Showing papers in "The American Naturalist in 1986"

Does herbivory benefit plants? A review of the evidence

Abstract: The potential benefits of herbivory to plants have been debated over the last decade. Several investigators claim that removal of or damage to the productive, absorptive, or reproductive tissue of plants by herbivores benefits some plant species by increasing their net primary productivity, seed production, or longevity, and that these changes increase plant fitness and result in the evolution of herbivore-plant mutualisms. Although more than 40 papers have been cited as presenting experimental evidence in support of these benefits and mutualisms, strong evidence is lacking. Increased plant biomass as a result of tissue removal has been found only under growth-chamber conditions and in cultivated crops. Although herbivores may benefit certain plants by reducing competition or removing senescent tissue, no convincing evidence supports the theory that herbivory benefits grazed plants.

Amphibian metamorphosis growth rate predation risk and the optimal size at transformation

Abstract: Many taxa have evolved complex life cycles featuring a dramatic shift in habitat or resource use at metamorphosis. Despite their prevalence and unique characteristics, we understand little about the adaptive properties and evolution of these life histories. I offer a conceptual framework that considers how size-specific growth and mortality rates in both habitats interact with size at metamorphosis to affect lifetime fitness. This model predicts the size at metamorphosis that maximizes fitness, and I use this framework to interpret the wide variation in the life history structure of the amphibians. In particular, I speculate on the adaptive significance of the tadpole stage of the anurans and on the cause of variation in the size at metamorphosis both between and within anuran families. Further, I predict the conditions under which direct development or paedomorphosis will be selected for, and I offer hypotheses on the selective factors that may contribute to the three-stage life history of the newts. Fin...

The Nature of Nutrient Limitation in Plant Communities

Abstract: The concept of nutrient limitation, as developed in agriculture, applies well to wild plants grown under controlled conditions, although plants adapted to infertile soils are less responsive to nutrient addition than are most crop species. There are serious difficulties in transferring the concept of nutrient limitation directly to plant communities, however, because (1) in comparing communities with different dominant species, the species characteristic of nutrient-rich sites are inherently more responsive to nutrient supply and may be more strongly nutrient-limited than species in low-nutrient sites, and (2) ecosystem-level feedback complicates the analysis of experiments involving fertilization. We suggest that nutrient limitation in communities can be best measured by assessing the plants' response to large nutrient additions that are sufficient to saturate chemical and microbial immobilization processes and still meet plant nutrient requirements. The magnitude of community-level nutrient limitation i...

Sexual Selection for Aesthetic Traits in Species with Biparental Care

Abstract: Two timely evolutionary issues concern the potential for sexual selection in mating systems characterized by substantial parental investment by both sexes and the occurrence and significance of mate preferences for aesthetic characteristics. Here F explore these issues at their interface. First, I hypothesize two processes that may generate differential reproductive success (RS) resulting from the possession of aesthetic traits in species with parental investment (PI) by both sexes. The differential-access hypothesis posits that attractive individuals have preferential access to high-quality mates. The differential-allocation hypothesis states that mates of attractive individuals are "willing" to contribute greater than average PI to obtain and/or maintain their attractive mates. Second, I report on experiments that investigate the impact of aesthetic traits on fitness (long-term RS). The test organism is the zebra finch (Poephila glittata), a sexually dimorphic, typically monogamous estrildid. The aesthetic traits are plastic leg bands of various colors. Previous research has shown that zebra finches prefer oppositesex adults wearing certain colors (attractive traits) and avoid those wearing other colors (unattractive traits).

The Common Currency for Behavioral Decisions

Abstract: The study of optimal life histories involves the maximization of lifetime fitness but usually ignores the details of behavioral sequences. In contrast, the study of optimal behavioral sequences usu...

