Anim. Behav., 1995,49,240-243 Inter-group encounters in blue monkeys:how territorial must a territorial species be? MICHAEL J. LAWES* & S. PETER HENZIt *Department of Zoology, University of Natal, PIBag X01, Pietetmaritzburg 3209, South Africa TBehavioural Ecology Research Group, University of Natal, King George V Avenue, Durban 400, South Africa (Received 22 March 1994; initial acceptance 18 May 1994; final acceptance 13 July 1994; MS. number: ~~-997) Current primate socio-ecological modelling assumes that the patterning of social relationships reflects prevailing environmental conditions (Wrangham 1980). Populations of a species are assumed to have the same degree of response lability so that any observed variation in behav- iour can be attributed directly to variation in ecological parameters (Andehnan 1986; Isbell 1991). However, most of the data sets used in these analyses are constructed at specific, rather than the population, level, and persist in using standard typological, bipolar constructs (unimale/ multimale; territorial/non-territorial; despotic/ egalitarian; Crook & Gartlan 1966; Struhsaker 1969; Clutton-Brock & Harvey 1977), implying that the animals may have a circumscribed ca- pacity to respond to environmental change. While this view may be correct for at least one analytical dimension (tmimale/multimale; Henzi 1988), its appropriateness for others needs verification. Of these, territoriality (the creation of a formal territory through the defence of a ranging area; Dunbar 1988) is particularly interesting. The behavioural expression of such spatial defence (if it is a species characteristic) should take the form of untempered or invariant aggression (obligate territoriality) towards other groups, particularly if these cross some perceived boundary. Yet, as Mitani & Rodman (1979) and Dunbar (1988) have pointed out, such aggression will make econ- omic sense only under certain ecological con- ditions, so that one might expect it to be only one of a suite of contingent strategic responses.Work on at least one guenon species-group, Cercopithecus aethiops, has shown that ‘territorial’ behaviour is not a rigid maintenance of fixed boundaries, but a facultative response to variable conditions (Kavanagh 1981; Harrison 1983). The obvious question is, therefore, how territorial must a strictly territorial species be? We maintain that to argue convincingly that a species is ‘territorial (i.e. where members of a group demonstrate obligate aggression towards other groups when these are observed within the home range), one would need to show a consistency of inter-group response under a sulhciently broad range of conditions. Blue monkeys, Cercopithecus mitis, are ideally suited to assessing this argument: they are regarded as a typical territorial species, found in a wide variety of woodland/forest types across a broad latitudinal range, consequently experienc- ing markedly different local population densities (Cords 1987a; Butynski 1990; Lawes 1992). We studied a population living at high density (2.02 individuals per ha) at Cape Vidal, South Africa. Our study group of about 32 animals used a small home range (15 ha) and shared 45% of this area with three neighbouring groups. Mitani & Rodman (1979) suggested that the decision to defend the home range is primarily related to the frequency with which a group can patrol its territorial boundary. Given that the 45% overlap provides, on the surface, a clear incentive to be territorial, while the very small home range encourages the expression of territoriality (Mitani & Rodman 1979), we would expect our study group to respond aggressively to all observed intrusions. We defined an inter-group encounter by the presence of another group within sight of the study group, where members of one or both groups had indicated at least by visual fixation that the other group had been seen. A group was considered to be intruding when it was seen in a quadrat that had been previously entered at least once by the study group. Antagonistic behaviour ranged from head-bob threats and vocaliz- ations to conspicuous chases. We recorded 58 inter-group encounters (1.07 per diem) during