Niche convergence suggests functionality of the nocturnal fovea.

Gillian L Moritz,Perry S Ong,Amanda D Melin,Henry Bernard,Nathaniel J Dominy,Fred Tuh Yit Yu

doi:10.3389/fnint.2014.00061

Abstract

The fovea is a declivity of the retinal surface associated with maximum visual acuity. Foveae are widespread across vertebrates, but among mammals they are restricted to haplorhine primates (tarsiers, monkeys, apes, and humans), which are primarily diurnal. Thus primates have long contributed to the view that foveae are functional adaptations to diurnality. The foveae of tarsiers, which are nocturnal, are widely interpreted as vestigial traits and therefore evidence of a diurnal ancestry. This enduring premise is central to adaptive hypotheses on the origins of anthropoid primates; however, the question of whether tarsier foveae are functionless anachronisms or nocturnal adaptations remains open. To explore this question, we compared the diets of tarsiers (Tarsius) and scops owls (Otus), taxa united by numerous anatomical homoplasies, including foveate vision. A functional interpretation of these homoplasies predicts dietary convergence. We tested this prediction by analyzing stable isotope ratios that integrate dietary information. In Borneo and the Philippines, the stable carbon isotope compositions of Tarsius and Otus were indistinguishable, whereas the stable nitrogen isotope composition of Otus was marginally higher than that of Tarsius. Our results indicate that species in both genera consumed mainly ground-dwelling prey. Taken together, our findings support a functional interpretation of the many homoplasies shared by tarsiers and scops owls, including a retinal fovea. We suggest that the fovea might function similarly in tarsiers and scops owls by calibrating the auditory localization pathway. The integration of auditory localization and visual fixation during prey detection and acquisition might be critical at low light levels.

Highlights

The fovea centralis, or fovea, is an avascular declivity of the retinal surface
In the dipterocarp forests of Borneo, the δ15N values of predators are ca. 2.6‰ higher than those of omnivores, 3‰ higher than those of herbivores, and 3.7‰ higher than those of detritivores (Hyodo et al, 2010). These results suggest that variation in δ15N can discriminate trophic levels in the habitats used by tarsiers and scops owls, the isotopic baseline of tree leaves in northern Borneo can vary slightly as function of soil N availability (Kitayama and Iwamoto, 2001) and disturbance history (Woodcock et al, 2012)
We sampled the contour feathers of Sunda scops owls (Otus lempiji, formerly O. bakkamoena lempiji; n = 8) and Philippine scops owls (Otus megalotis, formerly O. bakkamoena megalotis; n = 11; taxonomy follows König and Weick, 2008)

Summary

Introduction

The fovea centralis, or fovea, is an avascular declivity of the retinal surface. It is aligned with the visual axis of the eye and contains a disproportionately high density of photoreceptors. The optics of foveae are an enduring interest (Walls, 1937; Weale, 1966; Locket, 1992; Ross, 2004) because the fovea has greater spatial resolving power than other retinal specialization (Inzunza et al, 1989; Moore et al, 2012). The tandem concept of sensory specialization and cortical overrepresentation, or magnification, is practically idiomatic: gymnotid and mormyrid fish have electrosensory “foveas”; (Castelló et al, 2000; Bacelo et al, 2008); echolocating bats have acoustic “foveas” (Neuweiler, 2003); and some haptic species have tactile or somatosensory “foveas” (Pettigrew and Frost, 1985; Catania and Remple, 2004; Hoffmann et al, 2004; Mancini et al, 2013)

Methods

Results

Conclusion