Flower constancy: definition, cause, and measurement

Abstract: A pollinator that restricts its visits to one flower type, even when other rewarding types are accessible, can be said to exhibit flower constancy. This usage distinguishes constancy from fixed preference or labile preference for the most rewarding flower type; I discuss a quantitative constancy index that is insensitive to preference changes. Because a constant visitor avoids flowers with acceptable rewards, the behavior is inefficient unless there are constraints such as an inability to learn quickly or to remember simultaneously how to deal with many flower types. If such constraints are the basis for constancy, it should be most pronounced when flowers in a mixture differ strongly in morphology or color. I observed bees foraging in outdoor flower arrays and found that constancy always increased with increasing differences among flower types; similar results can be gleaned from one other study. The available experimental evidence thus suggests that constancy reflects behavioral constraints.

Optimal egg size and clutch size: effects of environment and maternal phenotype

Abstract: In this paper, we develop a series of models for predicting optimal egg size and clutch size in different environments and for different maternal phenotypes. The models investigate the interaction between three components of fitness: (1) the intrinsic effect of egg size, (2) the density effect, the effect of the density of competing offspring (including competition with sibs and with non-sibs), and (3) the hierarchy effect, the effect of egg size relative to the sizes of competing eggs laid by other females. The environmental effects that we consider are the intensities of sib and non-sib competition, the number of egg-laying females, and some aspects of seasonal development. The particular aspects of maternal phenotype examined are foraging efficiency and the gametic reserve available at the time of egg laying (broadly equivalent to female size). Regarding environmental effects, increasing intensities of sib competition select for the production of smaller clutches of larger eggs, and increasing intensit...

Developmental and Physiological Determinants of Caste in Social Hymenoptera: Evolutionary Implications

Abstract: Ants, bees, and wasps form societies made up almost entirely of females. The societies are structured by differences among female members; these may be subtle, involving only physiology, or profound, involving gross size differences and hundreds of morphological characters. Since adult morphology is the product of development, the developmental systems that generate different female forms must be fundamental to social structure. Increased complexity of social organization requires changes in the underlying developmental programs that produce the members of a society. In this paper, I present a developmental view of the evolution of societies. These insights serve as a bridge between theories focused on the initial stages of the evolution of sociality in Hymenoptera (Hamilton 1964; Lin and Michener 1972; West-Eberhard 1975) and theories focused on the evolution of insect societies as superorganisms (Oster and Wilson 1978). In simple social systems, the struggle for control of the reproductive physiology of nest mates is resolved entirely in the adult stage; all females are potential queens. The evolution of higher sociality involves an enhancement of the differences between nonreproductive workers and reproductive queens. In what are termed primitively eusocial species, queens and workers cannot be distinguished on the basis of external structure. Size differences, however, often do exist. By contrast, queens and workers differ morphologically in the highly eusocial species (Michener 1974). (A eusocial society must, by definition, have both reproductive division of labor [reproductive castes] and overlap between generations, so that offspring assist parents in brood care [Wilson 1971].) The evolutionary divergence of queen and worker morphology raises two complementary questions regarding the relationship between social structure and the factors that determine physical or physiological differences among females. How far can the physiological and morphological gap between workers and queens be widened in simple systems

Size-Dependent Effects in the Analysis of Reproductive Effort in Plants

Abstract: Size-dependent variation in reproductive effort (RE) appears to be a common phenomenon in many plant species, but researchers have not fully appreciated the factors causing such variation and the potential significance of these patterns to plant ecology and life history. Data from desert winter annuals and the reexamination of published work indicate that many plant populations can be characterized by a significant linear relationship between absolute reproductive biomass and vegetative biomass. The form of this allometric relationship results in variable RE with size (monotonically increasing or decreasing) in many of these species. Both increasing and decreasing RE-with-size patterns are found in both iteroparous and semelparous species, but theoretical expectations for such patterns in different plants remain undeveloped. Some of the conclusions of previous studies of RE in plants may be in error if size-dependent effects were not taken into account. The graphical-regression approach presented here pro...

