Differences In Resource Holding Potential Research Articles

Differential dispersal costs and sex-biased dispersal distance in a cooperatively breeding bird

In most bird species, dispersal distance from the natal territory to a breeding territory is greater for females than for males. Two main hypotheses have been proposed to explain sex-biased dispersal: 1) it serves as an inbreeding-avoidance mechanism or 2) it is linked to a sex difference in resource-holding potential and territory establishment. Additionally, in species where individuals delay dispersal and become subordinates in a natal territory, differences in benefits of philopatry (e.g. territory inheritance, own reproduction) may also affect sex-biased dispersal. We show that in the group-living Seychelles warbler, Acrocephalus sechellensis, females disperse further to obtain a breeding position than males do. However, we found no evidence that female-biased dispersal distance can be explained by the above-mentioned hypotheses: further dispersal does not lead to less-related partners, both sexes defend and can inherit a territory, and subordinate females are more likely to obtain some reproduction than subordinate males. Instead, we provide evidence for a little-explored hypothesis based on sex differences in dispersal costs: namely that extra-territorial forays, pursued to search for limited vacancies, are more costly for males in terms of increased mortality, although the exact mechanism for this is unclear. In line with differential dispersal costs, males foray less far than females and often wait for local dispersal opportunities, ultimately resulting in a shorter average dispersal distance. Our results may help future studies in explaining sex-biased dispersal in social and perhaps also non-social species, and we suggest some mechanisms that may explain why sex-biased dispersal differs between species.

Age and reproductive status influence dominance in wild female Phayre's leaf monkeys

In group-living mammals, social dominance can be achieved and maintained via kin support, winner–loser effects and individual differences in resource-holding potential (RHP). When dominance is based on RHP, the likelihood of winning is also influenced by the incentive to compete. For females, this equation can be quite complex, as both age and reproductive state can alter the relative value of resources and the risks associated with fighting. For example, females that are young, pregnant or lactating may have high nutritional demands, while lactating females also face risks associated with offspring survival. To date, however, there have been few studies investigating how such factors influence female agonistic relationships. Here, we use 2 years of data to evaluate how age and reproductive state influence the probability of winning, focusing on two groups (5–9 and 10–12 females) of wild Phayre's leaf monkeys, Trachypithecus phayrei crepusculus, at Phu Khieo Wildlife Sanctuary, Thailand. Consistent with previous studies, we found that young females and pregnant females were more likely to win dyadic interactions in the smaller group, PA. By contrast, the likelihood of winning was unrelated to age in the larger group, PB. Moreover, differences between reproductive states were less pronounced in PB, with females that were pregnant or in early lactation being more likely to win compared to females that were cycling. The deviating results for PB were likely associated with a greater number of competitors and small age differences intensifying intrasexual competition. These results highlight the importance of individual as well as demographic factors in female dominance relationships. In this population, females with high nutritional demands are more likely to win agonistic encounters, suggesting that female–female competition occurs primarily over access to resources.

The first description of dominance hierarchy in captive giraffe: not loose and egalitarian, but clear and linear.

Wild giraffes live in extensive groups in the fission fusion system, maintaining long social distances and loose social bonds. Within these groups, resources are widely distributed, agonistic encounters are scarce and the dominance hierarchy was reported in males only, while never deeply analysed. In captivity, the possibility to maintain inter-individual distances is limited and part of the resources is not evenly distributed. Consequently, we suggest that agonistic encounters should be more frequent, leading to the establishment of the dominance hierarchy. Based on the differences in resource-holding potential, we suggested that the rank of an individual would be affected by age and sex. Based on hypotheses of prior ownership, we tested whether rank was positively affected by the time spent in a herd and whether it was stable in adult females, which were present long-term in the same herd. We originally monitored four herds of Rothschild giraffes (Giraffa camelopardalis rothschildii) in Dvůr Králové zoo (n = 8), Liberec zoo (n = 6), and two herds in Prague zoo: Prague 1 (n = 8) and Prague 2 (n = 9). The Prague 1 and Prague 2 herds were then combined and the resulting fifth herd was observed over three consecutive years (2009, 2010, and 2011) (n = 14, 13, and 14, respectively). We revealed a significantly linear hierarchy in Dvůr Králové, Prague 2 and in the combined herd in Prague. Rank was significantly affected by age in all herds; older individuals dominated the younger ones. In females, rank was positively affected by the time spent in the herd and adult females in Prague maintained their rank during three consecutive years. This study represents the first analysis of the dominance hierarchy in the captive giraffe, and discusses the behavioural flexibility of the social structure in response to monopolisable resources in a captive environment.

Frustrative reward omission increases aggressive behaviour of inferior fighters.

