The Demographic Benefits of Belligerence and Bravery: Defeated Group Repopulation or Victorious Group Size Expansion?

Intraspecific coalitional aggression between groups of individuals is a widespread trait in the animal world. It occurs in invertebrates and vertebrates, and is prevalent in humans. What are the conditions under which coalitional aggression evolves in natural populations? In this article, I develop a mathematical model delineating conditions where natural selection can favor the coevolution of belligerence and bravery between small-scale societies. Belligerence increases an actor''s group probability of trying to conquer another group and bravery increase the actors''s group probability of defeating an attacked group. The model takes into account two different types of demographic scenarios that may lead to the coevolution of belligerence and bravery. Under the first, the fitness benefits driving the coevolution of belligerence and bravery come through the repopulation of defeated groups by fission of victorious ones. Under the second demographic scenario, the fitness benefits come through a temporary increase in the local carrying capacity of victorious groups, after transfer of resources from defeated groups to victorious ones. The analysis of the model suggests that the selective pressures on belligerence and bravery are stronger when defeated groups can be repopulated by victorious ones. The analysis also suggests that, depending on the shape of the contest success function, costly bravery can evolve in groups of any size.     

The sociality of solitary foragers: a model based on resource dispersion

Under versions of what may broadly be called the Resource Dispersion Hypothesis (RDH) several authors have concluded that territory size and group size are limited respectively, and independently, by the dispersion and richness of patches of food. This paper presents a model that shows how the frequency distribution of resources available per unit time within a territory may permit the formation of groups even in the absence of any functional advantage to any individual from the presence of another. In this model, animals (called primary occupants) occupy territories containing sufficient resources to meet or exceed their requirements for a critical proportion of feeding periods. The availability of these resources is described in terms of their mean richness and their heterogeneity, and plots of these parameters indicate the circumstances within which individuals may share the minimum territory with the primary occupants. The model shows how, under plausible conditions of resource dispersion, a territory that provides almost total food security for two occupants could also provide, at no cost to the original occupants, substantial food security for an additional group member, even if it never used the same food patches as the originals. It is not therefore, as is sometimes supposed, a necessary condition for the RDH that members of a group often forage simultaneously in the same patch. Thus the model describes ecological circumstances whereby groups could evolve amongst species whose members neither forage communally, nor even meet frequently.     

Defence, intrusion and the evolutionary stability of territoriality

Territorial behaviour can only be adaptive if its costs are outweighed by its benefits. Territorial individuals incur costs by defending their territories against intruders. Usually these intruders are assumed to be non-territorial floaters attempting to take over the whole territory or neighbours trying to extend the borders of their own territory. We instead investigate how costs and benefits of territorial behaviour are affected by neighbours which invade to steal resources on a territory.We show analytically that in the absence of defence intrusion into neighbouring territories always pays and that even if territories are defended intrusion levels can still be high. Using a more detailed simulation model we find that territory defence usually disappears from the population even if owners have a strong advantage over intruders in terms of fighting costs or foraging efficiency. Defence and thus territoriality can only be evolutionarily stable if fighting costs for the intruder relative to the productivity of the territory are very high or if crossing the borders between territories carries additional costs.Our results show that stealing of resources by neighbours can have a considerable effect on the evolutionary stability of territory defence and thus territoriality itself. A more mechanistic model of territorial behaviour is needed to incorporate these kinds of mechanisms into a general theory on the evolution of territoriality.     

Genetic relatedness and group size in an aggregation economy

Summary We use Hamilton's Rule to investigate effects of genetic relatedness on the predicted size of social groups. We assume an aggregation economy; individual fitness initially increases with group size, but in sufficiently large groups each member's individual fitness declines with further increments in the size of the group. We model two processes of group formation, designated free entry and group-controlled entry. The first model assumes that solitary individuals decide to join groups or remain alone; group size equilibrates when solitaries no longer choose to join. The second model allows group members to regulate the size of the group, so that the predicted group size results from members' decisions to repel or accept intruding solitaries. Both the Nash equilibrium group size and any change in the equilibrium caused by varying the level of relatedness depend on the particular entry rule assumed. The largest equilibrium group size occurs when solitaries choose between joining or not joining and individuals are unrelated. Increasing genetic relatedness may reduce and can never increase, equilibrium group size when this entry rule applies. The smallest equilibrium group size occurs when group members choose between repelling or accepting intruders and individuals are unrelated. Under this entry rule, increasing genetic relatedness can increase and can never decrease, equilibrium group size. We extend the models' predictions to suggest when individuals should prefer kin vs non-kin as members of the same group.     

