Showing papers in "The American Naturalist in 1979"

A General Hypothesis of Species Diversity

Abstract: A new hypothesis, based on differences in the rates at which populations of competing species approach competitive equilibrium (reduction or exclusion of some species), is proposed to explain patte...

The statistics and biology of the species-area relationship

Abstract: Regional differences in species number have puzzled naturalists ince the early 1800's, and explanations account for a large part of modern ecological research. Two venerable observations form the cornerstone of our knowledge on the subject: The number of species within a taxonomic group tends to increase with decreasing latitude (see Fischer 1960; Pianka 1966); and the number of species within a taxonomic group tends to increase with increasing area (see Preston 1960, 1962; Williams 1964; MacArthur and Wilson 1967; Simberloff 1972). Despite early research on the latter trend (the species-area relationship), ecologists have studied it intensely only in the last 50 yr. The relationship was originally envisioned as an empirical tool and used in three principle ways: (1) to determine optimal sample size and sample number, (2) to determine the minimum area of a \"community,\" and (3) to predict the number of species in areas larger than those sampled. All three uses are discussed by Kilburn (1966). More recently interest in the species-area relationship has focused on mechanistic explanations, its precise mathematical descriptions, and interpretations of parameters derived from these mathematical descriptions. Williams (1964) and Preston (1960, 1962) have proposed that the exponential and power function models (\"exponential model\" throughout his paper also refers to the species/log area transformation, and \"power function\" also refers to the log species/log area transformation) of the species-area relationship result from the way in which individuals are distributed among species. Williams' (1964) exponential model, which emphasizes habitat heterogeneity, was considered important by many plant ecologists but is now largely ignored. Preston's (1960, 1962) power function model was based on the assumption of a dynamic equilibrium of species exchanges between islands in an archipelago. This assumption led to the equation of the power function model with the idea of a dynamic equilibrium as expounded by MacArthur and Wilson (1963, 1967), such that an adequate fit of this model to observed species numbers has been viewed as support of the equilibrium hypothesis (Grant 1970; Diamond 1973; Simpson 1974). The interplay of the equilibrium hypothesis and the power function model of the species-area relationship has led to interpretation of the slope and intercept of the power function model exclusively in the context of the equilibrium hypothesis. In particular, specific values of the slope of the power function are often construed to * Order of authorship determined by the toss of a coin. t Present address: Department of Biology, University of South Florida, Tampa, Florida 33620. Am. Nat. 1979. Vol. 113, pp. 791-833. c) 1979 by The University of Chicago. 0003-0147/79/1306-0002$03.26

Grazing as an Optimization Process: Grass-Ungulate Relationships in the Serengeti

Abstract: A substantial literature is reviewed which indicates that compensatory growth upon tissue damage by herbivory is a major component of plant adaptation to herbivores. Experiments in Tanzania's Serengeti National Park showed that net above-ground primary productivity of grasslands was strongly regulated by grazing intensity in wet-season concentration areas of the large ungulate fauna. Moderate grazing stimulated productivity up to twice the levels in ungrazed control plots, depending upon soil moisture availability. Productivity was maintained at control values even under very intense grazing, suggesting that conventional definitions of overgrazing may be inapplicable to these native plant-herbivore systems. A laboratory clipping experiment with a sedge abundant in one of the most intensely utilized regions resulted in a maximum net above-ground productivity of 11.6 g/m2 · day when clipped daily at a height of 4 cm. Few plant species have been reported with the ability to maintain a significant level of pr...

Some Reproductive Factors Affecting the Selection of Self-Fertilization in Plants

Abstract: The fitnesses of two phenotypes which differ in their frequency of self-fertilization are expressed exactly in terms of various parameters, including the relative fitness of progeny from selfing, i, and the proportion of available ovules fertilized with the aid of an external agent, e. Strategic models of natural selection find stationary conditions where the two phenotypes have the same fitness. Conditions when selfing is advantageous to populations and to individuals are not usually identical. Conditions favoring self-fertilization are more stringent when selfing competes with crossing (for individual selection, i > 1/2) and less stringent when selfing occurs prior to crossing (i > e/2). When selfing is delayed until after all opportunities for crossing, it is always advantageous if the parameters vary independently. Some functional interactions between parameters, as when an increase in selfing simultaneously reduces the efficiency of external pollinating agents, result in stationary conditions with mi...

