Evidence for a quiet revolution: seasonal variation in colonies of the specialist tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach) (Hemiptera: Aphididae) studied using microsatellite markers

In cyclical parthenogens, clonal diversity is expected to decrease due to selection and drift during the asexual phase per number of asexual generations. The decrease in diversity may be counteracted by immigration of new genotypes. We analysed temporal variation in clonal diversity in colonies of the monophagous tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach), sampled four times over the course of a growing season. In a related field study, we recorded aphid colony sizes and the occurrence of winged dispersers throughout the season. The number of colonies increased from April, when asexual stem mothers hatched from the sexually produced eggs, to the end of June. The proportion of colonies with winged individuals also increased over this period. After a severe reduction in colony sizes in late summer, a second expansion phase occurred in October when sexuals were produced. At the season's end, the only winged forms were males. A linked genetic study showed that the number of microsatellite multilocus genotypes and genetic variability assessed at three polymorphic loci per colony decreased from June to October. Overall, the relatedness of wingless to winged individuals within colonies was lower than average relatedness among wingless individuals, suggesting that winged forms mainly originated in different colonies. The results demonstrate that patterns of genetic diversity within colonies can be explained by the antagonistic forces of clonal selection, migration and genetic drift (largely due to midsummer population bottlenecks). We further suggest that the males emigrate over comparatively longer distances than winged asexual females.     

Evidence of prey depletion around lesser kestrel Falco naumanni colonies and its short term negative consequences

Predation may reduce prey numbers in such extent that prey may be depleted, which has negative effects on predator populations. Prey depletion would be more likely when the number of predators increase and/or concentrate their activity in a certain area, as is the case of colonial birds. As a matter of fact, food depletion due to intraspecific competition is considered a major cost of coloniality, and several studies have shown indirect evidence of this. However, no direct measures of food depletion have been provided along with its consequences for the fitness of the colony inhabitants. We carried out a field study with the lesser kestrel Falco naumanni, a raptor that breeds in colonies ranging from two to dozens of pairs. During the nestling period we sampled the main prey of the kestrels around different sized colonies at increasing distances. At the same time, we recorded hunting distances and prey delivery rates to the nest. In addition, we monitored the reproductive success in colonies of different sizes. Lesser kestrels feed their nestlings mainly with grasshoppers and these prey became depleted through the season in the surroundings of the large colonies. Prey depletion made kestrels fly longer distances to forage and prey delivery rates to the nest decreased. Lower feeding rates were not compensated by bringing larger prey, hence, the net amount of energy provided to the chicks decreased with the date in large colonies. By contrast, none of this occurred around small colonies, where both prey abundance and hunting distance remained constant throughout the season. As a consequence, the seasonal decline in the reproductive success (number of fledglings and fledgling body condition) was greater the larger the colony. Thus, these results evidence that food depletion and its fitness costs are related to colony size, as they are suffered by the kestrels breeding in large colonies but not by those settled at small ones. Finally, the consequences of prey depletion on the demographic dynamics and the regulation of colony size are discussed.     

Defense of Food Supply by Eusocial Colonies

Overdispersion of colonies exists in many eusocial insects.Overdispersion can be generated by direct attack on coloniesor founders, by defense of space, by defense of food resourcesbeing harvested, or by exploitative competition. When directcompetitive interactions lead to colony overdispersion, territorialityis said to occur. Whereas solitary territory holders typicallydefend space, most eusocial colonies defend resource patchesrather than space per se. Also unlike solitary territory holders,colonies with forager communication can simultaneously defendseveral spatially separated food patches. A model explores optimalnumbers of scouts (discoverers of patches) and recruits (followers)needed to maximize net rate of energy intake by the colony.Territorial costs are added to the model by requiring a higherinvestment of foragers per unit resource collected. Accordingto the model, optimal colony size and percentage scouts aremore sensitive to changes in patch size than in patch density.If patch defense is required for resource control, a declineoccurs in optimal percentage of scouts; the decline is greatestfor small colonies. Colonies that must defend patches in orderto harvest from them suffer a loss in net energy intake; theloss is greatest for small colonies. It is predicted that amongeusocial insects, those with territorial defense of resourcesshould preferentially visit large patches and have comparativelylarge colony sizes and relatively few scouts. Ways of testingthese predictions are discussed.     