Do Feeding Resources Induce the Adoption of Resource Defence Polygyny in a Lekking Butterfly?

Among polygynous species, males often compete for the possession of mating sites to increase their reproductive success. Weaker individuals frequently adopt alternative non‐territorial mate‐locating tactics, but the adoption of alternative territorial tactics may also occur. Although alternative tactics with territory defence are less common in arthropods, factors that drive its adoption may provide information to understand the organization of different territorial mating systems in the group. Here we investigate the adoption of resource‐based territoriality as an alternative to a non‐resource‐based one by males of the butterfly Paryphthimoides phronius. Male P. phronius commonly defend sunny clearings lacking feeding resources in the forest edge (non‐resource‐based territoriality). However, after experimentally offering fermenting fruit in previously undefended sites, we showed that males also adopt a resource defence tactic. Males in territories with fermenting fruits apparently feed on this resource when they are not defending the territory. However, males in sites without resources did not migrate to territories with resources when given the opportunity. To our knowledge, this is the first experimental investigation to show a non‐resource‐ and an alternative resource‐based mate‐locating tactic in a butterfly. We suggest that this behavioural flexibility may represent an important step to understand the ecological factors responsible for the organization and evolution of different territorial mating systems in insects.     

Territorial defense, territory size, and population regulation

The carrying capacity of an environment is determined partly by how individuals compete over the available resources. To territorial animals, space is an important resource, leading to conflict over its use. We build a model where the carrying capacity for an organism in a given environment results from the evolution of territorial defense effort and the consequent space use. The same evolutionary process can yield two completely different modes of population regulation. Density dependence arises through expanding and shrinking territories if fecundity is low, breeding success increases gradually with territory size, and/or defense is cheap. By contrast, when fecundity is high, breeding success sharply saturates with territory size, and/or defense is costly, we predict fixed territory sizes and regulation by floaters. These "surplus" individuals form a buffer against population fluctuations. Yet floaters can also harm breeder performance, and by comparing population growth of a territorial population to a nonterritorial (and individually suboptimal) alternative, we can quantify the harmful effect of evolutionary conflict on population performance. Territoriality has often been found to increase population stability, but this may come at a cost of reduced equilibrium densities.     

Factors affecting binary sex evolution with respect to avoidance of vertical transmission of deleterious intracellular parasites

In sexual reproductive systems, the number of sexes is generally binary, viz. male and female. Several theoretical studies have shown that the evolution of this system is possibly related to cytoplasmic DNA, including deleterious cytoplasmic symbionts. When organisms are infected by a symbiont that is transmitted vertically to offspring via gametes, the exclusion or degeneration of the latter may evolve as a characteristic of those organisms. If this necessarily results in the elimination of organelle DNA in gametes, a reciprocal preference between individuals, one transmitting organelles and the other not, may be favored. In this theoretical study, factors affecting such an evolutionary process, in which the symbiont is considered as a parasite infecting vertically, horizontally and naturally, are considered. In addition, host individuals are assumed to recover from the infection to some degree. According to the analysis, a binary sex system can evolve only when uninfected and infected host individuals co-exist in a single host population. This condition can be satisfied only if natural infection occurs. Although recovery from infection has both positive and negative effects on binary sex evolution, the latter is promoted only when natural infection exists. Accordingly, if natural infection does not exist, the evolution of binary sex system is unlikely with respect to deleterious cytoplasmic symbionts, in absent of heterozogotic advantage in vertical transmission.     

Manipulating daytime sleep in herring gulls (Larus argentatus)

The regular provision of a sufficient quantity of food on the territory of pairs of breeding herring gulls led to an increase in the time individuals spent on the territory during the day, relative to controls. This ‘extra’ time was not used to increase overall sleeping time, as predicted. Instead, the experimental gulls reduced their overall sleep time due to a decline in front-sleep. Back-sleep remained relatively unaffected, with both groups spending similar amounts of time in this posture. The consequences of the two postural types of sleep were examined with respect to the cost of allowing intruders to remain on an owner's territory. Intruders arrived more often on control territories and stayed longer when only one pair member was present. Intruders were seen on control territories for more consecutive scans when the solitary control gull was in a sleep posture, particularly front-sleep. It is concluded that these results imply a difference in the consequences resulting from the two sleep postures and give additional support for the multiple function hypothesis of sleep.     

