Retinal Structure and Visual Acuity in a Polyprotodont Marsupial, the Fat-Tailed Dunnart <i>(Sminthopsis crassicaudata)</i>

Abstract

The visual system of the fat-tailed dunnart (Sminthopsis crassicaudata), a small polyprotodont marsupial, has been examined both anatomically and behaviourally. The ganglion cell layer was examined in cresyl-violet stained wholemounts and found to contain a mean of 81,400 ganglion cells (SD ± 3,360); the identification of ganglion cells was supported by a correspondence to optic axon counts. Ganglion cells were distributed as a mid-temporally situated area centralis, embedded in a pronounced visual streak. Localised implants of horseradish peroxidase into retinal wholemounts revealed both A-type and B-type horizontal cells. Sections of the outer retina showed it to be rod-dominated, with a rod-to-cone ratio of 40:1 at the area centralis; cones were found to contain oil droplets but double cones were not a prominent feature. The retinal pigment epithelium consisted of squamous cells. Visual acuity, estimated from counts of peak ganglion cell density (8,300/mm², SD ± 1,180) and measurements of posterior nodal distance (2.9 mm), was found to be 2.30 cycles per degree. The value was close to that of 2.36 cycles per degree estimated by behavioural tests using a Mitchell jumping stand; values were similar at low, intermediate and high light levels. Our findings are discussed in relation to the lifestyle of the dunnart.

On the relationship between orbit orientation and binocular visual field overlap in mammals

“…Two cited advantages for binocular visual fields are enhanced light sensitivity and contrast discrimination, both of which would benefit nocturnal taxa (Lythgoe, 1979;Ross et al, 2005). Sminthopsis is cathemeral, active at all light levels, and Dasyurus is nocturnal (Arrese et al, 1999). Although speculative, one possible reason for the larger binocular zone in these two animals may relate to their nocturnal and predatory habits.…”

“…Although speculative, one possible reason for the larger binocular zone in these two animals may relate to their nocturnal and predatory habits. However, both of these taxa exhibit relatively low levels of visual acuity as well as retinal physiological traits similar to other mammals that spend some or their entire activity budget under nocturnal conditions (Arrese et al, 1999), so it is not apparent what benefit the expansion of binocularity would provide these animals. Yet, under certain conditions, functionally converging the visual axes may occasionally be required for some visual tasks, and the ophthalmoscopic measurements may reflect this ability.…”

“…Additional data were collected based on the retinotopic projections to cortical visual areas. The correspondence of ophthalmoscopic and the retinotopic organization of cell projection data has been explored elsewhere (Rodger et al, 1998;Arrese et al, 1999), but in general these provide similar estimates of the size of the binocular visual field.…”

Primate origins, human origins, and the end of higher taxa

“…Among living prosimians and small marsupials, insecteating forms (Tarsius, Microcebus, Sminthopsis) have markedly keener retinal resolution and greater pupillary mobility than do specialized fruit-and nectar-feeders (Cheirogaleus, Tarsipes). [103][104][105][106][107] Fruit-and nectar-feeders do not need keen vision, because fruits and flowers are not cryptic. After all, angiosperms evolved fruits and flowers for animals to eat, and they typically advertise them with shiny bright colors and sweet smells.…”

Dominant Vertical Orientation Processing without Clustered Maps: Early Visual Brain Dynamics Imaged with Voltage-Sensitive Dye in the Pigeon Visual Wulst

“…The references for these values were mainly derived from the study by Heesy (2004), his Table 1. Human (Vakkur and Bishop, 1963); macaque (Vakkur and Bishop, 1963;Ross, 1995); owl monkey (Allman and McGuinness, 1988); marmoset (Cartmill, 1971;Fritsches and Rosa, 1996); cat (Hughes, 1976;Arrese et al, 1999;Finarelli and Goswami, 2009); ferret (Garipis and Hoffmann, 2003); tree shrew (Hughes, 1977); squirrel (Kaas et al, 1972;Van Hooser et al, 2005); rat (Arrese et al, 1999); mouse (Dräger, 1978;Arrese et al, 1999); rabbit (Wall, 1942;Hughes and Vaney, 1982); barn owl (Iwaniuk et al, 2008); pigeon (Iwaniuk et al, 2008). more prominent at lower speeds because of the specific spatiotemporal bandwidth of visual neurons.…”