How much do army ants eat? On the prey intake of a neotropical top-predator

New World army ants (Ecitoninae) are nomadic group-predators that are widely thought to have a substantial impact on their prey. Nevertheless, quantitative data on prey intake by army ants is scarce and mostly limited to chance encounters. Here, I quantify the prey intake of the army ant Eciton hamatum at the contrasting scales of raid, colony (sum of simultaneous raids), and population. Like most army ants, E. hamatum conducts narrow ‘column raids’ and has a specialized diet of ant prey. I show that individual raids often had periods of no prey intake, and raid intake rates, calculated in g/min, differed significantly among colonies. Moreover, neither mean nor peak raid intake rates were correlated with colony size. Similarly, colony intake rates differed significantly among colonies, and mean colony intake rates were not correlated with colony size. However, mean colony intake rates were significantly higher than mean raid intake rates, and peak colony intake rate was correlated with colony size. Having multiple raids thus improves colony-level intake rates, and larger colonies can harvest more prey per unit time. Mean colony intake rate across colonies was 0.067 g/min dry weight and mean daily colony intake was calculated at 38.2 g. This intake is comparable to that of Eciton burchellii, which has a more generalized diet and conducts spectacular ‘swarm raids’ that are seen as having a greater impact on prey than column raids. Population size on Barro Colorado Island, Panama, was estimated to be 57 colonies, which extrapolates to a daily population intake of nearly 2 kg of prey dry weight, or 120 g/km². Broadly, these findings demonstrate that column raiding army ants experience considerable variation in prey intake for individual raids, but can still achieve notable impact at the larger scales of colony and population. Furthermore, they challenge the idea that swarm-raiding species necessarily have greater intake and thus impact on prey. Instead, I suggest that conducting multiple column raids may be a strategy that allows for comparable intake from a more specialized diet.     

Variation in queen size across a behavioral transition zone in the ant Messor pergandei

Summary. Ant colonies should be selected to optimally allocate resources to individual reproductive offspring so as to balance production costs with offspring fitness gains. Different modes of colony founding have different size-dependent fitness functions, and should thus lead to different optimal queen sizes. We tested whether a behavioral transition from solitary colony founding (haplometrosis) to group colony founding (pleometrosis) across the range of the ant Messor pergandei was associated with a difference in queen size or condition. Both winged gynes and founding queens were significantly smaller and lighter at pleometrotic than at haplometrotic sites, with an abrupt shift in these characters across the 8.5 km-wide behavioral transition zone. Both the mutualistic advantages of grouping and among-queen competition within associations are likely to be important in selecting for smaller queen size in pleometrotic populations.Received 16 January 2004; revised 13 August 2004; accepted 16 August 2004.     

Individual personalities shape task differentiation in a social spider

Deciphering the mechanisms involved in shaping social structure is key to a deeper understanding of the evolutionary processes leading to sociality. Individual specialization within groups can increase colony efficiency and consequently productivity. Here, we test the hypothesis that within-group variation in individual personalities (i.e. boldness and aggression) can shape task differentiation. The social spider Stegodyphus sarasinorum (Eresidae) showed task differentiation (significant unequal participation) in simulated prey capture events across 10-day behavioural assays in the field, independent of developmental stage (level of maturation), eliminating age polyethism. Participation in prey capture was positively associated with level of boldness but not with aggression. Body size positively correlated with being the first spider to emerge from the colony as a response to prey capture but not with being the first to attack, and dispersal distance from experimental colonies correlated with attacking but not with emerging. This suggests that different behavioural responses to prey capture result from a complex set of individual characteristics. Boldness and aggression correlated positively, but neither was associated with body size, developmental stage or dispersal distance. Hence, we show that personalities shape task differentiation in a social spider independent of age and maturation. Our results suggest that personality measures obtained in solitary, standardized laboratory settings can be reliable predictors of behaviour in a social context in the field. Given the wealth of organisms that show consistent individual behavioural differences, animal personality could play a role in social organization in a diversity of animals.     