Factors influencing leaf choice by howler monkeys: a test of some hypotheses of food selection by generalist herbivores

Abstract: Some theories on the foraging strategies of generalist herbivores suggest that the food choices of such animals are greatly influenced by the quality and antiquality components of potential foods. Many herbivores selectively feed on young rather than mature foliage. It has been hypothesized that young leaves are preferred because they have higher contents of protein and/or lower contents of toxic secondary compounds. These and several other hypotheses were tested with data from a long-term study of the feeding ecology of howler monkeys (Alouatta palliata), using a cost/benefit approach. Samples of young and mature leaves from the same species were analyzed for total protein, cell wall constituents, total phenolics, presence of condensed tannins, and total nonstructural carbohydrates. The results support the hypothesis that more than one factor determine howler leaf choices. The most important factors (relating to food content) appeared to be protein and fiber content, with perhaps some influence from seco...

Sexual Selection in Plants

Abstract: The concept of sexual selection (intrasexual competition for mates and mate preference) is used in revising the classical explanation of dioecy in plants. Male and female functions of hermaphroditic flowers can be subject to different sexual selection pressures, which may conceivably lead to the separation of male and female structures. This suggestion does not exclude genetic advantages that may accrue from outcrossing; the two aspects of selection probably operated together in the evolution of dioecy. The hypothesis regarding the importance of sexual selection could be tested by monitoring variation in individual fitness via pollen donation and via ovule maturation. The evolution of pollinia and other large pollen dispersal units is also viewed in terms of sexual selection. Availability of a reliable pollen vector is a prerequisite to the evolution of pollen packages. From the point of view of a flower functioning as a male, large PDU's ensure fertilization of more ovules and possible preemption of stig...

Competitive Networks: Nontransitive Competitive Relationships in Cryptic Coral Reef Environments

Abstract: Nonhierarchial sequences of interference competitive abilities, competitive networks, have been observed in Jamaican cryptic coral reef environments and also appear to exist in Jamaican open reef surface environments. These competitive networks are both numerous and complex; they appear more likely to be formed by interactions between than within major taxonomic groups. The exact spatial position an organism occupies and the rate at which organisms overgrow one another will be important determinants of patterns of species distribution on substrata supporting competitive networks. This will not be the case if a competitive hierarchy exists. The existence of a competitive network on a particular substratum will serve to increase the length of time required for single species resource monopolization relative to the time which would be required if a competitive hierarchy exists, assuming equivalent rates of overgrowth in both cases. The competitive networks situation provides a mechanism for the development o...

Offspring Quality and the Polygyny Threshold: "The Sexy Son Hypothesis"

Abstract: The foundation for current theories on the evolution of mating systems was laid by Darwin (1871) in his elucidation of the process of sexual selection. Darwin introduced this concept to explain the evolution of characters that were not related to an individual's ability to survive in the environment (and in fact probably reduced this ability), but enhanced its ability to obtain mates. Implicit in this concept is that individuals of each sex differ from one another with respect to characteristics which affect the fitness they will confer on another individual if chosen as a mate. Consequently, selection pressures will exist that promote the development of techniques both for advertising superior characteristics and for distinguishing good characteristics from bad. In his review of sexual selection and the role of parental investment, Trivers (1972) concludes that when the investment of each sex in its offspring is approximately equal, then sexual selection should affect both equally. However, when, for example, females invest more heavily than males, then competition among males to increase their frequency of mating will be high and, consequently, sexual selection among males will be strong. As Trivers (1972) points out, females initially have a much higher investment in terms of the energy supplied to each ovum relative to the very small amount of energy devoted by the male to each sperm. In highly polygynous mating systems the role played by the male in rearing young becomes extremely limited and thus the difference in the relative contributions of each sex increases. Consequently, in such systems sexual selection will act much more strongly on the male. A corollary to this is that selection will at the same time favor increased discrimination on the part of the female in her choice of a mate in order to protect her investment. A poor choice by the female has much greater repercussions on her fitness than a similar poor choice by a male has on his fitness (Orians 1969). A model explaining the evolution of polygynous mating systems based on the importance of female choice is presented by Orians (1969) based on the work of Verner (1964) and Verner and Willson (1966). The foundation of the model rests on the definition of the polygyny threshold (Verner and Willson 1966) as the point at which the difference in quality of two males' territories is great enough that a female could rear as many young alone or with limited male assistance in the better territory

