Function Of Group Size Research Articles

Social cohesion and foraging decrease with group size in fallow deer (Dama dama)

We studied the impact of group size on foraging behaviour and level of movement synchronisation among female herdmates of a fallow deer population in Central Italy. Both proportion of foraging events and movement synchronisation decreased with increasing group size. The proportion of foraging events was higher for animals on the edge of the group than for deer in the centre of the group; hence, there appears to be a trade-off between protection against predators and foraging interference, both of which decrease from the centre to the periphery of the group. This is the first time this type of behaviour has been recorded for wild ungulates. As expected, we also found that the movement of peripheral animals was less synchronised than that of central animals. Consequently, peripheral animals may lose contact with their herdmates and split off the group. We conclude that social inequalities may lead to conflicting requirements among group members and instability of large groups. Movement synchronisation (as a function of group size) appears to interact with habitat openness to produce variations of group size (which appear to be adaptive for individuals) as an emergent property of these aggregations.

A Test of the Multi‐Predator Hypothesis: Rapid Loss of Antipredator Behavior after 130 years of Isolation

AbstractMany species find themselves isolated from the predators with which they evolved. Isolation often leads to the loss of costly antipredator behavior, which may have adverse consequences if the population should later come into contact with predators. An understanding of both the mechanism (i.e. the degree to which antipredator behavior depends on experience), and of the time course of loss is important to be able to predict how a population will respond to future contact. We studied ‘group‐size effects’– the way in which animals change the time they allocate to antipredator vigilance as a function of group size – and visual and acoustic predator recognition in a population of tammar wallabies (Macropus eugenii), a cat‐sized (6–10 kg) macropodid marsupial. To study group size effects we observed wallabies foraging in four populations – three with some sort of predator and a New Zealand population that was isolated from all predators for about 130 yr. To study predator recognition, we observed the response of New Zealand wallabies to the presentation of a model or taxidermic mount of mammalian predators, and to the broadcast sounds of mammalian and avian predators. We compare these predator recognition experiments with results from a previous study of Kangaroo Island (South Australia) tammars. Complete isolation from all predators for as few as 130 yr led to the loss of group size effects and a rapid breakdown in visual predator recognition abilities. Our results are consistent with a key prediction of the multi‐predator hypothesis – namely, that the isolation from all predators may lead to a rapid loss of antipredator behavior.

Aggression and group size in domesticated pigs, Sus scrofa: ‘when the winner takes it all and the loser is standing small’

Formation of artificial groups of domestic species occurs frequently in today's husbandry systems. These groups are kept in a closed space where the individuals are not able to withdraw from the group, and resources in the environment are most commonly limited and defensible. This creates a competitive environment where aggression and social stress are often seen. We developed a theoretical model that describes how aggression among unacquainted pigs is a function of group size. As the number of potential competitors increases, more individuals will benefit from not getting involved in costly fights. The reason for this is that the probability of being able to monopolize resources diminishes as group size increases. To verify predictions from this model, we compared aggression among unacquainted, weaned pigs in three group sizes (6, 12 and 24 pigs). There were significantly more fights per individual in groups of six and 12 pigs than in groups of 24 pigs, and the proportion of pigs not participating in fights was significantly higher in the largest group. For pigs that were willing to take the risk of getting involved in fights in the largest group size, the duration of each fight was longer than for fights in the smaller groups. These results support the predictions from our theoretical model, and we conclude that pigs are able to alter their behaviour according to how the actual competitive situation changes with group size.

IMPACT OF HUMAN NUISANCE DISTURBANCE ON VIGILANCE AND GROUP SIZE OF A SOCIAL UNGULATE

In social ungulates, the proportion of time devoted to vigilance is a function of group size (known as the group size effect). We studied how varying levels of human disturbance influence this function in the mountain gazelle (Gazella gazella) along the southern coastal plain of Israel. Based on current theory, we predicted that changes in the slope of this function should be associated with changes in average group size. In heavily disturbed areas, individual vigilance levels increased in the bigger groups, causing the vigilance vs. group size curve to flatten, i.e., vigilance was high in all group sizes. Consequently, and in accordance with theory, we found a negative relationship between group size and human disturbance. Specifically, we found that in open areas with low disturbance levels, gazelles were in bigger groups than in open areas with high disturbance levels. In social species, the disruption of behavioral patterns by increased human presence can affect their social structure. Because social structure is a key component in the evolution and dynamics of social species, its disruption by human disturbance can have a considerable effect on population performance even if the disturbance does not directly impact survival and reproduction. Social disruption due to increased access to natural areas should be an important consideration in managing fragmented landscapes.

