The effect of tied rank numbers on the linearity of dominance hierarchies

The occurence of tied rank numbers in dominance hierarchies is discussed, especially its effect on the linearity of the hierarchy. This linearity is measured with Landau's index, that is calculated for several hierarchies with tied ranks on one, two of three levels. Linearity is mostly affected by ties in small groups with many ties. A distinction is made between a hierarchy of individuals and hierarchical levels. The phenomenon of despotism is called an extreme case of tied ranks. It is proposed to regard hierarchies with a linearity in a continuous scale.     

Socio-spatial levels in linearity analysis of dominance hierarchies: a case study on elephant seals

The analysis of linearity is a key aspect of the study of dominance hierarchies. To study the effect of the choice of socio-spatial level of analysis, we calculated linearity in a large set of southern elephant seal (Mirounga leonina) hierarchies from two populations (Valdés Peninsula and Falkland Islands). The socio-spatial level of analysis affects the observational effort, the completeness of matrices, and the frequency of unknown relationships. These factors, in turn, have a notable effect on linearity. We conclude that dominance should be studied at the local level, where the absence of structural zeros and the low incidence of observational zeros produce complete matrices, well rooted in the true spatial and social structure of the population. Depending on the specific social system, the extrapolation of dominance from the local level to higher levels may result in sparse matrices, and in biased estimates of linearity. The variation of the socio-spatial level of analysis may in part explain the contrasting results obtained in different studies of linearity of dominance hierarchies. Electronic Publication     

Sex Differences in the Steepness of Dominance Hierarchies in Captive Bonobo Groups

Bonobos have a reputation as a female-dominated and egalitarian species. We examined the 2 aspects of dominance in 6 captive bonobo groups. Females do not consistently evoke submission from all males in all contexts. Though females occupy the highest-ranking positions in the dominance hierarchy, there are in each group males that obtain rather high ranks and are able to dominate ≥1 female. Thus female dominance is not complete and hierarchies can be better described as nonexclusive female dominance. We studied egalitarianism by measuring linearity and steepness of dominance hierarchies. The hierarchies of all groups are highly linear. Hierarchies among males are steeper than among females. On average, male bonobos are more despotic than females, but females too can have despotic relations, both with other females and with males. Hence one can call bonobos in captivity semidespotic rather than egalitarian.     

Trade‐offs in primate grooming reciprocation: testing behavioural flexibility and correlated evolution

Primates may trade altruistic behaviours, such as grooming, either for itself or for different rank‐related benefits, such as tolerance or agonistic support. Ecological conditions are expected to affect competition and thus the steepness of dominance hierarchies. This, in turn, may influence the value of the different currencies that primates exchange. Thus, it can be hypothesized that, as the dominance hierarchy becomes steeper, more grooming is directed up the hierarchy (in exchange for tolerance or agonistic support) and less grooming is exchanged for other grooming. We assembled a large database of within‐group grooming distribution in primates (38 social groups belonging to 16 species and eight genera) and tested these hypotheses both within species (i.e. comparing different groups of the same species) and between species (using comparative methods that control for phylogenetic relatedness). We found within‐species evidence that steeper dominance hierarchies were associated with more grooming being directed up the hierarchy, and that a trade‐off occurred between the tendency to groom up the hierarchy and the degree of grooming reciprocation (although, in some analyses, only a nonsignificant trend was observed). By contrast, phylogenetically controlled comparisons between species did not reveal evidence of correlated evolution between the steepness of the dominance hierarchy, the tendency to direct grooming up the hierarchy, and the degree of grooming reciprocation. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 439–446.     

The ghost of social environments past: dominance relationships include current interactions and experience carried over from previous groups

Dominance hierarchies pervade animal societies. Within a static social environment, in which group size and composition are unchanged, an individual's hierarchy rank results from intrinsic (e.g. body size) and extrinsic (e.g. previous experiences) factors. Little is known, however, about how dominance relationships are formed and maintained when group size and composition are dynamic. Using a fusion-fission protocol, we fused groups of previously isolated shore crabs (Carcinus maenas) into larger groups, and then restored groups to their original size and composition. Pre-fusion hierarchies formed independently of individuals' sizes, and were maintained within a static group via winner/loser effects. Post-fusion hierarchies differed from pre-fusion ones; losing fights during fusion led to a decline in an individual's rank between pre- and post-fusion conditions, while spending time being aggressive during fusion led to an improvement in rank. In post-fusion tanks, larger individuals achieved better ranks than smaller individuals. In conclusion, dominance hierarchies in crabs represent a complex combination of intrinsic and extrinsic factors, in which experiences from previous groups can carry over to affect current competitive interactions.     

