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PubMed2003

Overt and covert competition in a promiscuous mammal: the importance of weaponry and testes size to male reproductive success.

Male contests for access to receptive females are thought to have selected for the larger male body size and conspicuous weaponry frequently observed in mammalian species. However, when females copulate with multiple males within an oestrus, male reproductive success is a function of both pre- and postcopulatory strategies. The relative importance of these overt and covert forms of sexual competition has rarely been assessed in wild populations. The Soay sheep mating system is characterized by male contests for mating opportunities and high female promiscuity. We find that greater horn length, body size and good condition each independently influence a male's ability to monopolize receptive females. For males with large horns at least, this behavioural success translates into greater siring success. Consistent with sperm-competition theory, we also find that larger testes are independently associated with both higher copulation rates and increased siring success. This advantage of larger testes emerges, and strengthens, as the number of oestrous females increases, as dominant males can no longer control access to them all. Our results thus provide direct quantitative evidence that male reproductive success in wild populations of mammals is dependent upon the relative magnitude of both overt contest competition and covert sperm competition.

Animals↗

PubMed2003

Competition asymmetry with taxon divergence.

Most organisms experience competition for resources, probably most of the time. As the structure and requirements of closely related species are generally liable to be more similar than in distantly linked species, Darwin suggested that the potential for competition was greater in the former. Since that time, studies have concentrated on interactions of either conspecifics or congeneric species. Shared critical resources, which organisms compete for, are generally mates, food and space (for access to the former). Whilst mates are valued only within species, in that the definition of a species requires it so, both food and space have the potential to be shared by very different organisms. It is now clear that vertebrates may compete with remotely related species: e.g. with squid for krill and with insects for nectar or seeds. Diamond suggested that (i) mutual aggression, (ii) displacement and (iii) evolutionary change in morphology would be increasingly asymmetric with competitor dissimilarity. Thus, with increasing taxonomic distance between two competitors (A and B), increasing aggression is exhibited between them and, increasingly, one consistently displaces the other. Here, Darwin's suggestion and Diamond's first two theories are tested across a taxonomic spectrum for the first time to the best of the author's knowledge. The proportion of spatial competitors in two different marine invertebrate groups demonstrating mutual aggression and displacement increases with taxon divergence (Nei's genetic identity). Congenerics were twice as likely to fight as conspecifics, and confamilial competitors were three times as likely to fight as conspecifics. This relationship seems robust to taxonomic and environmental variability. Competitors do not need to be as distant as birds and bees for complete asymmetry, a different family seems sufficient.

Aggression↗

PubMed2003

Multiple mating and sequential mate choice in guppies: females trade up.

The trade-up hypothesis outlines a behavioural strategy that females could use to maximize the genetic benefits to their offspring. The hypothesis proposes that females should be more willing to accept a mate when the new male encountered is a superior genetic source to previous mates. We provide a direct test of the trade-up hypothesis using guppies (Poecilia reticulata), and evaluate both behavioural and paternity data. Virgin female guppies were presented sequentially with two males of varying attractiveness, and their responsiveness to each male was quantified. Male attractiveness (ornamentation) was scored as the amount of orange coloration on their body. Females were generally less responsive to second-encountered males, yet responsiveness to second males was an increasing function of male ornamentation. These attractive second males also sired a greater proportion of the offspring. There was an overall tendency for last-male advantage in paternity, and this advantage was most exaggerated when the second male was more ornamented than the first. Finally, we found that our estimate of relative sperm number did not account for any significant variation in paternity. Our results suggest that female guppies may use pre-copulatory mechanisms to maximize the genetic quality of their offspring.

Animals↗

PubMed2003

Sibling competition and the evolution of prenatal development rates.

