Interspecific competition during transmission of two sympatric malaria parasite species to the mosquito vector.
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The role of species interactions in structuring parasite communities remains controversial. Here, we show that interspecific competition between two avian malaria parasite species, Plasmodium gallinaceum and P. juxtanucleare, occurs as a result of interference during parasite fertilization within the bloodmeal of the mosquito. The significant reduction in the transmission success of P. gallinaceum to mosquitoes, due to the co-infecting P. juxtanucleare, is predicted to have compromised its colonization of regions occupied by P. juxtanucleare and, thus, may have contributed to the restricted global distribution of P. gallinaceum. Such interspecies interactions may occur between human malaria parasites and, thus, impact upon parasite species epidemiology, especially in regions of seasonal transmission. (+info)
Kin competition within groups: the offspring depreciation hypothesis.
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Where relatives compete for the same resources (kin competition) and each obtains an equal share, this can favour the evolution of elevated dispersal rates, such that most resource competition is among non-relatives. We show that this effect evaporates as among-sibling dominance increases to the point where the allocation of resources is maximally unequal. We restore a kin-competition effect on emigration rates from dominance-ranked family groups by showing that where siblings form queues to inherit the breeding positions, the length of the queue affects the fitness of all individuals by depreciating the rank of subsequent offspring. Incorporating this 'offspring depreciation' effect decreases optimal queue lengths, increases dispersal rates and explains the otherwise paradoxical use of sinks by cooperatively breeding birds in stable environments. The offspring depreciation effect also favours the evolution of small, but consistent, clutch sizes and high reproductive skew, but constrains the evolution of alloparenting. (+info)
Sibling competition and the evolution of prenatal development rates.
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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. (+info)
Competition asymmetry with taxon divergence.
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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. (+info)
Overt and covert competition in a promiscuous mammal: the importance of weaponry and testes size to male reproductive success.
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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. (+info)
Heart rate responses of women aged 23-67 years during competitive orienteering.
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OBJECTIVES: To compare the heart rate responses of women orienteers of different standards and to assess any relation between heart rate responses and age. METHODS: Eighteen competitive women orienteers completed the study. They were divided into two groups: eight national standard orienteers (ages 23-67 years); 10 club standard orienteers (ages 24-67 years). Each participant had her heart rate monitored during a race recognised by the British Orienteering Federation. Peak heart rate (HR(PEAK)), mean heart rate (HR(MEAN)), standard deviation of her heart rate during each orienteering race (HR(SD)), and mean change in heart rate at each control point (DeltaHR(CONTROL)) were identified. The data were analysed using analysis of covariance with age as a covariate. RESULTS: National standard orienteers displayed a lower within orienteering race standard deviation in heart rate (6 (2) v 12 (2) beats/min, p<0.001) and a lower DeltaHR(CONTROL) (5 (1) v 17 (4) beats/min, p<0.001). The mean heart rate during competition was higher in the national standard group (170 (11) v 158 (11) beats/min, p = 0.025). The HR(MEAN) for the national and club standard groups were 99 (8)% and 88 (9)% of their age predicted maximum heart rate (220-age) respectively. All orienteers aged >55 years (n = 4) recorded HR(MEAN) greater than their age predicted maximum. CONCLUSIONS: The heart rate responses indicate that national and club standard women orienteers of all ages participate in the sport at a vigorous intensity. The higher DeltaHR(CONTROL) of club standard orienteers is probably due to failing to plan ahead before arriving at the controls and this, coupled with slowing down to navigate or relocate when lost, produced a higher HR(SD). (+info)
Sexual conflicts in spotted hyenas: male and female mating tactics and their reproductive outcome with respect to age, social status and tenure.
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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. (+info)
Influence of male relatedness on lethal combat in fig wasps: a theoretical analysis.
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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. (+info)