Coexistence in a Variable Environment

Abstract: A community which would not reach a stable equilibrium may nevertheless persist if there is temporal variation and nonlinear dynamics. A procedure is introduced for taking time averages of the rates of change. Since the average of a nonlinear function is not the function of the average, higher terms such as the variances of resources or covariances among species and environmental factors enter into the coexistence conditions. These measures behave as if they were resources. Therefore the number of consumer species cannot exceed the number of resources plus distinct nonlinearities. The nonlinearities arise from predator saturation, learning, group hunting, multiple nutritional requirements, or seasonally variable feeding rates. It is shown that there is no long term correlation between the abundance of a species and its rates of increase.

Predicting Amphibian Metamorphosis

Abstract: Data are presented to test predictions of amphibian metamorphosis by a differentiation based model Differentiation rates account for 95% of the variance in the length of the larval period, while growth rates account for less than 50% of the variance The probability of completing metamorphosis at a given time is a function of current developmental stage and of differentiation rate Growth parameters are poor predictors of metamorphosis, because the relationship between growth and differentiation varies as a function of the abiotic and biotic environment Differentiation rate, growth rate, and stage-specific growth can be expressed as functions of environmental parameters such as temperature and density Data presented support the hypothesis that temperature is a major proximal factor determining larval growth and differentiation patterns Temperature effects on developmental rates during metamorphosis can be expressed by a modification of Belehradek's equation, previously applied only to embryonic develo

Evolutionary consequences of seed pools

Abstract: The annual habit in plants is often accompanied by specialized physiological mechanisms which permit seeds to remain dormant for a few years to decades. This results in the establishment of a seed ...

Optimal Diets under the Energy Maximization Premise: The Effects of Recognition Time and Learning

Abstract: A model, developed previously by several authors, predicting the diet which maximizes a predator's rate of energy intake is generalized to include prey recognition times, the probability of prey misidentification, and learning by the predator. The model predicts that the most valuable prey are always eaten when encountered, but if their encounter rates are too low the diet expands to include less valuable prey. Less valuable prey may be eaten when their relative abundances are high even when the preferred prey are plentiful if there is a finite recognition time or, alternatively, if there is a significant probability of the predator mistaking them for the preferred prey. Crypsis may cause a predator to mistake a proportion of inedible objects for the real prey, thereby decreasing the expected value of the cryptic prey. When the cryptic prey becomes scarce the predator may specialize on intrinsically less valuable noncryptic prey. By learning to handle prey more efficiently, the predator may effectively tr...

Optimal Life Histories Under Age-Specific Predation

Abstract: If a predator crops prey of certain ages in preference to others, it may be acting as an agent of selection, causing genetic change in the prey population. This paper adapts a model of life-history evolution to investigate some selective consequences of several patterns of age-specific predation. The model allows allocation of resources to reproduction, growth, and maintenance in an age class i to be varied. Subject to several assumptions, it is shown that increasing predation on individuals younger than i leads to selection for lower rates of reproduction at age i. Conversely, increasing predation on individuals older than and/or equal to i leads to selection for greater rates of reproduction. If individuals from both these age ranges are cropped the optimal rate of reproduction may either increase or decrease. The only circumstance under which no change occurs arises if all individuals irrespective of age are liable to be cropped, or if the effects of predation on younger and older age classes exactly c...