Sibling competition has been proposed as an important evolutionary pressure driving interspecific variation in developmental rates. We tested this hypothesis using rates of extra-pair paternity and brood parasitism, as well as the degree of hatching asynchrony, as indices of sibling competition in a comparative analysis of 70 species of bird. We found mixed support for the influence of sibling competition on prenatal development. Brood parasitism was negatively correlated with the length of incubation, and hatching asynchrony was positively correlated with the length of incubation, but both correlations disappeared when phylogeny was controlled for. Extra-pair paternity, however, was negatively correlated with incubation length even when phylogeny was controlled for. The latter could represent support for the influence of sibling competition on prenatal development or indirect effects of correlated selection on both traits by adult mortality. The existence of these correlations demonstrates that life-history strategies include linkages among a larger suite of traits than previously recognized.

Animals↗

PubMed2003

Sexual conflicts in spotted hyenas: male and female mating tactics and their reproductive outcome with respect to age, social status and tenure.

We investigated the reproductive outcomes of male and female mating tactics in the spotted hyena, Crocuta crocuta, a female-dominated social carnivore with high maternal investment, an absence of paternal care and female control over copulation. Paternity was determined using microsatellite profiling of 236 offspring in 171 litters from three clans. We found little evidence that male tactics that sought to coerce or monopolize females were successful. Polyandry and sperm competition appeared to counter effectively pre-copulatory male tactics, such as harassment, monopolization and other tactics, such as infanticide, that were against the evolutionary interests of females, and may have contributed to the stability of the male dominance hierarchy, which operated as a social queue. At least 39% of 54 females mated multiply, and 35% of 75 twin litters were fathered by two sires. Polyandry may also serve to ensure fertilization, compensate for an initial poor-quality mate or ensure fertilization by genetically compatible mates. Female mate choice matched observed patterns of affiliative male-female behaviour, indicating that affiliative behaviour is a successful male mating tactic, and was consistent with the idea that male tenure may serve as an index of male quality, although male fertility may decline with extreme old age.

Aging↗

PubMed2003

Influence of male relatedness on lethal combat in fig wasps: a theoretical analysis.

In many fig wasp species, flightless males fatally fight with rivals. In a recent comparative analysis, West and colleagues found no influence of male relatedness on the frequency of fatal fighting. Inspired by this study, I used a simple theoretical model to examine the conditions that should influence the probability of fatal fighting in a closed mating system. I show that, without kin recognition, relatedness can be expected to have no influence on the frequency of fatal fighting. Under such conditions, males should fight if their chance of winning is greater than a threshold determined by the number of competitors. If males recognize kin, a non-linear relationship between relatedness and the frequency of fatal fighting can be expected. However, two other factors should also be important when there is kin recognition: male number and variance in male fighting strength can be shown to have decisive influences on the frequency of fatal fighting. My results thus corroborate and explain one finding of the empirical study--that there is no significant influence of relatedness on fatal fighting--but point to a role for other factors besides foundress number--male number and male variance in fighting strength. These are probably important factors determining the occurrence of lethal combat in fig wasps.

Aggression↗

PubMed2003

Detecting sexually antagonistic coevolution with population crosses.

The result of population crosses on traits such as mating rate, oviposition rate and survivorship are increasingly used to distinguish between modes of coevolution between the sexes. Two key hypotheses, erected from a verbal theory of sexually antagonistic coevolution, have been the subject of several recent tests. First, statistical interactions arising in population crosses are suggested to be indicative of a complex signal/receiver system. In the case of oviposition rates, an interaction between populations (x, y and z) would be indicated by the rank order of female oviposition rates achieved by x, y and z males changing depending upon the female (x, y or z) with which they mated. Second, under sexually antagonistic coevolution females will do 'best' when mated with their own males, where best is defined by the weakest response to the signal and the highest fitness. We test these hypotheses by crossing strains generated from a formal model of sexually antagonistic coevolution. Strains differ in the strength of natural selection acting on male and female traits. In our model, we assume sexually antagonistic coevolution of a single male signal and female receptor. The female receptor is treated as a preference function where both the slope and intercept of the function can evolve. Our results suggest that neither prediction is consistently supported. Interactions are not diagnostic of complex signal-receiver systems, and even under sexually antagonistic coevolution, females may do better mating with males of strains other than their own. These results suggest a reinterpretation of several recent experiments and have important implications for developing theories of speciation when sexually antagonistic coevolution is involved.

