Maximum Group Size Research Articles

Maximum group sizes for simultaneous testing in high potential risk scenarios

When tests are performed in scenarios such as reliability demonstration, two extreme possibilities are to perform all required tests simultaneously or to test all units sequentially. From the perspective of time for testing, the former is typically preferred, but in high potential risk scenarios, for example in the case of possibly disastrous results if a tested unit fails, it is better to have the opportunity to stop testing after a failure occurs. An analogon appears in medical testing, with patients being the ‘units’, if new medication is to be tested to confirm its functionality while possibly severe (side) effects are not yet known. There is a wide range of test scenarios in between these two extremes, with groups of units being tested simultaneously. This article discusses such scenarios in a basic setting, assuming that the total number of required tests has been set, for example based on other criteria or legislation. A new criterion for guidance on suitable test group sizes is presented. Throughout, the aim is for high reliability, with testing stopped in case any unit fails, following which the units will not be approved. Any consecutive actions, such as improvement of the units or dismissing them, are not part of the main considerations in this article. While in practice the development of complex models and decision approaches may appear to be required, a straightforward argument is presented, which leads to results that can be widely applied and easily communicated.

A ring-based multicast routing topology with QoS support in wireless mesh networks

Wireless mesh networking (WMN) is an emerging technology for future broadband wireless access. The proliferation of the mobile computing devices that are equipped with cameras and ad hoc communication mode creates the possibility of exchanging real-time data between mobile users in wireless mesh networks. In this paper, we argue for a ring-based multicast routing topology with support from infrastructure nodes for group communications in WMNs. We study the performance of multicast communication over a ring routing topology when 802.11 with RTS/CTS scheme is used at the MAC layer to enable reliable multicast services in WMNs. We propose an algorithm to enhance the IP multicast routing on the ring topology. We show that when mesh routers on a ring topology support group communications by employing our proposed algorithms, a significant performance enhancement is realized. We analytically compute the end-to-end delay on a ring multicast routing topology. Our results show that the end-to-end delay is reduced about 33 %, and the capacity of multicast network (i.e., maximum group size that the ring can serve with QoS guarantees) is increased about 50 % as compared to conventional schemes. We also use our analytical results to develop heuristic algorithms for constructing an efficient ring-based multicast routing topology with QoS guarantees. The proposed algorithms take into account all possible traffic interference when constructing the multicast ring topology. Thus, the constructed ring topology provides QoS guarantees for the multicast traffic and minimizes the cost of group communications in WMNs.

Rapid evolution of cooperation in group-living animals

BackgroundIt is often assumed that evolution takes place on very large timescales. Countering this assumption, rapid evolutionary dynamics are increasingly documented in biological systems, e.g. in the context of predator–prey interactions, species coexistence and invasion. It has also been shown that rapid evolution can facilitate the evolution of cooperation. In this context often evolutionary dynamics influence population dynamics, but in spatial models rapid evolutionary dynamics also emerge with constant population sizes. Currently it is not clear how well these spatial models apply to species in which individuals are not embedded in fixed spatial structures. To address this issue we employ an agent-based model of group living individuals. We investigate how positive assortment between cooperators and defectors and pay-off differences between cooperators and defectors depend on the occurrence of evolutionary dynamics.ResultsWe find that positive assortment and pay-off differences between cooperators and defectors differ when comparing scenarios with and without selection, which indicates that rapid evolutionary dynamics are occurring in the selection scenarios. Specifically, rapid evolution occurs because changes in positive assortment feed back on evolutionary dynamics, which crucially impacts the evolution of cooperation. At high frequencies of cooperators these feedback dynamics increase positive assortment facilitating the evolution of cooperation. In contrast, at low frequencies of cooperators rapid evolutionary dynamics lead to a decrease in assortment, which acts against the evolution of cooperation. The contrasting dynamics at low and high frequencies of cooperators create positive frequency-dependent selection.ConclusionsRapid evolutionary dynamics can influence the evolution of cooperation in group-living species and lead to positive frequency-dependent selection even if population size and maximum group-size are not affected by evolutionary dynamics. Rapid evolutionary dynamics can emerge in this case because sufficiently strong selective pressures allow evolutionary and demographic dynamics, and consequently also feedback between assortment and evolution, to occur on the same timescale. In particular, emerging positive frequency-dependent selection could be an important explanation for differences in cooperative behaviors among different species with similar population structures such as humans and chimpanzees.

