Asymmetric competition between Rana clamitans and Hyla versicolor tadpoles

When two organisms interact over a potentially limiting resource, several outcomes are possible (e.g. neither affected, both equally affected, one affected more than the other). The outcome depends on a variety of factors, both internal and external to the organism. We performed a laboratory experiment to investigate the outcome of potential competitive interactions between tadpoles of the green frog ( Rana clamitans ) and the gray treefrog ( Hyla versicolor ), and to examine the factors that may explain the outcome of the interaction. We found that competition between these two species was asymmetric. Hyla versicolor tadpoles were more affected by intraspecific competition than interspecific competition (i.e. grew faster when grown with heterospecifics than conspecifics); whereas, R . clamitans tadpoles were more affected by interspecific competition than intraspecific competition (i.e. grew faster when grown with conspecifics than heterospecifics). Based on behavioral observations, this result could not be explained on the basis of activity levels, a trait previously linked to competitive ability in tadpoles. However, differences in initial body size may help explain the observed results; the gray treefrog tadpoles were larger than the green frog tadpoles.     

Effects of Familiarity and Population Density on Competitive Interactions and Growth: An Experimental Study on a Territorial Salmonid Fish

The deleterious effect of competition for space and food in animals increases with increasing population density. In contrast, familiarity towards conspecifics can relax the intensity of interference competition. Here, we hypothesized that familiarity towards conspecifics mitigates the effect of density‐dependent growth and dispersal behaviour in territorial animals. To test this, wild‐captured juvenile brown trout were subjected to two consecutive laboratory experiments. First, growth and fin erosion were measured for 40 d in a 2 × 2 factorial design manipulating density and familiarity. The density was manipulated via size of experimental tanks, while per capita food abundance and fish number was constant. All fish were subsequently exposed to an emergence test, giving them the option to leave their group and disperse to a novel unoccupied environment. The results show that familiarity increases growth and decreases the level of fin erosion (i.e. proxy of intensity of aggressive interactions). We found no significant effect of population density on growth rate. However, there was a tendency towards higher fin erosion in fish kept under high density. The growth of individuals was also affected by their size rank within the group, with the largest individuals in each group growing disproportionally faster than the rest of the group, probably due to their high social rank. However, the second and third fish in the size rank did not grow significantly faster and tended to suffer higher mortality than the rest of the group. During the emergence test, the largest individuals in the familiar groups left the shelter either as the first (six of 12 groups) or last (five of 12 groups) individual in the group, while no such pattern was observed in unfamiliar groups. Our results suggest that individuals in familiar groups receive less aggression and stress (i.e. fin damage) and grow faster than fish in unfamiliar groups. The mechanisms indicated in this laboratory study may be especially important in highly fecund organisms like fish which undergo density‐dependent bottlenecks during early life.     

Sex, Rank and Age Differences in the Japanese Macaque (Macaca fuscata yakui) Participation in Inter-Group Encounters

In many species interactions among group are often characterized by agonistic behaviour. Although animals may participate in inter‐group encounters in different ways, depending on their energetic requirements, reproductive tactics, and/or developmental stage, the proximate causes affecting an animal's participation in inter‐group encounters are still poorly understood. Indeed, many studies have analysed the behaviour of males and females during inter‐group encounters without considering the importance of additional factors (e.g. rank). This study focuses on wild non‐provisioned Japanese macaques (Macaca fuscata yakui) living on Yakushima Island, Japan. It aims to determine how monkeys of different sex, age, and rank behave during inter‐group encounters and it discusses the implications and consequences of their behaviour on group composition and male dispersal. Males participated significantly more than females in inter‐group encounters, by displaying more aggressive or affiliative behaviour. High‐ranking and/or adult males were more aggressive than low‐ranking and/or subadult males during encounters occurring in the mating season and they also showed more herding behaviour. This trend was not found in inter‐group encounters occurring during the non‐mating season. Finally, males which then emigrated to new groups were low‐ranking and/or subadult individuals. Those males displayed more affiliative behaviour towards foreign males than males which did emigrate. These data indicate that in non‐territorial species with male dominance over female and high competition for mating partners males play an active, and often aggressive, role during inter‐group encounter while female participation is scarce. Factors such as age, rank and period of the year (in seasonally breeding species) have to be taken into considerations when analysing interactions between groups and their effects on group composition and social behaviour.     

