Character displacement of Cercopithecini primate visual signals

Abstract

Animal visual signals have the potential to act as an isolating barrier to prevent interbreeding of populations through a role in species recognition. Within communities of competing species, species recognition signals are predicted to undergo character displacement, becoming more visually distinctive from each other; however, this pattern has rarely been identified. Using computational face recognition algorithms to model primate face processing, we demonstrate that the face patterns of guenons (tribe: Cercopithecini) have evolved under selection to become more visually distinctive from those of other guenon species with whom they are sympatric. The relationship between the appearances of sympatric species suggests that distinguishing conspecifics from other guenon species has been a major driver of diversification in guenon face appearance. Visual signals that have undergone character displacement may have had an important role in the tribe's radiation, keeping populations that became geographically separated reproductively isolated on secondary contact.

Figure 1. Examples of guenon faces, illustrating interspecific diversity

Species: (A) Cercopithecus ascanius, (B) C. mitis, (C) Chlorocebus aethiops, (D) C. lhoesti, (E) C. neglectus, (F) Allenopithecus nigroviridis, (G) Ch. sabaeus, (H) C. sclateri, (I) C. diana, (J) C. campbelli, (K) C. hamlyni, (L) C. cephus, (M) Miopithecus talapoin, (N) Erythrocebus patas, (O) C. petaurista, (P) Ch. tantalus, (Q) C. erythrotis, (R) C. erythrogaster, (S) C. mona, (T) C. nictitans, (U) C. wolfi, (V) C. pogonias, (W) C. lomamiensis, (X) C. preussi. (W) by M. Emetshu, reproduced with permission of K. Detwiler and J. Hart, (R) with permission of N. Rowe, all other images are by the author W. Allen.

Figure 2. The first 11 frontal eigenfaces

Eigenfaces are illustrated by reconstructing the faces produced by eigenface scores +/− two standard deviations from the mean of the 22 species’ eigenface scores. Each dimension responds to features of guenon face patterns such as nose spots, eyebrow patches and beards of decreasing importance to overall variation in the set. For example the first eigenface mainly describes how light or dark the face is and the second eigenface contrasts a light beard and dark eyebrow patch with dark cheeks and a yellowish eyebrow patch. More detailed and less common features were quantified by later components. Together components 1 and 2 account for 52.89% of variance in estimates of guenon long, medium and short wavelength photoreceptor responses and the first 11 components describe 93.6% of variance. For the current application, the goal of achieving accurate reconstruction and recognition of new faces was more important than dimension reduction; we hence report the results of a guenon face-space based on all 21 eigenfaces.

Figure 3. Phylomorphospace of the first two eigenface dimensions

This shows the position of each species on the first two axes, giving an indication of the relative visual distinctiveness of species. Connections between points show species’ phylogenetic relationships. Branch colour indicates distance from the root.

Figure 4. The two consensus phylogenetic trees of the Cercopithecini study species used for phylogenetic analyses ^,