Intermediate Group Sizes Research Articles

Independent fitness consequences of group size variation in Verreaux’s sifakas

The costs and benefits of group living are also reflected in intraspecific variation in group size. Yet, little is known about general patterns of fitness consequences of this variation. We use demographic records collected over 25 years to determine how survival and reproductive success vary with group size in a Malagasy primate. We show that female reproductive rates of Verreaux’s sifakas (Propithecus verreauxi) are not affected by total group size, but that they are supressed by the number of co-resident females, whereas mortality rates are significantly higher in larger groups. Neither annual rainfall nor the adult sex ratio have significant effects on birth and death rates. Hence, these sifakas enjoy the greatest net fitness benefits at small, and not the predicted intermediate group sizes. Thus, independent fitness proxies can vary independently as a function of group size as well as other factors, leading to deviations from optimal intermediate group sizes.

Consensus driven by a minority in heterogenous groups of the cockroach Periplaneta americana

SummaryMany social species are able to perform collective decisions and reach consensus. However, how the interplay between social interactions, the diversity of preferences among the group members and the group size affects these dynamics is usually overlooked. The collective choice between odourous and odorless shelters is tested for the following three groups of social cockroaches (Periplaneta americana) which are solitary foragers: naive (individuals preferring the odorous shelter), conditioned (individuals without preference), and mixed (combining, unevenly, conditioned, and naive individuals). The robustness of the consensus is not affected by the naive individuals' proportion, but the rate and the frequency of selection of the odorous shelter are correlated to this proportion. In mixed groups, the naive individuals act as influencers. Simulations based on the mechanisms highlighted in our experiments predict that the consensus emerges only for intermediate group sizes. The universality of these mechanisms suggests that such phenomena are widely present in social systems.

The effect of group size on the speed of decision making depends on compromise and predation risk across populations in the guppy Poecilia reticulata

Abstract While larger groups tend to be better at making decisions, very few studies have explored how ecological variables, including predation pressure, shape how group size affects decision making. Our cross-population study of wild-caught guppies (Poecilia reticulata) shows that leading individuals from larger groups made faster decisions when deciding to leave the start area and reach the junction of a Y-maze, which allows for compromise over timing. However, at the junction of the Y when the fish needed to make a mutually exclusive decision that does not allow for compromise, there was no effect of group size in high predation fish on decision speed. In fish from low predation habitats, speed was fastest at the intermediate group size with a decline in speed in the largest group size. These results challenge the view that decision making always improves with group size and shows this effect depends on ecological and decision-making conditions.

When Group Dispersal and Allee Effect Shape Metapopulation Dynamics

The dispersal ability of a species will be critical for how population dynamics are realized in spatially structured systems. To date, the effect of group dispersal on metapopulation dynamics is poorly understood. Here, we investigate how group dispersal and Allee effects shape metapopulation dynamics identifying conditions in which group dispersal can be an advantage over independent dispersal. We approach this question by building and analysing a Markovian random walk for metapopulation dynamics including group dispersal and Allee effect. This Markovian random walk is analogous to the discrete-time Stochastic Patch Occupancy Model (SPOM). We find that intermediate group sizes may lead to larger and more sustainable metapopulations in the presence of an Allee effect. Hence, understanding how group size variation and realized (meta)population dynamics are linked offers an exciting future venue for research that is expected to yield key insights into the ecology and evolution of populations occupying spati...

Group size and phenotypic appearance: Their role on the social dynamics in pullets