Analysis of Paternity within a Natural Population of Chamaelirium luteum. 1. Identification of Most-Likely Male Parents

Abstract: The most-likely male parents were identified by using genetic markers and evaluating the statistical likelihoods of paternity for seed collected from known female parents within a natural population of the forest herb Chamaelirium luteum. Likelihoods were determined for a total of 2255 seed, of which male parents were assigned for a subset of 575 seed. These 575 seed could thus be sorted into male sibships as well as female sibships. Within these partial sibships, males had a higher variance in the number of mates, showing a potential for sex-specific selection. Intermate (pollen-flow) distances were found to show more nearby matings than expected on the basis of random mating; however, the incidence of long-distance pollen flow was high compared to findings from studies on other insect-pollinated species. Finally, the variance in the distance to mates was greater among females than among males, again showing a potential for sex-specific selection. The unique discriminatory power provided by the identific...

Attack Abatement: A Model for Group Protection by Combined Avoidance and Dilution

Abstract: An evolutionarily-stable-strategy model for the antipredator advantages of group living in animals is described and termed here the attack-abatement effect. The model considers the possible benefits to the individual of joining or not joining a group and is based on the relative fitnesses of these competing strategies in the same population. It is shown that neither avoidance of contact with the predator nor dilution of the effects of its attack once encountered is advantageous on its own, but in combination they reduce the risk to grouped prey. This is so even in the absence of predation at the margins of the group (as is assumed in Hamilton's selfish-herd model) and without the need for active group defense, evasion measures, or confusion of the predator. It is suggested that avoidance and dilution effects of group living cannot be considered separately, but should be regarded as effective only when in combination (the abatement effect).

Body size, diet, and population density of Neotropical forest mammals

Abstract: The population densities of Neotropical mammalian species are predictably related to their body masses and diets. In interspecific comparisons, population densities generally declined with increasing body mass, and declined with body mass within each of seven specified dietary categories. In our regression analyses, body mass alone accounted for approximately half of the variation in density in the general case, and a greater proportion of the variation in five of the regressions within dietary categories. Pairwise comparisons using stepwise multiple regression indicated that adding diet as well as body mass significantly increased the proportion of variance explained. Finally, the magnitude of the effect of body mass on population density varied with dietary class. These results indicate that, in general, larger-bodied species occur at lower densities than smaller-bodied species, and species with restricted diets and those at higher trophic levels occur at lower densities than species whose diet allows t...

Associational Plant Defenses and the Maintenance of Species Diversity: Turning Competitors Into Accomplices

Abstract: The palatable plants investigated in this study gained significant protection from herbivores by associating with abundant competitors that were less susceptible to herbivory. When herbivores were excluded, palatable species associated with unpalatable ones grew at only 14%-19% of the rate of palatable plants separated from unpalatable ones. When herbivores were present, however, palatable species appeared to depend completely on unpalatable competitors to provide microsites of reduced herbivory to prevent grazers from causing local extinction of the preferred species. For the species studied here, the cost of being associated with a larger, unpalatable competitor was much less than the cost of increased consumption in the absence of that competitor. Under these conditions, one competitor can have a strong positive effect on another. Associational defenses can provide an unappreciated mechanism for maintaining species richness within communities that are dominated by one or a few major species. In this co...

Sex Differences in Spatial Ability: An Evolutionary Hypothesis and Test

Abstract: Although sex differences in cognitive functioning have been studied from many perspectives, current explanations consistently appeal only to proximate causes. An extensive literature details the developmental, neurological, hormonal, genetic, and experiential bases of cognitive sex differences (see, e.g., Maccoby and Jacklin 1974; Harris 1978; McGee 1979; Wittig and Petersen 1979), but virtually no attempt has been made to investigate the evolutionary basis. In other words, we have made considerable progress in learning how such sex differences emerge, but we cannot say why they occur. This paper focuses on one of the best-studied cognitive sex differences-the differential ability to solve spatial problems-and combines field and laboratory techniques to test the evolutionary hypothesis that such differences are a consequence of sexual selection for particular ranging patterns. More than 50 years of psychological testing and research have yielded the consensus that "the most persistent of individual differences on multifactor tests of psychological functioning is a sex difference in spatial ability. Males have decidedly better spatial skill than females." (Harris 1978, p. 405; see additional reviews in Maccoby and Jacklin 1974; McGee 1979, 1982; Wittig and Petersen 1979.) In a detailed meta-analysis of the studies of cognitive sex differences, Rosenthal and Rubin (1982) concluded that, although the magnitude of such differences has declined in recent years, sex differences in cognitive functioning are still nontrivial and of real practical importance. The few nonhuman species for which there are comparative data suggest the generality of this phenomenon: among both wild and laboratory rodents males perform significantly better than

