Variation In Group Size Research Articles

Influence of food availability on the diet and activity budget of two western lowland gorilla (Gorilla gorilla gorilla) groups of differing size in the Dzanga-Ndoki National Park, Central African Republic

Variation in food availability, body size and group size are known to influence primate diet and activity budgets. Here we report how seasonal food availability shapes the diet and activity patterns of two habituated western lowland gorilla (WLG) groups of differing size. WLGs are ripe fruit opportunists, showing dietary flexibility when preferred foods are scarce. However, as fruit can be rare/ patchily distributed, as intra-group feeding competition increases with group size, access to individual patches may be limited. We thus predicted that frugivory decreases with increased group size and influences diet and activity budgets accordingly (increased diet breadth and time feeding since relying on alternative/lower quality food). First, we compared food availability between home-ranges by monitoring leafing/fruiting patterns of major gorilla food trees. Second, we compared the groups’ activity budgets and diet composition/diversity, testing for differences between high (HF) and low fruit seasons (LF). We measured gorilla activity over six months by continuous focal sampling of 16 target animals rotated daily from both groups (N=9 and N=15). Our results confirm that WLG diet consisted mainly of fruits (36%, then: stems 24%; leaves 21%; insects 14%; other food types 3%, and bark 2%) and spend most of their time feeding (39%, then: resting 33%; traveling 19%; social 5%, and other activities 4%). However, contrary to our predictions, we found no group differences within or between seasons: irrespective of group size both spent significantly less time feeding, but more time traveling and socializing, and consumed significantly more fruits, less leaves and bark during the HF compared to the LF. Our results show that WLG activity budgets and diet appear to vary more in response to ripe fruit availability than group size. We suggest that WLGs may cope with increased group size costs through other mechanisms such as group spread, rather than lowering diet quality or further changing activity patterns. Such flexibility may better allow WLG groups to track ripe fruits when available but, unlike sympatric chimpanzees, switch to more herbivorous diets when necessary, adjusting activity budgets accordingly ; WLGs thus may be considered more resilient faced with environmental change such as forest degradation.

Adjusting for Group Size Effects in Peer Nomination Data

AbstractAdjusting nomination‐based sociometric and peer assessment scores for biases due to variations in group size has been a long‐standing concern for peer relations researchers. The techniques that have been typically used to make these adjustments (e.g., proportion and standardized scores) are known to have fundamental problems that limit their utility. This study introduces a regression‐based procedure that adjusts nomination‐based scores for variations in group size and compares it with the standardization and proportion procedures. Analyses were conducted on sociometric and peer assessment scores of 1594 fourth, fifth, and sixth graders from 63 classrooms. The advantages of the regression‐based procedure over standardization and proportion transformations are outlined. Implications for the accuracy and validity of nomination‐based measures and the research findings based on them are discussed.

Epidemiological effects of group size variation in social species

Contact patterns in group-structured populations determine the course of infectious disease outbreaks. Network-based models have revealed important connections between group-level contact patterns and the dynamics of epidemics, but these models typically ignore heterogeneities in within-group composition. Here, we analyse a flexible mathematical model of disease transmission in a hierarchically structured wildlife population, and find that increased variation in group size reduces the epidemic threshold, making social animal populations susceptible to a broader range of pathogens. Variation in group size also increases the likelihood of an epidemic for mildly transmissible diseases, but can reduce the likelihood and expected size of an epidemic for highly transmissible diseases. Further, we introduce the concept of epidemiological effective group size, which we define to be the group size of a hypothetical population containing groups of identical size that has the same epidemic threshold as an observed population. Using data from the Serengeti Lion Project, we find that pride-living Serengeti lions are epidemiologically comparable to a homogeneous population with up to 20 per cent larger prides.