The co-evolution of culturally inherited altruistic helping and cultural transmission under random group formation

Limited migration results in kin selective pressure on helping behaviors under a wide range of ecological, demographic and life-history situations. However, such genetically determined altruistic helping can evolve only when migration is not too strong and group size is not too large. Cultural inheritance of helping behaviors may allow altruistic helping to evolve in groups of larger size because cultural transmission has the potential to markedly decrease the variance within groups and augment the variance between groups. Here, we study the co-evolution of culturally inherited altruistic helping behaviors and two alternative cultural transmission rules for such behaviors. We find that conformist transmission, where individuals within groups tend to copy prevalent cultural variants (e.g., beliefs or values), has a strong adverse effect on the evolution of culturally inherited helping traits. This finding is at variance with the commonly held view that conformist transmission is a crucial factor favoring the evolution of altruistic helping in humans. By contrast, we find that under one-to-many transmission, where individuals within groups tend to copy a “leader” (or teacher), altruistic helping can evolve in groups of any size, although the cultural transmission rule itself hitchhikes rather weakly with a selected helping trait. Our results suggest that culturally determined helping behaviors are more likely to be driven by “leaders” than by popularity, but the emergence and stability of the cultural transmission rules themselves should be driven by some extrinsic factors.     

Effects of Group Size on Approach to Novel Objects in Ravens (Corvus corax)

Neophobia may constrain explorative behaviour, learning and innovation, while social context may facilitate approach to novel objects and acceptance of novel food. We examined the effects of neophobia on the exploration of novel objects in relation to social context in ravens (Corvus corax). Ravens are suitable subjects for studying effects of social context, as they are highly neophobic scavengers that recruit conspecifics to food. We tested two groups of six and 11 hand‐raised birds in three conditions: single‐bird, in dyadic combinations and sibling sub‐groups of three to six birds. Contrary to expectation, individuals of both groups were quicker to approach novel objects when tested alone than when tested with conspecifics. However, they spent more time close to and manipulating the novel objects in the social conditions (dyadic and group) than when being alone. We discuss the possibility that the higher latencies of dyads and groups to approach novel objects may reflect a ‘negotiation’ process in a ‘war of attrition’ between the individuals over risk‐taking.     

Green woodhoopoe Phoeniculus purpureus territories remain stable despite group-size fluctuations

Cooperatively breeding groups may be constrained in size by the territory available to them, or territories may be expanded to accommodate extra group members. Here, we show that there was no relationship between the number of adult green woodhoopoes Phoeniculus purpureus in a group and the size of its territory. Furthermore, territories were remarkably stable between seasons, with no significant changes in area, despite fluctuating group sizes. These results suggest that food was not limiting at the group sizes found in this study: sufficient resources were available within existing territories for groups that were expanding in size. Following an increase in group membership, a larger proportion of the available area was utilised. Groups also used a larger area in the non-breeding season compared to when breeding: in the latter instance, foraging was concentrated in the vicinity of the nest.     

Variation and multilevel selection of SARS-CoV-2

The evolution of SARS-CoV-2 remains poorly understood. Theory predicts a group-structured population with selection acting principally at two levels: the pathogen individuals and the group of pathogens within a single host individual. Rapid replication of individual viruses is selected for, but if this replication debilitates the host before transmission occurs, the entire group of viruses in that host may perish. Thus, rapid transmission can favor more pathogenic strains, while slower transmission can favor less pathogenic strains. Available data suggest that SARS-CoV-2 may follow this pattern. Indeed, high population density and other circumstances that favor rapid transmission may also favor more deadly strains. Health care workers, exposed to pathogenic strains of hospitalized patients, may be at greater risk. The low case fatality rate on the Diamond Princess cruise ship may reflect the founder effect—an initial infection with a mild strain. A vaccine made with one strain may confer limited immunity to other strains. Variation among strains may lead to the rapid evolution of resistance to therapeutics. Finally, if less pathogenic strains are largely associated with mild disease, rather than treating all SARS-CoV-2 positive individuals equally, priority could be focused on testing and contact tracing the most seriously symptomatic patients.     

