Showing papers in "Proceedings of The Royal Society B: Biological Sciences in 2004"

Tolerance of pollination networks to species extinctions

Abstract: Mutually beneficial interactions between flowering plants and animal pollinators represent a critical 'ecosystem service' under threat of anthropogenic extinction. We explored probable patterns of extinction in two large networks of plants and flower visitors by simulating the removal of pollinators and consequent loss of the plants that depend upon them for reproduction. For each network, we removed pollinators at random, systematically from least-linked (most specialized) to most-linked (most generalized), and systematically from most- to least-linked. Plant species diversity declined most rapidly with preferential removal of the most-linked pollinators, but declines were no worse than linear. This relative tolerance to extinction derives from redundancy in pollinators per plant and from nested topology of the networks. Tolerance in pollination networks contrasts with catastrophic declines reported from standard food webs. The discrepancy may be a result of the method used: previous studies removed species from multiple trophic levels based only on their linkage, whereas our preferential removal of pollinators reflects their greater risk of extinction relative to that of plants. In both pollination networks, the most-linked pollinators were bumble-bees and some solitary bees. These animals should receive special attention in efforts to conserve temperate pollination systems.

Fitness consequences of avian personalities in a fluctuating environment

Abstract: Individual animals differ in the way they cope with challenges in their environment, comparable with variation in human personalities. The proximate basis of variation in personality traits has received considerable attention, and one general finding is that personality traits have a substantial genetic basis. This poses the question of how variation in personality is maintained in natural populations. We show that selection on a personality trait with high heritability fluctuates across years within a natural bird population. Annual adult survival was related to this personality trait (behaviour in novel environments) but the effects were always opposite for males and females, and reversed between years. The number of offspring surviving to breeding was also related to their parents' personalities, and again selection changed between years. The observed annual changes in selection pressures coincided with changes in environmental conditions (masting of beeches) that affect the competitive regimes of the birds. We expect that the observed fluctuations in environmental factors lead to fluctuations in competition for space and food, and these, in association with variations in population density, lead to a variation in selection pressure, which maintains genetic variation in personalities.

Identifying the role that animals play in their social networks.

Abstract: Techniques recently developed for the analysis of human social networks are applied to the social network of bottlenose dolphins living in Doubtful Sound, New Zealand. We identify communities and subcommunities within the dolphin population and present evidence that sex– and age–related homophily play a role in the formation of clusters of preferred companionship. We also identify brokers who act as links between sub–communities and who appear to be crucial to the social cohesion of the population as a whole. The network is found to be similar to human social networks in some respects but different in some others, such as the level of assortative mixing by degree within the population. This difference elucidates some of the means by which the network forms and evolves.

Evidence that disgust evolved to protect from risk of disease.

Abstract: Disgust is a powerful human emotion that has been little studied until recently. Current theories do not coherently explain the purpose of disgust, nor why a wide range of stimuli can provoke a similar emotional response. Over 40 000 individuals completed a web-based survey using photo stimuli. Images of objects holding a potential disease threat were reported as significantly more disgusting than similar images with little or no disease relevance. This pattern of response was found across all regions of the world. Females reported higher disgust sensitivity than males; there was a constant decline in disgust sensitivity over the life course; and the bodily fluids of strangers were found more disgusting than those of close relatives. These data provide evidence that the human disgust emotion may be an evolved response to objects in the environment that represent threats of infectious disease.

Tropical winter habitat limits reproductive success on the temperate breeding grounds in a migratory bird.

Abstract: Identifying the factors that control population dynamics in migratory animals has been constrained by our inability to track individuals throughout the annual cycle. Using stable carbon isotopes, w...

Do sexual ornaments demonstrate heightened condition-dependent expression as predicted by the handicap hypothesis?

Abstract: The handicap hypothesis of sexual selection predicts that sexual ornaments have evolved heightened condition-dependent expression. The prediction has only recently been subject to experimental investigation. Many of the experiments are of limited value as they: (i) fail to compare condition dependence in sexual ornaments with suitable non-sexual trait controls; (ii) do not adequately account for body size variation; and (iii) typically consider no stress and extreme stress manipulations rather than a range of stresses similar to those experienced in nature. There is also a dearth of experimental studies investigating the genetic basis of condition dependence. Despite the common claim that sexual ornaments are condition-dependent, the unexpected conclusion from our literature review is that there is little support from well-designed experiments.