Reduced α‐synuclein levels in cerebrospinal fluid in Parkinson's disease are unrelated to clinical and imaging measures of disease severity

The cerebrospinal fluid (CSF) concentration of α-synuclein may reflect the aggregation of α-synuclein in brain tissue that neuropathologically characterizes Parkinson's disease (PD). Although most studies in large cohorts report reduced CSF α-synuclein levels in PD, the available data to date are not consistent due to variation in group sizes, pre-analytical confounding factors and assay characteristics. Furthermore, it remains unclear whether CSF α-synuclein concentrations correlate with measures of disease severity. Acknowledging the methodological issues that emerged from previous studies, we evaluated whether CSF α-synuclein levels differ between patients with PD and controls, and relate to disease duration or severity. α-Synuclein levels were measured in CSF samples of 53 well-characterized patients with PD and 50 healthy controls employing a recently developed time-resolved Förster's resonance energy transfer assay. In addition, we studied the relationship of CSF α-synuclein levels with disease duration, clinical measures of disease severity and the striatal dopaminergic deficit as measured by dopamine transporter binding and single photon emission computed tomography. In patients with PD, we observed a decrease in mean CSF α-synuclein levels that was unrelated to disease duration or measures of disease severity. Using total protein normalized α-synuclein, a sensitivity and specificity of 70% and 74% could be reached for distinguishing between patients with PD and controls. CSF α-synuclein levels are reduced in patients with PD compared with healthy controls. However, sensitivity and specificity indicate that α-synuclein will not suffice as a single biomarker. CSF α-synuclein levels do not correlate with measures of disease severity, including striatal dopaminergic deficit.

Evolution of Helping and Harming in Viscous Populations When Group Size Varies

Recent years have seen huge interest in understanding how demographic factors mediate the evolution of social behavior in viscous populations. Here we study the impact of variation in group size on the evolution of helping and harming behavior. Although variation in group size influences the degree of relatedness and the degree of competition between groupmates, we find that these effects often exactly cancel, so as to give no net impact of variation in group size on the evolution of helping and harming. Specifically, (1) obligate helping and harming are never mediated by variation in group size, (2) facultative helping and harming are not mediated by variation in group size when this variation is spatial only, (3) facultative helping and harming are mediated by variation in group size only when this variation is temporal or both spatial and temporal, and (4) when there is an effect of variation in group size, facultative helping is favored in big groups and facultative harming is favored in little groups. Moreover, we find that spatial and temporal heterogeneity in individual fecundity may interact with patch-size heterogeneity to change these predictions, promoting the evolution of harming in big patches and of helping in little patches.

Group dynamics of Yellowstone pronghorn

Abstract Understanding mechanisms that influence the grouping tendencies of large herbivores is necessary to predict the influence of environmental and human factors on threatened populations. Locations of 53 adult female pronghorn (Antilocapra americana) in Yellowstone National Park during June 1999–April 2005 indicated that mean and typical group sizes and the variation in group size decreased during fawning when females secluded themselves, but became larger and more dynamic during fawn rearing and the rut and winter. Mixed-effects models indicated a strong effect of time of year on mean group sizes, with some evidence that predators negatively affected group sizes during winter. Within-animal variability (0.64) was substantially higher than between-animal variability (0.02). Pronghorn density, snow water equivalent, and predation apparently influenced variations in group size. Multiple regressions indicated effects of pronghorn density and snow water equivalent on typical group size, the size of the g...

DO PEERS AFFECT STUDENT ACHIEVEMENT? EVIDENCE FROM CANADA USING GROUP SIZE VARIATION

We provide the first empirical application of a new approach proposed by Lee (Journal of Econometrics 2007; 140(2), 333–374) to estimate peer effects in a linear-in-means model when individuals interact in groups. Assumingsufficient group size variation, this approach allows to control for correlated effects at the group level and to solve the simultaneity (reflection) problem. We clarify the intuition behind identification of peer effects in the model. We investigate peer effects in student achievement in French, Science, Mathematics and History in secondary schools in the Province of Québec (Canada). We estimate the model using conditional maximum likelihood and instrumental variables methods. We find some evidence of peer effects. The endogenous peer effect is large and significant in Mathematics but imprecisely estimated in the other subjects. Some contextual peer effects are also significant. In particular, for most subjects, the average age of peers has a negative effect on own test score. Using calibrated Monte Carlo simulations, we find that high dispersion in group sizes helps with potential issues of weak identification. Copyright © 2012 John Wiley & Sons, Ltd.

