This paper examines the structure and role of visual signals in African forest guenons with particular attention to face pattern. Anatomical and genetic configuration is sufficiently similar in Cercupithecus for hybridisation to be possible between most species and they share a large common repertory of gestures and expressions, yet cross-breeding is relatively rare in the wild, even along allopatric boundaries and in localities with up to six sympatric species. The most obvious differences between species are in voice and coat colour, It has been found that each of the major species making up a sympatric guenon community employs a different mode of visual signalling, each employing a distinct arrangement of pattern. The following types of pattern can be distinguished.
- 1Dull agouti pattern, minimal contrasts, possibly in interest of full repertoire of postures and facial expressions plus greater reliance on voice. Typified by large unspecialised monkeys of “blue monkey” or mitis group. Normally in relatively stable one-male groups, medium to low densities.
- 2Dull coat and face pattern, perhaps secondarily reduced in the interest of crypsis. Principal visual signal sex, and age-linked to scrotal area of adult male. Typified by sexually dimorphic C. hamlyni. Specialised, largely terrestrial; one-male groups at low densities.
- 3Bold, strongly simplified pattern. Signal over longer ranges. Genital and facial poles well differentiated. Typified by C. (diana), high canopy specialist, one-male groups at relatively high densities.
- 4Complex patterns concentrated in head, correlated with stereotyped head-flagging over medium ranges. Typified by C. (cephus) group, small, highly arboreal secondary growth specialists; high densities with relatively unstable male membership.
- 5Patterns with mosaic of separate features or mixture of earlier types (further categorisation probably possible here). C. (mona) group, C. neglectus and others.Ecological strategies and the environment especially influence the structuring of rally signals (which need to be species-specific in multi-specific communities). Correlations between a species' ecological/behavioural adaptations and the relative visibility in its favoured habitat are exemplified in comparisons between three divergent but related species, C. (diana), C. neglectus and C. hamlyni.
- (a). In arboreal C. (diana) reliance on visual communication evident in elaborate design while sharp tonal contrasts correlate with relatively long transmission ranges. Differentiated patterns at the front and back ends of the animal suggest facial and genital signals are functionally separate.
- (b). The flag-like chin panel of C. neglectus is appropriate to advertise rally call and ritualised side-to-side head movements with which it is associated. Beard matches white buttock patches of approximately similar shape and size, implying poor discrimination between genital and facial signals.
- (c). In more terrestrial and vulnerable C. hamlyni principal visual advertisement of caller is scrotal area. Discreet but eccentric design of the face pattern appropriate to short-range communication and staged movements.
Examples of species-specific stereotypy in behaviour correlating with patterns in appropriate parts of the body or face are given for some other species.
It is suggested that minor variations in a common cercopithecine behavioural repertoire are translated into highly specific visual signals through the elaboration of patterns.
Effective transmission demands selection for maximum optical clarity in the signal. The patterns of isolates with a common ancestry suggest that this principle determines how “traits” evolve into “patterns” but they also suggest numerous options for the elaboration of signals.
A concentration of pattern in the faces of the spot-nosed, red-tailed monkey group can be correlated with a transfer of signals from the genital to the facial pole. Factors influencing this development are size, diet, spatial dispersal and social organisation.
Detailed examination of their signalling behaviour and the structure of their facial patterns has indicated that these very diverse monkeys belong to a single allopatric species group which has necessitated a minor taxonomic revision and the allocation of all forms to the super-species Cercopithecus (cephus).
Stereotyped head movements in five members of this group have been filmed and subjected to time and motion analysis. They are unlike those observed and recorded in other species groups and appear to have a distinct derivation.
Head “flagging” appears in the context of courtship and appeasement in all observed members of C. (cephus). Variations in their motor patterns suggest that there may be regional (and in captives, individual) dialects which can, in some cases, be correlated with the structure and type of face marking.
It is suggested that the diversity of this group derives from past climatic changes and isolation at a critical time in their evolution. It is significant that genetic distinctness has been maintained in a large number of forms, despite the absence of contemporary geographic barriers.
The association of face patterns and idiosyncratic signalling with reproductive behaviour may constitute a major mechanism reinforcing the genetic isolation of a population. In this way the mechanism for reproductive isolation may be already operative in the contemporary allopatric populations of this species group.