雄性大仓鼠体重对其斗殴行为及社会等级的作用

在实验室条件下测定雄性大仓鼠体重对社会等级和斗殴行为的作用模式,检验体重对雄性大仓鼠社会等级及斗殴行为序列具有重要影响的假设。本实验以16只成年雄性大仓鼠为目标个体,采用等级内部的线性概率、组内循环三元组数量（d）和优势等级的线性度（K）,排列个体的社会等级序位。研究结果表明,雄性大仓鼠可形成近似线性的优势等级,体重与个体的优势等级,攻击行为和胁腺标记行为均呈显著的正相关关系,与防御行为和攻击潜伏期存在显著的负相关关系。说明独居性物种大仓鼠雄体间可形成优势等级关系,体重对此关系具有重要的作用。     

Within-brood social status and consequences for winter hierarchies amongst Mallard Anas platyrhynchos ducklings

Juveniles of many birds establish dominance hierarchies within family social units, only to leave and compete to acquire dominance status in new social groups. Little is known about the role of sex, body mass, size or experience during the duckling period on subsequent dominance rank and adult social relationships. We used captive Mallard Anas platyrhynchos ducklings to test for the role of individual characteristics and growth parameters in establishing within-brood hierarchies, the maintenance of within-brood hierarchies in the subsequent wintering group and differences in social ranks between broods. Strong stable linear hierarchies were present within each brood and, later, within each phase of the winter. There was a reorganisation of the hierarchical order between the duckling period and early winter, but only few modifications afterwards during the winter. None of the tested “hatching”, “duckling” and “adult” traits explained either the within-brood or the winter hierarchies, but winter rank was related to brood of origin with ducklings from the same brood having similar social ranks. These differences between broods were maintained through the whole winter in most cases, though one brood drastically progressed in the hierarchy during late-winter. These results suggest that the factors affecting the establishment of social relationships within broods differ from those in winter groups, and that brood-related mechanisms influence social relationships during winter. We discuss our results in the light of direct and indirect maternal influence.     

Aggression and dominance in matched groups of subadult Icelandic horses (<Emphasis Type="Italic">Equus caballus</Emphasis>)

We studied sex differences in the nature of aggression and dominance behaviour in two newly formed groups of 1-year-old Icelandic horses. One herd contained nine geldings, the other nine mares. The groups were matched with regard to dominance-determining traits such as age, weaning age, composition of native herd, social experience, genetic origin, body condition and maternal dominance status. High-ranking individuals of both sexes were more aggressive, high-ranking males were older, and high-ranking females had a better body condition. Frequencies of aggressions were similar in both groups. The mares reacted significantly more by showing submission upon an aggression rather than by not responding or by escalating the aggression. For the geldings, this difference was not observed due to a lower tendency to submit. A linear dominance hierarchy was found in both groups. David’s scores provided additional information regarding cardinal rank distances and were used to calculate steepness of hierarchies. The female hierarchy was somewhat steeper compared to the male hierarchy, suggesting somewhat lower despotism among males. This was mainly a consequence of the lower unidirectionality in male submission. Male contests occurred predominantly between dyads at top and mid positions, suggesting a low degree of acceptance of the hierarchy.     

Direct benefits of social dominance in juvenile crayfish

Crayfish are known for their innate aggressiveness and willingness to quickly establish dominance relationships among group members. Consequently, the formation of dominance hierarchies and the analysis of behavioral patterns displayed during agonistic encounters have mostly been tested in environments that provide no immediate resources besides space. We tested the hypothesis that social hierarchy formation in crayfish serves to determine access to future resources. Individuals within groups of three juvenile crayfish were allowed to form a social hierarchy in a featureless environment before a single food resource was presented. Higher dominance indices were significantly correlated with increased access to the food. The highest ranked crayfish spent more time in contact with the food than did medium-ranked and lowest ranked crayfish, and crayfish of medium rank spent more time in contact with the resource than did lowest ranked animals. The highest ranked crayfish consolidated their dominant status in the presence of food, indicated by a complete absence of any submissive behaviors during that period. The results of these experiments show that the disposition of crayfish to engage in fighting and formation of a dominance hierarchy in a featureless environment serves to determine future access to an emerging resource, thereby entailing greater benefits for animals of higher social rank.     