GROUP-SIZE AND AGE-SEX COMPOSITION OF NILGIRI LANGUR Trachypithecus johnii (PRIMATES: CERCOPITHECIDAE) IN INDIA

Group size and group composition of Nilgiri langur (Trachypithecus johnii) was studied in two habitats of Parambikulam Tiger Reserve, Kerala, India. Group size and age-sex composition data was collected during scan sampling, 18 monitoring transect lines, road-strip count, and direct encounter of the groups. Mean group size value significantly differ between moist deciduous forest and evergreen forest. Group size was varied from 2 to 22. The maximum group size, 22 was recorded in evergreen forest habitat. The mean group size of Nilgiri langur is less in moist deciduous forest and higher in evergreen forest.

Do Animals Living in Larger Groups Experience Greater Parasitism? A Meta-Analysis

Parasitism is widely viewed as the primary cost of sociality and a constraint on group size, yet studies report varied associations between group size and parasitism. Using the largest database of its kind, we performed a meta-analysis of 69 studies of the relationship between group size and parasite risk, as measured by parasitism and immune defenses. We predicted a positive correlation between group size and parasitism with organisms that show contagious and environmental transmission and a negative correlation for searching parasites, parasitoids, and possibly vector-borne parasites (on the basis of the encounter-dilution effect). Overall, we found a positive effect of group size (r = 0.187) that varied in magnitude across transmission modes and measures of parasite risk, with only weak indications of publication bias. Among different groups of hosts, we found a stronger relationship between group size and parasite risk in birds than in mammals, which may be driven by ecological and social factors. A metaregression showed that effect sizes increased with maximum group size. Phylogenetic meta-analyses revealed no evidence for phylogenetic signal in the strength of the group size-parasitism relationship. We conclude that group size is a weak predictor of parasite risk except in species that live in large aggregations, such as colonial birds, in which effect sizes are larger.

Intra-specific competition in foraging Griffon Vultures Gyps fulvus: 1. Dynamics of group feeding

Capsule Young Griffon Vultures Gyps fulvus suffer intense competition from elders at feeding sites and this reduces their intake rate.Aims To explore the costs and benefits of feeding in groups in Griffon VulturesMethods In a French population of individually marked vultures, we studied by video the dynamics of feeding events and the difference in competitive ability to access carrion between ages and sexes.Results When several carcasses were available, the distribution of the birds at the beginning of feeding event was generally different from an ideal free distribution. Depending on food availability, the potential group size was always higher than the maximum group size observed. However, the number of birds landed increased with food mass and was always higher than the potential group size, in accordance with the stable group size hypothesis. The timing of arrivals and departures of individuals differed with respect to their age, with old adults arriving and leaving before the arrival of other age-classes. Some birds, mostly old adults with a lower interaction rate, aggressiveness and dominance, regularly started the feeding events first.Conclusions At high-intensity feeding sites close to the colonies younger vultures probably access a lower quantity and quality of resources than old adults.