Competitive Altruism Explains Labor Exchange Variation in a Dominican Community

Smallholder farmers rely on labor exchange to generate agricultural work when cash is rare and credit unavailable. Reciprocal altruism, biased by genetic kinship, has been implicated as the mechanism responsible for labor exchange; however, few empirical tests confirm this proposition. Competitive altruism could be operating if people differ in ability and use this information as a criterion for partnership selection. Labor exchange data are presented from a Dominican smallholder village over a 10-month period within the village’s primary cash economic opportunity, bay oil production. Results indicate that competitive altruism better explains variation in labor exchange relationships and group size than reciprocal altruism and kinship, suggesting the presence of a biologic market for male exchange relationships. Bay oil laborers vary in altruistic behaviors, causing reputations for altruism to emerge. Men with reputations as high-quality altruists generate larger labor groups in bay oil production than do poor-quality ones. Larger groups induce bargaining wars, causing men to compete through altruistic acts, which allows high-quality individuals to discriminate potential partners for labor exchange relationships. Men with better reputations achieve more same-sex reciprocal partnerships but not a greater incidence of conjugal partnership, suggesting that male altruism is intra- but not intersexually selected.

Seasonal Changes In Group Size And Foraging Activity In An Urban Population Of Magpies ( Pica Pica )

Abstract We studied seasonal variation in group size and feeding activity in an urban population of Magpies in Zielona Góra, Western Poland. Each month from 1984 to 1987 we counted the species along a 10 km transect across the city and noted the number of single birds, pairs and flocks as well as flock size. In 1999 and 2000 we regularly observed Magpies foraging on the lawns of a park of Zielona Góra and recorded the number of pecks, as a measure for feeding activity. Magpies were most often encountered as single birds in April and May, as pairs in March, and in flocks in August. This pattern is predominantly dictated by the constraints of the breeding season and by reproductive success. The mean flock size was highest in August. Pecking rate showed a minimum in the summer months, with no sex-specific difference. However, males were much more active in foraging during winter, probably due to their larger body size and the respective higher energetic requirements.

Family values: Maternal care in rattlesnakes is more than mere attendance

AbstractParental care is important in many diverse taxa because of its positive impact on the parent's fitness. Relatively simple forms of parental care, such as female attendance and defense, which is present in many squamate reptiles, will likely provide insight into the evolution of more complex forms observed in crocodilians, birds, and mammals. Historically, observations of adult and newborn rattlesnakes in close proximity were dismissed as mere coincidence of live-birth, and only recently acknowledged as examples of parental care. In 2010, we monitored 11 pregnant Arizona black rattlesnakes (Crotalus cerberus): 4 solitary individuals and 2 aggregations composed of three and four individuals. We documented various forms of parental care, including active defense of neonates against potential predators. Parental care differed among solitary and aggregated females; solitary females spent more time attending and defending their progeny than individuals in aggregations. Natural variation in group size in C. cerberus provides an ancestral condition in which to examine how group membership may increase fitness through parental care.