Sexual Selection and Variance in Reproductive Success

Abstract: Bateman (1948) in his early studies of intrasexual selection in Drosophila melanogaster concluded that the variance in the number of mates is \"the only important cause of the sex difference in variance of fertility\" (p. 363). He also suggested that \". . . a sex difference in variance in fertility is, therefore, a measure of the sex difference in intensity of selection\" (p. 353). A more recent discussion of sexual selection (Trivers 1972) has emphasized the role of parental investment in offspring as the cause of sexual selection by means of competition for mates. \"What governs the operation of sexual selection is the relative parental investment of the sexes in their offspring. Competition for mates usually characterizes males because males usually invest almost nothing in their offspring\" (p. 141). Trivers specifically excludes those energies expended in intrasexual competition when evaluating parental investment. Trivers furthermore proposes a somewhat different measure of the \"potential for sexual competition\" (p. 140): the ratio of the number of offspring optimally produced by the sex investing less to the number of offspring the other sex optimally produces. The purpose of this letter is three-fold: (1) to derive a concise expression for the variance in male reproductive success in terms of the variance in reproductive success among females; (2) to show that Bateman correctly identified the variance in the number of mates as the only important cause of the sex difference in fertility; and (3) to illustrate that relative parental investment as defined by Trivers is not an adequate measure of the intensity of intrasexual selection. I will assume that the number of offspring produced by a female is independent of the number produced by any other female and that the female distribution of offspring numbers can be characterized by an arbitrary distribution with mean XV a, and variance o29 Y . Let Nag and NV y represent the number of breeding males and females, respectively, and let N &&IN = y S be the sex ratio. If pi is defined to be the fraction of males which obtain i mates (where i is any positive integer), then clearly J1=opi = 1 and 14=jpi = M, the mean number of mates per male. If all females are presumed to secure mates then N& (Y=ipi) = N y and i=tps 1/S. The assumptions that females reproduce independently of one another and that i is

The genetical evolution of patterns of sexuality: Darwinian fitness.

Abstract: R. A. Fisher (1930) was perhaps the first to realize that the key to sex ratio evolution lay in the almost trivial fact that (under diploidy) everyone has one mother and one father; that in terms of autosomal genes males and females contribute equally to any zygote formed. This paper shows that his observations proves a useful key to a host of other sex related problems. It is for this intuitive reason that fitness measures for the alteration of sex function are often of the general form W = m/m + f/f. In such a measure male and female function are assigned equal weight. It is somewhat surprising that this notion continues to hold under haplodiploidy (at least from the mother's viewpoint). There is much that this paper has ignored--inbreeding, fluctuating or stochastic environments, etc. A treatment of many of these is much beyond me. It will be quite interesting to know how well the m/m + f/f notion holds up to alterations in the basic models proposed here.

Seasonality and Patterns of Natural Selection for Life Histories

Abstract: I present a model to investigate the effects of density-dependent resource availability on per capita population growth rate. By considering a continuous time model of inclusive fitness, population losses as well as growth are essential determinants of phenotypic fitness. The outcome of natural selection for various life-history characteristics is dependent upon not only the shape of the life history-specific population growth curves, but also upon the form of the seasonality function. As the magnitude and length of a resource shortage period increases, the rate of decrease becomes a more important component of fitness. Seasonality is often characterized by (1) a period of population decrease, when natural selection favors functions which minimize somatic or embryonic mortality, and (2) a period of high resource availability when high somatic and gametic productivity are favored. Assuming that mean resource availability remains constant, seasonally deterministic and/or random variation in the availability...

Evolution of Life Histories in Response to Age-Specific Mortality Factors

Abstract: It has become increasingly common to view the life-history statistics of an organism as a coevolved unit. Regarded as an adaptation, these statistics summarize a good deal about the selective constraints "experienced" by the organism. Consequently, the lx and m. distributions reflect past environments in the sense that they have been molded by natural selection. As a result of MacArthur and Wilson's book (1967), an overemphasis was placed on density dependence as the main selective constraint molding the life history. This preoccupation with r and K-selection obscured some biological reality, and more recently writers have emphasized other constraints. For example, Wilbur et al. (1974) initiated a more community oriented perspective, while Istock, Zisfein, and Vavra (1976) and Istock, Vavra, and Zimmer (1976) emphasized environmental variation. Slobodkin (1957, 1959, 1961, 1968, 1972, 1974) initiated a theory of prey evolution, "prudent predation," which focused on the prey's life history. These papers, along with many others, are important because they help to identify a sufficient set of parameters which constrain the evolution of life histories. The motivation for the present work stems from the recurring interest in the literature about the relationship between age-specific mortality and Fisher's agespecific reproductive value (RV) function (e.g., Fisher 1958; Medawar 1952; Slobodkin 1968, 1974; Caughley 1966). These authors, among others, have either called attention to or suggested a mechanism for the commonly observed inverse relationship of RV and mortality. Questions concerning this relationship are contained in the more general problem of the evolution of life histories in response to age-specific extrinsic mortality factors or factors which originate outside of the organism (e.g., predators).

