In the forests of South America lives the unusual but aptly named owl monkey, or douroucouli. You could probably guess by looking at its large round eyes that it’s nocturnal, and indeed, it is the only monkey to be mostly active at night. But its eyes have many adaptations for such a lifestyle, beyond a large size.
The owl monkey’s retinas are 50% larger than those of a day-living monkey of similar size, like the brown capuchin. The proportions of different cells in their retina are also different. Owl monkeys have relatively few cone cells, which are responsible for colour vision and fewer ganglion cells, which process the signals from the cones. In contrast, they have many more rod cells, which are far more sensitive than cones and function best in low light, and rod bipolar cells, which transmit signals from the rods.
This is an eye that has sacrificed sharpness and colour for sensitivity. Nocturnal mammals the world over have developed a very similar suite of adaptations and according to Michael Dyer and Rodrigo Martins, these may be easier to evolve than you might think.
All of the cells in the retina are produced by a small group of stem cells called retinal progenitor cells (RPCs). As an embryo grows, its RPCs go through cycles of division, still maintaining their “stemness”. At some point, they leave this cycle and commit to becoming one of the various types of retinal cells. The fate they choose depends on when they leave the cycle. Those that are “born” early turn into cells that are important for daylight vision, such as cones and ganglions. Those that exit late become cells that play a greater role in night-vision, including rods and their bipolar cells.
This quirk of organisation means that the retina’s cells are always produced in a very specific order, with those that grant good night-vision cells appearing later. The upshot is that the owl monkey has been able to adapt its retina to see in the dark simply by tweaking the timing of its development. In its retinas, more RPCs commit to a particular fate later on in their cycle, producing fewer of the earlier types of cells and many more of the later ones. The result: an extra-sensitive retina with a complement of cells perfectly suited for nocturnal living, all triggered by a single change during development.
The eyes of the owl monkey hammer home an increasingly familiar message – you can get big results by very subtly tweaking the way that bodies develop, without any need for large-scale tinkering. Even the eye, an exceptionally complex organ, can be altered in a coordinated way, simply by shifting the timing of its development. It’s why the owl monkey, in a relatively short space of evolutionary time, has converted the daylight-loving eyes of its ancestor into a nocturnal model.
Nocturnal animals face an obvious challenge: collecting enough light to see clearly in the dark. We know about many of their tricks. They have bigger eyes and wider pupils. They have a reflective layer behind their retina called the tapetum, which reflects any light that passes through back onto it. Their retinas are loaded with rod cells, which are more light-sensitive than the cone cells that allow for colour vision.
But they also have another, far less obvious adaptation – their rod cells pack their DNA in a special way that turns the nucleus of each cell into a light-collecting lens. Their unconventional distribution is shared by the rods of nocturnal mammals from mice to cats. But it’s completely opposite to the usual genome packaging in the rods of day-living animals like primates, pigs and squirrels, and indeed, in almost all other eukaryotic cells.
In our cells, massive lengths of DNA are packaged into small spaces by wrapping them around proteins. These DNA-protein unions are known as chromatin, and they come in two different forms. Euchromatin is lightly packed and resembles a string of beads. Wrapping DNA in this way puts it within easy reach of other proteins and allows its genes to be actively transcribed. But imagine scrunching up that string of beads and you get heterochromatin – a tight, condensed ball of repressed genes that proteins cannot reach.
The two forms of chromatin are found in different areas, with euchromatin spread throughout the nucleus and heterochromatin concentrated at its edges. That pattern is nigh-universal and it applies from amoebae to plants to animals. There are only a few exceptions to this rule, including a minority of single-celled species and surprisingly, the rod cells in the eyes of nocturnal mammals. Now, Irina Solovei from the Ludwig-Maximilians University in Munich had found that this inverted distribution helps these species to see in the dark.
In the twilit waters of the deep ocean, beneath about 1000m of water, swims the brownsnout spookfish (Dolichopteryx longipes). Like many other deep-sea fish, the spookfish is adapted to make the most of what little light penetrates to these depths, but it does so with some of the strangest eyes in the animal kingdom.
For a start, each eye is split into two connected parts, so the animal looks like it actually has four. One half points upwards and gives the spookfish a view of the ocean above. The other points downwards into the abyss below and it’s this half that makes the spookfish unique. The eyes of all other back-boned animals use a lens to divert the path of incoming light and focus it onto a specific point of the retina. But the spookfish’s downward-facing eye uses mirrors instead, forgoing a lens in favour of hundreds of tiny crystals that collect and focus light.
This bizarre animal was first described 120 years ago, but no one had discovered its reflective eyes until now because a live animal had never been caught. Hans-Joachim Wagner from Tubingen University changed all of that by netting a live specimen off the Pacific island of Tonga.
The spookfish’s eyes are similar in structure to many other fish that swim in the ocean’s twilight zone, where darkness is heavy but not quite total. The main part of each eye is tube-shaped and points to the surface, like a vertically mounted telescope. In photos A and B below, this upward-facing half has a yellow-orange shine because the camera’s flash has bounced off a reflective layer at the back of the eye.