Cancer susceptibility and reproductive trade-offs: a model of the evolution of cancer defences

The factors influencing cancer susceptibility and why it varies across species are major open questions in the field of cancer biology. One underexplored source of variation in cancer susceptibility may arise from trade-offs between reproductive competitiveness (e.g. sexually selected traits, earlier reproduction and higher fertility) and cancer defence. We build a model that contrasts the probabilistic onset of cancer with other, extrinsic causes of mortality and use it to predict that intense reproductive competition will lower cancer defences and increase cancer incidence. We explore the trade-off between cancer defences and intraspecific competition across different extrinsic mortality conditions and different levels of trade-off intensity, and find the largest effect of competition on cancer in species where low extrinsic mortality combines with strong trade-offs. In such species, selection to delay cancer and selection to outcompete conspecifics are both strong, and the latter conflicts with the former. We discuss evidence for the assumed trade-off between reproductive competitiveness and cancer susceptibility. Sexually selected traits such as ornaments or large body size require high levels of cell proliferation and appear to be associated with greater cancer susceptibility. Similar associations exist for female traits such as continuous egg-laying in domestic hens and earlier reproductive maturity. Trade-offs between reproduction and cancer defences may be instantiated by a variety of mechanisms, including higher levels of growth factors and hormones, less efficient cell-cycle control and less DNA repair, or simply a larger number of cell divisions (relevant when reproductive success requires large body size or rapid reproductive cycles). These mechanisms can affect intra- and interspecific variation in cancer susceptibility arising from rapid cell proliferation during reproductive maturation, intrasexual competition and reproduction.     

Sampling scale can cause bias in positive assortative mating estimates: evidence from two intertidal snails

Assortative mating in the wild is commonly estimated by correlating between traits in mating pairs (e.g. the size of males and females). Unfortunately, such an approach may suffer from considerable sampling bias when the distribution of different expressions of a trait in the wild is nonrandom (e.g. when segregation of different size classes of individuals occurs in different microhabitats or areas). Consequently, any observed trait correlation in the wild can be an artefact of pooling heterogeneous samples of mating pairs from different microhabitats or areas rather than true nonrandom matings. This bias in estimating trait correlations as a result of sampling scale is termed the scale‐of‐choice effect (SCE). In the present study, we use two intertidal littorinid species from Hong Kong to show how the SCE can bias size‐assortative mating estimates from mating pairs captured in the wild, empirically demonstrating the influence of this effect on measures of positive assortative mating. This finding cautions that studies overlooking the SCE may have misinterpreted the magnitude and the cause of assortative mating, and we provide a new analytical approach for protecting against this potential bias in future studies.     

Sexual dimorphism and mating systems: jumping to conclusions

Previous studies have suggested that there is a strong relationship between a high degree of aggressive competition among males for access to fertile females and large body and canine size in males. It has further been suggested that such a relationship among living primates can be used to infer the social organization of extinct primate species from the degree of sexual dimorphism exhibited. Our field studies of patas (Erythrocebus patas) and blue monkeys (Cercopithecus mitis), two species which had previously been characterized as having one-male ‘harem’ group structures, indicate considerable variability in mating systems. We suggest, on the basis of our observations of these species, that factors other than male-male competition (e.g., predation) may also have influenced the degree of dimorphism in primates.     

Factors affecting aggression among females in captive groups of rhesus macaques (Macaca mulatta)

Captive groups of primates often exhibit higher rates of aggression than wild, free-ranging groups. It is important to determine which factors influence aggression in captivity because aggression, particularly intense aggression, can be harmful to animal health and well-being. In this study, we investigated the effect of ground substrate as well as season, rank, age, and group size on rates of agonistic interactions per female in seven captive groups of rhesus macaques (n = 70 females, 1,723 focal samples) at the California National Primate Research Center. Agonistic interactions were divided into three categories: displacements, mild aggression, and intense aggression. Females living in enclosures with gravel substrate were 1.7 times more likely to be involved in intense aggression (e.g. chases and physical contact) than females living in enclosures with grass (Poisson regression model: P < 0.001). High-ranking females were at least 1.3 times more likely to be involved in mild (e.g. threats and lunges) aggression than lower-ranking females (low rank: P = 0.03; mid rank: P = 0.001). Females of all ranks were 1.5-1.9 times more likely to be involved in both intense and mild aggression during the breeding season than other seasons. Age and group size did not affect rates of mild or intense aggression. These findings indicate that although some aggression appears to be natural and unavoidable, i.e. aggression during the breeding season, the well-being of captive macaques can be improved by developing grass substrate in outdoor enclosures.     