Lifetime Reproductive Success and Heritability: Empirical Support for Fisher's Fundamental Theorem

Abstract: LIFETIME REPRODUCTIVE SUCCESS AND HERITABILITY - EMPIRICAL SUPPORT FOR FISHER FUNDAMENTAL THEOREM

When Should Animals Tolerate Inbreeding

Abstract: We compare, in an initially outbred population, the number of offspring equivalents expected by an individual that avoids all inbreeding with that expected by an individual that tolerates one inbred mating. The model suggests that for most mating systems, the sole factor determining whether inbreeding tolerance spreads is the cost of inbreeding avoidance. Specifically, most forms of polygyny do not increase the payoff to inbreeding; the critical parameter is not the number of matings an individual engages in but rather how many outbred matings are forfeited when an individual chooses to mate with a relative. The model also suggests that dispersal is unlikely to have arisen primarily as a mechanism to avoid inbreeding, and that father-daughter inbreeding should be more common than mother-son inbreeding.

Complementary Models for Ecosystems

Abstract: The ecosystem level of ecological research is fraught with conceptual difficulties that have contributed to faltering progress in theoretical development. An acceptance of multiple models for different aspects of ecosystems may assist in reducing this confusion. Two complementary models of ecosystems are suggested: the first based on energy, the second based on matter. Whereas the first follows from the second law of thermodynamics, the second complementary model derives from the chemical stoichiometry of the biota. These models can be developed independently but have many points of interaction. These points yield another series of predictions and hypotheses. While these complementary models serve as theoretical structures for much of the body of ecosystem concepts, they are not adequate by themselves. Other complementary models, perhaps centered on explicit linkages at the population and community levels, are necessary.

Aggregation of risk: relationships among host-parasitoid models

Abstract: We explore the relationships among host-parasitoid models involving aggregation, with particular emphasis on May's negative binomial model. Models in which parasitoid density in a patch is strictly a function of host density in the patch, with no "error" about this function, are pure-regression models. Those in which there is random variation in the number of parasitoids per patch, with no relationship between local parasitoid density and local host density, are pure-error models. The key factor in these models is not the distribution of parasitoids per se, but the distribution of the relative risk of parasitism, p, which in the present formulation can result from variation in the number of parasitoids in a patch, Pj, or from variation in host vulnerability, a. We show that May's model, and the Bailey et al. (1962) model of which it is a special case, arises naturally from pure-error models. By contrast, it is very difficult to obtain May's model from biologically plausible pure-regression models. There a...

Canalization Versus Developmental Conversion in a Spatially Variable Environment

Abstract: A model is presented for the evolution of developmental control in a spatially variable environment. Individuals are assumed to disperse at random into one of two patches (one harsh and the other benign) and use one of three developmental strategies for the production of one of two discrete morphological types. Two of the strategies are unconditional (develop as either the stress-tolerant or the nontolerant morph), and the third strategy depends on the environment. Invasion criteria are used to determine the conditions under which each of the three pure strategies are evolutionarily stable and the conditions under which the population is expected to contain some mixture of these strategies at equilibrium. The results demonstrate that environmental control of development requires a cost to the stress-tolerant morphology, and that the average probability of making the right choice is greater than 50%. The range of patch frequencies over which environmental control is stable increases with increases in these...

