature and precipitation. Finally, we show how these patterns can be used to generate hypotheses about the processes that allow species coexistence.

Ontogenetic Diet Shifts Result in Niche Partitioning between Two Consumer Species Irrespective of Competitive Abilities

2010-09-17T12:34:00.000-05:00

By Tim Schellekens, Andre M. de Roos, and Lennart Persson

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656488

Tilman's theory predicts the outcome of competition between two consumers sharing two resources on the basis of the shape of zero net-growth isoclines (ZNGIs). In his theory, intraspecific differences in resource use are not accounted for. Here we extend this theory to include situations where organisms undergo ontogenetic diet shifts, as these characterize the life histories of many species. In a situation that without diet shifts would lead to neutral coexistence of consumer species, we investigate whether ontogenetic diet shifts lead to niche partitioning. We analyze a model describing competition for two resources between two competitors with distinctive diets over ontogeny, using copepods (showing ontogenetic diet shifts) and daphnids (not showing ontogenetic diet shifts) as appropriate representatives. We show that an ontogenetic diet shift affects the shape of the ZNGI, changing it from reflecting perfectly substitutable resources to reflecting essential resources. Furthermore, we show that resource supply determines population stage structure and stage-dependent resource consumption in copepods and influences the competitive outcome with daphnids. In particular, we show that in itself, an ontogenetic diet shift can provide a competitive advantage if the supply of the adult resource is lower than the supply of the juvenile resource but that it always causes a disadvantage if the supply of the adult resource exceeds that of the juvenile resource.

Hidden Consequences of Living in a Wormy World: Nematode-Induced Immune Suppression Facilitates Tuberculosis Invasion in African Buffalo

2010-09-17T12:33:00.002-05:00

By Vanessa O. Ezenwa, Rampal S. Etienne, Gordon Luikart, Albano Beja-Pereira, and Anna E. Jolles

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656496

Most hosts are infected with multiple parasites, and responses of the immune system to co-occurring parasites may influence disease spread. Helminth infection can bias the host immune response toward a T-helper type 2 (Th2) over a type 1 (Th1) response, impairing the host's ability to control concurrent intracellular microparasite infections and potentially modifying disease dynamics. In humans, immune-mediated interactions between helminths and microparasites can alter host susceptibility to diseases such as HIV, tuberculosis (TB), and malaria. However, the extent to which similar processes operate in natural animal populations and influence disease spread remains unknown. We used cross-sectional, experimental, and genetic studies to show that gastrointestinal nematode infection alters immunity to intracellular microparasites in free-ranging African buffalo (Syncerus caffer). Buffalo that were more resistant to nematode infection had weaker Th1 responses, there was significant genotypic variation in nematode resistance, and anthelminthic treatment enhanced Th1 immunity. Using a disease dynamic model parameterized with empirical data, we found that nematode-induced immune suppression can facilitate the invasion of bovine TB in buffalo. In the absence of nematodes, TB failed to invade the system, illustrating the critical role nematodes may play in disease establishment. Our results suggest that helminths, by influencing the likelihood of microparasite invasion, may influence patterns of disease emergence in the wild.

Adaptive Suppression of Subordinate Reproduction in Cooperative Mammals

2010-09-16T15:06:00.000-05:00

By Tim H. Clutton-Brock, Sarah J. Hodge, Tom P. Flower, Goran F. Spong, and Andrew J. Young

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656492

Attempts to account for observed variation in the degree of reproductive skew among cooperative breeders have usually assumed that subordinate breeding has fitness costs to dominant females. They argue that dominant females concede reproductive opportunities to subordinates to retain them in the group or to dissuade them from challenging for the dominant position or that subordinate females breed where dominants are incapable of controlling them. However, an alternative possibility is that suppressing subordinate reproduction has substantive costs to the fitness of dominant females and that variation in these costs generates differences in the net benefits of suppression to dominants which are responsible for variation in the frequency of subordinate breeding that is not a consequence of either reproductive concessions or limitations in dominant control. Here, we show that, in wild Kalahari meerkats (Suricata suricatta), the frequency with which dominants evict subordinates or kill their pups varies with the costs and benefits to dominants of suppressing subordinate breeding, including the dominants' reproductive status, the size of their group, and the relatedness of subordinates. We review evidence from other studies that the suppression of reproduction by subordinates varies with the likely costs of subordinate breeding to dominants.

