Sławomir Wacewicz, Juan Olvido Perea-García, Dariusz Danel, Przemysław Żywiczyński, Antonio Benítez-Burraco, Theresa Matzinger (organizers): Introduction – state of current research & open questions

Two decades after the publication of Kobayashi & Kohshima’s seminal work on primate external eye appearance, the field has moved away from the certainty and conclusiveness with which it was inaugurated. While horizontal elongation of the eye outline has received support as a trait that sets humans apart from other primates, more recent examinations cast doubt on the categorical division between “signaling” and “camouflaged” eyes based on pigmentation patterns alone. There is little doubt that human eye appearance affords communicative functions, but to what extent is this also true in other primate species? In which species-specific ways do other primate species rely on their external eye appearance for (perhaps also species-unique) communicative functions? And, more generally, can we trace the evolutionary history of our species-typical external eye appearance?

The recent surge of interest in the topic has provided methodological tools to examine these questions with renewed vigor. Quantitative measurements, phylogenetically informed statistics, comparative genomics, comparative empirical studies, and modeling of visual systems have advanced a picture of the origins and functions of external eye appearance in different primate lineages that is much more complex than we initially thought. This new stream of evidence has also broadened the range of hypotheses under consideration beyond the seminal view of an enhancer of communicative functions: depigmentation as a by-product of selection against aggression; adaptation to species-typical spectral conditions; drift; and sexual selection. It is now pertinent to reflect on the state-of-the-art to decide the directions in which we should be investing time and effort. In our opening talk, we lay the grounds for this discussion by reviewing the results of recent studies, by ourselves and others, that bear on the hypotheses listed above.