On the Diversity-Multiplexing Tradeoff Under Groupwise Successive Interference Cancelation

In this paper, we study the diversity-multiplexing tradeoff assuming groupwise successive interference cancelation (GSIC). GSIC with layered codes in multiple-input multiple-output (MIMO) can vary from the plain SIC to the maximum likelihood (ML) decoding according to the group sizes, thus is flexible in both complexity and performance. The maximum group size is a dominating factor for the computation complexity of GSIC when the processing complexity is polynomially dependent on the group size with a high order. In this paper, we show the tradeoff among the maximum group size, diversity gain, and multiplexing gain (GDMT) under the GSIC and under the GSIC combined with antenna selection (GSICAS). For a given group-size bound c, we show that the number of groups achieving the optimal DMT under the GSIC with α selected transmit antennas is equal to ⌈α/c⌉ and a larger group should have a higher priority in decoding order. The optimal grouping is obtained in a systematic way. Based on these results, we find a limited set of groupings that contains the grouping achieving the optimal GDMT under GSIC and under GSICAS. The optimal multiplexing gain allocation for the optimal grouping is then found systematically and efficiently within the limited set.

Group Size in Animal Societies: The Potential Role of Social and Ecological Limitations in the Group-Living Fish, Paragobiodon xanthosomus

The number of group members in an animal society can have a major influence on group members’ life history, survival, and reproductive success. Identifying the factors that limit group size is therefore fundamental for a complete understanding of social behavior. Here, I examined the relationships between resource availability, social conflict, and group size in the coral-dwelling fish, Paragobiogon xanthosomus (Gobiidae). The size of the largest (breeding) female and the minimum size difference required for hierarchy stability strongly but not perfectly predicted maximum group size, suggesting that social conflicts and hierarchy structure set the upper limit on group size. Deviations in group size around the predicted maximum were explained by variation in average body size ratios but not by variation in coral size, suggesting that coral size does not directly influence group size. In contrast, coral size was a significant predictor of body size ratios, and possible explanations for this relationship are discussed. Group size may be limited by the social conflicts that characterize size-based dominance hierarchies. Ecological factors, namely coral size, may in turn play an indirect role via an effect on body size ratios.

A queueing model for general group screening policies and dynamic item arrivals

Classification of items as good or bad can often be achieved more economically by examining the items in groups rather than individually. If the result of a group test is good, all items within it can be classified as good, whereas one or more items are bad in the opposite case. Whether it is necessary to identify the bad items or not, and if so, how, is described by the screening policy. In the course of time, a spectrum of group screening models has been studied, each including some policy. However, the majority ignores that items may arrive at random time epochs at the testing center in real life situations. This dynamic aspect leads to two decision variables: the minimum and maximum group size. In this paper, we analyze a discrete-time batch-service queueing model with a general dependency between the service time of a batch and the number of items within it. We deduce several important quantities, by which the decision variables can be optimized. In addition, we highlight that every possible screening policy can, in principle, be studied, by defining the dependency between the service time of a batch and the number of items within it appropriately.

Establishment of an aerosol challenge model of tuberculosis in rhesus macaques and an evaluation of endpoints for vaccine testing.

The establishment of an aerosol challenge model in nonhuman primates (NHPs) for the testing of vaccines against Mycobacterium tuberculosis would assist the global effort to optimize novel vaccination strategies. The endpoints used in preclinical challenge studies to identify measures of disease burden need to be accurate and sensitive enough to distinguish subtle differences and benefits afforded by different tuberculosis (TB) vaccine regimens when group sizes are inevitably small. This study sought to assess clinical and nonclinical endpoints as potentially sensitive measures of disease burden in a challenge study with rhesus macaques by using a new protocol of aerosol administration of M. tuberculosis. Immunological and clinical readouts were assessed for utility in vaccine evaluation studies. This is the first example of TB vaccine evaluation with rhesus macaques where long-term survival was one of the primary endpoints. However, we found that in NHP vaccine efficacy studies with maximum group sizes of six animals, survival did not provide a valuable endpoint. Two approaches used in human clinical trials for the evaluation of the gamma interferon (IFN-gamma) response to vaccination (enzyme-linked immunospot [ELISpot] assay and enzyme-linked immunosorbent assay [ELISA]) were included in this study. The IFN-gamma profiles induced following vaccination were found not to correlate with protection, nor did the level of purified protein derivative (PPD)-specific proliferation. The only readout to reliably distinguish vaccinated and unvaccinated NHPs was the determination of lung lesion burden using magnetic resonance (MR) imaging combined with stereology at the end of the study. Therefore, the currently proposed key markers were not shown to correlate with protection, and only imaging offered a potentially reliable correlate.