Sex Allocation in a Colobine Monkey

In polygynous, sexual dimorphic species with higher variance in male reproductive success compared with females, females are expected to invest more heavily in sons than daughters within the constraints imposed by their physical condition (Science 1973; 179:90). Mothers in good condition, usually those of high rank, should produce more sons than females in poor condition or of low rank. We investigated sex allocation and sex‐biased maternal investment in a population of wild Hanuman langurs using rank and group size as approximations of female physical condition. Our results show that reproductive costs of sons were higher with both significantly longer interbirth intervals following male births and longer lactational periods for sons. Not in all groups did analyses of rank‐dependent sex allocation reveal the expected pattern of high‐ranking mothers producing more sons. However, sex ratio was significantly influenced by group size, with females from larger groups, i.e., in worse physical condition, producing a daughter‐biased sex ratio. In fact, only females of population‐wide superior physical condition can be expected to produce sons, because in Hanuman langurs males disperse and compete population‐wide. Thus, our results support the Trivers–Willard model and may explain the mixed evidence accruing from studies of single groups. We present a graphical model of how group size and dominance‐related differences in energy gain may influence sex allocation under different competitive regimes relative to overall resource availability. Tests of adaptive sex allocation models should consider whether reproductive competition of the preferred sex takes place primarily within a group or within the population.     

More plant biomass results in more offspring production in annuals,or does it?

Competitive ability in plants has been previously measured almost exclusively in terms of traits related to growth (biomass) or plant size. In this study, however, we used a multi‐species competition experiment with six annuals to measure relative competitive ability in terms of reproductive output, i.e. the number of offspring produced for the next generation. Under greenhouse conditions, plants of each species were started in pots from germinating seeds and were grown singly (free of competition) and at high density in both monocultures and in mixtures with all study species. Several traits traditionally regarded as determinants of competitive ability in plants were recorded for each species grown singly, including: seed mass, germination time, early growth rate and potential plant size (biomass and height). Under competition, several traits were recorded as indicators of relative performance in both monocultures and mixtures, including: biomass of survivors, total number of survivors, number of reproductive survivors, and reproductive output (total seed production) of the survivors. As expected, species that grew to a larger biomass in isolation had higher seed production in isolation. However, none of the traditional plant growth/size‐related traits, measured either in isolation or under competition, could predict between species variation in reproductive output under competition in either monocultures or mixtures. In mixtures, 97% of this variation in reproductive output could be explained by between‐species variation in the number of reproductive survivors. The results indicate that traits measured on plants grown singly may be poor predictors of reproductive output under competition, and that species’ rank order of competitive ability in terms of the biomass of survivors may bear no relationship to their rank order in terms of the number of offspring produced by these survivors. This has important implications for the interpretation of mechanisms of species coexistence and community assembly within vegetation.     

Intracommunity Coalitionary Killing of an Adult Male Chimpanzee at Ngogo,Kibale National Park,Uganda

Intercommunity coalitionary killing of adult and adolescent males has been documented in two chimpanzee communities in the wild, and it was strongly suspected in a third. It may increase survivorship for the attackers, their mates, and their offspring by reducing the combined strength of hostile neighbors and/or by increasing territory size and food availability, and it may help the attackers to attract mates. Lethal coalitionary attacks by males on other male members of their own communities would not provide these benefits and are not expected, given the importance of cooperation among male community members in contests for dominance rank and in both defense and offense against neighboring males. Nevertheless, intracommunity coalitionary killings associated with struggles for alpha rank occur in the wild and in captivity, and observers have seen serious gang attacks on maturing adolescent and young adult males at Mahale and Budongo: the victim in the Budongo case was killed (Fawcett and Muhumuza, 2000). I describe a lethal attack on a young adult male by a large coalition of males from his community at Ngogo, Kibale National Park, Uganda. The Ngogo community is the largest known for chimpanzees and has an unusually large number of males. The attack was not related to a struggle for alpha rank: the victim was low-ranking and the community had a well-established alpha at the time. However, the victim had risen substantially in the male hierarchy over the past few years and might have appeared threatening to many higher-ranking males. Simultaneously, he associated relatively little with most other adult males, had relatively few grooming partners and was not well integrated into the male grooming network, and had no influential allies. The combination of these social factors with the unusual demographic circumstances – which presumably meant that mating competition was relatively high and the cost of losing one male relatively low – might have triggered the attack.     