The Influence of Seasonal Variation on Chimpanzee (Pan troglodytes schweinfurthii) Fallback Food Consumption, Nest Group Size, and Habitat Use in Gishwati, a Montane Rain Forest Fragment in Rwanda

The increased number of primates living in fragmented habitats necessitates greater knowledge of how they cope with large-scale changes to their environment. Chimpanzees (Pan troglodytes) are exceptionally vulnerable to forest fragmentation; however, little is known about chimpanzee feeding ecology in fragments. Although chimpanzees have been shown to prefer fruit when it is available and fall back on more abundant lower quality foods during periods of fruit scarcity, our understanding of how chimpanzees use fallback foods in forest fragments is poor. We examined how chimpanzees cope with periods of fruit scarcity in Gishwati Forest Reserve, a disturbed montane rain forest fragment in Rwanda. We assessed seasonal changes in chimpanzee diet and the use of preferred and fallback foods through fecal and food site analysis. We also examined seasonal variation in nest group size and habitat use through marked nest censuses. We found that chimpanzees experienced a seasonal reduction in preferred fruit availability, which led to a seasonal diet shift to more fibrous foods, including several that functioned as fallback foods. Our results suggest that during periods of fruit scarcity the chimpanzees also reduced nest group size. However, we found that the chimpanzees did not alter their habitat use between high- and low-fruit seasons, which suggests that the small size of the forest limits their ability to change their seasonal habitat use. Consequently, fallback foods appear to be particularly important in small food-impoverished habitats with limited ranging options.

Ecological drivers of group living in two populations of the communally rearing rodent, Octodon degus

Intraspecific variation in sociality is thought to reflect a trade-off between current fitness benefits and costs that emerge from individuals' decision to join or leave groups. Since those benefits and costs may be influenced by ecological conditions, ecological variation remains a major, ultimate cause of intraspecific variation in sociality. Intraspecific comparisons of mammalian sociality across populations facing different environmental conditions have not provided a consistent relationship between ecological variation and group-living. Thus, we studied two populations of the communally rearing rodent Octodon degus to determine how co-variation between sociality and ecology supports alternative ecological causes of group living. In particular, we examined how variables linked to predation risk, thermal conditions, burrowing costs, and food availability predicted temporal and population variation in sociality. Our study revealed population and temporal variation in total group size and group composition that covaried with population and yearly differences in ecology. In particular, predation risk and burrowing costs are supported as drivers of this social variation in degus. Thermal differences, food quantity and quality were not significant predictors of social group size. In contrast to between populations, social variation within populations was largely uncoupled from ecological differences.

Identifying hotspots of chimpanzee group activity from transect surveys in Taï National Park, Côte d'Ivoire

Abstract:Locating activity hotspots – areas of higher density, more intense use, or distinct social units – is a prerequisite for answering many questions in animal ecology. However, for many species, carrying out such research from direct observations in tropical habitat is time-consuming and unrealistic for non-habituated animals. This study aimed to locate chimpanzee home ranges from transect nest counts. For validation purposes, 233 line transects were sampled within the home ranges of four habituated social groups of chimpanzees in Taï National Park, Côte d'Ivoire. In total, 373 km of transects were surveyed over 188 days and 683 nests of chimpanzee were recorded. First, we characterized heterogeneity of nest distribution patterns, including variation in density and group size within the area. Second, we used scan statistics, a likelihood-based cluster technique to locate chimpanzee social groups and compared them with the known home range boundaries. Chimpanzee nest distribution was characterized by a positive density and group-size gradient away from the range periphery. Furthermore, nest distribution clusters corresponding to the four groups could be successfully identified, although additional clusters for, for example, low-density areas between social groups seem to be an unavoidable by-product. The approach taken can be extended to a wide spectrum of data stemming from direct observations, camera traps, acoustic or genetic sampling to derive information about structure and patchiness of wild animal populations.