A theory of leadership in human cooperative groups

Two types of models aim to account the origins of rank differentiation and social hierarchy in human societies. Conflict models suggest that the formation of social hierarchies is synonymous with the establishment of relationships of coercive social dominance and exploitation. Voluntary or 'integrative' models, on the other hand, suggest that rank differentiation – the differentiation of leader from follower, ruler from ruled, or state from subject – may sometimes be preferred over more egalitarian social arrangements as a solution to the challenges of life in social groups, such as conflict over resources, coordination failures, and free-riding in cooperative relationships. Little formal theoretical work, however, has established whether and under what conditions individuals would indeed prefer the establishment of more hierarchical relationships over more egalitarian alternatives. This paper provides an evolutionary game theoretical model for the acceptance of leadership in cooperative groups. We propose that the effort of a leader can reduce the likelihood that cooperation fails due to free-riding or coordination errors, and that under some circumstances, individuals would prefer to cooperate in a group under the supervision of a leader who receives a share of the group's productivity than to work in an unsupervised group. We suggest, in particular, that this becomes an optimal solution for individual decision makers when the number of group members required for collective action exceeds the maximum group size at which leaderless cooperation is viable.

Abundance, population structure and seasonally changing social organization of argali Ovis ammon karelini in West-Central Tian-Shan of Kyrgyzstan

The main aim of this study was to determine the numbers, population structure and seasonal changes in group structure of argali Ovis ammon karelini Severtzov, 1873 in the Tian-Shan of Kyrgyzstan. The study was carried out within two adjoining areas: the Baralbas River region, and the Ak-Tash River region. Data were collected during three seasons: winter, spring and summer. This population consisted of 42.3% females, 22.2% males, 13.4% yearlings, and 22.1% lambs. Composition and numbers of groups were seasonally changing. Argali occurred predominantly in mixed groups during winter and exclusively in separated groups during summer. The maximum group size decreased from 25% from winter to spring, however, increased during summer.

Unavoidable limits on group size in a body size-based linear hierarchy

Competition between individual group members is a key force in shaping social group structure and size. In animal societies, within-group competition may be structured by linear dominance hierarchies, which can be stable if there are minimum differences in competitive ability between adjacently ranked individuals. This requirement constrains maximum group size because only a certain number of clearly differentiated ranks can be fit into the range of competitive abilities between the top and bottom of the hierarchy. We investigated this hypothesis in the body size-based linear dominance hierarchy of the angelfish Centropyge bicolor. Unlike in previous studies, we found that maximum group size in natural C. bicolor groups was not always strictly limited by the range of possible body sizes and the average size difference between adjacent ranks. Oversized groups displayed a compressed body size hierarchy with smaller size differences between adjacent ranks, less effective regulation of subordinate foraging rates to maintain size differences, and greater spatial segregation between adjacently ranked individuals. Our results suggest that when spatial segregation compromises the regulatory mechanisms that maintain clear size hierarchies, groups can become larger than expected by slotting more individuals into a compressed size hierarchy. However, we also found that oversized groups tended to fission into smaller groups, suggesting that they are transient entities and that ultimately the group size limits imposed by the need to maintain a well-defined hierarchy are unavoidable. Key words: Centropyge bicolor, group size, linear dominance hierarchy, maximum group size. [Behav Ecol]

Species differences in group size and electrosensory interference in weakly electric fishes: Implications for electrosensory processing