Physiological Stress as a Fundamental Mechanism Linking Predation to Ecosystem Functioning

2010-09-16T09:42:00.000-05:00

By Dror Hawlena and Oswald J. Schmitz

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656495

We present a framework to explain how prey stress responses to predation can resolve context dependency in ecosystem properties and functions such as food chain length, secondary production, elemental stoichiometry, and cycling. We first describe the major nonspecific physiological stress mechanisms and their ecologically relevant consequences. We next synthesize the evidence for prey physiological responses to predation risk and demonstrate that they are similar across taxa and fit well within the general stress paradigm. We then illustrate the utility of our idea by applying our understanding of the ecological consequences of stress to explain how herbivore-prey physiological antipredator responses affect ecosystem dynamics. We hypothesize that stressed herbivores should forage on plant species with higher digestible carbohydrates than should unstressed herbivores to meet heightened energy demands. Increased consumption of carbohydrate-rich plants should reduce their relative abundance in the community, hence altering the quantity and quality of plant litter entering the detrital pool. We further hypothesize that stress should change the elemental composition and energy content of prey excreta, egesta, and carcasses that enter the detrital pool. Finally, prey stress should lower energy and nutrient conversion efficiency and hence the transfer of materials and energy up the food chain, which should, in turn, weaken the association between ecosystem productivity and food chain length.

Floral Mimicry Enhances Pollen Export: The Evolution of Pollination by Sexual Deceit Outside of the Orchidaceae

2010-09-15T15:36:00.000-05:00

By Allan G. Ellis and Steven D. Johnson

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656487

Although the majority of flowering plants achieve pollination by exploiting the food-seeking behavior of animals, some use alternative ploys that exploit their mate-seeking behavior. Sexual deception is currently known only from the Orchidaceae and almost always involves pollination by male hymenoptera. An outstanding problem has been to identify the selective factors in plants that favor exploitation of mating versus feeding behaviors in pollinators. Here we show that the insectlike petal ornaments on inflorescences of the daisy Gorteria diffusa elicit copulation attempts from male bombyliid flies and that the intensity of the mating response varies across geographical floral morphotypes, suggesting a continuum in reliance on feeding through mating responses for pollination. Using pollen analogues applied to a morphotype with prominent insectlike petal ornaments, we demonstrate that mate-seeking male flies are several-fold more active and export significantly more pollen than females. These results suggest that selection for traits that exploit insect mating behavior can occur through the male component of plant fitness and conclusively demonstrates pollination by sexual deception in Gorteria, making this the first confirmed report of sexual deception outside of the Orchidaceae.

Diet Mixing: Do Animals Integrate Growth or Resources across Temporal Heterogeneity?

2010-09-15T10:19:00.000-05:00

By James M. Hood and Robert W. Sterner

The American Naturalist, Volume 176, Issue 5
DOI: 10.1086/656489

Animals commonly experience spatial and temporal variation in resource quality, thus experiencing temporally variable diets. Methods for scaling up growth in component patches to long-term growth across heterogeneity are seldom explicitly considered. Long-term growth is sometimes considered to be a weighted average of growth rates on component diets (growth integration). However, if animals integrate resources across high- and low-quality diets, their long-term growth may be greater than predicted from diet-specific growth rates (resource integration). We measured biomass growth rates of seven Daphnia species exposed to different types of diel variation in algal phosphorus (P) content. Support for resource integration was found for four of the seven species, which achieved near maximal growth when high-P food was available for at least 12 h. In contrast, no support for resource integration was found for the other three species. These three species achieved only one-half maximal growth rate under the same conditions and could be considered growth integrators. The type of integration could be predicted from the degree of stoichiometric homeostasis. Species with weak homeostatic regulation exhibited a capacity for resource integration. Resource integrators should have an advantage in heterogeneous environments.

Matrisibs, Patrisibs, and the Evolution of Imprinting on Autosomes and Sex Chromosomes

2010-08-26T12:18:00.000-05:00

By Yaniv Brandvain

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656272

The conflict theory of genomic imprinting argues that parent-of-origin effects on allelic expression evolve as a consequence of conflict between maternally and paternally derived genomes. I derive explicit population-genetic models of this theory when individuals in a cohort with an arbitrary and variable number of sires and dams interact. I show that the evolution of imprinting is governed by the reciprocal of the harmonic mean number of fathers but the reciprocal of the arithmetic mean number of mothers per cohort. Thus, a few monandrous females in a polyandrous population decrease the strength of the genetic conflict and the opportunity for conflict-driven paternal imprinting. In contrast, in populations in which few males control large harems, rare males with small harems do not have such a disproportionate effect on genetic conflicts and maternal imprinting. Additionally, I demonstrate that under the conflict theory, selection for imprinted expression on paternally derived X chromosomes is much weaker than it is on maternally derived X chromosomes or autosomes.