Group size versus individual group size frequency distributions: a nontrivial distinction

Understanding group size variation is a major challenge in animal ecology. However, we argue that understanding group sizes from an individual point of view (i.e. individual group sizes) and the relationship with population group sizes may be even more important. This may seem redundant, but in the present study we show that it is not. We analysed colony sizes of 20 seabird species breeding in Britain and Ireland from the Seabird 2000 project (19 978 colonies; 3 779 919 nests) comparing group (=colony) size frequency distributions (GSFDs) with their individual group size frequency distribution (IndGSFD) counterparts. We did so for the first time for a number of species with semilogarithmic plots, and correlated eight statistics from each GSFD–IndGSFD pair. Shape-related variables (e.g. skewness) of GSFD–IndGSFD pairs were highly unrelated with only 1–15% of redundancy. In fact, species with similar GSFDs had individuals concentrating in either the largest or the medium-sized groups. There was a trend towards those species with higher group size variation having individuals living in a narrower range of group sizes. Some group size-related measures (e.g. mean group size) showed a tight linear correlation in log–log scatterplots between GSFDs and IndGSFDs. However, this correlation disappeared in linear scatterplots for two of the four measures. Moreover, group size-related measures were always a poor surrogate of corresponding individual group size measures. We discuss how animal grouping research could benefit from similar comparisons between GSFDs and IndGSFDs and how this can be carried out in a meaningful way.

Seasonality of group size, feeding, and breeding in wild red‐shanked douc langurs (Lao PDR)

In Asian colobines, small one-male groups (OMG) seem to predominate alongside all-male groups (AMG), while larger multimale groups (MMG) are rare, but are reported for Hanuman langurs and red-shanked douc langurs. Recently, however, it has been speculated that the genus Pygathrix could have multilevel societies based on (1) a theoretical extension of the multilevel societies found in Rhinopithecus to all odd-nosed colobines and (2) first data for black-shanked douc langurs. This assumes bands composed of small OMG with a skewed adult sex ratio. Band size may vary with seasonal food availability resulting in smaller bands when feeding competition is increased. To investigate the social organization of red-shanked douc langurs and potential seasonal influences, we observed 2 unhabituated groups at Hin Namno National Protected Area, Lao PDR from March 2007 to August 2008 for 803 hr. We recorded births and performed group counts and scan sampling of feeding behavior. Most births (79% of N = 15) occurred from June-September, indicating a 4-month peak conception season from November to February. Group size averaged 24.5 individuals (range 17-45) with 2.45 adult males (range 1-4). Although the smaller group remained at a stable size (about 18 individuals), the larger group reduced from about 45 to 25 individuals during the 7-months long lean season, when less than 50% of the feeding time was spent on fruits. This suggests feeding competition as a potential cause of seasonal variation in group size. With 1.9 females per male the skew in adult sex ratio was much lower compared with Rhinopithecus, indicating MMG rather than multilevel societies. However, data on the spacing and interaction patterns between recognized individuals need to be collected and analyzed before the social organization can be determined. Detailed ecological data are furthermore required to investigate the basis for the seasonal changes in group size found.

Boundary nature induces greater group size and group density in habitat edges: an agent‐based model revealed

AbstractApart from the amount and distribution of food resources and predation pressure, we sought to clarify habitat spatial features that affect group size and group density. To simulate animal groups interacting with one another, we made an agent‐based model in a computational space. Assuming no predation pressure, we change the resource conditions between the habitat (comprising ‘interior’ and ‘edges’) and the ‘exterior’. The results are as follows: when the group density is high throughout the habitat, large groups tend to frequent the edges (i.e., the boundaries between the interior and exterior) even if food resources are scarce in the exterior. Additionally, when the group density is high and the group size variation is small, both group size and group density increase at the edges. These findings are discussed with reference to primates, particularly the Japanese macaque (Macaca fuscata) as an example of group‐living animal.

