The influence of geographic isolation on the skull of the green monkey ( Cercopithecus aethiops sabaeus ) V. The degree and pattern of differentiation in the cranial dimensions of the St Kitts green monkey

Abstract

Some 300 years ago, free-ranging colonies of green monkeys from the West African species Cercopithecus aethiops sabaeus became established on the Caribbean island of St Kitts. Today, the cranium of the St Kitts monkey is, on an average, 4% bigger than in the present-day mainland descendants of the parent stock. This overall divergence has been compared on the one hand with the differences between C. aethiops sabaeus and each of two distinct sub­species of C. aethiops , and on the other with the divergence between C. aethiops sabaeus and representatives of each of two additional full species of Cercopithecus . The average difference between the cranial dimensions of the St Kitts green monkey and its parent species, C. aethiops sabaeus , is of approximately the same size as that between C. aethiops sabaeus and C. cephus cephus , (the moustached monkey) but is less than the difference between the cranial dimensions of C. aethiops sabaeus and those of either C. aethiops aethiops (the grivet) or C. nictitans nictitans (the white-nosed monkey). On the other hand, while in common with the St Kitts monkey, C. nictitans nictitans is, on an average, in its cranial dimensions, bigger than C. aethiops sabaeus , both C. cephus cephus and C. aethiops aethiops are smaller. The cranium of C. aethiops pygerythrus (the vervet) does not, on an average, differ in size from that of C. aethiops sabaeus . In both the St Kitts green monkey and in C. aethiops aethiops the most marked differences from C. aethiops sabaeus are in the transverse dimensions of the palate and mandible. In C. nictitans nictitans both the antero-posterior and transverse dimensions of the palate and mandible differ from C. aethiops sabaeus to a greater extent than do those of other cranial regions, while in C. cephus cephus , the antero-posterior dimensions of all parts of the skull differ from C. aethiops sabaeus to a greater extent than do the transverse dimensions. Many of these differences in overall cranial size contrast with those previously established for the teeth, and when cranial and dental dimensions are considered together, it is found that the subgroups of Cercopithecus cannot be readily separated on the basis of differences in size and general proportions. As the pelage of the St Kitts monkey is apparently indistinguishable from that of C. aethiops sabaeus , it would be inappropriate to assign the island monkey to a distinct taxonomic group.