Advantages of having a lateralized brain.

Abstract: Brain lateralization is common among vertebrates. However, despite its implications for higher-order cognitive functions, almost no empirical evidence has been provided to show that it may confer any advantage to the functioning of the brain. Here, we show in the domestic chick (Gallus gallus domesticus) that cerebral lateralization is associated with an enhanced ability to perform two tasks simultaneously: finding food and being vigilant for predators. This finding suggests that cerebral lateralization enhances brain efficiency in cognitive tasks that demand the simultaneous but different use of both hemispheres.

The role of subspecies in obscuring avian biological diversity and misleading conservation policy

Abstract: Subspecies are often used in ways that require their evolutionary independence, for example as proxies for units of conservation. Mitochondrial DNA sequence data reveal that 97% of continentally distributed avian subspecies lack the population genetic structure indicative of a distinct evolutionary unit. Subspecies considered threatened or endangered, some of which have been targets of expensive restoration efforts, also generally lack genetic distinctiveness. Although sequence data show that species include 1.9 historically significant units on average, these units are not reflected by current subspecies nomenclature. Yet, it is these unnamed units and not named subspecies that should play a major role in guiding conservation efforts and in identifying biological diversity. Thus, a massive reorganization of classifications is required so that the lowest ranks, be they species or subspecies, reflect evolutionary diversity. Until such reorganization is accomplished, the subspecies rank will continue to hinder progress in taxonomy, evolutionary studies and especially conservation.

Rapid population decline in red knots: fitness consequences of decreased refuelling rates and late arrival in Delaware Bay

Abstract: Most populations of migrant shorebirds around the world are in serious decline, suggesting that vital condition-dependent rates such as fecundity and annual survival are being affected globally. A striking example is the red knot (Calidris canutus rufa) population wintering in Tierra del Fuego, which undertakes marathon 30,000 km hemispheric migrations annually. In spring, migrant birds forage voraciously on horseshoe crab eggs in Delaware Bay in the eastern USA before departing to breed in Arctic polar deserts. From 1997 to 2002 an increasing proportion of knots failed to reach threshold departure masses of 180-200 g, possibly because of later arrival in the Bay and food shortage from concurrent over-harvesting of crabs. Reduced nutrient storage, especially in late-arriving birds, possibly combined with reduced sizes of intestine and liver during refuelling, had severe fitness consequences for adult survival and recruitment of young in 2000-2002. From 1997 to 2002 known survivors in Delaware Bay were heavier at initial capture than birds never seen again, annual survival of adults decreased by 37% between May 2000 and May 2001, and the number of second-year birds in wintering flocks declined by 47%. Population size in Tierra del Fuego declined alarmingly from 51,000 to 27,000 in 2000-2002, seriously threatening the viability of this subspecies. Demographic modelling predicts imminent endangerment and an increased risk of extinction of the subspecies without urgent risk-averse management.

Large–scale geographical variation confirms that climate change causes birds to lay earlier

Abstract: Advances in the phenology of organisms are often attributed to climate change, but alternatively, may reflect a publication bias towards advances and may be caused by environmental factors unrelated to climate change. Both factors are investigated using the breeding dates of 25 long-term studied populations of Ficedula flycatchers across Europe. Trends in spring temperature varied markedly between study sites, and across populations the advancement of laying date was stronger in areas where the spring temperatures increased more, giving support to the theory that climate change causally affects breeding date advancement.

Realized heritability and repeatability of risk-taking behaviour in relation to avian personalities

Abstract: Personalities are general properties of humans and other animals. Different personality traits are phenotypically correlated, and heritabilities of personality traits have been reported in humans and various animals. In great tits, consistent heritable differences have been found in relation to exploration, which is correlated with various other personality traits. In this paper, we investigate whether or not risk-taking behaviour is part of these avian personalities. We found that (i) risk-taking behaviour is repeatable and correlated with exploratory behaviour in wild-caught hand-reared birds, (ii) in a bi-directional selection experiment on ‘fast’ and ‘slow’ early exploratory behaviour, bird lines tend to differ in risk-taking behaviour, and (iii) within-nest variation of risk-taking behaviour is smaller than between-nest variation. To show that risk-taking behaviour has a genetic component in a natural bird population, we bred great tits in the laboratory and artificially selected ‘high’ and ‘low’ risk-taking behaviour for two generations. Here, we report a realized heritability of 19.3 ± 3.3% (s.e.m.) for risk-taking behaviour. With these results we show in several ways that risk-taking behaviour is linked to exploratory behaviour, and we therefore have evidence for the existence of avian personalities. Moreover, we prove that there is heritable variation in more than one correlated personality trait in a natural population, which demonstrates the potential for correlated evolution.

