Eyes that see in the dark: nocturnal dinosaurs

  • Post category:News

With new fossils being discovered, new information about dinosaurs comes to light all the time. Just today, a study reveals evidence of dinosaurs adapted to hunt in the dark. Stig Walsh, an author on this study, illuminates further.

Barn owls are
incredible birds. They can hunt in total darkness, making no sound as they
glide slowly to catch their small mammalian prey unaware of the deadly claws
and beak above. They look pretty amazing too. Those huge eyes seem to face
forward just like ours do, giving them a large degree of visual overlap between
each eye. This is called stereopsis, and it gives them an excellent ability to
judge distance.

There is no
doubt that barn owls have evolved some pretty extreme adaptations to their
nocturnal life. Their eyes are large because they have to house huge numbers of
light receptors, called ‘rods’, making their eyes as light-sensitive as a
sniper’s image-intensifying night sight. In fact, these owls’ eyes are so large
and packed with light receptors that they have become fused to their eye
sockets by a ring of bones called the scleral ossicles, found under the surface
of each eye. That’s why these owls are able to turn their heads through such an
extreme range of motion: their eyes cannot move, so their heads have to be able
to turn instead.

Photo of a barn owl in flight.
One of our barn owl (Tyto alba) specimens.

As striking as
their eyes are, the real story is about their ears. Unlike mammals, birds (and
their dinosaur ancestors) have no external ear to help funnel sound waves into
the hearing apparatus. However, the facial feathers of barn owls actually
perform much the same function. Not only that, but the skull of these owls is
asymmetrical. This helps them to judge the direction and distance of a sound
made by their prey. They can even control the opening of their outer ears to
localise sound behind them.

Inside their
skulls, their inner ears have the longest cochlear canal of any bird. This canal
houses the organ of hearing, known as the basilar papilla. The longer the
basilar papilla, the more sensitive a bird’s ear is to a wider range of hearing
frequencies. The barn owl needs this to detect the high-pitched sounds made by its
small rodent prey.

3D model of a Barn owl skull showing the scleral ring and cochlea canal. (Courtesy of University of the Witwatersrand, South Africa.)

OK, so living owls
are pretty special birds, but when did these adaptations evolve? The oldest
known fossil owls are from around 55 million years ago, and it’s very unlikely
that those early owls would have had anything like the adaptations we see in
owl species today. If I had to stick my neck out and estimate when these
adaptations did first appear, I would probably go for some point in the last 30
to 20 million years. In the evolutionary scheme of things, that’s fairly

Could any of the dinosaur ancestors of owls have had such specialised senses? If owls really only developed their sensory specialities over the last few tens of millions of years, how could dinosaurs have been anywhere near as complex?

Photo of a man holding a specimen.
Prof. Jonah Choiniere, lead author on the study, holding a 3D printed model of the lagena of Shuvuuia deserti.

Over the past
two decades or so, science has been able to learn more and more about dinosaur
vision, hearing, smell and balance through the use of X-ray micro computed
tomography (micro-CT). Micro-CT allows palaeontologists to see structures
inside a fossil skull in 3D. The target of these studies has often been the
inner ear.

Back in 2009, I
was part of a study that used micro-CT to test whether the
dimensions of the cochlear canal could be used to estimate hearing sensitivity
in extinct birds and dinosaurs. It turned out that the length of the cochlear
canal is related to this sensitivity, but when our technique has been used to
estimate hearing in dinosaurs, the results usually show dinosaurs to have had pretty
average hearing. This is what we would expect, given owl-like adaptations are
probably very recent in evolutionary terms.

A study published today in the journal Science pretty much turns this view on its head.

Photo of fossilised remains in brown substrate.
Photograph of Shuvuuia deserti (MGI 100/0977) by Mick Ellison, American Museum of Natural History.

One group of theropods
(two-legged meat-eating dinosaurs) had indeed evolved some pretty amazing
hearing abilities rivalling those of the barn owl. Alvarezsaurs were a group of
very small theropods that had unusual arms and hands. If you think the arms and
hands of Tyrannosaurus rex were inadequate with their two digits,
alvarezsaur hands go one step further in being short and having only one digit.
However, the arms appear to have been very strong and are thought to be adapted
to digging, possibly into termite mounds. Perhaps the sensitive ears of
alvarezsaurs were adapted for hearing the sounds of their tiny insect prey.

Photo of the ear canals of the dinosaur and the owl. They are white, longish shapes against black.
Side by side comparison of the cochlea canal of a barn owl and Shuvuuia deserti. (Courtesy of University of the Witwatersrand, South Africa.)

this study also found that, like owls, alvarezsaur eyes were unusual too. We
can estimate how large dinosaur eyes might have been by measuring the eye
socket. But learning anything more specific about dinosaur vision is trickier. For
one thing, a large eye might be packed with light-gathering rod-cell light
sensors for allowing the owner of the eyes to see in low light, but it might
also be packed with cone-cells that give the owner sharp colour vision. Unfortunately,
these sensory cells rarely leave a trace in fossils.

However, the
internal diameter of the sclerotic ring in alvarezsaurs is particularly large. This
shows that the pupils of their eyes could open much wider than in other
dinosaurs to let more light in. This brings us back to the exciting study
published today, that used our specimens as part of the analysed dataset. The
two pieces of evidence taken together (the large, light-gathering eye and
sensitive hearing of alvarezsaurs) must surely mean that these small theropods
were adapted to hunt in the dark.

3D model of a Shuvuuia deserti skull illustrating the cochlea canal. (Courtesy of University of the Witwatersrand, South Africa.)

This strongly
suggests that alvarezsaurs must also have been adapted to live in cooler night
conditions. We already know that most, if not all, theropods were able to
generate their own body heat, and many were insulated by feathers, just as
living birds are. We know that these small alvarezsaurs had feathers, although
we cannot be sure just how well insulated they were from fossil evidence.

This study was
very careful not to speculate too much about these dinosaurs, but this is a
blog, and I like to throw things out there in blogs. Can we deduce anything
else about alvarezsaurs from the results of this study?

Well, there is evidence that at least some theropods had iridescent feathers. In other words, that they had pigments in their feathers that made them glow in the presence of ultraviolet light, like the feathers of many bird species do today. Species of theropods that had iridescent feathers must have been able to detect reflected ultraviolet light and actually see this iridescence. In birds, this is used to attract mates and for recognising members of their own species.

Illustration of a small dinosaur with reduced forelimbs, feathers and large eyes exploring in the dark.
Artist’s reconstruction of Shuvuuia deserti by Viktor Radermaker.

Because there is very little ultraviolet light available at night, I’d hazard a guess that alvarezsaurs didn’t have iridescent feathers, and also wouldn’t have been able to see ultraviolet light. In fact, if they had eyes full of rod light sensors, they probably couldn’t detect much in the way of colour at all.

This important discovery has shed light on previously unknown dinosaur behaviour (or maybe we’re just getting better at seeing in the dark!). As we find more fossils of dinosaurs, and as the number of tools to analyse those fossils grows, who knows what more we’ll be able to find out about these amazing extinct animals in the future?