Sex-biased dispersal of adults mediates the evolution of altruism among juveniles

Population viscosity has been proposed as an important mechanism for the evolution of cooperation. The idea is that if individuals do not disperse far during the course of their lives, they will tend to interact with their genealogical relatives, which may give kin-selected benefits for cooperation. However, in the simplest model of population structure, the evolution of cooperation is unaffected by the rate of dispersal, owing to dispersal also mediating competition between social partners. This surprising result has generated much research interest in recent years. Here I show that dispersal does matter if there is a sex difference in dispersal rate, even when the expression of cooperation is not conditional upon the actor's dispersal status or sex. In particular, I show that cooperation among juveniles is relatively favoured when there is a small sex bias in adult dispersal in favour of the sex with the greatest variance in reproductive success, and is relatively disfavoured when this sex bias is large or in the opposite direction. This is because dispersal by individuals of each sex can have different consequences for the genetic structure of the population.     

Why long-lived species are more likely to be social: the role of local dominance

Recent studies have shown that individuals of species that livein groups tend to have high annual survival, but this link haslacked a theoretical explanation. We evaluate two hypothesesthat explain how longevity could have led to the evolution ofgroup living. The first is the territory inheritance hypothesis,and it proposes that longevity increases the probability ofnonbreeding subordinates surviving long enough to have the opportunityof inheriting their natal territory. Second, we propose a novelhypothesis, the reciprocal altruism hypothesis, which is thatlongevity increases local dominance by favoring nonaggressionpacts among neighboring residents because longevity increasesthe likelihood of reciprocal altruism. Birds thus accept subordinateresidency because the exclusion of nonlocal birds will meanthat, if they survive long enough, they will be likely to actuallyachieve territory inheritance. The reciprocal altruism hypothesisis supported by a wider array of evidence; becomes progressivelymore powerful as longevity increases, thus producing a positivefeedback; explains the evolution of local dominance (whereasthe territory inheritance hypothesis assumes its existence);and provides an explanation for why cooperative breeding shouldbe found more often in aseasonal environments.     

Population structure inhibits evolutionary diversification under competition for resources

A model is presented that explores how population structure affects the evolutionary outcome of ecological competition for resources. The model assumes that competition for resources occurs within groups of a finite number of individuals (interaction groups), and that limited dispersal of individuals between groups (according to Wright's island model of population structure) results in genetic structuring of the population. It is found that both finite-sized interaction groups and limited dispersal can have substantial effects on the evolution of resource exploitation strategies as compared to models with a single, infinitely large, well-mixed interaction group. Both effects, in general, tend to select for less aggressive competitive strategies. Moreover, both effects also tend to reduce the likelihood of the evolutionary diversification of resource exploitation strategies that often occurs in models of resource competition with infinite populations. The results are discussed in the context of theories of the evolutionary diversification of resource exploitation strategies and speciation.     

Intruder Pressure Affects Territory Size and Foraging Success in Asymmetric Contests in the Water Strider Gerris lacustris

The theory of evolutionarily stable strategies (ESS) in asymmetric contests predicts that the propensity of an individual to expose itself to risk during contests depends on the individual's resource-holding-potential (RHP) and on the value of the disputed resource (V) for the individual compared with that for an opponent. If encounters of a territory owner with individuals of high RHP and high food demands (V) increase in frequency, one should expect a decrease in total aggressiveness of the territory owner, and in consequence a decrease of its territory size. Such a decrease should result in a lower amount of food consumed by the territory owner. Using natural variability in RHP and V in Gerris lacustris, I experimentally tested these predictions. The average prey item has higher value (V) for reproductive female water striders (which probably transform most of their food into eggs), than for nonreproductive females and for males. Because males are smaller, they have lower RHP than females, as RHP depends on size. Thus the reproductive females are the class of individuals of high RHP and high food demands (high V). Most nonreproductive females defend food-based territories. I observed two groups of water striders in a seminatural laboratory setting. As predicted, there was a negative correlation between the rate of encounter with reproductive females and size of the territory, and a positive correlation between territory size and number of Drosophila flies consumed by the owner. Territories were smaller in the group with high rates of encounter between territory owners and reproductive females. Territory owners caught the same number of Drosophila flies as non-territorial individuals in this group. In contrast, in the group with fewer encounters between territory owners and reproductive females, territories were larger, and territory owners gained more food than non territorial water striders.     