Sexual Selection in Raphanus sativus: Experimental Data on Nonrandom Fertilization, Maternal Choice, and Consequences of Multiple Paternity

Abstract: We determined the paternity of seeds in the multiseeded fruits of Raphanus sativus in order to measure the potential mechanisms of sexual selection after experimental pollination with mixed pollen loads. We also compared singly and multiply sired fruits. We found effects of pollen-donor identity on the number of ovules fertilized, the position of ovules fertilized, the fruit set, seeds per fruit, and seed weight. One pollen donor was inferior by all measures and had the slowest pollen germination. The effect of pollen-donor identity may be attributed to male-male competition because it is uniform over the maternal plants, or the effect may be caused by all females "choosing" in the same way. Another effect, that multiply sired fruits have a greater total weight of seeds per fruit than do fruits sired singly by any of the pollen donors, appears to be a case of maternal choice. Comparison of the seed sizes of two of the pollen donors suggests that competition between seeds for resources may be more intense ...

Introduced Species and Vacant Niches

Abstract: The idea of \"vacant niches\" has undergone a small resurgence recently (e.g., Lawton 1984; Price 1984). Walker and Valentine (1984) developed a model for estimating the number of unoccupied niches in ecosystems and predicted, for example, 150,000 empty niches in the \"marine biosphere.\" The application of their model to the real world depends on the assumption that vacant niches do exist. As the sole support for this assumption, they cited a review by Simberloff (1981) on the effects of introduced species. Simberloff (1981, p. 66, and in Walker and Valentine 1984) concluded, \"The most striking result is that in so many instances (678 of 854), an introduced species has no effect whatever on species in the resident community, or on the structure and function of the community. Perhaps the second most striking result is the scarcity of extinctions apparently attendant on introductions.\" Our purpose is to demonstrate that introduced species have not provided unequivocal evidence for the existence of vacant niches because the conclusions of Simberloff (1981) resulted from his special definitions of effect and from his methods of analyzing the available information. We review the reasons for rejecting the concept of vacant niches, concluding with an analysis of the effects of introduced species of fish on the aquatic communities of California. The concept of vacant niches in natural communities i of interest o conservationists and resource managers, as well as to ecologists, because it has been used to justify numerous introductions ofplants and animals, many of which have led to environmental disasters (see discussions in Elton 1958; Courtenay and Stauffer 1984). Contrary to the conclusions of Simberloff (1981), the reviews on which he based his analysis how that successful introductions have either altered the structure and function of natural communities or have occurred in habitats highly modified by humans. In either case, the existence of vacant niches in natural systems was not supported by species invasions.

Measuring diffuse competition along an environmental gradient: results from a shoreline plant community

Abstract: We propose a method for measuring variation in diffuse competition along an environmental gradient. This approach has two advantages over pairwise-competition experiments conducted in homogeneous environments. First, it may be more realistic, since individuals in nature are usually confronted with a variety of neighbors, that is, with diffuse competition. Second, this approach allows us to test whether the variation in diffuse competition is correlated with gradients in environmental factors. We used a field experiment to test whether diffuse competition is correlated with standing crop and organic-matter content in the soil of a lakeshore plant community. Diffuse competition was correlated significantly and positively with both of these factors. Further, standing crop was correlated positively with organic-matter content in the soil, suggesting that a general measure of habitat productivity may be indirectly related to the intensity of diffuse competition. These results support models of species diversit...

Evolutionary ecology of seed-bank annuals in temporally varying environments

Abstract: The production of long-lived seeds by annual plants introduces a unique form of age structure. In a temporally varying environment the dormant seed may experience many years with different weather, whereas the germinating individual experiences only the weather conditions of a single growing season. Natural selection operates on both the between-year dormancy and on non-seed-bank traits that affect the degree of specialization to conditions pertaining in different year types. We have used an integrated model that permits these two aspects of the life history to evolve simultaneously. This leads to predictions that are not attainable by considering the evolution of each in isolation. Changes in the survival probability of the between-year seed bank select for reinforcing changes in between-year dormancy and specialization. Changes in the probability of occurrence of different year types select for damping changes in between-year dormancy and specialization. Across a gradient in environmental quality, we pr...

