In the Ruff two groups of males can be distinguished: independent males and satellite males. This classification is based upon differences in territoriality and behaviour, and is highly correlated with differences in the nuptial plumage. Independent males are mostly dark coloured; their behaviour contains much fighting and related activities. They can be subdivided into resident males and marginal males. Resident males defend territories (residences), marginal males do not. Satellite males are mostly white. They behave peacefully and do not defend a territory. Satellite males have access to the residences on an arena (cfr. HOGAN-WARBURG, 1966). In this study an attempt has been made to throw some more light upon the behavioural dimorphism (independent males versus satellites). On the one hand I tried to analyse the causation of this phenomenon, on the other hand I was interested in its biological significance. I have tackled these problems by means of an examination of film material, and by a quantitative study of the behaviour on different arenas (leks) in the field. The film supplied me with detailed data about the behaviour of resident males, satellite males, and females in different external situations. On the basis of these data it was possible to analyse the temporal associations between the various behavioural units, for this purpose divided into "postures", "locomotion sequences", "action sequences", and "actions". The observed relations were largely similar for the three categories of individuals. For this reason I was able to compose one model for the hypothetical causal mechanism underlying the behaviour of resident males, satellite males, and females. The display of postures was supposed to be controlled by the values of two variables, the display of the other units by the value of the same variables and directly by the external situation. The values of both variables depended on external stimuli and on the internal situation (partly determined by previous external stimulation) I could not obtain evidence for the existence of different mechanisms underlying the different groups of behavioural units serving one function, such as aggression, flight, and sex. These groups of behavioural units were fully integrated in the entire causal system. They each appeared at distinctive combinations of values of both variables. Aggressive and sexual behaviour appeared each at only one cluster of combinations, however, protective behaviour appeared at three clusters. The suitability of the drive concept for both variables has been discussed. Normal behaviour of the three categories distinguished seemed to correspond to category-specific ranges of both variables. Female behaviour represented the one extreme, resident male behaviour the other one, and satellite behaviour was mainly intermediate. This intermediate behaviour has been ultimately ascribed to the light plumages of satellites and the genetical factors connected with that; the lack of a territory and distinct components in the behaviour of resident males towards satellites have also been emphasized as causal factors. I presented arguments for the hypothesis that the status (independent or satellite) of an individual depends on genetical and environmental factors. The importance of genetical factors appears from the correlation between behaviour and plumage colour. The influence of the environment follows from (a) the inconstancy of the status of some individuals (particularly young ones), and (b) the relation between status of an individual and plumages of the other individuals present on the same arena. It has been emphasized that some of the plumage colours occur exclusively in satellite males (satellite plumages), and others exclusively in independent males (independent plumages). Males with untypical plumages (intermediate colours) do not necessarily belong to one and the same category. They generally belong to the independent category when there are many males with satellite plumages on the arena, and relatively often to the satellite category when independent plumages are common. This phenomenon could not be ascribed to differential reproduction in previous seasons; it seemed to be connected with migration and changes of status, probably caused by low copulation frequences. It is argued that males with independent plumages are homozygotes with respect to a particular gene, that males with satellite plumages represent the same condition but with alternative alleles, and that males with untypical plumages are heterozygotes. This implies that both the behaviour and the plumage are influenced by one pair of genes (pleiotropy). Resident males can enlarge their reproductive output when tolerating satellite males on their residences in particular situations. The presence of satellite males on residences promotes the attraction of females. However, the presence of satellites has no influence on the duration of female visits, moreover, it has a negative effect on the copulation frequency of resident males. It could be demonstrated that the behaviour of resident males towards satellites is either tolerant or intolerant. Intolerant behaviour occurs when the resident male acquires a high density of female visits on his residence; it also occurs during and after high copulation frequencies of the male concerned. In all other circumstances tolerant behaviour will be shown. Both kinds of behaviour regularly alternate with a period of some days up to some weeks. This causes a large fluctuation in the density of satellite visits to the residence. The density of female visits also fluctuates, but less extremely. The other factors influencing female and satellite visits to residences have been analysed. This enabled me to formulate a model by which the oscillations in the densities of female and satellite visits could be simulated. Furthermore the effects of satellites on copulating of resident males could be studied. Satellite males appeared to play a very important role. A resident male cannot attract enough females for copulating without co-operation with satellites. The copulation success also depends on the tactics of the resident male, particularly on the adjustment of the tolerant-intolerant transition. Furthermore the degree of intolerance plays an important role. In the Ruff we are probably concerned with a balanced polymorphism. The genetical diversity may be a consequence of superiority of heterozygotes (males with untypical plumage colours). Another explanation is provided by the mutual dependence between independent males and satellites. It is likely that the fitness of individuals of either category changes during oscillations of their relative frequencies, because of the change in the effectiveness of the co-operation with the other category. It is suggested that the success of satellite males on an arena is negatively correlated with the number of stable relations between resident males. In the early part of the season, on recently established arenas, and after changes in the occupation of residences the copulation frequency of satellite males is high. This phenomenon is probably caused by the low number of attacks on non-territorial males in such situations. The high number of attacks on marginal males in the other situations implies that arenas seldom grow when stable relations are common. Hence the relative frequencies of independent males and satellites are supposed to be controlled by the influx of new independent males, and thus by the establishment of new arenas. This hypothesis implies that independent males are favoured during periods with low numbers of males, and that satellites have the advantage when there are many males.