Dispersal-Dormancy Relationships in Annual Plants: Putting Model Predictions to the Test

2010-08-25T15:46:00.000-05:00

By Wolfgang Siewert and Katja Tielborger

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656271

Bet hedging is a means to increase fitness in environments that vary unpredictably in space and time. In such environments, models predict a trade-off between the bet-hedging strategies dispersal and dormancy, while the increasing importance of risk reduction with decreasing predictability should lead to an increase in dispersal and dormancy along gradients of environmental predictability. However, so far there has been no experimental study to test these predictions in the field. Here, we used a set of novel field experiments that enabled us to quantify and separate seedling recruitment from three sources: local reproduction, dormancy, and dispersal. The study included the entire plant community from five environments differing considerably in predictability. Evidence for both the existence of a trade-off between dispersal and dormancy within environments and their increased use in unpredictable environments was very weak. The importance of dispersal for population and community dynamics in our system was extremely low relative to dormancy and local reproduction. This indicates that the role of dispersal for buffering environmental variation may be negligible compared with other risk-reducing strategies. Our findings highlight the urgent need for multispecies and multisite experiments in empirical tests of theoretical predictions.

How Demography, Life History, and Kinship Shape the Evolution of Genomic Imprinting

2010-08-25T15:44:00.004-05:00

By Jeremy Van Cleve, Marcus W. Feldman, and Laurent Lehmann

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656277

How phenomena like helping, dispersal, or the sex ratio evolve depends critically on demographic and life-history factors. One phenotype that is of particular interest to biologists is genomic imprinting, which results in parent-of-origin-specific gene expression and thus deviates from the predictions of Mendel's rules. The most prominent explanation for the evolution of genomic imprinting, the kinship theory, originally specified that multiple paternity can cause the evolution of imprinting when offspring affect maternal resource provisioning. Most models of the kinship theory do not detail how population subdivision, demography, and life history affect the evolution of imprinting. In this work, we embed the classic kinship theory within an island model of population structure and allow for diverse demographic and life-history features to affect the direction of selection on imprinting. We find that population structure does not change how multiple paternity affects the evolution of imprinting under the classic kinship theory. However, if the degree of multiple paternity is not too large, we find that sex-specific migration and survival and generation overlap are the primary factors determining which allele is silenced. This indicates that imprinting can evolve purely as a result of sex-related asymmetries in the demographic structure or life history of a species.

Bighorn Ewes Transfer the Costs of Reproduction to Their Lambs

2010-08-24T15:34:00.000-05:00

By Julien G. A. Martin and Marco Festa-Bianchet

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656267

Several studies of large mammals report no direct reproductive costs for females. Individual heterogeneity may hide fitness costs of reproduction, but mothers could also transfer some costs to their offspring. Using data on 442 lambs weaned by 146 bighorn sheep (Ovis canadensis) ewes at Ram Mountain, Alberta, we studied how reproductive effort varied with environmental and maternal conditions. During summer, lactating ewes should gain enough mass to survive the winter and to support their next gestation, while nursing their current lamb. We measured reproductive effort as summer mass gain by lambs corrected for maternal mass in June and maternal mass gain during summer. Females lowered their reproductive effort when population density increased and if they had weaned a lamb the previous year. A reduction in reproductive effort led to lower winter survival by lambs. Bighorn ewes have a conservative reproductive tactic and always favor their own body condition over that of their lambs. When resources are limited, ewes appear to transfer reproductive costs to their lambs, as expected from the much greater relative fitness consequences of a reduction in maternal than in offspring survival.

Macroecology: Does It Ignore or Can It Encourage Further Ecological Syntheses Based on Spatially Local Experimental Manipulations?

2010-08-24T15:33:00.000-05:00

By Robert T. Paine

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656273

Detailed natural history coupled to experimental ecology has provided a rich harvest of insights into how natural communities in all ecosystems function, insights that cannot be gleaned from macroecological analyses. That detail, generated by small-spatial-scale but often lengthy experiments, is essential to managing and even restoring ecosystems. My essay focuses primarily on the ecology of exposed rocky intertidal shores, but I believe the derived implications are generalizable to all ecosystems. A mainly experimental approach has tended to avoid a preoccupation with niches but instead has focused on the ecological roles exercised by particular species. Attention to roles has produced a growing appreciation for trophic cascades and their consequences, with obvious implications for the management of fisheries and the conservation significance of apex predators. Some studies are more phenomenological and others more reductionist in focus, but all provide pathways toward understanding abundance and body size variation or a miscellany of indirect effects. Microecology in all ecosystems should continue to prosper independently of a macroecological, predominately terrestrial perspective.