Changing partners in the dark: isotopic and molecular evidence of ectomycorrhizal liaisons between forest orchids and trees.

Abstract: In the mycorrhizal symbiosis, plants exchange photosynthates for mineral nutrients acquired by fungi from the soil. This mutualistic arrangement has been subverted by hundreds of mycorrhizal plant species that lack the ability to photosynthesize. The most numerous examples of this behaviour are found in the largest plant family, the Orchidaceae. Although non-photosynthetic orchid species are known to be highly specialized exploiters of the ectomycorrhizal symbiosis, photosynthetic orchids are thought to use free-living saprophytic or pathogenic fungal lineages. However, we present evidence that putatively photosynthetic orchids from five species that grow in the understorey of forests (i) form mycorrhizas with ectomycorrhizal fungi of forest trees and (ii) have stable-isotope signatures indicating distinctive pathways for nitrogen and carbon acquisition approaching those of non-photosynthetic orchids that associate with ectomycorrhizal fungi of forest trees. These findings represent a major shift in our understanding of both orchid ecology and evolution because they explain how orchids can thrive in low-irradiance niches and they show that a shift to exploiting ectomycorrhizal fungi precedes viable losses of photosynthetic ability in orchid lineages.

Large breasts and narrow waists indicate high reproductive potential in women

Abstract: Physical characteristics, such as breast size and waist-to-hip ratio (WHR), function as important features used by human males to assess female attractiveness. Males supposedly pay attention to these features because they serve as cues to fecundity and health. Here, we document that women with higher breastto-underbreast ratio (large breasts) and women with relatively low WHR (narrow waists) have higher fecundity as assessed by precise measurements of daily levels of 17-β-oestradiol (E2) and progesterone. Furthermore, women who are characterized by both narrow waists and large breasts have 26% higher mean E2 and 37% higher mean mid-cycle E2 levels than women from three groups with other combinations of body-shape variables, i.e. low WHR with small breasts and high WHR with either large or small breasts. Such gains in hormone levels among the preferred mates may lead to a substantial rise in the probability of conception, thus providing a significant fitness benefit.

An ecological perspective on bacterial biodiversity.

Abstract: Bacteria may be one of the most abundant and species-rich groups of organisms, and they mediate many critical ecosystem processes Despite the ecological importance of bacteria, past practical and theoretical constraints have limited our ability to document patterns of bacterial diversity and to understand the processes that determine these patterns However, recent advances in molecular techniques that allow more thorough detection of bacteria in nature have made it possible to examine such patterns and processes Here, we review recent studies of the distribution of free-living bacterial diversity and compare our current understanding with what is known about patterns in plant and animal diversity From these recent studies a preliminary picture is emerging: bacterial diversity may exhibit regular patterns, and in some cases these patterns may be qualitatively similar to those observed for plants and animals

Different algal symbionts explain the vertical distribution of dominant reef corals in the eastern Pacific

Abstract: Symbiotic reef corals occupy the entire photic zone; however, most species have distinct zonation patterns within the light intensity gradient. It is hypothesized that the presence of specific symbionts adapted to different light regimes may determine the vertical distribution of particular hosts. We have tested this hypothesis by genetic and in situ physiological analyses of the algal populations occupying two dominant eastern Pacific corals, over their vertical distribution in the Gulf of California. Our findings indicate that each coral species hosts a distinct algal taxon adapted to a particular light regime. The differential use of light by specific symbiotic dinoflagellates constitutes an important axis for niche diversification and is sufficient to explain the vertical distribution patterns of these two coral species.

Beaked whales echolocate on prey

Abstract: Beaked whales (Cetacea: Ziphiidea) of the genera Ziphius and Mesoplodon are so difficult to study that they are mostly known from strandings. How these elusive toothed whales use and react to sound...