Isaac, Ishmael, and Janus: past and future lessons regarding the ethnic categorization of faces

AbstractThe events of 9/11/2001 have stimulated an unprecedented degree of attention to, and efforts to identify, individuals of Arab descent. Following the Holocaust, social psychologists studied the extent to which Jewish and Non‐Jewish people could accurately categorize faces as Jewish vs. Non‐Jewish. This study provides a systematic analysis of the results of these earlier studies of the categorization of Jewish faces, with an eye toward informing the current cultural fixation upon the categorization of Arab faces. A meta‐analysis was conducted upon all available tests of the accuracy of categorization of Jewish and Non‐Jewish faces, allowing the empirical scrutiny of three models of facial perception. Judges can categorize Jewish faces to a significant, albeit extremely small degree; judges can categorize Non‐Jewish faces to a much greater degree. Results are most consistent with a model of facial perception that incorporates variations of cognitive representations of groups as a function of group size. These results suggest that training protocols for airport security guards should engage exemplar cognitive representations, emphasizing distinguishing features (rather than ethnic category membership) of faces. Copyright © 2003 John Wiley & Sons, Ltd.

Habitat preference reflects social organization of humpback whales (Megaptera novaeangliae) on a wintering ground

Habitat preference is driven by a complex interaction among behavioural patterns, biological requirements, and environmental conditions. These variables are difficult to determine for any species but are further complicated for migratory marine mammals, such as humpback whales Megaptera novaeangliae. Patterns of habitat use in relation to social organization potentially exist for this species on their wintering grounds. Using an integrated GIS approach, we examined the degree to which spatial patterns of habitat stratification are correlated within different humpback whale group types from 6 years of sighting data (1996–2001) collected on the Antongil Bay, Madagascar, wintering ground. Stratification of humpback whale sightings by behavioural classification showed significant variation in depth and distance from shore. Distribution by depth could not be described as a function of group size but could be described as a function of social organization, with mother–calf pairs showing a strong preference for shallower water compared to all other group types. Group size and social organization seem to be factors in distribution by distance from shore. Significant diurnal patterns in distribution by depth and distance from shore also exist, where mother–calf groups maintain a relatively stable distribution and pairs and competitive groups are the most variable. Patterns of habitat preference on this wintering ground appear to be guided by social organization, where distribution by depth and distance from shore highlight areas critical to conservation.

GROUP SIZE BUT NOT DISTANCE TO COVER INFLUENCES AGILE WALLABY (MACROPUS AGILIS) TIME ALLOCATION

Despite their widespread distribution and relative abundance, little is known about the antipredator behavior of agile wallabies (Macropus agilis). The ability to modify time allocation as a function of group size is an antipredator behavior that has been reported in some, but not all, previous studies of members of the genus Macropus. We studied group-size effects and space use for agile wallabies foraging in the open to develop a better understanding of behavior of mid-sized macropods. We found that agile wallabies modified time allocated to foraging and vigilance as a function of group size but were insensitive to distance from cover. We contrast the group size results from agile wallabies with those from other macropodids and speculate that predation by saltwater crocodiles (Crocodylus porosus) may have a profound effect on how agile wallabies perceive and manage predation risk.

