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.

Introduction

Generally stated, living in social groups can be beneficial for individual and species level survival for several reasons and in several circumstances, both in the short and long term (see [1]). Wolves and domestic dogs are Canidea species known for their high degree of sociality [2,3], but there is little quantitative data concerning the dynamics of their social organisation, dominance hierarchy and dominance style, affiliative relationships, coalition/alliance formationf and reconciliation behaviour. Such behaviour, if the social organisation in wolves can be taken as an example, appears to assist in coping with continuous change in several areas. For example, they contribute to hunting efficiency: prey size, availability of prey, prey detection, and to the care offered in a pack: learning opportunities, alloparental care; and territory defence [2,4]. At the same time, living in social groups may enhance competition for highly valued resources such as food and mates [5]. This competition can lead to conflicts that may compromise group stability and may even result in chronic stress and physical harm [6]. It can be alleviated by dominance hierarchies built from stable dyadic dominance relationships, allowing regulation of priority of access to highly valuable resources, and preventing fierce or recurring conflicts. The peacefulness of interactions within wild wolf packs impressed Mech, an experienced wolf observer [7]. Nevertheless, Mech’s remarks to this effect has led to a series of articles in the ongoing debate on whether or not dominance plays a role in the society of the wild wolf and consequently, whether dominance could constitute an important element in structuring social relationships between dogs and also between dogs and their owners (for review [8]). Unfortunately, in this debate the term dominance is widely used, often without reference to an underlying model or definition.

In our research, we rely on the model devised by van Hooff and Wensing [9], which has been applied to primates by de Waal [10] and used in many other studies of social living animals (free-ranging dogs: [11], bonobos: [12], macaque species: [13], wolves: [14], plains zebras: [15], Icelandic horses: [16], domestic pigs: [17]). The model runs as follows:

Members of a social group may differ in many aspects, including asymmetries regarding physical power, stamina, personality, weight, weaponry, age, and so on (see for detailed descriptions on criteria:[18]). These differences in personal properties are likely to influence the relationships between individuals (see also [19]) and may be stable for some time. Stable relationships between individuals may be correlated with more or less predictable differences in behaviours and predictable outcomes of conflicts. However, motivation may interfere and this may lead to some variation in the outcomes of conflicts over resources. It is thus only useful to speak of a dominance relationship between two individuals when a number of (behavioural) asymmetries correspond. Thus a number of different behaviours exchanged within each pair of animals should show corresponding main directions: e.g. individual A shows some relevant dominance related behaviours more frequently towards individual B than vice versa, and consequently some submission related behaviours consistent with these main directions could be shown more frequently by B towards A. If this is the case, then dominance may be regarded as a so called intervening variable that summarizes a set of behavioural differences between individuals [20,21]. If it then turns out that the individual group members can be ranked according to this intervening variable, the concept of dominance as defined above is applicable.

It should be made clear that animals do not need to have a notion of the concept of dominance in order to establish a dominance relationship and with it, a hierarchy. As computer simulations have shown, rank orders may arise automatically [22], when a few simple rules of giving or taking precedence are followed. Self-organisation is an underestimated aspect of social organisation in animal species. It arises through repeated encounters among group members, which are opportunities to gain information on the actual strength of opponents and help to avoid losing fights in the future [23]; thus learning plays a role in the formation of dominance relationships.

It’s important here to understand what dominance actually is. The definition of dominance by Drews [24] is analogous to the original definition by Schelderup-Ebbe [25]: the outcomes of agonistic dyadic interactions result in consistent winners being dominant and losers being subordinate. But dominance based on winning conflicts in agonistic contexts is not the only way to view it. Two more types of dominance, distinguished by primatologist de Waal [10], are based on either formal dominance or competitive ability. Formal dominance develops via the exchange of status information through ritualized and/or greeting signals that are independent of context. Competitive ability considers the motivation of animals to obtain or to possess resources. In canids, this has been measured using pairwise competition tests over bones or toys [26]. Competitive orders based on priority of access to food or water, however, are not necessarily in agreement with agonistic dominance [27] or formal dominance [28,11], although these are usually correlated.

