Irradiance levels across land environments span eight to nine orders of magnitude, and eyeball morphology will often reflect the diel activity pattern of an animal (Schmitz and Motani, 2010). Diurnal species are active by day; their nocturnal counterparts are active by night. One common mechanism inherent in the natural world is the existence of a 24-hour rhythm which dictates daily and nightly organism activity. Even when cut off from abiotic factors, such as light or temperature, organisms still closely follow their usual pattern of activity (Burton, 1982). However, a distinct line that differentiates diurnal and nocturnal crumbles when looking at many species: the night time is not completely black, as the moon provides excellent lighting for some animals. By all means a full moon can offer decent lighting for some human activities. The day is not always radiantly bright: the lighting beneath a rainforest canopy on a cloudy day can be dark and gloomy. Many animals also share increased activity patterns during the early morning hours and the late evening, a behavioral pattern defined as crepuscular. “The fact that mammals do not strictly conform to their biorythmical assignments means that the visual capabilities of diurnal versus nocturnal vision potentially overlap" (Ankel-Simons and Rasmussen, 2008).

Primates

Primates exhibit a wide range of diel activity patterns: from being diurnal to nocturnal, and a couple species that are active both day and night (Fernandez-Duque et al., 2010). Morphologically, primates and in general mammals with large optical orbits tend to have nighttime activity patterns. For example, among the smaller primates, those with larger eyes tend to be active at night. One notable monkey is the nocturnal Tarsius, whose eyeballs amount to 4.5% of its body weight. Compared to large-bodied chimpanzees and humans, this is a significant difference. Our eyeballs amount to only 0.03% of our body weight. However the Tarsius is an extreme, for most other nocturnal primates the percentage is around 0.3% and for diurnals 0.15%. Cathemeral, or crepuscular, mammals tend to fall between 0.3 and 0.15% (Ankel-Simons and Rasmussen, 2008).

The Central and South American night or owl monkey, falling under the genus Aotus, is the only extant nocturnal anthropoid (Fernandez-Duque et al., 2010). Some authors note that Aotus does not have a fovea, but rather many more rods. Also, hundreds of these rods might be connected together with their ends all feeding into the same couple of neural cells, increasing the chance for stimulation of the neuron. Because of its lack in cones, the visual acuity of the Aotus falls short in comparison to it diurnal relatives (Cawthon, 2005); nevertheless, its larger eyeballs, densely packed ganglion cells (the nervous tissue) and rods give this animal an advantage at night (Ankel-Simons and Rasmussen, 2008, Dyer et al., 2009).

Figure 1. The owl moneky. Photo courtesy of Mono Nocturno Aotus. Juan Carlos Pérez