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Hierarchies and roles






 

In the wild, a great deal of diversity of hierarchically organised animal communities exists, and specific structures are often initialized by evolutionary histories and species predispositions as well as environmental conditions.

Individual dominance, or the “pecking order”, serves as a universal mechanism of maintaining an order within animal communities. The history of the discovery of the pecking order by a Norwegian zoologist, Thorleif Schjelderup-Ebbe (1922, 1935), is fascinatingly described by deWaal (2001). Since a boy, Schjelderup-Ebbe was keeping detailed notebooks keeping track of who pecked whom in the flock of domestic chickens. He was particularly interested in exceptions to the hierarchy, so called " triangles, " in which hen A is master over B, and B over C, but C over A. So, from the start, like a real scientist, he was interested in not only the regularities but also the irregularities of the rank order. The social organization that he discovered is now so obvious to us that we cannot imagine how anyone could have missed it, but no one had described it before. After Sehjelderup-Ebbe received a degree in zoology, he published the chicken observations of his youth introducing the term Hackordnung, German for pecking order. His classic paper, which appeared in 1922, describes dominants as " despots" and demonstrates the elegance of hierarchical arrangements in which every individual has its place.

The basis of the linear dominance is individual recognition. In linear hierarchy, a single individual generally dominates all the others. His immediate subordinate dominates all others except him, and so on. The last animal in the order is subordinate to all the other animals and may lead a frenetic life indeed. Certain features associated with threat may determine one’s position in hierarchy. In chickens, if the comb and wattle (structures used in aggressive signalling) are removed from a hen, she will begin to lose more fights and may end up at the end of the pecking order (Marler and Hamilton, 1966). Being not smart in solving many cognitive problems (as we have seen earlier in this book), chickens display remarkable abilities to remember their partners in a flock. Moreover, they can recognise members of several flocks and estimate social circumstances. In the experiments, hens were placed into different groups that were formed in such a way that a certain chicken occupied different levels in them. Linear hierarchy turned out to be important for public tranquillity within flocks. Experimenters compared two sets of groups: in one set of groups dominates were removed each week and replaced by other individuals in order to disturb social order, whereas in another set of groups social structure was not disturbed. In control groups clashes were observed rarer, chickens ate more and laid much more eggs (Guhl and Alle, 1944).

In 30-th, very impressive results were obtained concerning variants of social ranking in animal communities, in particular, by Lorenz (1931, 1937) on ranking in jackdaws and other birds, and Zuckerman (1932), and Maslow (1937) on dominance in monkeys. A concept of domination had been elaborated as a possibility for a dominant to do all things not suiting convenience of other members of a group. Correspondingly, subordinating individuals have to adjust all behaviours (such as sexual, feeding, social) with dominant’s “opinion”.

The discovery of pecking order influenced ecological and evolutionary sciences because the idea that a dominating individual has immediate access to limited resources was concordant with the view of the animal world as the “gladiator’s show” tracing to Thomas Henry Huxley (1888). Wynn Edwards (1962) and Lack (1966) suggested that hierarchies may act as “social guillotines” when food becomes scarce by causing the quick elimination of certain, less vigorous, segments of the population.

More recently it was shown that although the linear dominance model is a useful construct to analyse the social organisation within groups of animals, there are limitations in this model concerning linearity of ranks itself as well as correlation between ranking and living standards in animal communities.

First of all, even in controlled laboratory conditions when researchers use the method of pair wise interaction (very popular in physiological experiments), it is difficult to find correlation between parameters of ranking when animals struggle for different limited resources such as food, water, refuges, and sexual partner. So it is impossible to reveal an absolute leader. In the wild, the qualities of leadership do not always depend on animal’s size and even on their fighting prowess. In some species not only are individuals ranked according their own attributes, but also according to whose mates they are and whose offspring. Besides, factors other than pugilistic skill may be determinant of one’s place in the pecking order, and, as we will see further, intelligence is not the least among these factors. Finally, even if one reaches the top rank this does not automatically reflect one’s fitness. For instance, it is known that the highest ranking males of polygamous species do not necessarily fertilise more females than other males.