Quantitative Genetics in Natural Plant Populations: A Review of the Theory

Abstract: The methods of quantitative genetics may be useful for quantifying constraints on the rate and direction of response to selection in natural plant populations We review the assumptions necessary for accurate estimation and significance testing of quantitative-genetic parameters, and consider when such estimates may accurately predict a response to selection Estimates of heritabilities and genetic correlations can provide good predictions of a long-term response to selection if (1) such estimates are reasonably accurate; (2) many genes contribute to genetic variances and covariances; (3) genetic variance-covariance matrices remain approximately constant through time (when major genes are important sources of variation, when selection is strong, or when populations are small, then rapid alterations in gene frequencies may occur, with consequent changes in the quantitative-genetic parameters); (4) genotype-environment interaction does not alter genetic parameters in new or unmeasured environments; and (5)

Sex-ratio selection in species with helpers-at-the-nest.

Abstract: The data presented by Gowaty and Lennartz (1985) for red-cockaded woodpeckers, Picoides borealis, provide the first empirical evidence of a skewed sex ratio at the termination of parental care for a cooperatively breeding bird. Their findings take on added significance because they suggest that helping behavior per se may be a driving force selecting for an imbalanced sex ratio, rather than vice versa. It is well known that males outnumber females in adult populations of many, if not most, species of cooperatively breeding birds (Brown 1978, 1983; Emlen 1978, 1984). It is also true that among birds in general, and cooperative breeders in particular, males usually are the more philopatric sex, whereas females are the dispersers (Rowley 1965; Zahavi 1974; Gaston 1978; Woolfenden and Fitzpatrick 1978; Greenwood 1980; Reyer 1980; Koenig et al. 1983). The result is that males are more likely to remain in or near their natal groups and to act as helpers at the nest. These facts have led several authors to hypothesize that the skew in the adult sex ratio is brought about by heightened mortality associated with female-biased dispersal. This imbalance, in turn, imposes a demographic constraint on the ability of males to find mates, with the effect that a proportion of the male

On Plants and Herbivores

Abstract: Belsky's (1986) recent paper titled \"Does herbivory benefit plants? A review of the evidence\" represents a fundamental material fallacy characterized by Aristotle as plurimum interrogationum, the fallacy of many questions. By posing a question in a fashion demanding a single answer when only a complex answer can be correct, she developed an argument hat is neither internally consistent nor supportable by the scientific evidence. Her criticisms of the literature are too farreaching to be addressed fully in a note; I confine myself to major points that bear on my own research, review articles, and statements about plant-herbivore relations. I believe that commentary is also relevant o many of her statements about the publications of other scientists. I explicitly identify logical errors, papers uncited by Belsky that refute her principal points, a misrepresentation f the methods that I used in field studies, and errors of fact and interpretation that flow inevitably from her approach. 1. Regarding the benefit hat grasses might derive from grazing, I have nowhere written that the mere act of herbivory isbeneficial to any affected plant. I wrote, \"Although these experiments indicate that compensatory growth responses of the plants are a significant factor in this ecosystem's energy flow, this does not imply that the relationship between the plants and herbivores is symbiotic by any conventional definition of that term\" (McNaughton 1979b, p. 698). And, more recently, \"Although the animals and plants are interdependent, compensatory growth of the grasslands did not compensate fully for removal by herbivores, and the grasses have evolved levels of silicification, an antiherbivore d fense, more pronounced than have been recorded in any other ecosystem. Thus, it is improper to conclude that grazing is strictly advantageous to the plants.\" (McNaughton 1985a, p. 284.) These caveats notwithstanding, grasses (as species, ecotypes, or genotypes) that dominate grazed grasslands but disappear in the absence of the grazers (Clements 1920; Stapledon 1928; McNaughton 1979a,b, 1984) are obligate grazophiles (McNaughton 1979b) and would not exist in the absence of grazers. Belsky concluded, from species-composition changes when grazed ranges are fenced, \"In each of these cases the individual plants depend upon the grazers to reduce the biomass of their competitors or prevent the build-up of senescent tissue in the community. Normal community processes such as competition and succession adequately explain the persistence or disappearance of these species.\" (Belsky 1986, p. 884.) The first sentence quoted is as clear and comprehensive a statement of benefits accorded plants by animals as any I have read. The second sentence is, simply, a non sequitur. Its context also implies that herbivory isnot a normal community process. Rather, I believe, it is a lack of grazing and not its