Cooperation among germinating spores facilitates the growth of the fungus, Neurospora crassa

Fusions between individuals are a common feature of organisms with modular, indeterminate life forms, including plants, marine invertebrates and fungi. The consequences of fusion for an individual fungus are poorly understood. We used wild-type and fusion mutant strains of the genetic model Neurospora crassa to chronicle the fitness in two different laboratory habitats, and in each experiment started colonies from multiple different densities of asexual spores. On round Petri dishes, fusion enabled wild-type colonies to grow larger than mutant (soft) colonies; but in linear 'race tubes', the soft mutant always grew more quickly than the wild-type. Starting a colony with more spores always provided an advantage to a wild-type colony, but was more often neutral or a cost to the soft mutant. The ability to fuse does not provide a consistent advantage to wild-type colonies; net benefits are shaped by both habitat and initial spore densities.     

Adaptive advantages and the evolution of colony formation in Cyrtophora (Araneae: Araneidae)

Cyrtophora moluccensis (Doleschall) in New Guinea and C. citricola Forskal in the Mediterranean and Africa form persistent aggregations of individuals of all ages. Characteristic of this "communal" organization (as defined by Wilson, 1971) in C. moluccensis are low level aggressive interactions during prey capture and during web repair and renewal. Such interactions may serve as a spacing mechanism, ensuring an adequate fool supply for each individual
Disadvantages resulting from conspicuousness and persistence of C. moluccensis colonies are a high incidence of parasitism and a large degree of colony avoidance by flying insects (potential prey). Advantages of communal organization postulated for C. moluccensis includes increased protection of egg-cases and young, greater web building efficiency, greater prey capture efficiency, and the ability to exploit habitats that are unavailable to solitary species. Evidence is presented for the first and last of these hypotheses.
It is suggested that due to its durable, 3-dimensional web Cyrtophora species could invade grassland habitats exposed to frequent and heavy wind and rain (Lubin, 1973). Colony formation then evolved in the larger species, C. moluccensis and C. citricola , as a means for exploiting large, open spaces that are rich in flying insects and poor in competing araneids. Under such optimal conditions, low level aggression occurring during prey capture and web construction may be interpreted as instances of reciprocal altruism among related colony members.     

Relationship between prey consumption and colony size in an orb spider

Summary I studied the relationship between prey consumption and colony size in the orb spiderPhiloponella semiplumosa. Observations of unmanipulated colonies showed that prey biomass per juvenile spider was positively correlated with colony size, indicating that prey consumption was highest in the largest colonies observed. In contrast, the relationship between prey biomass per adult female and colony size was curvilinear; prey consumption tended to be highest in intermediatesized colonies. Adult female cephalothorax width was positively correlated with colony size. Number of egg sacs per adult female tended to be highest in intermediate-sized colonies. Prey biomass per juvenile was lower in experimentally reduced colonies than in large control colonies. Aerial-arthropod abundance was not correlated with colony size, and experimental prey supplementation did not affect colony size. Thus, the relationship between prey consumption and colony size was influenced by coloniality directly, rather than by a correlation between prey abundance at a site and colony size.     

Foraging decisions of individual workers vary with colony size in the greenhead ant Rhytidoponera metallica (Formicidae, Ectatomminae)

Summary. The ability of worker ants to adapt their behaviour depending on the social environment of the colony is imperative for colony growth and survival. In this study we use the greenhead ant Rhytidoponera metallica to test for a relationship between colony size and foraging behaviour. We controlled for possible confounding ontogenetic and age effects by splitting large colonies into small and large colony fragments. Large and small colonies differed in worker number but not worker relatedness or worker/brood ratios. Differences in foraging activity were tested in the context of single foraging cycles with and without the opportunity to retrieve food. We found that workers from large colonies foraged for longer distances and spent more time outside the nest than foragers from small colonies. However, foragers from large and small colonies retrieved the first prey item they contacted, irrespective of prey size. Our results show that in R. metallica, foraging decisions made outside the nest by individual workers are related to the size of their colony.Received 23 March 2004; revised 3 June 2004; accepted 4 June 2004.     