Generality of the Size-Distance Relation in Models of Optimal Feeding

Abstract: If pursuit or provisioning time is unrelated to prey size, the following size-distance properties are expected for a variety of prey-handling functions. 1. The best prey size increases with distance. 2. For given values of profit (measured as energy per unit time), profit changes less with distance, the larger the prey. 3. The range of sizes taken shifts toward larger sizes, as distance increases. If pursuit or provisioning time increases with prey size, these effects can be reversed.

A Critical Evaluation of Intrinsic and Extrinsic Factors Responsible for Niche Restriction in Parasites

Abstract: Intrinsic and extrinsic factors responsible for niche restriction in parasites and particularly in Monogenea of fish are discussed. Interspecific competition may result in competitive exclusion of one or several species, or it may lead to a change in the microhabitat of one or all co-occurring species (interactive site segregation), but there is no evidence that such effects lead to evolutionary changes and avoidance of competition, i.e., to selective site segregation. That extrinsic factors, particularly competition, are not of such great evolutionary significance among parasites as is usually assumed, is indicated by the following observations. Most effects of competitive exclusion may also be caused by intraspecific crowding. Interspecific effects may also be positive, enhancing the chances of species co-occurring. Host-mediated effects are usually more harmful to the species evoking them than to competing species. Parasite species with coinciding or overlapping microhabitats often show no interactions...

Spiral Chaos in a Predator-Prey Model

Abstract: Cox, G. W., and R. E. Ricklefs. 1977. Species diversity, ecological release, and community structuring in Carribbean land bird faunas. Oikos 28:113-122. Crowell, K. L. 1962. Reduced interspecific competition among the birds of Bermuda. Ecology 43:75-88. Gause, G. F. 1934. The struggle for existence. Williams & Wilkins, Baltimore. Grant, P. R. 1972. Interspecific competition among rodents. Pages 74-106 in R. F. Johnson, P. W. Frank, and C. D. Michener, eds. Annual review of ecology and systematics. Vol. 3. Annual Reviews, Palo Alto, Calif. Harcourt, D. G. 1960. Biology of the diamond back moth, Plutella maculipennis (Curt.) Lepidoptera: Plutellidae in Eastern Ontario. III. Natural enemies. Can. Entomol. 92: 419-429. Huffaker, C. B., P. S. Messenger, and P. DeBach. 1971. The natural enemy component in natural control and the theory of biological control. Pages 16-67 in C. B. Huffaker, ed. Biological control. Plenum, New York. MacArthur, R. H. 1972. Geographical ecology: patterns in the distribution of species. Harper & Row, New York. MacArthur, R. H., J. M. Diamond, and J. R. Karr. 1972. Density compensation in island faunas. Ecology 53:330-342. Miller, R. S. 1967. Pattern and process in competition. Pages 1-74 in J. B. Cragg, ed. Advances in ecological research. Vol. 4. Academic Press, New York. Morris, R. F. 1976. Influences of genetic change and other variables on encapsulation of parasites by Hyphantria cunea. Can. Entomol. 108: 673-684. Park, T. 1962. Beetles, competition, and populations. Science 138:1369-1375. Price, P. W. 1971. Niche breadth and dominance of parasitic insects sharing the same host species. Ecology 52: 587-596. Raizenne, H. 1952. Forest lepidoptera of southern Ontario and their parasites. Canada Department of Agriculture, Division of Forestry Biology. Syme, P. D. 1975. The effects of flowers on the longevity and fecundity of two native parasites of the European pine shoot moth in Ontario. Environ. Entomol. 4:337-346. Zwolfer, H. 1963. Untersuchungen uber die Struktur von Parasitenkomplexen bei einigen Lepidoptera. Z. Angew. Entomol. 51:346-357.

