White Sclera Is Present In Chimpanzees & Other Mammals

Humans can communicate with a subtle glance. This is due, in part, to our eyes having a visible white sclera that contrasts with the iris, thereby enabling humans to effectively attend to others' gaze direction (Kobayashi and Kohshima, 1997, 2001; Tomasello et al., 2007; Yorzinski et al., 2021). Both our white sclera and complex social cognition have long been considered unique among mammals, strengthening the hypotheses that these traits are linked—i.e. cooperative eye hypothesis (Tomasello et al., 2007), gaze signaling, and gaze camouflage hypotheses (Kobayashi and Kohshima, 2001). However, the last two decades have revealed that our closest ape relatives, especially chimpanzees, exhibit complex perspective-taking abilities (Hare et al., 2001, 2006; Krupenye et al., 2016) and that there may be greater variability in the eye color of apes and other primates than previously thought (Perea-García et al., 2022). Some gorillas exhibit white sclera (Mayhew and Gómez, 2015; Caspar et al., 2021), and many Sumatran orangutans (Perea-García, 2016; Caspar et al., 2021) and bonobos (Perea-García et al., 2019) exhibit light brown sclera. Recent studies have sparked debate over the extent to which the sclera needs to be white to effectively contrast with the iris and enhance the visibility of gaze direction (Perea-García et al., 2019; Caspar et al., 2021; Kano et al., 2021, 2022; Mearing and Koops, 2021; Whitham et al., 2022, Whitham et al., 2022). 

A critical piece of information is missing from this debate: studies continue to report chimpanzees, our closest living relatives along with bonobos, as having dark sclera. Early reports of the occasional wild chimpanzee with white sclera have been considered pathologies or anomalies (Goodall, 1986; Boesch and Boesch-Achermann, 2000). More recent studies have not addressed the few cases that diverge from the assumed norm of uniformly dark sclera (e.g. Perea-García et al., 2019; Kano et al., 2022). Is white sclera an anomaly among chimpanzees, or might prior studies have failed to detect variation due to small samples? 

Here, we examine the prevalence and development of white sclera from photographs in the largest sample of wild chimpanzees studied to date (n = 230 individuals). We then explore the presence of white sclera in 70 zoo mammal species.

- Full paper here

Jeez! Reminds of George Orwell's famous lines

To see what is in front of one’s nose needs a constant struggle.