Dominance and association among members of a captive and a free-ranging group of grey kangaroos (Macropus giganteus)

Dominance relationships and the degree of association between members of groups of eastern grey kangaroos (Macropus giganteus) were investigated by observing a small group in captivity and a larger free-ranging group. Only decisive aggressive interactions were used as a criterion of dominance, and the number of times individuals occurred together were used as a measure of association. A dominance hierarchy in which the male assumed the highest rank was found in the captive group, while in the wild hierarchies were found separately among both males and females in the group. A low level of positive association existed between members of both the groups studied and the data indicated that in general the free ranging animals tended to move about at random with respect to each other. In the captive group dispersion about the enclosure was regular but became more random during the day in winter and spring.     

Adolescent male chimpanzees do not form a dominance hierarchy with their peers

Dominance hierarchies are a prominent feature of the lives of many primate species. These hierarchies have important fitness consequences, as high rank is often positively correlated with reproduction. Although adult male chimpanzees strive for status to gain fitness benefits, the development of dominance relationships is not well understood. While two prior studies found that adolescent males do not display dominance relationships with peers, additional research at Ngogo in Kibale National Park, Uganda, indicates that adolescents there form a linear dominance hierarchy. These conflicting findings could reflect different patterns of rank acquisition across sites. An alternate possibility arises from a recent re-evaluation of age estimates at Ngogo and suggests that the report describing decided dominance relationships between adolescent males may have been due to the accidental inclusion of young adult males in the sample. To investigate these issues, we conducted a study of 23 adolescent male chimpanzees of known age during 12 months at Ngogo. Adolescent male chimpanzees exchanged pant grunts, a formal signal of submission, only 21 times. Recipients of pant grunts were late adolescent males, ranging between 14 and 16 years old. In contrast, younger adolescent males never received pant grunts from other males. Aggression between adolescent males was also rare. Analysis of pant grunts and aggressive interactions did not produce a linear dominance hierarchy among adolescent males. These data indicate that adolescent male chimpanzees do not form decided dominance relationships with their peers and are consistent with the hypothesis that the hierarchy described previously at Ngogo resulted from inaccurate age estimates of male chimpanzees. Because dominance relationships develop before adulthood in other primates, our finding that adolescent male chimpanzees do not do so is surprising. We offer possible explanations for why this is the case and suggest future studies that may help clarify the matter.     

Fight for your breeding right: hierarchy re-establishment predicts aggression in a social queue

Social aggression is one of the most conspicuous features of animal societies, yet little is known about the causes of individual variation in aggression within social hierarchies. Recent theory suggests that when individuals form queues for breeding, variation in social aggression by non-breeding group members is related to their probability of inheriting breeding status. However, levels of aggression could also vary as a temporary response to changes in the hierarchy, with individuals becoming more aggressive as they ascend in rank, in order to re-establish dominance relationships. Using the group-living fish, Neolamprologus pulcher, we show that subordinates became more aggressive after they ascended in rank. Female ascenders exhibited more rapid increases in aggression than males, and the increased aggression was primarily directed towards group members of adjacent rather than non-adjacent rank, suggesting that social aggression was related to conflict over rank. Elevated aggression by ascenders was not sustained over time, there was no relationship between rank and aggression in stable groups, and aggression given by ascenders was not sex-biased. Together, these results suggest that the need to re-establish dominance relationships following rank ascension is an important determinant of variation in aggression in animal societies.     

Food Competition and Linear Dominance Hierarchy Among Female Chimpanzees of the Taï National Park

Dominance rank in female chimpanzees correlates positively with reproductive success. Although a high rank obviously has an advantage for females, clear (linear) hierarchies in female chimpanzees have not been detected. Following the predictions of the socio-ecological model, the type of food competition should affect the dominance relationships among females. We investigated food competition and relationships among 11 adult female chimpanzees in the Taï National Park, Côte d'Ivoire (West Africa). We detected a formal linear dominance hierarchy among the females based on greeting behaviour directed from the subordinate to the dominant female. Females faced contest competition over food, and it increased when either the food was monopolizable or the number of competitors increased. Winning contests over food, but not age, was related to the dominance rank. Affiliative relationships among the females did not help to explain the absence of greetings in some dyads. However comparison post hoc among chimpanzee study sites made differences in the dominance relationships apparent. We discuss them based on social relationships among females, contest competition and predation. The cross-site comparison indicates that the differences in female dominance hierarchies among the chimpanzee study sites are affected by food competition, predation risk and observation time.     