Recombination in a Simulated Predator-Prey Interaction

The influence of a co-evolved predator-prey interaction on theoutcome of competition between sexual and asexual prey was examinedby computer simulation. Intraspecific competition in both predatorand prey populations is governed by relative success in theinterspecific interaction. Predators key on a certain variableprey character under the control often independently assorting,diploid loci. Individual predators vary genetically in theirability to handle prey of the various states of this character.Predator fitness is proportional to the prey available to them.Prey fitness is inversely proportional to the frequency of predatorsto which they are susceptible. In addition, sexual prey of agiven phenotype are assigned half the fitness of the correspondingasexual form, representing a "cost of meiosis." Mating is randomand there is no facultative response allowed to changes in thephenotypic distribution of the interacting species. Three parametersare varied: (1) the maximum contribution of a phenotype to thenext generation (fecundity limit), (2) population size, and(3) clonal diversity of the asexual prey form. Most realisticcombinations of these variables favor sexual reproduction. Accordingto the model, it is particularly unlikely that an asexual mutantcould spread in an existing sexual population. However, theconditions found to favor asexuality are consistent with observationsfrom nature where reduced recombination is relatively common.The model predicts asexual forms should persist in very large,low fecund populations, as in the prokaryotes, and where theirphenotypic diversity is high, as near zones of recurrent hybridization.It is also consistent with the occurrence of parthenoforms inregions of reduced interspecific interaction where the assumptionsof this model, and therefore the advantages to sex, are lesslikely to be realized.     

Differences in aggressive behaviour between male mice (Mus musculus L.) in colonies of different sizes

Using cages of uniform size, colonies of three, four, five, nine and twelve unfamiliar male mice were established and observed over a 21-day period. A dominant mouse emerged in every colony, but in the two largest sizes five to six changes in dominance were recorded. The per capita aggression did not increase with colony size. There was a general linear decline in the number of attacks recorded in colony sizes nine and twelve whilst in the three smaller colony sizes the dominant showed an exponential decline in aggressiveness. Differences in hierarchical organization were interpreted in terms of the number of attacks shown by the dominant in relation to the number of subordinates present.     

Contrast Between Solitary and Clonal Lifestyles in the Sea Anemone Anthopleura elegantissima

The common intertidal sea anemone Anthopleura elegantissima(Brandt) occurs in two forms the clonal aggregating form andthe solitary form The obvious differences between the two apparentlyresult from the presence of asexual reproduction in the clonalaggregating form and its absence in the solitary form resultingdifferences in growth form suit the two anemones to differentlifestyles The clonal form is well suited to life higher inthe mid intertidal Asexual reproduction resulting in moderatelysmall individual size and close association with clonematesimproves its resistance to physical stress (drag and desiccation)and makes it a superior competitor in exposed habitats higherin the intertidal where species diversity and predator pressureare low The larger solitary animals live in more protected microhabitatslower in the intertidal and subtidal and are probably more resistantto predation and less able to withstand physical stress or intenseintraspecific competition The two forms also have differentbiogeographic ranges While the clonal aggregations are quitecommon at rocky sites at least as far north as Vancouver Islandin British Columbia populations of solitaries have not beenfound north of Point Reyes California Apparently the two formsare re productively isolated phenotype frequencies are verydifferent for clonals and solitaries living at the same locationswhile the between-sample variation within each form is relativelysmall These differences in phenotype frequencies biogeographicrange and microhabitat suggest that the clonal anemones andthe solitary anemones known as Anthopleura elegantissima areactually a sibling species pair.     

