On The Origin of Nocturnality.

The Philippine Tarsier's large eyes give it excellent night vision

Large mammals, like elephants or lions are active during the day, good light allowing them to feed or mate. At night they rest to regain lost energy. Small mammals, however, are typically the opposite. As the sun goes down, tall grasses and hedges rustle as rodents scurry along well used trails or emerge from the safety of their burrows in search of food.

Their large brains with enlarged ocular, auditory and olfactory processing and highly attuned senses, make them ideal for functioning at night; but their night vision is perhaps their most exceptional trait.

Snakes have the ability to see the infra-red part of the spectrum. This allows them to see their prey as glowing patches perfectly contrasted against the cool, dark night time environment. Mammals see the visible part of the spectrum only, utilizing immensely sensitive eyes to collect what little light remains in the near pitch black night. These are remarkable evolutionary designs. Now a recent discovery has shown that their nocturnal heritage extends back even further than the first mammals on Earth.

Mammals evolved from a group of reptiles known as the synapsids around 200 million years ago. Using new fossil evidence a team of researchers has shown that the synapsids themselves possessed decent night vision. 'Synapsids are most common in the fossil record between 315 million years ago and 200 million years ago. The conventional wisdom has always been that they were active during the day (or diurnal), but we never had hard evidence to say that this was definitely the case,' said Kenneth Angielczyk, a curator at The Field Museum in Chicago and leader of the study.

The black arrow indicates the position of the scleral ossicles
in Dimetrodon, a synapsid from the Permian period

Scouring museum collections in South Africa and the United States, they found that synapsids, such as Dimetrodon, had a small ring of bones around each eye known as scleral ossicles. They allow researchers to make predictions about the size and shape of different parts of the eye. By analysing the scleral ossicles and comparing them to those from snakes and birds they were able to infer the day and night-time activity patterns of the synapsid family. 'The idea of a nocturnal Dimetrodon was very surprising,' said Angielczyk, 'but it shows how little we really known about the daily lives of some of our oldest relatives.'

'This is the first time we can make informed predictions about the activity patterns of synapsids,' added Lars Schmitz, a professor of biology at Claremont McKenna, Pitzer, and Scripps Colleges near Los Angeles. 'As we discover more fossils, we can continue to test these predictions and start to address questions such as how many times nocturnality evolved in synapsids and whether the synapsids most closely related to mammals were also nocturnal.'

Mammals made their début on the evolutionary stage 200 million years ago, but only in a minor fashion thanks to the dominance on the dinosaurs, which restricted the number of ecological niches they could exploit. Their diminutive size and night vision, however, gave them a nocturnal advantage, allowing them to survive past the extinction of the dinosaurs and to become a dominant animal group. Unlocking their evolutionary heritage is vital if we are to understand their success.