Do pretty women inspire men to discount the future

Abstract: Organisms 'discount the future' when they value imminent goods over future goods. Optimal discounting varies: selection should favour allocations of effort that effectively discount the future relatively steeply in response to cues promising relatively good returns on present efforts. However, research on human discounting has hitherto focused on stable individual differences rather than situational effects. In two experiments, discounting was assessed on the basis of choices between a smaller sum of money tomorrow and a larger sum at a later date, both before and after subjects rated the 'appeal' of 12 photographs. In experiment 1, men and women saw either attractive or unattractive opposite-sex faces; in experiment 2, participants saw more or less appealing cars. As predicted, discounting increased significantly in men who viewed attractive women, but not in men who viewed unattractive women or women who viewed men; viewing cars produced a different pattern of results.

Ice sheets promote speciation in boreal birds

Abstract: The premise that Pleistocene ice ages played an important role in generating present-day species diversity has been challenged by genetic data indicating that most of the youngest terrestrial species on Earth coalesced long before major glacial advances. However, study has been biased towards faunas distributed at low latitudes that were not directly fragmented by advancing ice sheets. Using mitochondrial sequence divergence and a molecular clock, we compared the coalescence times of pairs of avian species belonging to superspecies complexes from the high-latitude boreal forest with those of sub-boreal and tropical avifaunas of the New World. Remarkably, all coalescence events in boreal superspecies date to the Pleistocene, providing direct evidence that speciation was commonly initiated during recent glacial periods. A pattern of endemism in boreal superspecies plausibly links the timing of divergence to the fragmentation of the boreal forest by ice sheets during the Mid- and Late Pleistocene. In contrast to the boreal superspecies, only 56% of sub-boreal and 46% of tropical superspecies members coalesced during the Pleistocene, suggesting that avifaunas directly fragmented by ice sheets experienced rapid rates of diversification, whereas those distributed farther south were affected to a lesser extent. One explanation for the absence of pre-Pleistocene superspecies in boreal avifaunas is that strong selection pressures operated in boreal refugia, causing superspecies members to achieve ecological differentiation at an accelerated rate.

Random drift and culture change

Abstract: We show that the frequency distributions of cultural variants, in three different real–world examples—first names, archaeological pottery and applications for technology patents—follow power laws that can be explained by a simple model of random drift. We conclude that cultural and economic choices often reflect a decision process that is value–neutral; this result has far–reaching testable implications for social–science research.

Do positive interactions increase with abiotic stress? A test from a semi-arid steppe

Abstract: Theoretical models predict that the relative importance of facilitation and competition may vary inversely across gradients of abiotic stress. However, these predictions have not been thoroughly tested in the field, especially in semi-arid environments. In this study, we evaluated how the net effect of the tussock grass Stipa tenacissima on the shrub Pistacia lentiscus varied across a gradient of abiotic stress in semi-arid Mediterranean steppes. We fitted the relationship between accumulated rainfall and the relative neighbour index (our measures of abiotic stress and of the net effect of S. tenacissima on P. lentiscus, respectively), which varied across this gradient, to a quadratic model. Competitive interactions dominated at both extremes of the gradient. Our results do not support established theory. Instead, they suggest that a shift from facilitation to competition under high abiotic stress conditions is likely to occur when the levels of the most limiting resource are so low that the benefits provided by the facilitator cannot overcome its own resource uptake.

Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: evidence for a mechanism.

Abstract: Evolutionary theory predicts that mothers of different condition should adjust the birth sex ratio of their offspring in relation to future reproductive benefits. Published studies addressing variation in mammalian sex ratios have produced surprisingly contradictory results. Explaining the source of such variation has been a challenge for sex-ratio theory, not least because no mechanism for sex-ratio adjustment is known. I conducted a meta-analysis of previous mammalian sex-ratio studies to determine if there are any overall patterns in sex-ratio variation. The contradictory nature of previous results was confirmed. However, studies that investigated indices of condition around conception show almost unanimous support for the prediction that mothers in good condition bias their litters towards sons. Recent research on the role of glucose in reproductive functioning have shown that excess glucose favours the development of male blastocysts, providing a potential mechanism for sex-ratio variation in relation to maternal condition around conception. Furthermore, many of the conflicting results from studies on sex-ratio adjustment would be explained if glucose levels in utero during early cell division contributed to the determination of offspring sex ratios.

Global hot spots of biological invasions: evaluating options for ballast-water management.