Non-caged production systems offer greater freedom of movement and behavioural opportunities to pullets, which may also include the occurrence of undesired behaviours. The incidence of such behaviours may be affected by group size but also by the group memberś phenotype. This study was designed to explore the effects of group size and phenotypic appearance in the social dynamics of pullets. A total of 1050day old Hy-line Brown was randomly assigned to 45 pens at 3 group sizes (GS), 10, 20 and 40 birds (constant density 8 hens/m2). For all GS treatments, the phenotypic appearance (PA) of different bird proportions was modified with a black mark at the back of their head. Two types of populations were studied: homogenous (0, 100%) with all pen members presenting the same marked (M) or unmarked (U) phenotype, and heterogeneous (30, 50, and 70%) were the two phenotypes (M and U) coexisted in the same pen. All pens were observed during rearing on alternate weeks. Aggressive and affiliative social interactions performed among group members were registered together with their PA (M or U). The observed and expected mean frequencies for all phenotype combinations involved in the social interaction were calculated: MM, MU, UU and UM, where the giveŕs phenotype is indicated by the first letter and the receiveŕs phenotype by the second letter. Data were analysed with mixed model ANOVAs that included GS and PA as fixed factors for the general model, and GS, PA and type of interaction (MM, MU, UU, UM) to determine the direction of the interaction in the case of heterogeneous groups. Both aggressive (threats) and affiliative interactions (exploratory pecking and beak pecking) were more frequent in small (10) than in large (20, 40) GS (P<0.05) regardless of PA (PA, P>0.05; GS×PA, P>0.05). Aggressive interactions however, occurred at a higher than expected frequency in heterogeneous GS 20, and were predominantly from U towards M birds (UM; P<0.05), as compared to MU or UU. The significant interaction among GS×PA× Type of interaction for affiliative behaviours did not show a clear relationship. Overall, the results obtained showed that both aggressive and affiliative interactions occurred at a higher frequency in the smallest GS 10. Although the effects of PA were not as clear as expected, these results provide evidence of the directionality of the aggressive interactions from U towards M birds especially at intermediate GS 20.

When is bigger better? The effects of group size on the evolution of helping behaviours.

Understanding the evolution of sociality in humans and other species requires understanding how selection on social behaviour varies with group size. However, the effects of group size are frequently obscured in the theoretical literature, which often makes assumptions that are at odds with empirical findings. In particular, mechanisms are suggested as supporting large-scale cooperation when they would in fact rapidly become ineffective with increasing group size. Here we review the literature on the evolution of helping behaviours (cooperation and altruism), and frame it using a simple synthetic model that allows us to delineate how the three main components of the selection pressure on helping must vary with increasing group size. The first component is the marginal benefit of helping to group members, which determines both direct fitness benefits to the actor and indirect fitness benefits to recipients. While this is often assumed to be independent of group size, marginal benefits are in practice likely to be maximal at intermediate group sizes for many types of collective action problems, and will eventually become very small in large groups due to the law of decreasing marginal returns. The second component is the response of social partners on the past play of an actor, which underlies conditional behaviour under repeated social interactions. We argue that under realistic conditions on the transmission of information in a population, this response on past play decreases rapidly with increasing group size so that reciprocity alone (whether direct, indirect, or generalised) cannot sustain cooperation in very large groups. The final component is the relatedness between actor and recipient, which, according to the rules of inheritance, again decreases rapidly with increasing group size. These results explain why helping behaviours in very large social groups are limited to cases where the number of reproducing individuals is small, as in social insects, or where there are social institutions that can promote (possibly through sanctioning) large-scale cooperation, as in human societies. Finally, we discuss how individually devised institutions can foster the transition from small-scale to large-scale cooperative groups in human evolution.

Evolution of public cooperation in a monitored society with implicated punishment and within-group enforcement.

Monitoring with implicated punishment is common in human societies to avert freeriding on common goods. But is it effective in promoting public cooperation? We show that the introduction of monitoring and implicated punishment is indeed effective, as it transforms the public goods game to a coordination game, thus rendering cooperation viable in infinite and finite well-mixed populations. We also show that the addition of within-group enforcement further promotes the evolution of public cooperation. However, although the group size in this context has nonlinear effects on collective action, an intermediate group size is least conductive to cooperative behaviour. This contradicts recent field observations, where an intermediate group size was declared optimal with the conjecture that group-size effects and within-group enforcement are responsible. Our theoretical research thus clarifies key aspects of monitoring with implicated punishment in human societies, and additionally, it reveals fundamental group-size effects that facilitate prosocial collective action.