Commuting, territoriality and variation in group and territory size in a black‐backed jackal population reliant on a clumped, abundant food resource in Namibia

This study examines the effect of a clumped, non-defendable and abundant year-round food resource (Cape fur seals) for black-backed jackal Canis mesomelas social structure and spatial organization at Cape Cross Seal Reserve and the National West Coast Recreation Area in Namibia during the jackals' denning period in 2004 and 2005. Geo-referenced observations of behaviour and space-use were used to test for territoriality, and to assess commuting distances, territory size, group size and within-territory density on the Namibian coast. Jackals displayed behaviour indicative of territoriality to within 50 m of the fur seal colony. In accordance with optimal foraging theory, jackals commuted between 0.45 and 20.03 km from their territory (low prey availability) to the seal colony (high prey availability). The observed within-population variation in group size (two to eight adults), territory size (0.20–11.11 km2) and within-territory density (0.31–9.80 jackals km−2) was unprecedented and strongly associated with distance from the food resource. Group and territory size increased, while within-territory density declined with increasing distance from the fur seal colony. We discuss the relative importance of the food resource and other factors in determining jackal social and spatial organization.

Intraspecific variation in space use, group size, and mating systems of caviomorph rodents

Intraspecific variation in social systems is widely recognized across many taxa, and specific models, including polygamy potential, resource defense, and resource dispersion, have been developed to explain the relationship between ecological variation and social organization. Although mammals from temperate North America and Eurasia have provided many insights into this relationship, rodents from the Neotropics and temperate South America have largely been ignored. In this review we focus on reports documenting intraspecific variation in spacing systems, group size, and mating systems of caviomorphs. This large group of New World hystricognath rodents occupies a diverse array of habitats; thus, members of the same species potentially exhibit different social systems in response to different ecological conditions. Spatial patterns vary in response to a diverse array of factors, including predation, food availability, population density, and soil characteristics. Changes in group size typically correlate with changes in resource availability, particularly food. Mating systems generally reflect the ability of males to control access to females, which may depend on population density or food distribution. In general, social organization in caviomorphs fits predictions of resource-based models; however, most studies have been purely observational, involving small numbers of animals over short time periods and reporting qualitative rather than quantitative levels of ecological correlates. In future studies the use of molecular techniques and controlled, experimental manipulations can increase our understanding of intraspecific variation in caviomorph social systems. This understudied group of rodents offers excellent opportunities to provide insights into the influence of ecological conditions on behavior such as social systems.

Investigating the impact of observation errors on the statistical performance of network-based diffusion analysis

Experiments in captivity have provided evidence for social learning, but it remains challenging to demonstrate social learning in the wild. Recently, we developed network-based diffusion analysis (NBDA; 2009) as a new approach to inferring social learning from observational data. NBDA fits alternative models of asocial and social learning to the diffusion of a behavior through time, where the potential for social learning is related to a social network. Here, we investigate the performance of NBDA in relation to variation in group size, network heterogeneity, observer sampling errors, and duration of trait diffusion. We find that observation errors, when severe enough, can lead to increased Type I error rates in detecting social learning. However, elevated Type I error rates can be prevented by coding the observed times of trait acquisition into larger time units. Collectively, our results provide further guidance to applying NBDA and demonstrate that the method is more robust to sampling error than initially expected. Supplemental materials for this article may be downloaded from http://lb.psychonomic-journals.org/content/supplemental.

Spacing in Colonial Spiders: Effects of Environment and Experience

Colonial orb-weaving spiders from Mexico (Metepeira spp.), show considerable geographic variation and temporal flexibility in group size and social spacing. A series of laboratory studies was conducted to test whether the variation observed in the field is the result of behavioral plasticity, or the result of genetic mechanisms inherent in different populations. Spiders from source populations in desert and moist tropical habitats were collected as eggs and raised in the laboratory under identical controlled conditions. To test for the effect of experience on conspecific tolerance, experiments were conducted rearing tropical and desert spiderlings in isolation and in communal groups. Measurements of spacing in laboratory colonies and observations of web building behavior show significant differences between populations, suggesting genetic differences between them. Desert spiders show an initial effect of isolation on tolerance of conspecifics — greater inter-individual distance that is eventually modified by communal adult experience. Tropical spiders put together after isolation show spacing patterns opposite those observed for desert spiders, and nearest neighbor distances similar to those seen in the communally reared (tropical) groups. These findings support the hypothesis that these populations represent separate species that differ with respect to their level of sociality.