Animals use aggressive behaviour to gain access to resources, and individuals adjust their behaviour relative to resource value and own resource holding potential (RHP). Normally, smaller individuals have inferior fighting abilities compared with larger conspecifics. Affective and cognitive processes can alter contest dynamics, but the interaction between such effects and that of differing RHPs has not been adjudged. We investigated effects of omission of expected reward (OER) on competing individuals with contrasting RHPs. Small and large rainbow trout (Oncorhynchus mykiss) were conditioned to associate a light with reward. Thereafter, the reward was omitted for half of the fish prior to a contest between individuals possessing a 36-40% difference in RHP. Small control individuals displayed submissive behaviour and virtually no aggression. By contrast, small OER individuals were more aggressive, and two out of 11 became socially dominant. Increased aggression in small OER individuals was accompanied by increased serotonin levels in the dorsomedial pallium (proposed amygdala homologue), but no changes in limbic dopamine neurochemistry were observed in OER-exposed individuals. The behavioural and physiological response to OER in fish indicates that frustration is an evolutionarily conserved affective state. Moreover, our results indicate that aggressive motivation to reward unpredictability affects low RHP individuals strongest.

Increased aggression during human group contests when competitive ability is more similar

Theoretical analyses and empirical studies have revealed that conflict escalation is more likely when individuals are more similar in resource-holding potential (RHP). Conflicts can also occur between groups, but it is unknown whether conflicts also escalate more when groups are more similar in RHP. We tested this hypothesis in humans, using data from two professional sports competitions: football (the Bundesliga, the German first division of football) and basketball (the NBA, the North American National Basketball Association). We defined RHP based on the league ranks of the teams involved in the competition (i.e. their competitive ability) and measured conflict escalation by the number of fouls committed. We found that in both sports the number of fouls committed increased when the difference in RHP was smaller. Thus, we provide what is to our best knowledge the first evidence that, as in conflicts between individuals, conflicts escalate more when groups are more similar in RHP.

Owner positional disadvantage in contests over mating prevents monopolization of females

The act of mating may render individuals vulnerable by inhibiting mobility and defence. While the consequences of this vulnerability for predation risk are well known, the implications for male–male contests over mating have been largely ignored. We examined the influence of vulnerability during mating by assessing the effectiveness of asymmetries in resource-holding potential (RHP) and subjective resource value (female mating status) in determining access to females in a resident–intruder scenario. In the earwig Euborellia brunneri, mating males are vulnerable to attack because they cannot use their armaments, and may risk breakage of their extremely elongated intromittent organs. We expected agonistic outcomes and ‘ownership’ of females to be decided entirely according to imposed asymmetries, since defeat normally induces a strong loser effect and flight responses in losers. Using behavioural assays of dyads of males contesting one female we show that, although small differences in RHP strongly affected agonistic outcomes, dominant males could not monopolize females. Through recurrent harassment, subordinate males exploited the vulnerability of mating opponents to break up copulations and generate mating opportunities. Dominant males also employed this tactic and were more effective at it. While female mating status affected resident mating duration and investment in conflict, only asymmetry in RHP affected fighting and mating outcomes. Resource owners in E. brunneri hence experience a positional disadvantage specific to a mating context. This previously unrecognized phenomenon may provide a proximate explanation for the taxonomically widespread occurrence of attacks during copulation and the avoidance thereof.

Information, variance and cooperation: minimal models

There is increasing awareness that inter-individual variation can affect the propensity of individuals to cooperate. Here we formally examine the role of variation in the context of animal conflict, assuming that individuals vary in resource holding potential (RHP) and are capable of adopting strategies (Hawk if strong, Dove if weak) based on this trait. Our models allow winners and/or losers of contests to pay costs of fighting that vary with the difference in RHP between contestants. In contrast to earlier studies, we identify a broad range of conditions under which increasing variation in RHP serves to reduce rather than to enhance the proportion of conflicts resolved without fighting. Moreover, we show that in many cases ignorance of one’s RHP can lead to a higher tendency for individuals to cooperate than awareness of one’s RHP. Equally counterintuitively, precluding fights between individuals strong enough to want to fight and individuals not strong enough to want to fight decreases the overall extent to which conflicts are settled without conflict, by enhancing the incentive to act like a Hawk. Collectively, our results indicate that the effect of RHP variation on the tendency of individuals to resolve conflicts without fighting is more subtle than previously anticipated.

Dilemma of the Equality: an All-pay Contest with Individual Differences in Resource Holding Potential

An all-pay contest in which many players compete for an indivisible resource and each player continuously maintains a different resource holding potential (RHP) is analysed. There exists the unique pure ESS function, which is common sense; that is, a higher RHP induces a higher level of investment, and, as a consequence, a player with a greater RHP always wins. As the variance in distribution of RHP converges to zero, the ESS becomes equal to the symmetric mixed Nash-equilibrium reported by Rose, which does not satisfy the condition of ESS. This suggests that some unstable symmetric Nash equilibria change to ESS functions in some games when we extend the games by assuming one more random continuous parameter of the player's condition. Moreover, the smaller the individual differences in RHP, the more intense the competition becomes, and the smaller becomes the expected payoff for almost all individuals as well as the average payoff of the population. This negative correlation between equality in RHP and the payoff in the population was first found by Kura & Kura in a war-of-attrition game and named the “dilemma of equality”. Its biological implications are also discussed.