Animals↗

PubMed2003

Absence of nepotism in the harassment of duelling queens by honeybee workers.

Nepotism shapes interactions among the members of almost every animal society. However, clear evidence of nepotism within highly cooperative insect societies, such as ant, wasp and honeybee colonies, is rare. Recent empirical findings suggest that nepotism occurs within honeybee colonies where kin-selection theory most strongly predicts its existence: during the lethal queen-queen duels that determine which of several young queens will become the colony's next queen. In this study, I test whether worker bees act nepotistically by hindering duelling queens that are distantly related to themselves. I accomplished this by observing labelled workers harassing duelling queen bees in observation hives and subsequently by determining worker-queen relatedness using DNA microsatellites. I show that the workers that harassed duelling queens were neither more-closely nor more-distantly related to them than were workers selected randomly from the colony. Thus, workers did not behave nepotistically by hindering half-sister queens more than full-sister queens. These results demonstrate that under certain conditions, natural selection limits the evolution of nepotism within animal societies despite strong theoretical predictions for its existence.

Aggression↗

PubMed2003

Soay rams target reproductive activity towards promiscuous females' optimal insemination period.

Female promiscuity is thought to have resulted in the evolution of male behaviours that confer advantages in the sperm competition that ensues. In mammalian species, males can gain a post-copulatory advantage in this sperm 'raffle' by inseminating females at the optimal time relative to ovulation, leading to the prediction that males should preferentially associate and copulate with females at these times. To the best of our knowledge, we provide the first high-resolution test of this prediction using feral Soay sheep, which have a mating system characterized by male competition for access to highly promiscuous females. We find that competitive males time their mate guarding (and hence copulations) to occur close to the optimal insemination period (OIP), when females are also increasingly likely to 'cooperate' with copulation attempts. Subordinate males practice an alternative mating tactic, where they break the integrity of the consort pair and force copulations on females. The timing of these forced copulations is also targeted towards the OIP. We thus provide quantitative evidence that female promiscuity has resulted in the evolution of reproductive strategies in which males 'load' the sperm raffle by targeting their mating activity towards female OIPs, when the probability of sperm-competition success is at its greatest.

Animals↗

PubMed2003

Competition and predation in simple food webs: intermediately strong trade-offs maximize coexistence.

Competition and predation are fundamental interactions structuring food webs. However, rather than always following these neat theoretical categories, mixed interactions are ubiquitous in nature. Of particular importance are omnivorous species, such as intra-guild predators that can both compete with and predate on their prey. Here, we examine trade-offs between competitive and predatory capacities by analysing the entire continuum of food web configurations existing between purely predator-prey and purely competitive interactions of two consumers subsisting on a single resource. Our results show that the range of conditions allowing for coexistence of the consumers is maximized at intermediately strong trade-offs. Even though coexistence under weak trade-offs and under very strong trade-offs is also possible, it occurs under much more restrictive conditions. We explain these findings by an intricate interplay between energy acquisition and interaction strength.

Animals↗

PubMed2003

A fitness cost of learning ability in Drosophila melanogaster.

Maintenance of substantial genetic variation for learning ability in many animal populations suggests that learning ability has fitness costs, but there is little empirical evidence for them. In this paper, we demonstrate an evolutionary trade-off between learning ability and competitive ability in Drosophila melanogaster. We show that the evolution of an improved learning ability in replicated experimental fly populations has been consistently associated with a decline of larval competitive ability, compared with replicated control populations. The competitive ability was not affected by crossing of the replicate populations within each selection regime, excluding differential inbreeding as a potential confounding factor. Our results provide evidence for a constitutive fitness cost of learning ability, i.e. one that is paid irrespective of whether or not the learning ability is actually used.