Iteroparous Animals that Skip Opportunities for Reproduction

Abstract: A recent interest in life-history evolution has focused on patterns of reproduction (Cole 1954; Stearns 1976). One major type of reproduction is iteroparity, in which adults reproduce repeatedly during life. In most iteroparous animals each adult reproduces every year. Here we describe a widespread form of iteroparity in which reproduction occurs in the population every year, but each individual reproduces only in alternate years or less often, thereby neglecting half the opportunities for reproduction. We term this behavior a \"low frequency of reproduction\" (LFR) for lack of a more precise term; it is understood that the \"low frequency\" refers to the opportunities passed up, rather than to any absolute time scale. The species known to have a LFR among its adults are vertebrate ctotherms, uch as fish, amphibians, and reptiles (table 1). These species are characterized by moderate to large clutch sizes and asynchronous reproduction in the population. The LFR is usually restricted to the female, although in some species the male also reproduces at a low frequency. The species and sexes with a LFR often have a major \"accessory\" activity associated with reproduction (some activity in addition to the usual production of eggs or fertilization of females). The three types of these activities observed are breeding migrations, egg brooding (throughout incubation), and live-bearing (table 1). This association of accessory activity and LFR is apparently not random: In urodele amphibians (salamanders) the proportion of maternal egg-brooders among the LFR species (.67) is significantly higher than the proportion of maternal egg-brooding species among urodeles in general (.37, data from Brame 1967; Salthe and Mecham 1974; 302 species, P < .005, Fisher's exact test); and in snakes, livebearing is more common in LFR species (.80) than among snakes in general (.31, data from Fitch 1970; 329 species, P < .005, Fisher's exact test). Also, males do not reproduce at a low frequency unless they also have some accessory activity (breeding migration). The reverse association is not so strong, however. Many reptiles and amphibians are either live-bearers or brood eggs, yet reproduce yearly. The frequency of reproduction varies between different populations in some of these species (table 2). In all but one case (Salmo) the lower frequency is found in the habitat of poorer quality or shorter growing season. In Salmo the lower frequency may be associated with an increased difficulty in the breeding migration.

Scaling of Skeletal Mass to Body Mass in Birds and Mammals

Abstract: New data on the allometric relationship of skeletal mass to body mass of birds and mammals indicate that the avian skeleton is not proportionately lighter than that of mammals. Pneumatization may make some birds bones lighter, but the leg bones of birds are more robust than those of mammals. This results in an internal redistribution rather than a reduction of skeletal mass.

Stability under Environmental Stress: Resistance, Resilience, Persistence, and Variability

Abstract: Four main contributions are made toward understanding stability and clarifying the ambiguity that has surrounded the term stability in the past. (1) Four different aspects of the response of populations to environmental stress-resistance, resilience, persistence, and variability-are given mathematical formulations. (2) Methods are given to analyze a differential equation model of population growth for resistance and persistence, while resilience corresponds to the traditional Lyapunov asymptotic stability. (3) The relation of resistance and resilience to each other depends on the underlying characteristics of negative feedback to population changes and sensitivity of the growth rate to the environment. Strong feedback mechanisms which are independent of the environmental factor causing the stress increase both resilience and resistance, but if the feedback mechanism makes the population growth rate sensitive to an environmental factor, the amount of resilience and resistance under changes in that factor w...

The Allometry of Reproduction: An Empirical View in Salamanders

Abstract: In order to distinguish important variations in some life history traits, body size must be subtracted by constructing allometric null hypotheses. We present interspecific empirical models that describe the relationship among salamanders between body size and three variables: clutch volume, clutch size, and egg size. The relationship between log clutch volume and log body volume for 74 species of salamanders has a correlation of .899 and a slope of 0.64. The significance of this interspecific slope and lack of identity between it and intraspecific slopes is discussed. The relationships between log clutch size versus log body volume and log egg volume versus log body volume are largely determined by the mode of reproduction of the species. Data for four populations of salamanders of the genus Ambystoma are presented and compared with the models. The usefulness and necessity of first determining how a particular reproductive trait is expected to change with body size is made clear, for example, by observing...

A Model for the Evolution of Distyly

Abstract: Reasons are given for believing that morphological distyly is unlikely to evolve before the incompatibility system. The first problem studied is therefore the evolution of incompatibility of the type found in distyly, with two pollen and two stigma types. It is argued that the first step, starting from self-compatibility, is a mutation to a new pollen type, such that the mutant cannot self-fertilize or fertilize other individuals. Conditions for the spread of such a mutation (assumed recessive) are given. Next, a dominant mutation to a new stigma type, incompatible with the original pollen type but compatible with the new type, is studied. Such a mutation is almost certain to be eliminated if it occurs first, but can spread in a population polymorphic for the pollen mutation provided that it occurs at a linked locus. With tight linkage, this model generates a population with two incompatibility types, one dominant to the other, with only rare recombinant types. It is shown by computer runs that modifiers ...