The ritualized communication patterns that may serve a relative riskless establishment of dominance relationships (primates: [10], wolves: [29]) are of specific interest in the study on dominance and hierarchy formation. Such communication signals seem to deescalate conflicts and reduce the risk of physical injuries or worse [10]. A variety of ritualized signals, mostly body postures and facial expressions, were found to be shown in only one direction in dyadic encounters. These are described in primate species to serve as formal signals of dominance or submission (e.g. bare-teeth display in rhesus monkeys: [28]), and the same has been seen in wild wolves, captive wolves and free-ranging dogs [3,30–36]. This one-directional pattern is also seen in Van Hooff & Wensing [9], who were the first to study body postures as dominance indicators in a captive pack of wolves. Their findings showed that in a captive wolf pack, two postural displays (namely high posture and low posture), accompanied by seven agonistic and affiliate behaviours, were displayed in the dyads in mainly one direction and were therefore better indicators of formal dominance than the seven behaviours per se. On the basis of these two postures, it was possible to construct a rank order that was not only transitive but also linear (Landau’s linearity index h>0.9). In this way, formal dominance was proven and the significant correspondence between the rank orders constructed on the basis of several formal status indicators justified the application of the dominance concept in a captive wolf pack.

A different set of indicators was used by Bradshaw and co-workers [37] in their unpublished qualitative study in which they examined a semi-permanent all-male group of 19 neutered domestic dogs, searching for a dominance hierarchy based on “confident” (e.g. growl, inhibited bite, stand over, stare at, chase, bark at, mount) and “submissive” (e.g. crouch, avoid, displacement lick/yawn, run away) behaviours. These behaviours were presumed to be useful as rank indicators but not investigated for their usefulness, as Van Hooff & Wensing had done in their captive wolf study [9]. Since they found no linear hierarchy, Bradshaw et al. [37] concluded that dominance does not play a role in the domestic dog and that dogs do not strive for a dominant position. Their study excluded posture as a behavioural variable previously shown to be an important variable for dominance relationship assessment in wolves [9].

Dominance hierarchies based on aggression and submission were found in two packs of Indian free-ranging dogs [38], but the properties linearity, directional consistency and coverage were not assessed. A later recalculation of these properties for these behaviours ([39], p.78), showed high levels of linearity in the two packs. Linearity is also shown in a study on Italian free-ranging dogs [11] in which three types of dominance were investigated and compared: the formal dominance, agonistic dominance and competitive ability [10]. In that study, presumed behaviours belonged to one of the three clusters (aggressive, dominance and submissive behaviour) and these clusters were investigated to determine whether they could be used to fit dogs into a linear dominance hierarchy. The findings showed that (1) the agonistic-dominance hierarchy was substantially linear, (2) the submissive-affiliative patterns fulfilled the criteria of formal submission signals (but were not observed among all dog pairs) and (3) the competitive rank order (based on gaining access to food) was predicted reasonably well by the agonistic rank order.

So far, nearly linear hierarchies constructed from systematic, quantitative data on submissive (but not agonistic) behavioural measures, appear in one study on captive wolves [9] and in two studies on free-ranging dogs [11,39] but not in group housed domestic dogs [37]. These contradictory results concerning the role of dominance in canids have led us to investigate this aspect of social organisation in a group of domestic dogs in more depth. We hypothesize that the dominance model as sketched above is applicable in our group of domestic dogs, just as in captive wolves [9] and in free-ranging dogs [11,39]. More specifically, we expect that postural communication and submissive behaviours play a major role in status communication, since these have been shown to be the best indicators in wolves and free-ranging dogs. Since wolves and dogs are genetically very close [40,41], despite domestication we expect to find the same social organisation, meaning a (nearly) linear hierarchy based on formal dominance with stable relationships.

In the present research, we firstly address the question of the usefulness of postural and behavioural variables as status indicator in domestic dogs. To this end, we strictly distinguished postures (including ear, tail, and body positions) from all the other behavioural variables (e.g. bark, growl or pilo-erection). Subsequently, we compared these behaviours and postures with respect to their qualities as indicators of dominance or submission in dyadic relationships and characterised the constructed rank orders in terms of linearity. As an additional element, we investigated the steepness of the one rank order that stood out in terms of linearity, transitivity and coverage [42,43]. Steepness provides a measure of the strength of the asymmetry between neighbouring ranked individuals concerning a relevant behavioural measure for instance, the summed dyadic proportions with which a group member receives submissive acts or wins dyadic encounters indicates overall individual dominance success. Finally, cluster analysis on a subset of postures and behaviours was used to reveal clusters possibly indicative of different aspects of dominance and aggression that would justify the use of the concept of dominance as an intervening (= summarizing) variable [9,20].