In animals with active sensory systems, group size can have dramatic effects on the sensory information available to individuals. In “wave-type” weakly electric fishes there is a categorical difference in sensory processing between solitary fish and fish in groups: when conspecifics are within about 1 m of each other, the electric fields mix and produce interference patterns that are detected by electroreceptors on each individual. Neural circuits in these animals must therefore process two streams of information—salient signals from prey items and predators and social signals from nearby conspecifics. We investigated the parameters of social signals in two genera of sympatric weakly electric fishes, Apteronotus and Sternopygus, in natural habitats of the Napo River valley in Ecuador and in laboratory settings. Apteronotus were most commonly found in pairs along the Napo River (47% of observations; maximum group size 4) and produced electrosensory interference at rates of 20–300 Hz. In contrast, Sternopygus were alone in 80% of observations (maximum group size 2) in the same region of Ecuador. Similar patterns were observed in laboratory experiments: Apteronotus were in groups and preferentially approached conspecific-like signals in an electrotaxis experiment whereas Sternopygus tended to be solitary and did not approach conspecific-like electrosensory signals. These results demonstrate categorical differences in social electrosensory-related activation of central nervous system circuits that may be related to the evolution of the jamming avoidance response that is used in Apteronotus but not Sternopygus to increase the frequency of electrosensory interference patterns.

Distributed scalable multi-robot learning using particle swarm optimization

Designing effective behavioral controllers for mobile robots can be difficult and tedious; this process can be circumvented by using online learning techniques which allow robots to generate their own controllers online in an automated fashion. In multi-robot systems, robots operating in parallel can potentially learn at a much faster rate by sharing information amongst themselves. In this work, we use an adapted version of the Particle Swarm Optimization algorithm in order to accomplish distributed online robotic learning in groups of robots with access to only local information. The effectiveness of the learning technique on a benchmark task (generating high-performance obstacle avoidance behavior) is evaluated for robot groups of various sizes, with the maximum group size allowing each robot to individually contain and manage a single PSO particle. To increase the realism of the technique, different PSO neighborhoods based on limitations of real robotic communication are tested and compared in this scenario. We explore the effect of varying communication power for one of these communication-based PSO neighborhoods. To validate the effectiveness of these learning techniques, fully distributed online learning experiments are run using a group of 10 real robots, generating results which support the findings from our simulations.

Size and Place in the Construction of Indigeneity in the Russian Federation

Within the Russian Federation there are nearly 200 recognized "nationalities," approximately 130 of which could claim to be "indigenous." However, only 45 peoples are officially recognized as "indigenous small-numbered peoples of the Russian Federation" and thereby qualify for the rights, privileges, and state support earmarked for indigenous peoples. This status is conditioned upon a maximum group size of 50,000. While experts insist that this numerical criterion is provisional and without serious political implications, our fieldwork demonstrates that it has become a social fact that cannot be ignored, especially in light of the 2002 All-Russia Census and the release of its results in 2004. This numerical benchmark forces a dichotomization into small-numbered versus non-small-numbered peoples and creates a peculiar type of identity politics based on ethnic-group size. The "indigenous small-numbered" status is also conditioned upon a set of overlapping but often contradictory residency requirements. Using case studies from southern Siberia and the north of European Russia, we document the dynamic interplay between these dimensions of identity and the opportunities for maneuvering in the competition for the benefits that attach to certain categories. However, indigenous peoples who engage in such identity politics run the risk of becoming "incarcerated" within the confines of those categories.

Vigilance and fitness in grey partridges Perdix perdix: the effects of group size and foraging‐vigilance trade‐offs on predation mortality