Female Choice of Mates: A General Model for Birds and Its Application to Red-Winged Blackbirds (Agelaius phoeniceus)

Abstract: A model of female choice in birds is developed with special reference to red-winged blackbirds. Characteristics of the male are related to female fitness, and it is argued that these characteristics should influence mate choice only if they (a) have an important effect on female fitness, (b) are variable, and (c) are accurately assessable prior to mating. Using these criteria, it is concluded that female choice in redwings (1) should not be influenced by the female's estimate of the male's future nest defense effort because nest defense seems to have little effect on the severity of predation and parasitism losses; (2) should not be influenced by an estimate of the male's future time investment in feeding the young because this investment is small in redwings, and the female's estimate must be inaccurate; (3) should be influenced by the vulnerability of the territory to predation because predation rates are high, variable from area to area, and predictable; (4) should not be influenced by the vulnerabilit...

Optimal Foraging: On Flower Selection by Bees

Abstract: A model based on the assumption that bees are efficient foragers is presented that predicts the visitation frequency of bees foraging at two randomly patterned flower types. The important parameters of the model considered are the densities, the relative caloric values, and the handling times of the alternative flower types. The predictions of the model are tested by observing the behavior of 41 honeybees foraging at seven different 1.22 x 1.22 m (1.5 m2) patches of artificial "flowers." The densities of the two flower types were equal and held constant at the patches, and the relative caloric values were varied. The observed foraging behavior of the bees is used to reject the model as originally stated. It is believed that heterogeneity among the bees accounted for the rejection. The foraging strategies of individual bees remain stationary over successive foraging bouts. When a variance component to the foraging behavior is added to the model the results are consistent with the predictions.

The Cost of Reproduction in Annual Meadow Grass

Abstract: Studies of the evolution of life histories assume that there are costs to reproduction, although there is little experimental evidence for their existence. In this paper some populations of annual meadow grass (Poa annua L.) are analyzed, to test for the existence of these costs. The results show that there is a price to be paid for high rates of reproduction early in life, in lower rates and smaller plant size subsequently. There are also signs that high rates of reproduction very early in life increase subsequent risks of mortality. A possible form for the relationship is considered from a model of reproduction, growth, and survival. The implications of the observed and predicted costs to the evolution of life histories in annual meadow grass are discussed.

The Evolution of Concealed Ovulation

Abstract: Several hypotheses have been proposed to explain the occurrence of continual sexual receptivity and concealed ovulation in human females. In view of the large number of benefits that would accrue to females if they could sense their own ovulation, these explanations appear insufficient to explain why ovulation is concealed from females as well as from males. The hypothesis presented here is that the phenomenon occurs because of a hominid female tendency to avoid conception in biologically nonadaptive ways. This tendency was countered by natural selection by making ovulation virtually undetectable to women. The sequence of evolutionary adaptations culminating in concealed ovulation is most likely as follows. First, olfactory, visual, and pronounced behavioral cues to ovulation were lost to conspecifics. Coincidentally females evolved continuous receptivity, frequently copulating at times other than when ovulating. Finally, females lost conscious cues to their own fertility. This last step was predicated up...

Parental Care Patterns of Fishes

Abstract: Sexual roles in brood care have been examined for a number of animal groups. Most authors have related these patterns to general ecological characteristics of the animal's environment, such as distribution and character of food, nature of predation, and difficulty of finding and keeping a mate (e.g., Verner and Willson 1966, for birds; Orians 1969, for birds and mammals; Crook 1970, for primates; Kleiman and Eisenberg 1973, for canids and felids; Alexander 1974, for social animals). A second approach has been that of Trivers (1972) and Williams (1975), who explored the relation between parental care patterns and mating systems. Trivers pointed out that reproductive effort is divided between courtship (i.e., efforts to get mates) and parental investment, (PI), that is, efforts to improve the probability of an individual offspring's survival. The latter limits the parent's ability to invest in other offspring, current and future. Trivers also suggested that maximum reproductive success would often result in different courtship and parental roles for the sexes, and that this provided the most basic explanation for the variety of mating patterns found in nature. Williams reached similar conclusions, and provided examples from a variety of taxonomic groups illustrating differences between the sexes in the form of parental investment. The roles of the sexes in brood care in fishes differ among species, and although some authors have considered them peripherally, at present no comprehensive theory has been developed to explain them. In this paper we develop a theory to account for the sexual roles in different fish parental care patterns based on the approaches of Trivers (1972) and Williams (1975).