Sexual selection and canine dimorphism in New World monkeys

Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologist interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys. Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism. Species with fission-fusion social structures and transitory intermale breeding-season competition fall between these extremes. Among platyrrhines there is a significant positive correlation between body size and canine dimorphism However, within levels of competition, no significant correlation was found between the two. Also, with increased body size, body-size dimorphism tends to increase, and this correlation holds in some cases within competition levels. In an analysis of covariance, once the level of intermale competition is controlled for, neither molar size nor molar-size dimorphism accounts for a significant part of the variance in canine dimorphism. A similar analysis using body weight as a measure of size and dimorphism yields a less clear-cut picture: body weight contributes significantly to the model when the effects of the other factors are controlled. Finally, in a model using head and body length as a measure of size and dimorphism, all factors and the interactions between them are significant. We conclude that intermale competition among platyrrhine species is the most important factor explaining variations in canine dimorphism. The significant effects of size and size dimorphism in some models may be evidence that natural (as opposed to sexual) selection also plays a role in the evolution of increased canine dimorphism.     

Egg Size, Intrinsic Competition, and Lethal Interference in the Parasitoids Encarsia pergandiella and Encarsia formosa

Recent population dynamic theory predicts that disruption of biological control may occur when one parasitoid species' superiority in intrinsic competition is associated with a lower ability to find and exploit hosts (i.e., ability in extrinsic competition). One might expect such a trade-off, for instance, if parasitoids with larger (and fewer) eggs are more likely to prevail in intrinsic competition than species with smaller (and more numerous) eggs. We tested the idea that relative egg size could be used to predict the outcome of intrinsic competition in two closely related endoparasitoids, Encarsia pergandiella Howard and Encarsia formosa Gahan. Contrary to expectation, the parasitoid species with smaller eggs, E. pergandiella, prevailed in intrinsic competition, regardless of the order that hosts were exposed to the two species. In a literature survey, we found four studies of competing pairs of endoparasitoid species for which: (a) egg size estimates were available and (b) one species was consistently superior in intrinsic competition. In three of the four studies, the small-egged species prevailed in intrinsic competition, as we also found. Although E. formosa lost in intrinsic competition, this species negatively affected E. pergandiella's progeny production by host feeding on and killing hosts containing E. pergandiella eggs. E. formosa females also host fed on conspecific-parasitized hosts. As a mechanism of both intra- and interspecific interference competition, host feeding on parasitized hosts contradicts assumptions about the nature of interference competition in existing population dynamics models.     

Predation and competition within an assemblage of larval newts (Triturus)

The impact of crested newts (Triturus cristatus) on the smaller-bodied palmate and smooth newts (T helveticus and T vulgaris) was studied during the larval stages using a combination of field and laboratory experiments In pond enclosures T cristatus larvae had no effect on the two smaller species over the first four weeks of development By eight weeks, however. T cristatus had achieved a size advantage which enabled it to eliminate T helveticus and severely reduce T vulgaris by predation In laboratory trials under food-limited conditions, T helveticus and T vulgaris were slightly smaller when raised with T cristatus, suggesting that this predatory effect was complemented by interspecific competition during early development Predation of the smaller species started when T cristatus reached a threshold size of c 27 mm No reciprocal effects on T cristatus growth or survival were observed Although T cristatus may be a significant predator of congeneric species in natural ponds, other factors, such as differences in microhabitat selection, higher-order predator-prey interactions and the occasional desiccation of pond habitats may facilitate coexistence between the species     

Facilitation and competition interact with seed dormancy to affect population dynamics in annual plants