1. Vigilance increases fitness by improving predator detection but at the expense of increasing starvation risk. We related variation in vigilance among 122 radio-tagged overwintering grey partridges Perdix perdix (L.) across 20 independent farmland sites in England to predation risk (sparrowhawk Accipiter nisus L., kill rate), use of alternative antipredation behaviours (grouping and use of cover) and survival. 2. Vigilance was significantly higher when individuals fed in smaller groups and in taller vegetation. In the covey period (in early winter when partridges are in flocks), vigilance and use of taller vegetation was significantly higher at sites with higher sparrowhawk predation risk, but tall vegetation was used less by larger groups. Individuals were constrained in reducing individual vigilance by group size and habitat choice because maximum group size was determined by overall density in the area during the covey period and by the formation of pairs at the end of the winter (pair period), when there was also a significant twofold increase in the use of tall cover. 3. Over the whole winter individual survival was higher in larger groups and was lower in the pair period. However, when controlling for group size, mean survival decreased as vigilance increased in the covey period. This result, along with vigilance being higher at sites with increasing with raptor risk, suggests individual vigilance increases arose to reduce short-term predation risk from raptors but led to long-term fitness decreases probably because high individual vigilance increased starvation risk or indicated longer exposure to predation. The effect of raptors on survival was less when there were large groups in open habitats, where individual partridges can probably both detect predators and feed efficiently. 4. Our study suggests that increasing partridge density and modifying habitat to remove the need for high individual vigilance may decrease partridge mortality. It demonstrates the general principle that antipredation behaviours may reduce fitness long-term via their effects on the starvation-predation risk trade-off, even though they decrease predation risk short-term, and that it may be ecological constraints, such as poor habitat (that lead to an antipredation behaviour compromising foraging), that cause mortality, rather than the proximate effect of an antipredation behaviour such as vigilance.

Evolution of cooperation by multilevel selection.

We propose a minimalist stochastic model of multilevel (or group) selection. A population is subdivided into groups. Individuals interact with other members of the group in an evolutionary game that determines their fitness. Individuals reproduce, and offspring are added to the same group. If a group reaches a certain size, it can split into two. Faster reproducing individuals lead to larger groups that split more often. In our model, higher-level selection emerges as a byproduct of individual reproduction and population structure. We derive a fundamental condition for the evolution of cooperation by group selection: if b/c > 1 + n/m, then group selection favors cooperation. The parameters b and c denote the benefit and cost of the altruistic act, whereas n and m denote the maximum group size and the number of groups. The model can be extended to more than two levels of selection and to include migration.

A new perspective on size hierarchies in nature: patterns, causes, and consequences

Many plant and animal aggregations have size hierarchies within which a variety of sizes of individuals, from large to small, can be found. Size hierarchies are thought to indicate the existence of competition amongst individuals within the aggregation, but determining their exact cause is difficult. The key to understanding size hierarchies lies in first quantifying the pattern of size and growth of individuals. We conducted a quantitative investigation of pattern in the size hierarchy of the clown anemonefish Amphiprion percula, in Madang Lagoon, Papua New Guinea. Here, groups of A. percula occupy sea anemones (Heteractis magnifica) that provide protection from predators. Within each anemone there is a single group composed of a breeding pair and zero to four non-breeders. Within each group there is a single size hierarchy; the female is largest (rank 1), the male is second largest (rank 2), and the non-breeders get progressively smaller (ranks 3-6). We demonstrate that individuals adjacent in rank are separated by body size ratios whose distribution is significantly different from the distribution expected under a null model-the growth of individuals is regulated such that each dominant ends up being about 1.26 times the size of its immediate subordinate. We show that it is decisions about growth at the individual level that generate the size hierarchy at the group level, and thereby determine maximum group size and population size. This study provides a new perspective on the pattern, causes and consequences of size hierarchies.

Optimal grouping algorithm for a group decision feedback detector in synchronous CDMA communications

The group decision feedback (GDF) detector is studied in this letter. Given the maximum group size, a grouping algorithm is proposed. It is shown that the proposed grouping algorithm maximizes the symmetric energy of the multiuser detection system. Furthermore, based on a set of lower bounds on asymptotic group effective energy (AGEE) of the GDF detector, it is shown that the proposed grouping algorithm, in fact, maximizes the AGEE lower bound for every group of users. The theoretical analysis of the grouping algorithm enables the offline estimation of the computational cost and the performance of a GDF detector. The computational complexity of a GDF detector is exponential in the largest size of the groups. Simulation results are presented to verify the theoretical conclusions. The results from this letter can be applied to the decision feedback detector by setting the maximum group size to one.