Arms Races, Conflict Costs and Evolutionary Dynamics

In conflicts and fights, the winner is often determined by the difference in resource-holding potential, e.g. size, weaponry, strength (RHP). I model the evolution of RHP in a symmetric game with continuous strategies. I show that there is a convergence stable ESS level of RHP if the cost of the trait increases faster than linearly, and that this is the only solution if the cost increases fast enough with RHP. Otherwise with slowly increasing cost, the solution is a cyclically fluctuating level of RHP. It is also shown that if the cost increases linearly with RHP, the only solution to the game is neutrally stable cycles, the amplitude determined by the initial conditions. The cycles come about because in a population drawn into an arms race of RHP, individuals after a while suffer costs so high that mutants with the lowest possible armament level may invade.

War of Attrition with Individual Differences on RHP

The fact that there always exists various kinds of almost continuous mutations for any animal population implies that players in competitions can never be perfectly symmetric in any sense. To develop a model to fit this reality, we consider war of attrition games in which players have continuously different resource holding potential (RHP). The RHP of each opponent is not known in our settings.Pure ESS functions and Nash equilibria are obtained under sufficiently rotational conditions as unique solutions of certain differential equations among the class of Lebesgue measurable functions. They are normal in that a higher RHP induces a longer attrition time, which implies that a player with greater RHP always wins. This model includes as the limit the conclusions of Maynard Smith (1974,J. theor. Biol.47, 209–221) and Normanet al. (1977,J. theor. Biol.65, 571–578), which did not consider individual differences in RHP. Our results suggest that, by changing each player's qualitative differences to continuous quantitative differences, some of the mixed ESS solutions previously found in discrete games may degenerate into pure ESS functions. Moreover, we found that the smaller the individual differences of RHP, the smaller is the mean pay-off of most individuals as well as the total pay-off of the population.

The influence of size differntial on agonistic behaviour in the freshwater crayfish, Cherax cuspidatus (Decapoda: Parastacidae)

The agonistic behaviour of juvenile pairs of intermoult freshwater crayfish Cherax cuspidatus, interacting over space in experimental containers, was examined. The study investigated the effects of differences in Resource Holding Potential (RHP) on social rank, patterns of interaction initiation, and the importance of fighting during encounters between pairs of crayfish. Crayfish were paired with unfamiliar opponents at size classes of 1 : 1, 1 : 0.9, 1 : 0.8, I : 0.7 and 1 : 0.6 using standard carapace length as the RHP index.Larger animals were dominant during significantly more than 50% of replicates at all size classes. Larger animals were dominant at all size ratios lower than 1 : 0.89. Seven smaller animals were dominant at size ratios between 1 : 0.89 and 1 : 0.99. The dominance of smaller animals could not be explained on the basis of morphological measures or age. The influence of prior experience as dominants or subordinates on the behaviour of contestants during these encounters appeared the most likely explanation.Carapace length, body length, and weight were equally reliable RHP indices. Age, number of injuries, and sex were not reliable RHP indices.The probability of initiating the first agonistic bout was independent of relative size for three of the five size classes. Larger initiators of the first bout invariably were successful, while smaller initiators rarely won. The winner of the first bout was the overall dominant during significantly more than 50% of replicates at each size class. The dominant individuals initiated the majority of bouts at all size classes, including the seven replicates where the smaller crayfish was dominant.Fighting (fight time, fight frequency, average bout length), a high intensity behaviour, was an important component of contests at the 1 : 1, 1 : 0.9, and 1 : 0.8 size classes. The significance of fighting was greatly reduced at the 1 :0.7 and 1 : 0.6 classes, indicating that fighting was correlated with the relative size of opponents.

Territory size in the moorhen ( Gallinula chloropus): An outcome of RHP asymmetry between neighbours

Territory size can be considered as the outcome of contests between an owner and its neighbours for the resource units in the defended area. Variation in territory size could be dependent upon three possible asymmetries: (1) difference in resource holding potential (RHP); (2) difference in resource value to the competitors; and (3) difference in ‘ownership’ status. These possibilities were tested in a 3-year study of moorhens which defend linear territories along ditches in a grazing marsh habitat. Of the three, relative difference in RHP between an owner and its two neighbours provided the best correlation with territory size. There was no relationship between the amount of resources per unit area and territory size nor between the time of establishing a territory and its later size. Heavier moorhens are more likely to win contests in winter flocks and are thought to have a greater RHP. The ratio of the weight of a male territory owner to the weight of both its male neighbours was highly correlated with territory size. Birds with larger territories may gain more potential nesting sites with good cover from predators.