The role of group size in group lending

This paper explores group size in joint liability lending, primarily in the adverse selection framework with local borrower information. A single, standardized contract that imposes full joint liability subject to a limited liability cap is optimal. Further, if gross returns to borrowing are moderately high, this contract results in perfectly efficient lending if groups are large enough. However, raising group size accomplishes nothing if there is no local borrower information. These results show that more is required for efficient lending than full within-group insurance, and highlight a complementarity between group size and social capital. Very similar results are shown in two different settings, ex ante and ex post moral hazard, though the type of social capital that complements group size varies across the settings. Taking a step toward modeling drawbacks of larger groups, it is shown that if information deteriorates sufficiently with group size, an intermediate group size does better than either extreme. Simulations suggest that most of the efficiency gains from larger groups are realized in group sizes below ten, and that outreach and efficiency can increase dramatically when a moderate group size threshold is crossed.

Growing Groups, Cooperation, and the Rate of Entry

We study the stability of voluntary cooperation in response to varying rates at which a group grows. Using a laboratory public-good game with voluntary contributions and economies of scale, we construct a situation in which expanding a group’s size yields potential efficiency gains, but only if the group overcomes the challenge that growth poses for sustained cooperation. We then study the effect on cooperation of exogenously varying rates of entry. Slow growth yields higher cooperation rates and welfare than fast growth, both for incumbents and entrants. This is consistent with slow growth allowing the persistence of optimistic self-reinforcing beliefs. We also study growth rates determined by incumbent group members. While such endogenous growth generally also produces high cooperation levels, growth stalls at intermediate group sizes, leaving potential efficiency gains from growth unrealized.

Nonlinear effects of group size on collective action and resource outcomes

For decades, scholars have been trying to determine whether small or large groups are more likely to cooperate for collective action and successfully manage common-pool resources. Using data gathered from the Wolong Nature Reserve since 1995, we examined the effects of group size (i.e., number of households monitoring a single forest parcel) on both collective action (forest monitoring) and resource outcomes (changes in forest cover) while controlling for potential confounding factors. Our results demonstrate that group size has nonlinear effects on both collective action and resource outcomes, with intermediate group size contributing the most monitoring effort and leading to the biggest forest cover gain. We also show how opposing effects of group size directly and indirectly affect collective action and resource outcomes, leading to the overall nonlinear relationship. Our findings suggest why previous studies have observed differing and even contradictory group-size effects, and thus help guide further research and governance of the commons. The findings also suggest that it should be possible to improve collective action and resource outcomes by altering factors that lead to the nonlinear group-size effect, including punishing free riding, enhancing overall and within-group enforcement, improving social capital across groups and among group members, and allowing self-selection during the group formation process so members with good social relationships can form groups autonomously.

The volunteer's dilemma and the optimal size of a social group

If one or few individuals are enough to perform an action that produces a collective good and if this action has a cost, living in group can be beneficial because the cost can be shared with other individuals. Without coordination, however, the production of a collective good by the contribution of one or few individuals is inefficient and can be modelled as a volunteer's dilemma. In the volunteer's dilemma the individuals that pay the cost for the production of the collective good benefit from their action if nobody else volunteers, but the cost is wasted if too many individuals volunteer. Increasing group size reduces the need of volunteering for each member of the group; the overall benefit for the group, however, decreases too because the larger the group is, the less likely it is that the collective good is produced. This problem persists even with a high degree of relatedness between group members; an optimal, intermediate group size exists that maximizes the probability to produce the collective good.

Small-world properties emerge in highly compartmentalized networks with intermediate group sizes and numbers

Many recent studies have focused on two statistical properties observed in diverse real-world networks: the small-world property and compartmentalization [D. J. Watts and S. H. Strogatz, Nature 393, 440 (1998); M. Girvan and M. E. J. Newman, Proc. Natl. Acad. Sci. 99, 7821 (2002)]. Models that include group affiliations have been shown to produce networks with high clustering coefficients, a necessary condition for small-world properties [M. E. J. Newman, Phys. Rev. E, 68, 026121 (2003); M. E. J. Newman and J. Park, Phys. Rev. E 68, 036122 (2003)]. However, the consequences of varying the number and size of groups in a network are not well understood. In order to investigate the consequences of group organization, we examined sets of networks that varied simultaneously in the size and number of groups, while maintaining the same overall size and average degree. Here we show that the small-world property arises in maximally compartmentalized and clustered networks that occur in the intermediate region between few, very large groups and many, very small groups.