Allometric Scaling of Metabolism, Growth, and Activity in Whole Colonies of the Seed-Harvester Ant Pogonomyrmex californicus

2010-08-24T15:31:00.002-05:00

By James S. Waters, C. Tate Holbrook, Jennifer H. Fewell, and Jon F. Harrison

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656266

The negative allometric scaling of metabolic rate with body size is among the most striking patterns in biology. We investigated whether this pattern extends to physically independent eusocial systems by measuring the metabolic rates of whole functioning colonies of the seed-harvester ant Pogonomyrmex californicus. These intraspecific scaling data were compared to the predictions of an additive model developed to estimate collective metabolic rates. Contrary to the prediction of the additive model, colony metabolic rate allometry resembled the pattern commonly observed interspecifically for individual organisms, scaling with colony mass^0.75. Among the same-aged colonies, net growth rate varied by up to sevenfold, with larger colonies exhibiting higher net growth efficiency than smaller colonies. Isolated worker groups exhibited isometric metabolic rate scaling, suggesting that the social environment of the colony is critical to regulating individual patterns of work output. Within the social environment, individual worker locomotor velocities exhibited power-law distributions that scaled with colony size so that larger colonies exhibited a greater disparity between active and inactive ants than did smaller colonies. These results demonstrate that behavioral organization within colonies may have a major influence on colony-level metabolism and in generating intraspecific variation in growth trajectories.

Games in Tetrads: Segregation, Recombination, and Meiotic Drive

2010-08-20T14:45:00.000-05:00

By David Haig

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656265

The two alleles at a heterozygous locus segregate during meiosis, sometimes at meiosis I and sometimes at meiosis II. The timing of segregation is determined by the pattern of crossing-over between a locus and its attached centromeres. Genes near centromeres can exploit this process by driving against spores from which the genes separated at meiosis I. Other genes, located distal to centromeres, can benefit from driving against spores from which they separated at meiosis II. Asymmetric female meiosis is particularly susceptible to such forms of drive. Selection on modifiers of recombination favors changes in the location of chiasmata that increase the proportion of tetrads of high average fitness by changing the timing of segregation. Such changes increase the frequency of driving alleles. This source of selection on recombination does not depend on effects on linkage disequilibrium. Recombinational responses to meiotic drive may contribute to sex differences in overall recombination and sex differences in the localization of chiasmata.

Evolution of Cryptic Coloration in Ectoparasites

2010-08-19T15:07:00.000-05:00

By Sarah E. Bush, Dukgun Kim, Michelle Reed, and Dale H. Clayton

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656269

Cryptic coloration is a classic example of evolution by natural selection. However, it has been studied almost exclusively in predator-prey systems, despite the fact that it may evolve in other groups, such as ectoparasites. The principle defense of hosts against ectoparasites is grooming behavior, which has a visual component. Host-imposed selection should lead to the evolution of background matching if it helps ectoparasites escape from grooming. Here we use sister taxa comparisons to show that avian feather lice (Phthiraptera: Ischnocera) have evolved coloration that matches the host's plumage, except in the case of head lice, which are protected from grooming. We also show covariation of parasite and host color within a single species of louse. Thus, cryptic coloration has evolved both within and between species of feather lice. Other examples of the evolution of crypsis presumably exist among the 70,000 known species of ectoparasites that collectively represent five animal phyla.

The Physiological Basis of Geographic Variation in Rates of Embryonic Development within a Widespread Lizard Species

2010-08-18T16:05:00.000-05:00

By Wei-Guo Du, Daniel A. Warner, Tracy Langkilde, Travis Robbins, and Richard Shine

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656270

The duration of embryonic development (e.g., egg incubation period) is a critical life-history variable because it affects both the amount of time that an embryo is exposed to conditions within the nest and the seasonal timing of hatching. Variation in incubation periods among oviparous reptiles might result from variation in either the amount of embryogenesis completed before laying or the subsequent developmental rates of embryos. Selection on incubation duration could change either of those traits. We examined embryonic development of fence lizards (Sceloporus undulatus) from three populations (Indiana, Mississippi, and Florida) that occur at different latitudes and therefore experience different temperatures and season lengths. These data reveal countergradient variation: at identical temperatures in the laboratory, incubation periods were shorter for lizards from cooler areas. This variation was not related to stage at oviposition; eggs of all populations were laid at similar developmental stages. Instead, embryonic development proceeded more rapidly in cooler-climate populations, compensating for the delayed development caused by lower incubation temperatures in the field. The accelerated development appears to occur via an increase in heart mass (and, thus, stroke volume) in one population and an increase in heart rate in the other. Hence, superficially similar adaptations of embryonic developmental rate to local conditions may be generated by dissimilar proximate mechanisms.