Abstract: Biological invasions from ballast water are a severe environmental threat and exceedingly costly to society. We identify global hot spots of invasion based on worldwide patterns of ship traffic. We then estimate the rate of port-to-port invasion using gravity models for spatial interactions, and we identify bottlenecks to the regional exchange of species using the Ford-Fulkerson algorithm for network flows. Finally, using stochastic simulations of different strategies for controlling ballast-water introductions, we find that reducing the per-ship-visit chance of causing invasion is more effective in reducing the rate of biotic homogenization than eliminating key ports that are the epicentres for global spread.

Modelling strategies for controlling SARS outbreaks

Abstract: Severe acute respiratory syndrome (SARS), a new, highly contagious, viral disease, emerged in China late in 2002 and quickly spread to 32 countries and regions causing in excess of 774 deaths and 8098 infections worldwide. In the absence of a rapid diagnostic test, therapy or vaccine, isolation of individuals diagnosed with SARS and quarantine of individuals feared exposed to SARS virus were used to control the spread of infection. We examine mathematically the impact of isolation and quarantine on the control of SARS during the outbreaks in Toronto, Hong Kong, Singapore and Beijing using a deterministic model that closely mimics the data for cumulative infected cases and SARS-related deaths in the first three regions but not in Beijing until mid-April, when China started to report data more accurately. The results reveal that achieving a reduction in the contact rate between susceptible and diseased individuals by isolating the latter is a critically important strategy that can control SARS outbreaks with or without quarantine. An optimal isolation programme entails timely implementation under stringent hygienic precautions defined by a critical threshold value. Values below this threshold lead to control, but those above are associated with the incidence of new community outbreaks or nosocomial infections, a known cause for the spread of SARS in each region. Allocation of resources to implement optimal isolation is more effective than to implement sub-optimal isolation and quarantine together. A community-wide eradication of SARS is feasible if optimal isolation is combined with a highly effective screening programme at the points of entry.

Interpreting the von Bertalanffy model of somatic growth in fishes: the cost of reproduction.

Abstract: We develop a model for somatic growth in fishes that explicitly allows for the energy demand imposed by reproduction. We show that the von Bertalanffy (VB) equation provides a good description of somatic growth after maturity, but not before. We show that the parameters of the VB equation are simple functions of age at maturity and reproductive investment. We use this model to show how the energy demands for both growth and reproduction trade off to determine optimal life-history traits. Assuming that both age at maturity and reproductive investment adapt to variations in adult mortality to maximize lifetime offspring production, our model predicts that: (i) the optimal age of maturity is inversely related to adult mortality rate; (ii) the optimal reproductive effort is approximately equal to adult mortality rate. These predictions are consistent with observed variations in the life-history traits of a large sample of iteroparous freshwater fishes.

Only six kingdoms of life.

Abstract: There are many more phyla of microbes than of macro-organisms, but microbial biodiversity is poorly understood because most microbes are uncultured. Phylogenetic analysis of rDNA sequences cloned after PCR amplification of DNA extracted directly from environmental samples is a powerful way of exploring our degree of ignorance of major groups. As there are only five eukaryotic kingdoms, two claims using such methods for numerous novel 'kingdom-level' lineages among anaerobic eukaryotes would be remarkable, if true. By reanalysing those data with 167 known species (not merely 8-37), I identified relatives for all 8-10 'mysterious' lineages. All probably belong to one of five already recognized phyla (Amoebozoa, Cercozoa, Apusozoa, Myzozoa, Loukozoa) within the basal kingdom Protozoa, mostly in known classes, sometimes even in known orders, families or genera. This strengthens the idea that the ancestral eukaryote was a mitochondrial aerobe. Analogous claims of novel bacterial divisions or kingdoms may reflect the weak resolution and grossly non-clock-like evolution of ribosomal rRNA, not genuine phylum-level biological disparity. Critical interpretation of environmental DNA sequences suggests that our overall picture of microbial biodiversity at phylum or division level is already rather good and comprehensive and that there are no uncharacterized kingdoms of life. However, immense lower-level diversity remains to be mapped, as does the root of the tree of life.