Red kangaroos ( Macropus rufus ) receive an antipredator benefit from aggregation

For species that cannot seek cover to escape predators, aggregation becomes an important strategy to reduce predation risk. However, aggregation may not be entirely beneficial because aggregated animals may compete for access to limited resources and might even attract predators. Available evidence suggests that foraging competition influences time allocation in large-bodied macropodid marsupials, but previous studies have focused primarily on species in areas with protective cover. We studied red kangaroos, a species often found in open country without noticeable cover, to determine whether they experienced a net benefit by aggregation. Red kangaroos varied their time allocation as a function of group size and, importantly, more variation in time allocation to vigilance and foraging was explained by non-linear models than by linear models. This suggests red kangaroos directly translated the reduction of predation risk brought about by aggregation into greater time foraging and less time engaged in vigilance. We infer that red kangaroos received a net benefit by aggregation. Social species living in the open may be generally expected to rely on others to help manage predation risk.

Conformity effects in memory as a function of group size, dissenters and uncertainty

AbstractIn two experimental studies we investigated how false memory reports provided by anonymous group members affect individuals' recognition. Participants were presented with a series of salient and non‐salient objects and recognition memory was subsequently assessed for presented and non‐presented stimuli. In this recognition test, participants were confronted with (in)correct responses from bogus group members. The findings indicate that the bogus answers influenced memory reports only in the absence of a clear recollection and if the stimulus was not judged to be particularly memorable. Moreover, we found no evidence that motivational influences affected participants' judgements. However, the influence of the bogus group decreased with the presence of dissenters who broke the unanimity of the group, and it increased with higher uncertainty. Copyright © 2002 John Wiley & Sons, Ltd.

A macroscopic analytical model of collaboration in distributed robotic systems.

In this article, we present a macroscopic analytical model of collaboration in a group of reactive robots. The model consists of a series of coupled differential equations that describe the dynamics of group behavior. After presenting the general model, we analyze in detail a case study of collaboration, the stick-pulling experiment, studied experimentally and in simulation by Ijspeert et al. [Autonomous Robots, 11, 149-171]. The robots' task is to pull sticks out of their holes, and it can be successfully achieved only through the collaboration of two robots. There is no explicit communication or coordination between the robots. Unlike microscopic simulations (sensor-based or using a probabilistic numerical model), in which computational time scales with the robot group size, the macroscopic model is computationally efficient, because its solutions are independent of robot group size. Analysis reproduces several qualitative conclusions of Ijspeert et al.: namely, the different dynamical regimes for different values of the ratio of robots to sticks, the existence of optimal control parameters that maximize system performance as a function of group size, and the transition from superlinear to sublinear performance as the number of robots is increased.

Comparative patterns of philopatry and dispersal in two common mole‐rat populations: implications for the evolution of mole‐rat sociality

Summary 1. The aridity food‐distribution hypothesis (AFDH) maintains that ecological constraints and foraging risks in arid environments curtail dispersal and promote the evolution of cooperatively foraging social groups within African mole‐rat populations. To assess the influence of habitat constraints upon mole‐rat social behaviour and hence the validity of the AFDH, we investigated the differential patterns of dispersal and philopatry in two populations of the common mole‐rat, Cryptomys hottentotus hottentotus Lesson, one from a more ‘optimal’ habitat (mesic) and another from a ‘suboptimal’ habitat (arid). 2. Rates of immigration and emigration were markedly lower in the arid relative to the mesic site, reflecting divergence in the ecological constraints upon dispersal. 3. The rates of dispersal at both sites increased as a function of group size. However, for any given group size, colony attrition was significantly greater at the mesic site than at the arid site, probably reflecting the relaxation of constraints upon dispersal in mesic areas. 4. There was no sex bias in dispersal at either study site. 5. At the mesic sites dispersing and nondispersing individuals were of comparable mass, whilst at the arid site there was a trend for dispersing animals to be heavier than their nondispersing counterparts. 6. This investigation revealed marked differences between the study populations in the nature of dispersal. This reflects adaptive variation in social behaviour between the regions, and the results suggest that delayed dispersal and cooperation may be more crucial to individual survival in arid than in mesic areas. As such, these findings provide support for the underlying contention of the AFDH that ecological constraints on successful dispersal and colony formation in arid areas have promoted a greater degree of social cohesion in mole‐rats occurring in these regions, ultimately advancing the evolution of group‐living and cooperative foraging in the Bathyergidae.

