Fernandez-Juricic, E. | bioRxiv Science

bioRxiv2019

A novel cellular structure in the photoreceptors of insectivorous birds

The keen visual systems of birds have been relatively well-studied. The foundations of avian vision rest on their cone and rod photoreceptors. Most birds use four cone photoreceptor types for color vision, a fifth cone for achromatic tasks, and a rod for low-light levels. The cones, along with their oil droplets, and rods are conserved across birds - with the exception of a few shifts in spectral sensitivity - despite taxonomic, behavioral and ecological differences. Here, however, we describe a novel photoreceptor in a group of New World flycatchers (Empidonax spp.) in which the traditional oil droplet is replaced with a complex of electron-dense megamitochondria surrounded by hundreds of small, orange oil droplets. These photoreceptors were unevenly distributed across the retina, being present in the central region (including in the fovea), but absent from the retinal periphery and the area temporalis. Many bird species have had their oil droplets and photoreceptors characterized, but only the two flycatchers described here (E. virescens and E. minimus) possess this unusual structure. We discuss the potential functional significance of the unique sub-cellular structure in these photoreceptors providing an additional visual channel for these small predatory songbirds.

zoology↗

bioRxiv2019

The visual social environment affects non-additively neighbor spacing and interaction time in zebrafish

Many fish form schools and visually track the position of their neighbors in a 3D environment. In this study, we assessed whether zebrafish modified their spacing behavior and interaction time in an additive or multiplicative way relative to multiple sources of visual social information using video playbacks. We simultaneously manipulated: (a) the magnitude of the social cues (by varying the size of the virtual fish), (b) the level of social risk (low, high based on the position of the virtual fish in the water column), and (c) the perceived depth of the social cues (visual horizon absent or present). Each of these factors independently affected spacing behavior (zebrafish increased the separation distance with larger virtual fish, under lower visual social risk, and when depth perception was enhanced), but they did not affect interaction time. However, some of these factors interacted significantly, such that their effects on social behavior depended on each other. For instance, zebrafish decreased their separation distance under high vs. low risk conditions when the virtual fish was the same or smaller size, but this risk effect disappeared with larger virtual fish likely to avoid aggression. Also, zebrafish increased their separation distance when perceived depth was enhanced under low risk, but the perceived depth effect became less pronounced under high risk probably due to dilution effects. Overall, the effects of certain visual social parameters depend on the intensity of other visual social parameters, ultimately tuning up or down different social behavioral responses. We discuss the implications for the spatial organization of fish schools.\n\nSignificance StatementMany fish form schools and visually track the position of their neighbors in a 3D environment. We found that zebrafish consider multiple visual social sources of information simultaneously to modify their neighbor distance. Thus, their spacing behavior appears to follow multiplicative rules, whereby the spacing response to a visual social parameter depend on the intensity of a different visual social parameter.

animal behavior and cognition↗