Animals↗

PubMed2004

Post-copulatory sexual selection and female fitness in Scathophaga stercoraria.

Whether sexual selection increases or decreases female fitness is determined by the occurrence and relative importance of sexual-conflict processes and the ability of females to choose high-quality males. Experimentally enforced polyandry and monogamy have previously been shown to cause rapid evolution in the yellow dung fly Scathophaga stercoraria. Flies from polyandrous lines invested more in reproductive tissue, and this investment influenced paternity in sperm competition, but came at a cost to immune function. While some fitness consequences of enforced polyandry or monogamy have been examined when flies mate multiply, the consequences for female fitness when singly copulated remain unexplored. Under a good-genes scenario females from polyandrous lines should be of higher general quality and should outperform females from monogamous lines even with a single copulation. Under sexual conflict, costly adaptations will afford no advantages when females are allowed to mate only once. We investigate the lifetime reproductive success and longevity of females evolving under enforced monogamy or polyandry when mating once with males from these selection regimes. Females from polyandrous lines were found to have lower fitness than their monogamous counterparts when mating once. They died earlier and produced significantly fewer eggs and offspring. These results suggest that sexual conflict probably drove evolution under enforced polyandry as female fitness did not increase overall as expected with purely good-genes effects.

Animals↗

PubMed2004

Virulence reaction norms across a food gradient.

Host-parasite interactions involve competition for nutritional resources between hosts and the parasites growing within them. Consuming part of a host's resources is one cause of a parasite's virulence, i.e. part of the fitness cost imposed on the host by the parasite. The influence of a host's nutritional conditions on the virulence of a parasite was experimentally tested using the mosquito Aedes aegypti and the microsporidian parasite Vavraia culicis. A condition-dependent expression of virulence was found and a positive relation between virulence and transmissibility was established. Spore production was positively influenced by host food availability, indicating that the parasite's within-host growth is limited by host condition. We also investigated how the fitness of each partner varied across the nutritional gradient and demonstrated that the sign of the correlation between host fitness and parasite fitness depended on the amount of nutritional resources available to the host.

Aedes↗

PubMed2004

The influence of contests on optimal clutch size: a game-theoretic model.

We develop a game-theoretic model to predict the effect of size-dependent contest outcomes on optimal-clutch-size decisions. We consider the case where larger individuals develop from smaller clutches and, as adults, are advantaged in competition for limiting resources. The relationship between fitness and size thus depends on the sizes of other members of the population. We show that clutch-size optima are decreased by body-size-dependent contest outcomes, with larger effects when body size is most affected by clutch size, when prior resource ownership has less influence on contest outcome and when contests occur more frequently. We also show the existence of polymorphisms in clutch-size optima and that clutch-size driven changes in population density can, via an effect on the probability of host finding, further influence optimal clutch size. Our model is formulated to match the life history of a parasitoid wasp, in which clutch size affects offspring size and females engage in direct contests for host ownership, which larger females tend to win; we confirm that female-female competition is likely to influence clutch size in this species. However, the model is also relevant to clutch size in other taxa and supports recent suggestions concerning reproductive decisions in great tits.

Animals↗

PubMed2004

Spatial variation and density-dependent dispersal in competitive coexistence.