Seed dormancy increases population size via bet-hedging and by limiting negative interactions (e.g., competition) among individuals. On the other hand, individuals also interact positively (e.g., facilitation), and in some systems, facilitation among juveniles precedes competition among adults in the same generation. Nevertheless, studies of the benefits of seed dormancy typically ignore facilitation. Using a population growth model, we ask how the facilitation–competition balance interacts with seed dormancy rate to affect population dynamics in constant and variable environments. Facilitation increases the growth rate and equilibrium size (in both constant and variable environments) and reduces the extinction rate of populations (in a variable environment), and a higher rate of seed dormancy allows populations with facilitation to reach larger sizes. However, the combined benefits of facilitation and a high dormancy rate only occur in large populations. In small populations, weak facilitation does not affect the growth rate, but does induce a weak demographic Allee effect (where population growth decreases with decreasing population size). Our results suggest that facilitation within populations can interact with bet-hedging traits (such as dormancy) or other traits that mediate density to affect population dynamics. Further, by ensuring survival but limiting reproduction, ontogenetic switches from facilitation to competition may enable populations to persist but limit their maximum size in variable environments. Such intrinsic regulation of populations could then contribute to the maintenance of similar species within communities.     

Invasion under a trade-off between density dependence and maximum growth rate

The invasion of alien species and genotypes is an increasing concern in contemporary ecology. A central question is, what life-history traits enable invasion amidst populations of wild species and conventional cultivars? In order to invade, the initially rare species must perform better than their resident competitors. We conducted a mathematical analysis and simulation of a two-species extension of the Maynard Smith and Slatkin model for population dynamics in discrete time to study the role of density dependence as different types of competition in the invasion of new species. The type of density dependence ranged from scramble to contest competition. This led to intrinsic dynamics of the species range from point equilibrium to cycles and chaos. The traits were treated either as free parameters or constrained by a trade-off resulting from a common fixed strength of density dependence or equilibrium density. Resident and intruder traits had up to ten-fold differences in all of the parameters investigated. Higher equilibrium density of the intruder allowed invasion. Under constrained equilibrium density, an intrinsically stable intruder could invade an unstable resident population. Scramble competition made a population more susceptible to invasion than contest competition (e.g., limitation by light or territory availability). This predicts that a population which is mainly limited by food (or nutrients in plants) is more likely to be invaded than a population limited by a hierarchical competition, such as light among plants. The intruder population may have an effect on the resident population's dynamics, which makes the traditional invasion analysis unable to predict invasion outcome.     

Contrasting effects of large density changes on relative testes size in fluctuating populations of sympatric vole species

Across species, there is usually a positive relationship between sperm competition level and male reproductive effort on ejaculates, typically measured using relative testes size (RTS). Within populations, demographic and ecological processes may drastically alter the level of sperm competition and thus, potentially affect the evolution of testes size. Here, we use longitudinal records (across 38 years) from wild sympatric Fennoscandian populations of five species of voles to investigate whether RTS responds to natural fluctuations in population density, i.e. variation in sperm competition risk. We show that for some species RTS increases with density. However, our results also show that this relationship can be reversed in populations with large-scale between-year differences in density. Multiple mechanisms are suggested to explain the negative RTS–density relationship, including testes size response to density-dependent species interactions, an evolutionary response to sperm competition levels that is lagged when density fluctuations are over a certain threshold, or differing investment in pre- and post-copulatory competition at different densities. The results emphasize that our understanding of sperm competition in fluctuating environments is still very limited.     

Cheek pouch use, predation risk, and feeding competition in blue monkeys (Cercopithecus mitis stuhlmanni)

The adaptive function of cheek pouches in the primate subfamily Cercopithecinae remains unresolved. By analyzing the circumstances of cheek pouch use, we tested two hypotheses for the evolution of cercopithecine cheek pouches proposed in earlier studies: (1) cheek pouches reduce vulnerability to predation, and (2) cheek pouches increase feeding efficiency by reducing competition. We studied two groups of wild blue monkeys (Cercopithecus mitis stuhlmanni) in the Kakamega Forest, Kenya, conducting focal observations of feeding individuals. Monkeys were less exposed while emptying their cheek pouches than filling them, supporting the predation-avoidance hypothesis. We investigated several measures of competitive threat, but only one supported the competition-reduction hypothesis: when the nearest neighbor's rank increased, subjects were more likely to increase than to decrease cheek pouch use. Overall, our findings supported the predation-avoidance hypothesis more strongly than the competition-reduction hypothesis. We suggest that variation in cheek pouch use may reflect differing behavioral strategies used by cercopithecines to mitigate competition and predation, as well as factors such as resource size and distribution, home range size, and travel patterns.     