Testosterone and Linear Social Dominance Status in Captive Male Dabbling Ducks in Winter

Dominance hierarchies play an important role in avoidance and/or solving conflicts in gregarious species. In dabbling ducks (Anas species), dominance allows for feeding‐site monopolization in winter quarters where resources are generally limited. In addition, male social rank should theoretically favour access to mates. Dominance rank can be associated with morphological traits, and is often correlated with aggressiveness, a behavioural trait generally related to high testosterone levels. In this study, we investigated the existence of a winter group structure based on dominance relationships and tested for a linear hierarchy, in three species of captive male dabbling ducks (mallard Anas platyrhynchos, pintail A. acuta and wigeon A. penelope). We then analysed the relationship between dominance ranks, morphological parameters and testosterone levels measured in early (Oct.) and mid‐winter (Dec./Jan.). We found that the three male groups of the three species exhibited a linear hierarchy. Testosterone levels differed during winter and between species. Morphologic measurements, body mass and body condition were not correlated with individual dominance ranks, whereas dominant males had higher testosterone levels than subordinates. The slopes of the relationships were similar between species and winter period, but the y‐intercepts differed between species and between early and mid‐winter phases. The linear hierarchy found in the three species indicates that dominance relationships strongly structure dabbling duck groups in winter. Lack of correlation between rank and morphological characters, but correlation of rank with testosterone levels suggests that social rank is more dependent on behavioural traits such as aggressive behaviour. The differences between species and winter periods are discussed in relation to migration and wintering phenology.     

Dominance,Age and Aggression among Female Pronghorn,Antilocapra americana (Family: Antilocapridae)

The relationships among dominance, age and aggressive behaviour of marked, female pronghorn (Antilocapra americana; Family Antilocapridae) were studied in north central Colorado. Females formed dominance hierarchies with few circular relationships. Unlike other female ungulates, dominance rank was not correlated with age. Dominance rank was negatively correlated with rate of aggression initiated in summer and winter; however, aggressiveness as an individual trait was not related to dominance. Older females received higher rates of aggression than younger females in summer and winter. During the rut, dominance rank was correlated with rate of aggression received but not with rate of aggression initiated. Patterns of aggression that differ by season within a given hierarchy of dominance relationships suggest a different cause or function of aggressive behaviour in different seasons.     

The peck orders of chickens: How do they develop and why are they linear?

The development of peck orders in mixed sex groups of domestic chickens was observed to determine how linearity occurred. Based on threats, head-pecks and submission, dominance relationships emerged in a virtual peck right form. Leaping by males, however, did not closely conform to dominance relationships. There were no rank reversals in 50% of male-male and 80% of female-female relationships, and only single changes occurred in most of the others. These resulted from the movements of individuals up the hierarchy rather than from any general reorganization of relationships. Reversals did not necessarily occur between rank neighbours, and stable triangles were sometimes introduced. The initial status of males and females depended upon the age at which they first showed aggression, while the final, stable status of males depended upon the age at which they were first submitted to. Sexual maturity of the males produced a number of changes, with earlier-maturing birds tending to rise in status above their later-maturing companions. Linear hierarchies therefore appear to result from birds developing at different rates.     

Reevaluation of dominance hierarchy in bonobos (Pan paniscus)

While dominance relationships have been widely studied in chimpanzees, in bonobos, dominance style and linearity of hierarchy are still under debate. In fact, some authors stated that bonobo hierarchy is nonlinear/ill-defined, while others claimed that it is fairly linear. In this paper, we test the hypothesis that a shift in group composition determines changes in linearity of hierarchy. To test this hypothesis, we collected data on one of the largest captive groups in the world, in the Apenheul Primate Park (The Netherlands). We investigated the linearity of the hierarchy in two different periods, with a shifting group composition. We used the corrected Landau's index and David's scores to estimate which animals were most dominant. The major overall result of our study is that hierarchy is fairly nonlinear in this group: during the first study period (eight adults), the hierarchy was nonlinear, whereas during the second one (six adults), it failed to reach statistical linearity. We argue that the reduction of the number of adults is the principal factor affecting linearity. We also found that dominance interactions were evenly distributed across sex classes in both study periods. Furthermore, no correlation was observed between age/body weight and rank. As for the overall dominance relationship between males and females, our results suggest that there is no exclusive female dominance in the Apenheul group. The dominance style of bonobos may be loose and differentially expressed in diverse groups or in the same group, along with shifting conditions.     