It is well known that dispersal from localities favourable to a species' growth and reproduction (sources) can prevent competitive exclusion in unfavourable localities (sinks). What is perhaps less well known is that too much emigration can undermine the viability of sources and cause regional competitive exclusion. Here, I investigate two biological mechanisms that reduce the cost of dispersal to source communities. The first involves increasing the spatial variation in the strength of competition such that sources can withstand high rates of emigration; the second involves reducing emigration from sources via density-dependent dispersal. I compare how different forms of spatial variation and modes of dispersal influence source viability, and hence source-sink coexistence, under dominance and pre-emptive competition. A key finding is that, while spatial variation substantially reduces dispersal costs under both types of competition, density-dependent dispersal does so only under dominance competition. For instance, when spatial variation in the strength of competition is high, coexistence is possible (regardless of the type of competition) even when sources experience high emigration rates; when spatial variation is low, coexistence is restricted even under low emigration rates. Under dominance competition, density-dependent dispersal has a strong effect on coexistence. For instance, when the emigration rate increases with density at an accelerating rate (Type III density-dependent dispersal), coexistence is possible even when spatial variation is quite low; when the emigration rate increases with density at a decelerating rate (Type II density-dependent dispersal), coexistence is restricted even when spatial variation is quite high. Under pre-emptive competition, density-dependent dispersal has only a marginal effect on coexistence. Thus, the diversity-reducing effects of high dispersal rates persist under pre-emptive competition even when dispersal is density dependent, but can be significantly mitigated under dominance competition if density-dependent dispersal is Type III rather than Type II. These results lead to testable predictions about source-sink coexistence under different regimes of competition, spatial variation and dispersal. They identify situations in which density-independent dispersal provides a reasonable approximation to species' dispersal patterns, and those under which consideration of density-dependent dispersal is crucial to predicting long-term coexistence.

Animals↗

PubMed2004

Life-history trade-offs and ecological dynamics in the evolution of longevity.

Longevity is a life-history trait that is shaped by natural selection. An unexplored consequence is how selection on this trait affects diversity and diversification in species assemblages. Motivated by the diverse rockfish (Sebastes) assemblage in the North Pacific, the effects of trade-offs in longevity against competitive ability are explored. A competition model is developed and used to explore the potential for species diversification and coexistence. Invasion analyses highlight that life-history trait trade-offs in longevity can mitigate the effects of competitive ability and favour the coexistence of a finite number of species. Our results have implications for niche differentiation, limiting similarity and assembly dynamics in multispecies interactions.

Animals↗

PubMed2004

Male-male competition and nuptial-colour displacement as a diversifying force in Lake Victoria cichlid fishes.

We propose a new mechanism for diversification of male nuptial-colour patterns in the rapidly speciating cichlid fishes of Lake Victoria. Sympatric closely related species often display nuptial colours at opposite ends of the spectrum with males either blue or yellow to red. Colour polymorphisms within single populations are common too. We propose that competition between males for breeding sites promotes such colour diversification, and thereby speciation. We hypothesize that male aggression is primarily directed towards males of the common colour, and that rare colour morphs enjoy a negatively frequency-dependent fitness advantage. We test our hypothesis with a large dataset on the distributions and nuptial colorations of 52 species on 47 habitat islands in Lake Victoria, and with a smaller dataset on the within-spawning-site distributions of males with different coloration. We report that territories of males of the same colour are negatively associated on the spawning site, and that the distribution of closely related species over habitat islands is determined by nuptial coloration in the fashion predicted by our hypothesis. Whereas among unrelated species those with similar nuptial colour are positively associated, among closely related species those with similar colour are negatively associated and those with different colour are positively associated. This implies that negatively frequency-dependent selection on nuptial coloration among closely related species is a sufficiently strong force to override other effects on species distributions. We suggest that male-male competition is an important and previously neglected agent of diversification among haplochromine cichlid fishes.

Aggression↗

PubMed2004

Risk allocation and competition in foraging groups: reversed effects of competition if group size varies under risk of predation.

Animals often feed more quickly when in larger groups. This group-size effect is often explained by safety advantages for groups but an alternative explanation is that animals feed faster in larger groups because of greater scramble competition for limited food. We show that predation risk enhances the group-size effect if groups vary in size. By contrast, competition leads to the group-size effect only when individuals feed in groups of constant size. When individuals feed in groups that vary in size, the best strategy for dealing with competition is to feed intensely when in smaller groups and feed little when in larger (more competitive) groups. In all situations, the effects of competition interact with the effects of predation risk in a simple multiplicative way. Our results suggest that scramble competition is not a general explanation for the group-size effect on vigilance in situations where group size changes relatively rapidly.

Animals↗