A permutation procedure for testing the equality of pattern hypotheses across groups involving correlation or covariance matrices

This paper describes a permutation procedure to test for the equality of selected elements of a covariance or correlation matrix across groups. It involves either centring or standardising each variable within each group before randomly permuting observations between groups. Since the assumption of exchangeability of observations between groups does not strictly hold following such transformations, Monte Carlo simulations were used to compare expected and empirical rejection levels as a function of group size, the number of groups and distribution type (Normal, mixtures of Normals and Gamma with various values of the shape parameter). The Monte Carlo study showed that the estimated probability levels are close to those that would be obtained with an exact test except at very small sample sizes (5 or 10 observations per group). The test appears robust against non-normal data, different numbers of groups or variables per group and unequal sample sizes per group. Power was increased with increasing sample size, effect size and the number of elements in the matrix and power was decreased with increasingly unequal numbers of observations per group.

How White-Throated Magpie-Jay Helpers Contribute during Breeding

We investigated the mechanisms by which helpers contribute to breeder reproduction in a Costa Rican population of White-throated Magpie-Jays (Calocitta formosa). Helpers provided a substantial proportion of all feedings to female breeders and their offspring, proportionately more than most species of cooperatively breeding New World jays. Breeding males typically fed breeding females and offspring less frequently than expected, however. There was little evidence of brood division in the sense of individual provisioners (breeders or helpers) preferentially feeding particular fledglings within a brood. The rate of provisioning per recipient increased as a function of group size only during the pre-incubation period (provisioning of the laying female). Provisioning rates per nestling and per fledgling were not correlated with group size, and the number of offspring fledged per successful nest did not increase with group size. Helpers did reduce the provisioning burden on breeders, however, and occasionally were the primary care-providers of fledglings, which allowed breeders to renest. More successful nests were produced in groups with many helpers than few, resulting in more fledged young per year. Mechanisms contributing to this "helper-effect" included more nesting attempts per year and a higher likelihood of renesting after a successful attempt. We conclude that the contributions of magpie- jay helpers increased breeder fitness, and the indirect and direct benefits gained by helping probably favored its expression by nonbreeding group members.

Group size and aggression: ‘recruitment incentives’ in a cooperatively breeding primate

In many cooperatively breeding birds and mammals, group size is positively correlated with reproductive success. In marmoset and tamarin monkeys, species that display cooperative breeding, the presence of helpers appears to be critical for offspring survival, and breeders might be expected to display social strategies that would regulate group size. This study investigated the association between group size and aggression towards strangers in Wied's black tufted-ear marmosets,Callithrix kuhlifrom small groups (with no helpers present) and large groups (with helpers present). Residents were exposed to multiple presentations of male and female strangers. Breeding females from large groups spent more time in close proximity to strangers, showed higher levels of agonistic displays and engaged in higher levels of aggressive behaviour towards intruders than did breeding females from small groups. Breeding male behaviour did not dramatically differ as a function of group size. After the removal of the intruder, female breeders from large groups showed higher levels of scent-marking relative to baseline observations, and time spent in close proximity to the partner increased after exposure to female intruders in large, but not small, groups. The results reveal that breeders from small groups are tolerant of strangers, which may facilitate the recruitment of additional group members, whereas breeders from large groups, particularly females, are intolerant of strangers, which may inhibit immigration. These findings have implications for understanding mechanisms that regulate immigration in cooperatively breeding animals, and suggest of how breeders from small groups might reconcile the serious limitation of having few or no helpers.

Activation pattern reproducibility: Measuring the effects of group size and data analysis models

The reproducibility of patterns from brain activation experiments has been examined only for suprathreshold spatially localized foci. Scatter plots comparing signal levels across all pairs of Talairach voxels for pairs of functional activation images provide an alternative approach for assessing reproducibility. Image-wide, signal-level reproducibility may be quantitatively summarized using pattern similarity measures such as the Pearson product-moment correlation, rho. Empirical population distributions of rho for many pair-wise image comparisons, generated using statistical resampling techniques, may be used to examine the impact of a wide range of experimental variables. We demonstrate the use of such empirical rho-histograms to measure changes in reproducibility for [(15)O]-water PET scans of a simple motor task as a function of group size and data analysis model.