This paper has tried to answer the question of how the independent and the satellite strategy of the Ruff originated and has been maintained during evolution. It is not intended as a report of a piece of completed research but gives a number of tentative, but testable hypotheses. Data are presented that independent males and satellites are equally successful in copulating, provided that almost all copulations occur on leks, and independent males and satellites spend an equal proportion of their time on leks. The extent that males with the different strategies are attached to leks is analysed. Within the group of independent males, considerable differences exist: a resident male is strongly attached to only one particular lek and a marginal male seems to sample many leks. A satellite male behaves intermediately: he is attached to a limited number of leks. The mechanism for the maintenance of both strategies could not be explained by competition for resources (copulations). Contests between independent males and satellites are asymmetric, the satellite being the weaker contestant. The mechanism seems to be due to enlarging the accessibility of the resource by cooperation between both strategies. The ideas on the origin of the system are based mainly on the behaviour of females. There is no relation between the distribution of leks and the size of a female's foraging area, except that most leks are situated along the migration route. It further seems that the number of copulating females in the Netherlands is larger than the number of males on leks, but the number of breeding females is much lower. It is suggested that many females copulate on migration, which is related to extreme breeding conditions in the north of the range. Apart from a change in the sex ratio from south to north, a change in the proportion of satellites is also predicted. The origin of the system is ascribed to the incompatibility between favourable food conditions for chicks and further possibilities for inter‐male competition on leks in the same area. The satellite strategy is considered as being derived from a non‐competitive strategy accompanying females on migration. In the phase of cooperation between resident and satellite males, individual recognition became important. This could be the factor underlying the evolution of white plumages in satellites and the extreme plumage diversity in independent males.

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This paper discussed the information content of threat signals. It was an attempt to explain the inconsistencies between the points of view of model-builders using game theory as a tool (represented in a paper by CARYL in 1979 in this journal), and of the (mainly field-) ethologists spending a considerable part of their time observing animals in groups (expressed for instance in the paper by BOSSEMA & BURGLER in this issue). Arguments were presented for the transfer by threat signals of both information about intentions (motivation), and information about "resource-holding potential" (strength, ownership, etc.). Individual recognition was expected to be associated with honest signals about intentions. CARYL'S deviating findings could not be attributed to an absence of individual recognition in the animals he considered. His findings could also not be explained very well by the fact that he hardly considered subtle signals, although the present paper argued that information about intentions is mainly given by subtle signals, and information about resource-holding potential by elaborate action patterns. Imperfect methods in the papers cited by CARYL were considered as the most important source for the deviations. Finally it has been discussed to what extent observational data as presented by BOSSEMA and BURGLER help in solving problems raised by the model-builders. The occurrence of frequent, short escalations has been suggested as a mechanism for preventing bluff. The evolution of graded warning-signals could be related to (1) the low cost of a warning as compared to an attack, and (2) the settlement of a conflict on the basis of differing motivations. This paper considered the possibility of heterogeneous summation of (already known) information about the resource-holding potential of an opponent, and the information about its intentions (from the displays).