Production-related traits of layers reared in different sized flocks: the concept of problematic intermediate group sizes

Laying hens were reared from 1 d of age in four replicates each of four different group sizes: 15, 30, 60, and 120 birds. To maintain stocking density at a constant 5 birds/m2, they were housed in litter floor pens of 3, 6, 12, and 24 m2, respectively. The allocation of feeder space, drinker nipples, and perch space was also constant per bird, irrespective of group size, as was the arrangement of resources in the pens. Birds were individually weighed at 3, 7, 12, 15, and 18 wk of age, and comb length and height were measured with calipers. At 24 and 39 wk, a sample of 30 eggs from each pen was weighed to determine mean egg weight. Results show a significant effect of group size on BW, with birds in the groups of 30 and 120 being lighter than birds in groups of 15 and 60. Eggs from birds in groups of 30 were significantly smaller than those from birds in the other group sizes. Comb size was unaffected by group size. We propose that these results support the theory that the hierarchical social structure based on individual recognition in small groups breaks down in large groups as birds become less aggressive and more tolerant. The results suggest that this transition occurs at a group size of around 30 birds, and that this ‘intermediate’ group size presents social problems for birds which, in turn, has consequences for production. The practical implication of this research is to avoid keeping birds in flocks of this size.

Why Lions Form Groups: Food is Not Enough

Extensive observations of foraging female lions reveal that two group sizes maximize foraging success during the season of prey scarcity: one female and five or six females. Foraging success does not vary significantly with group size when prey is abundant. Female lions live in fission-fusion social units (prides) and forage only with members of their own pride. If lion grouping patterns were primarily related to group-size-specific feeding efficiency, females in prides containing fewer than five females should forage alone when prey is scarce, whereas females in larger prides should forage alone or in groups of five or six. However, extensive data on the grouping patterns of radio-collared females show that females in small prides most commonly forage in as large a group as possible, even at the expense of foraging efficiency. Females in large prides most often forage in intermediate group sizes of four or five. However, mothers keep their cubs in a creche and form highly stable maternity groups that are effective in defending the cubs against infanticidal males. Most large prides contain a creche involving four or five mothers, and in the absence of a creche, large prides show no preference for any group size. Females also compete aggressively against neighboring prides, and larger groups successfully repel smaller ones in territorial disputes. Small prides appear to be excessively gregarious in order to compete against larger neighboring prides.

The Relationships Among Population Density, Individual Size, Mating Tactics, and Reproductive Success in a Hermaphroditic Fish, Serranus Fasciatus

AbstractThe hermaphroditic reef fish Serranus fasciatus exhibits three types of social systems. The size of a social group is correlated with the local density of conspecifics. At very low densities, isolated pairs of individuals reciprocally spawn with each other, achieving equal current reproductive success. At intermediate group sizes, harems form, with the largest individual typically losing all of its female function and becoming a functional male. In harems, subordinate hermaphrodites obtain little male reproductive success through streaking, an alternative male mating tactic. The lone pure male maintains almost total monopolization of male reproductive success in harems, apparently due to aggressive domination of subordinates. At high group sizes, the ability of the male to monopolize all of the matings in a social group decreases, and some of the larger hermaphrodites obtain some male-role reproductive success by pair spawning with smaller subordinate hermaphrodites while continuing to spawn as females with the male. Mating partners stay relatively constant through time, resulting in a pattern of small 'sub-harems' within harems. These mating tactics are consistent with the hypothesis that dominant individuals increase their current reproductive success in this species by restricting male mating opportunities of conspecifics. Subordinate individuals spawn as males when the dominant is unable to restrict interactions between hermaphrodites that are potential mates, or when they successfully streak. The increased male reproductive success of hermaphrodites in isolated pairs and complex harems compared with hermaphrodites in harems appears to be important in maintaining a hermaphroditic subordinate phenotype in this largely non-reciprocating species.