The evolution of brain lateralization: a game-theoretical analysis of population structure

Abstract: In recent years, it has become apparent that behavioural and brain lateralization at the population level is the rule rather than the exception among vertebrates. The study of these phenomena has so far been the province of neurology and neuropsychology. Here, we show how such research can be integrated with evolutionary biology to understand lateralization more fully. In particular, we address the fact that, within a species, left- and right-type individuals often occur in proportions different from one-half (e.g. hand use in humans). The traditional explanations offered for lateralization of brain function (that it may avoid unnecessary duplication of neural circuitry and reduce interference between functions) cannot account for this fact, because increased individual efficiency is unrelated to the alignment of lateralization at the population level. A further puzzle is that such an alignment may even be disadvantageous, as it makes individual behaviour more predictable to other organisms. Here, we show that alignment of the direction of behavioural asymmetries in a population can arise as an evolutionarily stable strategy when individual asymmetrical organisms must coordinate their behaviour with that of other asymmetrical organisms. Brain and behavioural lateralization, as we know it in humans and other vertebrates, may have evolved under basically 'social' selection pressures.

Predators select against high growth rates and risk-taking behaviour in domestic trout populations.

Abstract: Domesticated (farm) salmonid fishes display an increased willingness to accept risk while foraging, and achieve high growth rates not observed in nature. Theory predicts that elevated growth rates in domestic salmonids will result in greater risk-taking to access abundant food, but low survival in the presence of predators. In replicated whole-lake experiments, we observed that domestic trout (selected for high growth rates) took greater risks while foraging and grew faster than a wild strain. However, survival consequences for greater growth rates depended upon the predation environment. Domestic trout experienced greater survival when risk was low, but lower survival when risk was high. This suggests that animals with high intrinsic growth rates are selected against in populations with abundant predators, explaining the absence of such phenotypes in nature. This is, to our knowledge, the first large-scale field experiment to directly test this theory and simultaneously quantify the initial invasibility of domestic salmonid strains that escape into the wild from aquaculture operations, and the ecological conditions affecting their survival.

Balancing food and predator pressure induces chronic stress in songbirds

Abstract: The never-ending tension between finding food and avoiding predators may be the most universal natural stressor wild animals experience. The ‘chronic stress’ hypothesis predicts: (i) an animal’s stress profile will be a simultaneous function of food and predator pressures given the aforesaid tension; and (ii) these inseparable effects on physiology will produce inseparable effects on demography because of the resulting adverse health effects. This hypothesis was originally proposed to explain synergistic (inseparable) food and predator effects on demography in snowshoe hares (Lepus americanus). We conducted a 2 � 2, manipulative food addition plus natural predator reduction experiment on song sparrows (Melospiza melodia) that was, to our knowledge, the first to demonstrate comparable synergistic effects in a bird: added food and lower predator pressure in combination produced an increase in annual reproductive success almost double that expected from an additive model. Here we report the predicted simultaneous food and predator effects on measures of chronic stress in the context of the same experiment: birds at unfed, high predator pressure (HPP) sites had the highest stress levels; those at either unfed or HPP sites showed intermediate levels; and fed birds at low predator pressure sites had the lowest stress levels.

The Prisoner's Dilemma and polymorphism in yeast SUC genes.

Abstract: The SUC multigene family of the single-celled yeast Saccharomyces cerevisiae is polymorphic, with genes varying both in number and activity. All of the genes encode invertase, an enzyme that is secreted to digest sucrose outside of the cell. This communal endeavour creates the potential for individual cells to defect (cheat) by stealing the sugar digested by their neighbours without contributing the enzyme themselves. We measured the fitness of a defector, with a deleted suc2 gene, relative to an otherwise isogenic cooperator, with a functional SUC2 gene. We manipulated the level of social interaction within the community by varying the population density and found that the defector is less fit than the cooperator at low levels of sociality but more fit in dense communities. We propose that selection for antisocial cheating causes SUC polymorphism in nature. The infamous Prisoner's Dilemma game shows that social behaviour is generally unstable, and the success of both cooperation and defection can vary continuously in time and space. The variation in SUC genes reflects constant adaptation to an ever-changing biotic environment that is a consequence of the instability of cooperation. It is interesting that social interactions can have a direct effect on molecular evolution, even in an organism as simple as yeast.

Social networks in the guppy (Poecilia reticulata).

Abstract: Social network theory is used to elicit details of the social structure of a population of free-ranging guppies, Poecilia reticulata. They were found to have a complex and highly structured social network, which exhibited characteristics consistent with the 'small world' phenomenon. Stable partner associations between individuals were observed, a finding that fulfils the basic prerequisite for the evolution of reciprocal altruism. The findings are discussed in relation to the ecology and evolution of the wild population, highlighting the potential application of network theory to social associations in animals.