The Equivocal Relationship Between Territoriality and Scent Marking in Wild Saddleback Tamarins (<Emphasis Type="Italic">Saguinus fuscicollis</Emphasis>)

Researchers have often assumed that scent marking serves a territorial function in callitrichines, although some controversy exists. To fulfill such a function, scent marks should 1) prevent intrusions, 2) ensure access to feeding resources, 3) enable avoidance of intergroup encounters, or 4) play an important role in the aggressive encounters between groups. We studied 13 saddleback tamarins (Saguinus fuscicollis) belonging to 3 free-ranging groups, which formed mixed-species troops with moustached tamarins (S. mystax) in the Amazonian rain forest of Peru. None of the predictions were confirmed. The tamarins used a border-marking strategy, marking more on the periphery of their territory. However, feeding trees in overlap and encounter areas received more scent marking but were still visited by neighboring groups. Intergroup encounters occurred more often than expected, and scent-marking frequency was not higher during them than when no other group was present. It appears that instead of defending a territory in the classic sense, the tamarins are optimizing signal transmission by depositing their scents where the probability of detection by neighbors is higher. Saddleback tamarins may use shared areas of their home ranges to exchange information with neighboring groups, perhaps regarding reproductive opportunities.     

Increased produce enrichment reduces trauma in socially‐housed captive rhesus macaques (Macaca mulatta)

Due to primate adaptations for sociality, captive rhesus macaques have optimal welfare and utility as a biomedical model when they can be maintained in outdoor social groups. As a despotic species; however, aggression can result in costly injuries and may result in temporary or permanent removal of specific individuals from social housing. Enrichment items, such as toys, climbing structures, and foraging material, are employed to keep captive animals occupied. We hypothesized that produce enrichment that requires more processing to extract may reduce socially‐derived injuries by keeping animals occupied. We tested the effects of additional weekly produce (corn‐in‐husk, whole melon, or whole squash) on trauma incidence in an outdoor social group of rhesus macaques across two distinct seasons (mating and birthing seasons) at the California National Primate Research Center. Aggression and status behavioral data, food resource use and proximity, and trauma incidence were collected over two 16‐week periods, with eight control and treatment conditions alternating biweekly. Mixed‐effects regression modeling was used to determine the best predictors of trauma risk and severe aggression at the group level and at an individual level. We found that food resource use was an important predictor of trauma risk at both group and individual levels; greater use of food resources reduced trauma risk. Produce enrichment did not; however, reduce severe aggression. We suggest that other captive social groups of rhesus macaques with high levels of trauma may benefit from supplemental produce enrichment that increases animal engagement with food resources.     

Female mate-choice behavior and sympatric speciation

Many models have investigated how the process of speciation may occur in sympatry. In these models, individuals are either asexual or mate choice is determined by very simple rules. Females, for example, may be assumed either to compare their phenotype to that of a potential mate, preferring to mate with similar males (phenotype matching), or to possess preference genes that determine which male phenotype they prefer. These rules often do not reflect the mate-choice rules found in empirical studies. In this paper, we compare these two modes of female choice with various types of sexual imprinting. We examine the efficacy of different mate-choice behavior in causing divergence in male traits under simple deterministic one-locus population genetic models as well as under polygenic, individual-based simulations based on the models of Dieckmann and Doebeli (1999). We find that the inheritance mechanism of mate choice can have a large effect on the ease of sympatric speciation. When females imprint on their mothers, the result of the model is similar to phenotype matching, where speciation can occur fairly easily. When females imprint on their fathers or imprint obliquely, speciation becomes considerably less likely. Finally, when females rely on preference genes, male trait evolution occurs easily, but the correlation between trait and preference can be weak, and interpreting these results as speciation may be suspect.     

Relatedness and resource diversity interact to influence the intensity of competition

When resource competition occurs between close relatives, the negative effects of competition are potentially amplified. However, kin selection theory predicts that natural selection should promote the evolution of mechanisms that minimize the intensity of competition between kin. Experimental tests of these hypotheses are mixed, however. Moreover, there is little consensus regarding the generality of either outcome, suggesting that the conditions important in determining the effects of competition between kin are likely complex and not fully understood. We performed two experiments using spadefoot toad tadpoles (Spea multiplicata) to evaluate the hypothesis that individuals can minimize the negative effects of exploitative competition by using alternative resources when competing with close relatives. Supporting our hypothesis, we found that only when individuals had access to alternative resources were the negative effects of competition between siblings less than between unrelated competitors. We suggest that mechanisms to lessen kin competition may be more likely to evolve in environments where alternative resources are available, and that selection to minimize exploitative competition between kin may promote the evolution of resource polyphenism. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 689–695.