Dominance in Domestic Dogs: A Quantitative Analysis of Its Behavioural Measures

A dominance hierarchy is an important feature of the social organisation of group living animals. Although formal and/or agonistic dominance has been found in captive wolves and free-ranging dogs, applicability of the dominance concept in domestic dogs is highly debated, and quantitative data are scarce. Therefore, we investigated 7 body postures and 24 behaviours in a group of domestic dogs for their suitability as formal status indicators. The results showed that high posture, displayed in most dyadic relationships, and muzzle bite, displayed exclusively by the highest ranking dogs, qualified best as formal dominance indicators. The best formal submission indicator was body tail wag, covering most relationships, and two low postures, covering two-thirds of the relationships. In addition, both mouth lick, as included in Schenkel’s active submission, and pass under head qualified as formal submission indicators but were shown almost exclusively towards the highest ranking dogs. Furthermore, a status assessment based on changes in posture displays, i.e., lowering of posture (LoP) into half-low, low, low-on-back or on-back, was the best status indicator for most relationships as it showed good coverage (91% of the dyads), a nearly linear hierarchy (h’ = 0.94, p<0.003) and strong unidirectionality (DCI = 0.97). The associated steepness of 0.79 (p<0.0001) indicated a tolerant dominance style for this dog group. No significant correlations of rank with age or weight were found. Strong co-variation between LoP, high posture, and body tail wag justified the use of dominance as an intervening variable. Our results are in line with previous findings for captive wolves and free-ranging dogs, for formal dominance with strong linearity based on submission but not aggression. They indicate that the ethogram for dogs is best redefined by distinguishing body postures from behavioural activities. A good insight into dominance hierarchies and its indicators will be helpful in properly interpreting dog-dog relationships and diagnosing problem behaviour in dogs.     

Agonistic behavior and dominance relationships in female Phayre's leaf monkeys -- preliminary results

Socioecological theory suggests a link between the strength of competition for food/safety, rates of agonism, structure of dominance hierarchies, and dispersal among group-living females. This study presents preliminary data on agonistic behavior and dominance relationships for female Phayre's leaf monkeys (Trachypithecus phayrei), a species in which females routinely disperse. Behavioral observations were conducted on two groups (four adult females, and five adult females plus two juvenile females, respectively) at Phu Khieo Wildlife Sanctuary, northeast Thailand. Rates of agonistic behavior were analyzed from focal continuous recordings, while dominance hierarchies were constructed from all agonistic behaviors (focal and ad libitum sampling). Overall, female-female agonistic behaviors (aggression, submission, and displacements) occurred at a rate of < 0.25 interactions per hour. Agonistic interactions involving food occurred more frequently than expected based on feeding time. Females in both groups exhibited linear dominance hierarchies with some reversals, and possibly an age-inversed hierarchical structure in the larger group. The results fit well with previous results for colobine monkeys regarding frequency of interactions, displacements predominating agonistic behavior, and the possibility of an age-inversed hierarchy. The results contradict the suggested link between linearity of hierarchies and female philopatry. Future studies should consider the notion that female dispersal may coexist with linear dominance hierarchies.     

The physiology of rainbow trout in social hierarchies: two ways of looking at the same data

Salmonids form dominance hierarchies in environments, where space or food are limiting. Our first objective was to investigate the physiology of individual rainbow trout in 4-fish hierarchies. Our second was to compare conclusions drawn from grouping physiological data on the basis of social rank with those based on relating individual physiology to individual aggressive behavior. To create a social hierarchy, groups of 4 juvenile trout were fed (1 % ration) using a darkened feeding container, twice daily (morning and evening). Each morning feeding was videotaped to record aggressive behavior, thereby facilitating the assignment of a social status rank to each fish. On days 5 and 10–11, physiological parameters were measured in fish fasted for 24 h. Social hierarchies formed in all tested groups. One fish would become dominant, whereas the three subordinate individuals would each assume a stable social rank. When classified according to this social rank, the three subordinate individuals all displayed similar physiology, different from the physiology of the dominant fish. The latter included higher ammonia excretion rate, greater protein utilization in aerobic metabolism, greater feeding, higher specific growth rate, greater increase in condition factor, and lower routine oxygen consumption rate. However, when individual aggression was taken into account, a continuous gradient was observed between aggression and physiology for most parameters, regardless of social status. These relationships could be improved by normalizing the aggression score to the overall level of aggression in each hierarchy. We argue that individual behavior should be considered instead of just social rank when studying the physiology of trout in social hierarchies.