Dominance concepts have been largely replaced by the concepts of roles, that is, behavioural set including the sum of reactions on environmental and social stimuli. For example, in macaque communities there is a control male, central subgroup males (competitive with others in that group, but dominant to peripheral males), peripheral males, isolate males, and central and peripheral females. Such roles are not fixed and immutable, and changes of position in the role hierarchy are usually correlated with changes in general behaviour. Males may shift their roles many times in a lifetime, while females are socially far more stable. Their stability thus gives them a certain “baseline” importance in determining the roles of males of the group (Crook, 1970; Clutton-Brock, 1974, 2002). Poyarkov (1986) described the role hierarchy in packs of stray dogs. There are certain members of the pack that are responsible for making an itinerary for the whole group to search for food, whereas other dogs are responsible for the efficient running of contacts with other packs, and with humans.

Certain roles, as would be expected, are characteristic of some species and not of others. For instance, the harem “overlord” exists in hamadryas baboons (Papio h. hamadryas), but not in gelada baboons (Theropithecus gelada) or in macaques (Rowell, 1966). At the same time, as it was already noted many species that belong to remote orders show similarities in their social structures. For example, many species of Old World monkeys typically live in groups which include several matrilineal families arranged in a stable, linear, dominant order (Silk et al., 1999). Hyenas also live in social groups comprising matrilines in which offspring inherit their mother’s dominance rank (Smale et al., 1993).

However, within species-specific profile of social structure variants can exist depending on animals’ social experience as well as their personal attributes. Animals of the same species can use different social strategies in similar situations. In order to compare various dominance styles, patterns of conflicts and reconciliation have been used as criteria over last decades (deWaal and Yoshihara, 1983; Thierry, 1985; Butovskaya et al., 1996; deWaal, 2003).

Flexibility of social behaviour and learning may help animals to change social strategies in concrete situations. This can be illustrated by an experiment of Cords and Thurnheer (1993). Pairs of longtail macaques were trained to obtain rewards by acting in a coordinated fashion: The only way to obtain popcorn would be for two monkeys to sit side by side at a dispenser, a procedure that attached significant benefits to their relationship. After this training, subjects showed a three times greater tendency to reconcile after an induced fight than subjects that had not been trained to cooperate.

One can assume that strong and despotic linear hierarchy is the inherited “blank” that is characteristic for many social animals. An acquired ability to estimate and use social affiliations makes social structures much more flexible and often more effective in changeable environment.

This hypothesis can be supported by experimental comparison of dominance styles within groups of socially naive and experienced animals. For example, Anderson and Mason (1978) experimentally formed two groups of rhesus macaques: one was composed of young individuals raised in social isolation carried by “artificial mothers”, whereas the other, control, group was composed of young animals raised in normal social environment. Experimenters then forced the animals to compete for drinking bowls. In the control group composed of socially experienced macaques animals displayed flexible and crafty behaviour. They could, for instance, form alliances and “ask” dominating male for protection in a conflict that was framed up just to draw dominant’s attention away from a desired drinking bowl. Socially naive monkeys formed a stable linear peck order ranking individuals strongly in accordance to their fighting prowess.

A similar experiment was conducted in our laboratory by Harkiv (1997). He formed two laboratory families of Formica sanguinea ants. The first group was composed by “wild” ants from a basic natural ant-hill. The second group included socially naive ants hatched from cocoons (taken from the same ant-hill) under laboratory controlled conditions. Ants in both groups were individually labelled by colour marks and housed in laboratory transparent nests so it was easy to observe their contacts within the nests and on large arenas where both families had possibility to walk and search for food. It turned out that although individual relationships in natural ant family are not so complex as in primates, behavioural patterns of members of this group are characterised by flexibility, lacking of self-confidence and propensity to cooperation. Absence of clashes proves that such tactics were efficient. The strong linear hierarchy was established in the group composed of socially naive ants, after numbers of aggressive encounters during which the ants sorted their relationships out, and the primary problem was which ant has rights to enter the arena first.

In general, we can suggest that linear ranking reflects one-dimensionality and unambiguity of individual inter-relations within communities whereas complexity and flexibility of dominance styles in animal communities are tied with variety of living problems. The more diverse problems to be solved by members of communities, the fuller the colour of social palette in animal societies.

 






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