Correlates of Male Consortship Rate in Free-Ranging Rhesus Macaques (Macaca mulatta)

Male–male competition for access to receptive females can take the form of nonrecurring fights and/or a sustained contest over mating opportunities. Male physical condition has been linked to dominance rank and reproductive success in species characterized by intrasexual fights for dominance and access to females. In group-living species characterized by endurance rivalry, however, factors contributing to male reproductive success are less well understood. In such species, particularly seasonally breeding ones with low sexual dimorphism and seniority-based rank, age and social factors other than rank may prove important. In the absence of genetic data, male mate guarding or consortship can serve as an indicator of male reproductive success. To evaluate the contribution of age and intragroup sociality to male consortship rate, I collected behavioral data during one nonmating and one mating season in two social groups of free-ranging rhesus macaques that experience no predation or food scarcity. Higher-ranking males, younger males, or males that exhibited lower rate of intrasexual aggression had higher consortship rates. Male–female dyads that groomed outside consortship did not form consortships as often as dyads that did not engage in nonconsort grooming. This is the first study to identify the significance of male–male aggression and male–female affiliation to male consortship rate in a species characterized by endurance rivalry, high male rank stability, and strong female mate choice. Social behaviors and male age may be particularly important in determining male reproductive success in populations experiencing high food availability and a lack of predation, which are typical of an increasing number of vertebrates in areas densely populated by humans.     

Experimental excursions on adaptive landscapes: density-dependent selection on egg size

Abstract. Theories of density-dependent natural selection suggest that intraspecific competition will favor juveniles of high competitive ability. Empirical evidence has been provided from laboratory selection experiments, but field studies are lacking due to the logistical difficulties of experimentally manipulating population densities in natural settings. Here, we present data from a decade-long experimental field study of side-blotched lizards, Uta stansburiana that overcomes these difficulties. We tested the hypothesis that density-dependent natural selection causes egg size to increase from early to late clutches in this and many other species. Using a novel combination of environmental manipulations of hatchling density and phenotypic manipulations of egg size, we demonstrate that the nature of selection on egg size changes dramatically in the absence of older competitors. The strength of selection on egg size among later-clutch hatchlings released in areas without competitors from early clutches became almost doubled in magnitude, compared to that among hatchlings released in the presence of older competitors. These experimental findings demonstrate density-dependent natural selection on egg size; however, they contradict the classical idea that egg size increases during the reproductive season because of competition between early and late hatchlings. The results indicate that competitive age or size asymmetries between early and late hatchlings can override within-cohort asymmetries due to egg size. We suggest that competition could be an important mediator of oscillating selection pressures in this and other systems. Finally, we discuss the utility of "double-level," simultaneous experimental manipulation of both phenotypic traits that are targets of selection (e.g., egg size) as well the environmental agents of selection (e.g., population density).     

RUNAWAY EVOLUTION TO SELF-EXTINCTION UNDER ASYMMETRICAL COMPETITION

We analyze a popular model of the evolution of traits related to performance in exploitative competition. This model has previously been used to explain a mechanism by which interspecific competition can cause taxon cycles. We show that purely intraspecific competition can cause evolution of extreme competitive abilities that ultimately result in extinction, without any influence from other species. The only change in the model required for this outcome is the assumption of a nonnormal distribution of resources of different sizes measured on a logarithmic scale. This suggests that taxon cycles, if they exist, may be driven by within- rather than between-species competition. Self-extinction does not occur when the advantage conferred by a large value of the competitive trait (e.g., size) is relatively small, or when the carrying capacity decreases at a comparatively rapid rate with increases in trait value. The evidence regarding these assumptions is discussed. The results suggest a need for more data on resource distributions and size-advantage in order to understand the evolution of competitive traits such as body size.