Choice of colony size in birds

Most populations of colonial birds exhibit extensive variation in colony size. Field studies over the last decade have shown that individual birds breeding in colonies of certain sizes are apparently more successful than those settling in colonies of other sizes, yet size variation persists. Enough information is now available to suggest four explanations for how birds choose colonies and why colonies vary in size.     

Colony size affects collective decision-making in the ant Temnothorax albipennis

Social insects are well-known for their ability to achieve robust collective behaviours even when individuals have limited information. It is often assumed that such behaviours rely on very large group sizes, but many insect colonies start out with only a few workers. Here we investigate the influence of colony size on collective decision-making in the house-hunting of the ant Temnothorax albipennis. In experiments where colony size was manipulated by splitting colonies, we show that worker number has an influence on the speed with which colonies discover new nest sites, but not on the time needed to make a decision (achieve a quorum threshold) or total emigration time. This occurred because split colonies adopted a lower quorum threshold, in fact they adopted the same threshold in proportion to their size as full-size colonies. This indicates that ants may be measuring relative quorum, i.e. population in the new nest relative to that of the old nest, rather than the absolute number. Experimentally reduced colonies also seemed to gain more from experience through repeated emigrations, as they could then reduce nest discovery times to those of larger colonies. In colonies of different sizes collected from the field, total emigration time was also not correlated with colony size. However, quorum threshold was not correlated with colony size, meaning that individuals in larger colonies adopted relatively lower quorum thresholds. Since this is a different result to that from size-manipulated colonies, it strongly suggests that the differences between natural small and large colonies were not caused by worker number alone. Individual ants may have adjusted their behaviour to their colony’s size, or other factors may correlate with colony size in the field. Our study thus shows the importance of experimentally manipulating colony size if the effect of worker number on the emergence of collective behaviour is to be studied. Received 13 December 2005; revised 9 May 2006; accepted 15 May 2006.     

An energetic correlate between colony size and foraging effort in seabirds, an example of the Adélie penguin Pygoscelis adeliae

Central-place foraging seabirds alter the availability of their prey around colonies, forming a "halo" of reduced prey access that ultimately constrains population size. This has been indicated indirectly by an inverse correlation between colony size and reproductive success, numbers of conspecifics at other colonies within foraging range, foraging effort (i.e. trip duration), diet quality and colony growth rate. Although ultimately mediated by density dependence relative to food through intraspecific exploitative or interference competition, the proximate mechanism involved has yet to be elucidated. Herein, we show that Adélie penguin Pygoscelis adeliae colony size positively correlates to foraging trip duration and metabolic rate, that the metabolic rate while foraging may be approaching an energetic ceiling for birds at the largest colonies, and that total energy expended increases with trip duration although uncompensated by increased mass gain. We propose that a competition-induced reduction in prey availability results in higher energy expenditure for birds foraging in the halo around large colonies, and that to escape the halo a bird must increase its foraging distance. Ultimately, the total energetic cost of a trip determines the maximum successful trip distance, as on longer trips food acquired is used more for self maintenance than for chick provisioning. When the net cost of foraging trips becomes too high, with chicks receiving insufficient food, chick survival suffers and subsequent colony growth is limited. Though the existence of energetic studies of the same species at multiple colonies is rare, because foraging metabolic rate increases with colony size in at least two other seabird species, we suggest that an energetic constraint to colony size may generally apply to other seabirds.     