Carnivore Life History Patterns: Allometric, Phylogenetic, and Ecological Associations

Abstract: Previous approaches to life history problems are considerably varied, and reviews may be found in Stearns (1976, 1977, 1980), Southwood (1976), Horn (1978), Horn and Rubenstein (1984), Western and Ssemakula (1982), and Calder (1984). In general, three factors have been shown to correlate with interspecific differences in life histories across mammals: size, phylogeny (taxonomy), and ecology. Size (primarily body weight) appears as an important variable in analyzing life history variation because most life history traits correlate with the rate of physiological activity and consequently with size (Huggett and Widdas 1951; Leitch et al. 1959; Millar 1977, 1981; Western 1979; McNab 1980; Lindstedt and Calder 1981; Calder 1984). Assuming that size must be considered when analyzing life histories, two problems should be addressed. First, the allometric relationship of many life history variables is not constant (Gould 1966, 1971). Numerous studies (e.g., Sacher and Staffeldt 1974; Millar 1977, 1981; Case 1978) have drawn conclusions about mammalian life history evolution by combining taxa (e.g., rodents, ungulates) that are heterogeneous in slope and therefore have differences in scaling. Certainly, extreme care must be taken before comparing trends between variables that are different in form among taxa (see Clutton-Brock and Harvey 1984). When allometric effects of life histories are considered, some index of size must be removed. Most studies have used body weight as the independent variable, arguing that weight reflects metabolic rate, which in turn regulates reproductive effort (Calder 1984). Sacher (1959) and Sacher and Staffeldt (1974), however, found that brain weight, rather than body weight, accounted for a greater proportion of the variance in life span and gestation time. They argued that the slow growth of neural tissue constrains somatic cell proliferation, thus slowing the reproductive rate of large-brained mammals. Support for this hypothesis has been

The Ecology and Evolution of Inducible Defenses in a Marine Bryozoan: Cues, Costs, and Consequences

Abstract: Membranipora membranacea rapidly deploys defensive spines in response to waterborne cues from a trophically specialized nudibranch, Doridella steinbergae. The defensive response by unattacked colonies is enhanced by a high concentration of predators or an attack on a nearby colony. Small colonies require a higher concentration of inducer than larger colonies, perhaps because defensive costs increase as size decreases. Spines reduce the feeding rate of nudibranchs to approximately 40% of the normal level, but they are produced at a cost in colony growth: colonies producing spines initially grow at 85% of the rate of unspined colonies. In colonial animals, a growth decrement is directly translated into a reduced output of sexual propagules because fecundity is directly proportional to colony size. In addition, intraspecific competition is so intense in M. membranacea that an initial decrement in growth can severely reduce final size. This study supports predictions that the evolution of inducible defenses i...

Stable Underdominance and the Evolutionary Invasion of Empty Niches

Abstract: Complex adaptations are frequently polygenic; they cannot evolve in a single step, but rather must pass through several relatively poorly adapted intermediate stages. Evolutionists frequently invoke density- and frequency-dependent selection to explain this process. The initial poorly adapted morphs are assumed to have high fitness because they use an unexploited resource or otherwise occupy an "empty niche." We explore this process with a single-locus selection model based on the differential utilization of two distinct resources. Not surprisingly, we show that unexploited niches can be invaded by poorly adapted morphs. Less obviously, we show that heterozygotes often have the lowest fitness at stable polymorphic equilibria. We also show by numerical example that stable two-locus polymorphisms exist, in which the most heterozygous individuals are the least fit. In general, maladaptive phenotypes are frequently maintained in these models. Thus, they may present a biologically plausible framework in which ...