A marginal model of tolerated theft

Using marginal analysis to represent Blurton Jones's concept of tolerated theft, I show how equilibrium resource transfers among individuals might be affected by foraging behavior, resource qualities, and number of participants. The model applies to hominids and other species that exchange or share food or other resources. Among the results: Tolerated theft enhances the value to be derived from resources, packets intermediate in size are most likely to be subjected to tolerated theft, packet division is more likely to be unequal than equal, division is a function of group size, and tolerated theft is most likely in small groups. The model also suggests that among reciprocators the widest possible exchange or sharing is in the self-interest of the individual procuring the resource. In general, evolutionary cost-benefit accounting should track marginal changes in the value (fitness or utility) of resources. Marginal valuation is conceptually primary and may produce results that differ from direct measures of quantity.

Birth rate and survival in relation to group size in the lion-tailed macaque,Macaca silenus

Birth rate and immature survival rate in group living primates have been predicted to vary as a function of group size. These predictions were tested with data from a wild population of lion-tailed macaques in the Anamalai Wildlife Sanctuary, Tamil Nadu (India). Group size and composition, births, and mortality were monitored from eight groups for one to six years. The rate of growth of groups, birth rate, and survival rate were examined in relation to group size. The rate of growth of a group was a decreasing function of group size. Birth rate was also a decreasing function of group size and the number of adult females in the group. The sample size for survival rate of adults was too small to test for their dependence on group size. Immature survival was independent of group size, but the analysis was based on a very small sample. It is possible that the nature of dependence of net growth rate, birth rate, and survival rate on group size could be different outside the range of group sizes on which the analysis was based. The stronger relationship between birth rate and group size in the lion-tailed macaque, in relation to other primates, might be the result of its low birth rate as well as the relatively low temporal variation in resources in the rain forest. Limited data available from other sites indicate that the observed relationship between birth rate and group size might be spatially stable.

A performance comparison of asynchronous atomic broadcast protocols

Atomic broadcast ensures that concurrent updates to replicated data maintained by a process group are consistently delivered to all group members despite random communication delays and failures. By simplifying the programming of applications that use replicated data, atomic broadcast provides basic support for implementing fault-tolerance in distributed systems. This paper reports discrete event simulation results that compare the performance of four asynchronous atomic broadcast protocols. We investigate five performance indexes: average delivery time, average stability time, average number of physical messages sent per update broadcast, maximum buffer size, and distribution of processing load among group members. These indexes are measured as a function of group size and update interarrival time, both in the absence of failures and in the presence of a single communication failure. Our comparison shows that there is no overall best protocol. We identify those application areas where a protocol dominates the other protocols and we discuss some protocol design techniques for achieving good performance.

Cognitive Representation in Ethnophaulisms as a Function of Group Size: The Phenomenology of Being in a Group

Two studies examined cognitive representation for minority groups of varying sizes. Cognitive representation was operationalized as the degree of complexity in the ethnophaulisms, or ethnic slurs, for these minority groups. The results of these studies confirm the prediction that prototype cognitive representations will be more prevalent for smaller groups. The relation between group size and cognitive representation could not be dismissed as an artifact of a more fundamental association between prejudice and cognitive representation. Moreover, this basic effect did not seem to vary as a function of the region of origin of these ethnic groups or membership in the groups. The Discussion section considers the implications of these results for the study of stereotyping.

Extraction of Teamwork Determinants in Japanese Manufacturing Industries

A survey of a large sample of Japanese industrial plant personnel is conducted in order to investigate the determinants of effective teamwork. Affinity relationship and multivariate analysis methods were used to identify the underlying determinants of teamwork. The relationships between such determinants for group leaders and their subordinates as a function of group size are analyzed. Teamwork determinants are classified into two categories, namely Task Oriented which relate to group productivity, and People Oriented which relate to group cohesiveness. In general group leaders perceived the latter category of determinants as being necessary for effective teamwork, while their subordinates believed that task oriented determinants were more important.