Ant Colonies Do Not Trade-Off Reproduction against Maintenance

The question on how individuals allocate resources into maintenance and reproduction is one of the central questions in life history theory. Yet, resource allocation into maintenance on the organismic level can only be measured indirectly. This is different in a social insect colony, a “superorganism” where workers represent the soma and the queen the germ line of the colony. Here, we investigate whether trade-offs exist between maintenance and reproduction on two levels of biological organization, queens and colonies, by following single-queen colonies of the ant Cardiocondyla obscurior throughout the entire lifespan of the queen. Our results show that maintenance and reproduction are positively correlated on the colony level, and we confirm results of an earlier study that found no trade-off on the individual (queen) level. We attribute this unexpected outcome to the existence of a positive feedback loop where investment into maintenance (workers) increases the rate of resource acquisition under laboratory conditions. Even though food was provided ad libitum, variation in productivity among the colonies suggests that resources can only be utilized and invested into additional maintenance and reproduction by the colony if enough workers are available. The resulting relationship between per-capita and colony productivity in our study fits well with other studies conducted in the field, where decreasing per-capita productivity and the leveling off of colony productivity have been linked to density dependent effects due to competition among colonies. This suggests that the absence of trade-offs in our laboratory study might also be prevalent under natural conditions, leading to a positive association of maintenance, (= growth) and reproduction. In this respect, insect colonies resemble indeterminate growing organisms.     

Towards a general life-history model of the superorganism: predicting the survival,growth and reproduction of ant societies

Social insect societies dominate many terrestrial ecosystems across the planet. Colony members cooperate to capture and use resources to maximize survival and reproduction. Yet, when compared with solitary organisms, we understand relatively little about the factors responsible for differences in the rates of survival, growth and reproduction among colonies. To explain these differences, we present a mathematical model that predicts these three rates for ant colonies based on the body sizes and metabolic rates of colony members. Specifically, the model predicts that smaller colonies tend to use more energy per gram of biomass, live faster and die younger. Model predictions are supported with data from whole colonies for a diversity of species, with much of the variation in colony-level life history explained based on physiological traits of individual ants. The theory and data presented here provide a first step towards a more general theory of colony life history that applies across species and environments.     

Reproduction and life strategies in the Paleozoic tabulate coral Paleofavosites capax (Billings)

Asexual and sexual modes of colony formation in a tabulate coral Paleofavosites capax are recognized from the early Silurian Gun River Formation of Anticosti Island. Québee. Colonies produced by asexual fragmentation comprise monospecific 'clumped populations'. They are characterized by circular and concave bases, and lack a protocorallite origin of colony growth. Sexually produced colonies, where in situ , are always dispersed and characterized by conical bases with a definite protocorallite point origin of colony growth. Asexual colony formation by fragmentation in P. capax appears to have been an adaptation to a habitat of muddy substrates. Sexual reproduction in this species probably played a minor role but was necessary for the maintenance of gene diversity and long-distance dispersal. A comparison of corallite size distributions between populations demonstrates that intrapopulation variation in the 'dispersed population' and the conical colonies in 'transported populations' of P. capax is significantly larger than the variation in the 'clumped populations'. It is suggested that this difference reflects the two modes of reproduction. The above observations are significant to systematic studies because they show that estimates of species morphologic parameters can he seriously biased even when based on a relatively large sample size from a well-defined population if that population is largely a result of asexual colony formation.     

Colony size and individual fitness in the social spider Anelosimus eximius

ABSTRACT The effects of colony size on individual fitness and its components were investigated in artificially established and natural colonies of the social spider Anelosimus eximius (Araneae: Theridiidae). In the tropical rain forest understory at a site in eastern Ecuador, females in colonies containing between 23-107 females had india significantly higher lifetime reproductive success than females in smaller colonies. Among larger colonies, this trend apparently reversed. This overall fitness function was a result of the conflicting effects of colony size on different components of fitness. In particular, the probability of offspring survival to maturity increased with colony size while the probability of a female reproducing within the colonies decreased with colony size. Average clutch size increased with colony size when few or no wasp parasitoids were present in the egg sacs. With a high incidence of egg sac parasitoids, this effect disappeared because larger colonies were more likely to be infected. The product of the three fitness components measured-probability of female reproduction, average clutch size, and offspring survival-produced a function that is consistent with direct estimates of the average female lifetime reproductive success obtained by dividing the total number of offspring maturing in a colony by the number of females in the parental generation. Selection, therefore, should favor group living and itermediate colony sizes in this social spider.